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Lorenzo ROSA

Ricercatore t.d. art. 24 c. 3 lett. A presso: Dipartimento di Ingegneria "Enzo Ferrari"


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Pubblicazioni

2021 - Fractal Graphene Patch Antennas and the THz Communications Revolution [Relazione in Atti di Convegno]
Blackledge, J M; Boretti, A; Rosa, L; Castelletto, S
abstract

Fractal antennas have and are continuing to receive attention in regard to the future of wireless communications. This is because of their wide- and multi-band capabilities, the opportunity of fractal geometries to drive multiple resonances, and, the ability to make smaller and lighter antennas with fewer components and radiative elements with higher gains. Small scale (i.e. on the micro- and nano-scale) and ultra high frequency (in the Terahertz or THz range) fractal antennas composed of Graphene have the potential to enhance wireless communications at a data rate that is unprecedented, i.e. ~ 1012 bits per second. A Fractal Graphene antenna is a high-frequency tuneable antenna for radio communications in the THz spectrum, enabling unique applications such as wireless nano-networks. This is because (mono-layer) Graphene is a one-atom-thick two-dimensional allotrope of Carbon with the highest known electrical conductivity that is currently unavailable in any other material, including metals such as Gold and Silver. Thus, combining the properties of Graphene with the self-affine characteristics of a fractal at the micro- and nano-scale, provides the potential to revolutionise communications, at least in the near field (the order of a few metres) for low power systems. In this paper, we consider the basic physics and some of the principle mathematical models associated with the development of this new disruptive technology in order to provide a guide to those engaged in current and future research, a fractal Graphene antenna being an example of an advanced material for demanding applications. This includes some example simulations on the THz field patterns generated by a fractal patch antenna composed of Graphene whose conductivity is taken to scale with the inverse of the frequency according to a 'Drude' model. The approach to generating THz sources using Graphene is also explored based on Infrared laser pumping to induce a THz photo-current.


2020 - A Preliminary Study of a Graphene Fractal Sierpinski Antenna [Relazione in Atti di Convegno]
Boretti, A.; Rosa, L.; Blackledge, J.; Castelletto, S.
abstract

We provide a preliminary study of a Graphene fractal antenna operating at THz frequencies with the opportunity to modulate the emission. There are many advantages of the fractal design, namely multiband/wideband ability, and, a smaller, lighter and simpler configuration for higher gain, that can benefit from the coupling with Graphene, the thinnest and strongest of materials exhibiting very high electrical conductivity and tunability. This paper proposes a conceptual background for the study and presents some preliminary results on the electromagnetic emission simulations undertaken.


2020 - Micro-manipulation of nanodiamonds containing NV centers for quantum applications [Articolo su rivista]
Castelletto, S.; Rosa, L.; Boretti, A.
abstract

Micro-manipulations of nanodiamonds (NDs) containing Nitrogen-Vacancy (NV) centers are here reviewed. Various methods such as optical tweezers, electro-kinetic trap, ions traps, optofluidics and plasmonics applied to the specific nanomaterial are reviewed, focusing on the advantages and achievements in controlling the NDs positioning for magnetic sensing. These approaches are relevant to extend magnetic sensing and imaging in different fluid environments. Levitated NDs in vacuum using optical tweezers or ions traps are also reviewed for applications in cavity optomechanics towards establishing ultra-sensitive mechanical spin hybrid systems or nano-optomechanics devices. The current demonstrations of trapped or levitated NDs containing NV centers are relevant to fundamental studies of quantum spin-photon and mechanical systems coupling but have not yet been applied directly to biological systems, as these fields of applications present several challenges.


2020 - Mode phase variation and sensitivity to thermal load in three-core optical fibers [Articolo su rivista]
Rosa, Lorenzo; Pallangal, Shahul Hameed; Poli, Federica; Selleri, Stefano; Cucinotta, Annamaria
abstract

The phase of modes in three-core optical fibers under high thermal load is studied by means of combined thermal and optical models. In particular, the effects of Thermal dissipation in the doped cores, as a result of quantum defect heat generation, and the relative material refractive index variation are investigated. The induced index gradient can influence the guided mode propagation in multiple adjacent cores, even if their distance is enough to decouple them optically, showing a remarkable dephasing of the different fiber output beams. This effect brought by the thermal phenomenon inherent in the amplification process has to be carefully accounted for when designing multi-core high-power optical lasing and amplifying devices. The mode dephasing results also provide useful information for future evaluation of thermo-optic coupling effects that may occur along the fiber under high power regime.


2020 - Non-idealities in hollow core inhibited coupling fibers [Relazione in Atti di Convegno]
Melli, F.; Giovanardi, F.; Rosa, L.; Vincetti, L.
abstract

Hollow Core Inhibited Coupling Fibers are experiencing an impressive reduction of their propagation loss. Despite that, there is still a difference between experimental loss and the theoretical minimum defined by confinement loss. This difference is assumed to be due to additional losses caused by geometrical irregularities of the real fibers compared to the ideal geometry, due to the fabrication process. In this work, we numerically investigate additional losses coming from several kinds of geometrical imperfections, highlighting their impact in defining fiber total loss.


2020 - Plasmonic nanopapers: flexible, stable and sensitive multiplex PUF tags for unclonable anti-counterfeiting applications [Articolo su rivista]
Cheng, Hongrui; Lu, Yongfeng; Zhu, Dongyan; Rosa, Lorenzo; Han, Fei; Ma, Ming-Guo; Su, Wenyue; Francis, Paul S; Zheng, Yuanhui
abstract

Highly flexible and stable plasmonic nanopaper comprised of silver nanocubes and cellulose nanofibres was fabricated through a self-assembly-assisted vacuum filtration method. It shows significant enhancement of the fluorescence emission with an enhancement factor of 3.6 and Raman scattering with an enhancement factor of ~104, excellent mechanical properties with tensile strength of 62.9 MPa and Young’s modulus of 690.9 ± 40 MPa, and a random distribution of Raman intensity across the whole nanopaper. The plasmonic nanopapers were encoded with multiplexed optical signals including surface plasmon resonance, fluorescence and SERS for anti-counterfeiting applications, thus increasing security levels. The surface plasmon resonance and fluorescence information are used as the first layer of security and can be easily verified by the naked eye, while the unclonable SERS mapping is used as the second layer of security and can be readily authenticated by Raman spectroscopy using a computer vision technique.


2020 - Thermo-optic effects in multicore fibers for high-power lasers [Relazione in Atti di Convegno]
Rosa, Lorenzo; Mckee, Seyyedhossein; Vincetti, Luca; Poli, Federica; Selleri, Stefano; Cucinotta, Annamaria
abstract

Multi-core fibers (MCFs) are promising solutions for high power fiber based devices as they reduce nonlinearity and other unwanted detrimental effects, like transverse mode instability, by transporting, instead of a single high power beam, several low-powered ones to be coherently combined at the fiber output. This method relies on accurate evaluation of the phase differences between signals in different cores, which are significantly impacted by changes in the effective index of the propagating modes. For this to be effective, spatial heat generation must be accounted for. In particular, the heat flux from the doped cores to the external boundary causes a temperature gradient across the fiber, which affects the refractive index distribution, creating the chance for effective index change and thus dephasing of the output beams, which is harmful for beam combining. The results of in-depth numerical analysis on the performance of 9-core and 16-core MCFs under thermal effects are presented by studying the mode phase sensitivity to heat load and by introducing a coupled-mode theory model to study possible optical coupling effects. The effectively single-mode condition is also investigated by calculating the core modal overlap differences between fundamental and higher-order modes.


2019 - A simple analytical model for confinement loss estimation in hollow-core Tube Lattice Fibers [Articolo su rivista]
Vincetti, Luca; Rosa, Lorenzo
abstract

In this work, we propose an analytical model for estimating confinement loss in Tube Lattice Fibers. It is based on the single-tube model and the inhibited coupling waveguiding mechanism. The comparison with numerical simulations of tube lattice fibers having different geometrical parameters and dielectric refractive indexes demonstrates the model validity and effectiveness. Being based only on analytical closed formulas, it constitutes a useful tool for rapid estimation of TLF CL. It also gives a more in-depth insight into the TLF guiding mechanisms, confirming the inhibited coupling is an appropriate and effective model for such kind of fibers.


2019 - Chlorination disadvantages and alternative routes for biofouling control in reverse osmosis desalination [Articolo su rivista]
Al-Abri, Mohammed; Al-Ghafri, Buthayna; Bora, Tanujjal; Dobretsov, Sergey; Dutta, Joydeep; Castelletto, Stefania; Rosa, Lorenzo; Boretti, Albert
abstract

With an ever-increasing human population, access to clean water for human use is a growing concern across the world. Seawater desalination to produce usable water is essential to meet future clean water demand. Desalination processes, such as reverse osmosis and multi-stage flash have been implemented worldwide. Reverse osmosis is the most effective technology, which uses a semipermeable membrane to produce clean water under an applied pressure. However, membrane biofouling is the main issue faced by such plants, which requires continuous cleaning or regular replacement of the membranes. Chlorination is the most commonly used disinfection process to pretreat the water to reduce biofouling. Although chlorination is widely used, it has several disadvantages, such as formation of disinfection by-products and being ineffective against some types of microbes. This review aims to discuss the adverse effect of chlorination on reverse osmosis membranes and to identify other possible alternatives of chlorination to reduce biofouling of the membranes. Reverse osmosis membrane degradation and mitigation of chlorines effects, along with newly emerging disinfection technologies, are discussed, providing insight to both academic institutions and industries for the design of improved reverse osmosis systems.


2019 - Complex permittivity measurements of hydraulic oil at UHF and microwave frequencies [Relazione in Atti di Convegno]
Lenzini, Stefano; Rosa, L.; Vincetti, L.
abstract

The results about the measurements of complex permittivity of hydraulic mineral oil is here presented. Two different oil samples were analyzed using this method: a non-used one and a sample with 1000 hours of usage. The obtained data were collected by using the cavity perturbation technique exploiting a coaxial cavity resonator. To overcame the approximations that are usually made in this technique a particular measurement procedure based on 3D-full wave electromagnetic simulations has been performed.


2019 - Guidance properties and phase shift of a 9-core fiber amplifier for high power operation in presence of consistent thermal load [Relazione in Atti di Convegno]
Molardi, Carlo; Pallangal, Shahul H.; Rosa, Lorenzo; Vincetti, Luca; Poli, Federica; Selleri, Stefano; Cucinotta, Annamaria
abstract

Multi-core fiber capability to deliver several independent beams in a single structure has been deeply investigated to obtain spatial multiplexing in optical communication. Recently, the coherent beam multiplexing idea has been extended to high power fiber laser field, where multi-core fiber amplifiers, combining low power beams, promise to overcome thermal mode instability, which characterizes single-core fiber amplifiers. Although coherent output beam combination is advantaged in multi-core fiber, the understanding of core phase shifts is necessary to implement efficient beam combination. In presence of thermal load, induced by pump-to-signal conversion quantum defect, a refractive index gradient is formed on the multi-core fiber amplifier cross-section, thus changing core propagation properties and possibly creating unwanted core couplings. In this work a 9-core double-cladding fiber amplifier is numerically investigated by varying the core thermal load, from 2 to 15 W/m, in order to understand the structure propagation mismatch. The 9 cores are organized in a 3×3 regular grid, each core has a diameter of 19 μm and a spacing of 55 μm. Cores numerical aperture is 0.06. The outer cladding has a diameter of 340 μm. A comparison between a rod-type fiber amplifier configuration and a flexible fiber amplifier has been performed. Results show that the cores can be divided in three groups according to their propagation properties: central core, side cores, and corner ones. The phase shift between these groups, or equivalently the effective index difference, becomes higher with the increase of thermal load. These observations are fundamental to implement a model for beam propagation in presence of thermal effect, to investigate the amplification dynamics along z-direction.


2019 - Guidance properties and thermal effects in 9-core Yb-doped fiber for high power applications [Relazione in Atti di Convegno]
McKee, S.; Poli, F.; Selleri, S.; Cucinotta, A.; Rosa, L.; Vincetti, L.
abstract

In this abstract, the performance of optically uncoupled cores in 9-core multi core fibers for high-power fiber lasers is evaluated by considering thermal effects due to the heat load originated by quantum defect between pump and laser radiation. For high power applications, thermal effects are currently seen as the main constraint to power scaling of fiber lasers. The finite-element method is employed to model the optical field distribution for optical amplification and lasing, in order to analyze the effect of heat load on the modes of ytterbium-doped multi-core fibers.


2019 - Hollow Core Inhibited Coupling Fibers for Biological Optical Sensing [Articolo su rivista]
Giovanardi, Fabio; Cucinotta, Annamaria; Rozzi, Andrea; Corradini, Roberto; Benabid, Fetah; Rosa, Lorenzo; Vincetti, Luca
abstract

In this work, we report how tube lattice hollow-core fibers can be successfully used to build sensors for molecule detection. The inner silica surface of the fiber is functionalized and coated with a probe layer, which permits to bond only with a particular molecule (the target). When the fiber is infiltrated with a solution containing the target an additional layer is crated on the silica surface causing a redshift of the fiber transmission spectrum. The technique does not require any additional transducer component such as Bragg gratings, amplifying techniques such as nano-particles nor coherent sources. It simply consists of the measurement of the transmission spectrum of a piece of fiber some tens centimeters long. The principle is validated with experimental results showing the detection of streptavidin protein. A solution containing streptavidin was flowed trough the hollow core of the fiber coated with biotine. The measurement of the transmitted spectrum before and after the infiltration showed the presence of a some nanometer thick bio-layer.


2019 - Nitrogen-vacancy centers in diamond for nanoscale magnetic resonance imaging applications [Articolo su rivista]
Boretti, A.; Rosa, L.; Blackledge, J.; Castelletto, S.
abstract

The nitrogen-vacancy (NV) center is a point defect in diamond with unique properties for use in ultra-sensitive, high-resolution magnetometry. One of the most interesting and challenging applications is nanoscale magnetic resonance imaging (nano-MRI). While many review papers have covered other NV centers in diamond applications, there is no survey targeting the specific development of nano-MRI devices based on NV centers in diamond. Several different nano-MRI methods based on NV centers have been proposed with the goal of improving the spatial and temporal resolution, but without any coordinated effort. After summarizing the main NV magnetic imaging methods, this review presents a survey of the latest advances in NV center nano-MRI.


2019 - Phase controlled SERS enhancement [Articolo su rivista]
Zheng, Yuanhui; Rosa, Lorenzo; Thai, Thibaut; Ng, Soon Hock; Juodkazis, Saulius; Bach, Udo
abstract

Surface-enhanced Raman spectroscopy (SERS) has attracted increasing interest for chemical and biochemical sensing. Several studies have shown that SERS intensities are significantly increased when an optical interference substrate composed of a dielectric spacer and a reflector is used as a supporting substrate. However, the origin of this additional enhancement has not been systematically studied. In this paper, high sensitivity SERS substrates composed of self-assembled core-satellite nanostructures and silica-coated silicon interference layers have been developed. Their SERS enhancement is shown to be a function of the thickness of silica spacer on a more reflective silicon substrate. Finite difference time domain modeling is presented to show that the SERS enhancement is due to a spacer contribution via a sign change of the reflection coefficients at the interfaces. The magnitude of the local-field enhancement is defined by the interference of light reflected from the silica-air and silica-silicon interfaces, which constructively added at the hot spots providing a possibility to maximize intensity in the nanogaps between the self-assembled nanoparticles by changing the thickness of silica layer. The core-satellite assemblies on a 135 nm silica-coated silicon substrate exhibit a SERS activity of approximately 13 times higher than the glass substrate.


2019 - Phasing and guidance properties of multi-core fibers under heat load [Relazione in Atti di Convegno]
Rosa, L.; Mckee, S.; Molardi, C.; Poli, F.; Selleri, S.; Vincetti, L.; Cucinotta, A.
abstract

Multi-core fibers are a technology currently at the forefront of research due to the increasing need for efficient transport of high-power continuous and pulsed laser beams [1], Power scaling is gained by propagating independent beams through spatial multiplexing in different cores to avoid thermal instability issues [2], shifting the traditional trade-off involved in enlarging the mode area while preserving single-mode operation. Phase difference compensation between the various beams is achieved by a piezo mirror system, and experimental results show that onset of multi-modality in heated cores is one of the main causes of efficiency reduction [2].


2019 - Reassessing the projections of the World Water Development Report [Articolo su rivista]
Boretti, Alberto; Rosa, Lorenzo
abstract

The 2018 edition of the United Nations World Water Development Report stated that nearly 6 billion peoples will suffer from clean water scarcity by 2050. This is the result of increasing demand for water, reduction of water resources, and increasing pollution of water, driven by dramatic population and economic growth. It is suggested that this number may be an underestimation, and scarcity of clean water by 2050 may be worse as the effects of the three drivers of water scarcity, as well as of unequal growth, accessibility and needs, are underrated. While the report promotes the spontaneous adoption of nature-based-solutions within an unconstrained population and economic expansion, there is an urgent need to regulate demography and economy, while enforcing clear rules to limit pollution, preserve aquifers and save water, equally applying everywhere. The aim of this paper is to highlight the inter-linkage in between population and economic growth and water demand, resources and pollution, that ultimately drive water scarcity, and the relevance of these aspects in local, rather than global, perspective, with a view to stimulating debate.


2018 - Analytical estimation of confinement loss in tube lattice fibers [Relazione in Atti di Convegno]
Rosa, Lorenzo; Vincetti, Luca
abstract

In this work, we propose an analytical formula for estimating confinement loss in Tube Lattice Fibers. The formula is based on a single-tube model and the comparison with numerical simulations of three TLFs designed to work in the UV, NIR, and THz spectral ranges shows good agreement.


2018 - Thermally-driven mode coupling in multi-core optical fibers [Relazione in Atti di Convegno]
Rosa, L.; Mckee, H.; Poli, F.; Selleri, S.; Vincetti, L.; Cucinotta, A.
abstract

We present results on multi-core fibers (MCFs) for optical amplification/lasing obtained by combined thermal-optical finite-element method (FEM) simulations. Refractive index variation due to quantum defect caused heating is simulated to evaluate thermal dissipationinduced index gradients.


2017 - Advances in diamond nanofabrication for ultrasensitive devices [Articolo su rivista]
Castelletto, Stefania; Rosa, Lorenzo; Blackledge, Jonathan; Al Abri, Mohammed Zaher; Boretti, Albert
abstract

This paper reviews some of the major recent advances in single-crystal diamond nanofabrication and its impact in nano- and micro-mechanical, nanophotonics and optomechanical components. These constituents of integrated devices incorporating specific dopants in the material provide the capacity to enhance the sensitivity in detecting mass and forces as well as magnetic field down to quantum mechanical limits and will lead pioneering innovations in ultrasensitive sensing and precision measurements in the realm of the medical sciences, quantum sciences and related technologies.


2017 - Cup-Shaped Nanoantenna Arrays for Zeptoliter Volume Biochemistry and Plasmonic Sensing in the Visible Wavelength Range [Articolo su rivista]
Drevinskas, Rokas; Rakickas, Tomas; Selskis, Algirdas; Rosa, Lorenzo; Valiokas, Ramunas
abstract

Although three-dimensional shaping of metallic nanostructures is an important strategy for control and manipulation of localized surface plasmon resonance (LSPR), its implementation in high-throughput, on-chip fabrication of plasmonic devices remains challenging. Here, we demonstrate nanocontact-based large-area fabrication of a novel, LSPR-active Au architecture consisting of periodic arrays of reduced-symmetry nanoantennas having sub-50 nm, out-of-plane features. Namely, by combining nanosphere and molecular self-assembly processes, we have patterned evaporated polycrystalline Au films for chemical etching of nanocups with controlled aspect ratios (outer diameter d = 100 nm and void volumes = 18 or 39 zL). The resulting nanoantennas were highly ordered, forming a hexagonal lattice structure over centimeter-sized glass substrates, and they displayed characteristic LSPR absorption in the visible/near-infrared spectral range. Theoretical simulations indicated electric field confinement and enhancement patterns located not only around the rims but also inside the nanocups. We also explored how these patterns and the overall spectral characteristics depended on the nanocup aspect ratio as well as on electric field coupling in the arrays. We have successfully tested the fabricated architecture for detection of stepwise immobilization and interactions of proteins, thus demonstrating its potential for both nanoscopic scaffolding and sensing of biomolecular assemblies.


2017 - Gain competition in Yb-doped symmetry-free photonic crystal fibers under severe heat load [Relazione in Atti di Convegno]
Poli, Federica; Molardi, Carlo; Rosa, Lorenzo; Cucinotta, Annamaria; Selleri, Stefano
abstract

In the last few years Yb-doped Photonic Crystal Fibers (PCFs) have become the key component for the development of reliable and high-performance lasers [1]. Despite an effective cooling of the fiber medium, a significant heat density is generated when high pump power is involved, which alters the propagation of the modes, causing unwanted coupling and destroying the output beam quality [2]. Symmetry-Free PCFs (SF-PCFs), characterized by the absence of any mirror symmetry in the inner cladding, have shown interesting properties in terms of resilience to thermal effects [3]. In fact SF-PCFs provide a strong delocalization of the Higher-Order Mode (HOM) even under severe heat load, thus preventing its thermally-driven reconfinement in the core. As an unwanted side effect, a poor confinement of the Fundamental Mode (FM), due to its coupling with cladding modes, is obtained in the absence of thermal effects, which can negatively affect the fiber single-mode regime and potentially compromise the effectiveness of the amplification process.In this work the amplification properties of air-silica Yb-doped SF-PCFs have been analyzed with a simulation tool which comprises a FEM-based full-vector modal solver, to calculate the propagating mode fields, an amplifier model, which accounts for the power evolution, and a thermal model, to obtain the temperature distribution and the consequent refractive index change in the fiber cross-section [4]. The 19-cell core SF-PCF considered in this analysis, whose cross-section is shown in the inset of Fig. 1(a), is built over a triangular lattice with pitch Λ = 15 μm and normalized air-hole diameter d/Λ = 0.5. The interaction between FM and first HOM at 1032 nm, induced by both thermal effects and amplification process, has been studied for fibers with inner cladding diameter dIC between 15Λ and 19Λ, while the air-cladding thickness is fixed at 7 μm. A co-propagating pumping scheme, with 400 W of pump power at 976 nm, has been considered for the 1-m long fiber amplifier with a pump absorption of 27 dB/m. The input power for the FM and the first HOM, defined as the LP11 mode or the LP11-like mode with the highest core overlap integral Γ value at a certain heat density Q0, is 5 W and 50 mW, respectively.


2017 - Improved performances of photonic crystal fibers for high power laser operation [Relazione in Atti di Convegno]
Molardi, Carlo; Poli, Federica; Selleri, Stefano; Cucinotta, Annamaria; Rosa, Lorenzo
abstract

In recent years fiber laser systems have shown a rapid evolution in terms of beam quality and power. High pulse energy and peak power require large effective area, so new photonic crystal fiber designs have been proposed. Thermo-optical effects can lead to transverse mode instability, a nonlinear effect that sets in at a threshold power level. The influence of thermal effects on fiber guiding properties is here investigated. Improved fiber designs are proposed.


2017 - Inner cladding influence on amplification properties in fiber amplifiers based on symmetry free design [Relazione in Atti di Convegno]
Molardi, C.; Poli, F.; Tragni, K.; Rosa, L.; Petracek, J.; Selleri, S.; Cucinotta, A.
abstract

Yb-doped Symmetry-Free PCF based amplifier has been numerically characterized toinvestigate amplification properties and modal content under severe heat load. A custom software,providing amplification and thermal model, has been used for this analysis.


2017 - Mode discrimination criterion for effective differential amplification in Yb-doped fiber design for high power operation [Articolo su rivista]
Molardi, C.; Poli, F.; Rosa, L.; Selleri, S.; Cucinotta, A.
abstract

The mode discrimination criterion for single mode operation, usually considered in fiber amplifiers designed for high power operation, has been investigated and tested on three di erent fiber designs, a large pitch fiber and two symmetry free photonic crystal fibers. To have a significant collection of results, parameters like pump configuration, pump power, and amplifier length have been varied. The analysis has been carried out through the use of a custom numerical tool provided with e cient thermal and spatial amplification models. From the obtained results, it is possible to observe that the mode discrimination criterion is helpful but not strictly necessary to pledge an e ective single mode operation through di erential amplification. This fact extends the possibility for the study, as well as for the optimization, of di erent fiber designs. The use of advanced numerical analysis, which takes into consideration amplification along with thermally influenced modes guidance, becomes extremely useful for an e ective fiber design.


2017 - Nano-Magnetic Resonance Imaging (Nano-MRI) Gives Personalized Medicine a New Perspective [Articolo su rivista]
Rosa, Lorenzo; Blackledge, Jonathan; Boretti, Albert
abstract

This paper reviews some of the major and most recent advances in nanoscale-magnetic resonance imaging (nano-MRI) for personalized medicine (PM). Nano-MRI may drastically expand the capabilities of the traditional magnetic resonance images (MRI), down to the nanometer scale and possibly, in the near future, at the atomic scale. Nano-MRI is potentially able to observe structures which cannot be seen using today's molecular imaging, with sensitivities of many billions of times better than MRI as currently used in hospitals, for example. The paper briefly reports on the foremost research themes in nano-MRI.


2017 - Size-controlled gold nanoparticles on octahedral anatase particles as efficient plasmonic photocatalyst [Articolo su rivista]
Wei, Zhishun; Rosa, Lorenzo; Wang, Kunlei; Endo, Maya; Juodkazis, Saulius; Ohtani, Bunsho; Kowalska, Ewa
abstract

Octahedral anatase particles (OAPs), prepared by ultrasonication-hydrothermal reaction (US-HT), were modified with 2 wt% of gold by photodeposition. Conditions of US-HT process such as durations of US and durations of HT were varied to obtain OAPs products different by physicochemical and morphological properties. Au/OAPs samples were characterized by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). The photocatalytic activity was tested under UV irradiation for decomposition of acetic acid (CO2 system) and dehydrogenation of methanol (H2 system) under aerobic and anaerobic conditions, respectively, and for oxidation of 2-propanol under visible light irradiation. Photodeposition of gold was very fast for all OAPs samples (0.5–10 min) under Ar atmosphere, and the clear correlation between the content of electron traps (ETs) and the induction period, during which nanoparticles (NPs) of gold are formed, indicates that ETs in titania samples are a key-factor for rapidity of gold photodeposition on titania surface. It was found that better morphology of titania (larger content of faceted particles) resulted in formation of larger gold NPs, while small gold NPs were deposited on structural defects. Modification of OAPs with gold NPs resulted in significant enhancement of photocatalytic activity, being e.g., 1.5 (CO2 system), 7.7 (H2 system), and even more than 40 under vis irradiation. It was found that both the properties of titania and gold are crucial for resultant photocatalytic activity, but a direct correlation between one structural/physical property and photocatalytic activity could not be obtained since all structural properties changed simultaneously when conditions of photocatalyst preparation (US-HT) were changed. Therefore, gold NPs of controlled sizes were deposited on OAPs product with the best morphology by modified photodeposition method. Clear correlation between photocatalytic activity under visible light and the size of gold NPs indicates that gold properties are decisive for visible light activity rather than titania properties. 3D-FDTD simulations confirm that an increase in the size of gold NPs results in extended surface areas with field enhancement.


2017 - Thermal induced dynamics of gain competition in Yb-doped Symmetry-Free Photonic Crystal Fibers [Relazione in Atti di Convegno]
Tragni, Katia; Molardi, Carlo; Poli, Federica; Rosa, Lorenzo; Cucinotta, Annamaria; Selleri, Stefano
abstract

A fiber amplifier based on Yb-doped Symmetry-Free Photonic Crystal Fiber has been studied through the use of a numerical tool. The amplification properties and the modal content of the amplifier have been investigated in conditions of severe heat load, in order to understand the effectiveness of amplification dynamics. The numerical tool, used for this study, is based on a FEM modal solver combined with a model which allows to evaluate amplification and thermal effects. Results depict a scenario where it is possible to obtain a good mode discrimination and a robust amplification under critical thermal condition.


2016 - Femtosecond laser-induced hard X-ray generation in air from a solution flow of Au nano-sphere suspension using an automatic positioning system [Articolo su rivista]
Hsu, W. H.; Masim, F. C. P.; Porta, M.; Nguyen, M. T.; Yonezawa, T.; Balčytis, A.; Wang, X.; Rosa, Lorenzo; Juodkazis, S.; Hatanaka, K.
abstract

Femtosecond laser-induced hard X-ray generation in air from a 100-μm-thick solution film of distilled water or Au nano-sphere suspension was carried out by using a newly-developed automatic positioning system with 1-μm precision. By positioning the solution film for the highest X-ray intensity, the optimum position shifted upstream as the laser power increased due to breakdown. Optimized positioning allowed us to control X-ray intensity with high fidelity. X-ray generation from Au nano-sphere suspension and distilled water showed different power scaling. Linear and nonlinear absorption mechanism are analyzed together with numerical modeling of light delivery.


2016 - Full-vector modeling of thermally-driven gain competition in Yb-doped reduced symmetry photonic-crystal fiber [Articolo su rivista]
Rosa, Lorenzo; Coscelli, Enrico; Poli, Federica; Cucinotta, Annamaria; Selleri, Stefano
abstract

A new model to study gain competition in high-power fiber amplifiers, which accounts for the full-vector nature of the propagating modes and thermal effects on core overlap, is applied to simulate the performance of an active rod-type fiber under severe heat load. Model tests on a Yb-doped large-pitch reduced symmetry photonic-crystal fiber show a successful analysis of gain competition.


2016 - Latest Advances in the Generation of Single Photons in Silicon Carbide [Articolo su rivista]
Boretti, Albert; Rosa, Lorenzo
abstract

The major barrier for optical quantum information technologies is the absence of reliable single photons sources providing non-classical light states on demand which can be easily and reliably integrated with standard processing protocols for quantum device fabrication. New methods of generation at room temperature of single photons are therefore needed. Heralded single photon sources are presently being sought based on different methods built on different materials. Silicon Carbide (SiC) has the potentials to serve as the preferred material for quantum applications. Here, we review the latest advances in single photon generation at room temperatures based on SiC.


2016 - Modelling of thermal effects and gain competition in Yb-doped large mode area photonic crystal fibers [Relazione in Atti di Convegno]
Coscelli, Enrico; Poli, Federica; Rosa, Lorenzo; Cucinotta, Annamaria; Selleri, Stefano
abstract

In the last few years Yb-doped double cladding fibers have become the key component for the development of reliable and high-performance lasers. Despite an effective cooling of the fiber medium, a significant heat load is generated when high pump power is involved, which alters the mode propagation characteristics, causing unwanted coupling among the modes and destroying the output beam quality. This work presents a new tool for the analysis of the amplification and modal properties of Yb-doped double-cladding fibers, which comprises a full-vector modal solver, based on the finite-element method, an amplifier model and a thermal one. Simulation results, shown for two large pitch fiber designs, both in co-propagating and counter-propagating pumping schemes, have demonstrated the influence of the generated heat load on the overlap integral and on the power evolution of the guided modes.


2016 - Thermal effects and gain competition in Yb-doped large mode area fibers for high-power applications [Relazione in Atti di Convegno]
Poli, Federica; Coscelli, Enrico; Rosa, Lorenzo; Cucinotta, Annamaria; Selleri, Stefano
abstract

Currently, the increase of output power of Yb-doped large mode area photonic crystal fiber lasers, fundamental for many practical industrial applications, is mainly limited by thermal effects, which negatively affect the beam properties. In this work the influence of the inner cladding characteristics on the guiding properties of Yb-doped symmetry-free photonic crystal fibers in different heating conditions has been investigated using simulation tools based on the finite element method. Simulation results have shown that, by properly modifying the inner cladding characteristics, it is possible to facilitate the coupling of the guided modes with the cladding ones, thus affecting their confinement in different heating regime.


2015 - Asymmetric gold nanodimer arrays: electrostatic self-assembly and SERS activity [Articolo su rivista]
Zheng, Yuanhui; Rosa, Lorenzo; Thai, Thibaut; Ng Soon, Hock; Gomez, Daniel E.; Ohshima, Hiroyuki; Bach, Udo
abstract

A simple, scalable, low-cost and high-throughput nanofabrication method is developed to produce discrete gold nanoparticle (AuNP) dimer arrays. This method involves a two-step electrostatic self-assembly: (1) electrostatic immobilization of negatively charged AuNPs onto a positively charged surface and (2) electrostatic adsorption of a positively charged AuNP onto each pre-assembled AuNP. The latter requires a careful control of the electrostatic energy barrier originating from the interactions between the charged AuNPs and surfaces. This can readily be achieved by tuning the ionic strength of the self-assembly media. We calculate the interaction energies for immobilizing a single positively charged AuNP onto each pre-assembled NP at different ionic strengths and present successful experimental results on the synthesis of high-yield symmetric and asymmetric AuNP dimers (dimer yield: similar to 85%). A theoretical and experimental investigation of their optical properties is conducted to correlate the spectral properties of these dimers with their structure. We also study the SERS activity of the as-synthesized AuNP dimers using benzenethiol as a model analyte. It is found that, with the increase of the size dissimilarity between the two NPs in the dimers, the Raman intensities of the analyte increase gradually. This trend is completely different from those of both single AuNPs and AuNP aggregates with identical particle size.


2015 - Electrically Driven Quantum Light Sources [Articolo su rivista]
Boretti, A.; Rosa, Lorenzo; Mackie, A.; Castelletto, S.
abstract

Typical applications of quantum light require optical sources which generate either individual photons or entangled (correlated) photons. For the sake of practicality and scalability, these quantum sources should be easily produced, operate at room temperature, and be electrically excited and controlled. Here, recent research on quantum sources obtained from electrically driven (ED) devices constructed from p-n junctions integrated in planar optical cavities, micropillars, nanowires, photonic crystals, and active plasmonic elements is reviewed. Single-photon and entangled-photon sources are distinguished by their different roles in the development of either quantum cryptography or quantum computing protocols, and the different types of devices used to produce them are highlighted, with a focus on their spectral emission, brightness, and conditions of operation. Achievements to date are summarized and compared with prerequisites for the practical use of these sources. Important recent results that could provide future novel quantum sources are also in focus, where more practical requirements could be addressed by the judicious engineering of materials and careful device design.


2015 - Novel plasmonic materials to improve thin film solar cells efficiency [Relazione in Atti di Convegno]
Saiprasad, N.; Boretti, A.; Rosa, Lorenzo; Castelletto, S.
abstract

Thin film solar cells have been widely studied with the focus on how to improve light trapping mechanism and enhance the overall photon-electrons conversion efficiency. The effect of novel plasmonic materials based on wide bandgap semiconductors, such as metals heavily doped zinc oxides and metal nitrides, are here studied in relation to their potential use in thin film solar cells. Here, we show that metal nitrides can play similar roles as gold nanoparticles on a surface of a Si-thin film solar cell, possibly without introducing excessive dissipative absorption, while metals doped zinc oxide nanoparticles could significantly improve the efficiency of thin film solar cells.


2015 - Reversible gating of smart plasmonic molecular traps using thermoresponsive polymers for single-molecule detection [Articolo su rivista]
Zheng, Y.; Soeriyadi, A. H.; Rosa, Lorenzo; Ng, S. H.; Bach, U.; Gooding J., Justin
abstract

Single-molecule surface-enhanced Raman spectroscopy (SERS) has attracted increasing interest for chemical and biochemical sensing. Many conventional substrates have a broad distribution of SERS enhancements, which compromise reproducibility and result in slow response times for single-molecule detection. Here we report a smart plasmonic sensor that can reversibly trap a single molecule at hotspots for rapid single-molecule detection. The sensor was fabricated through electrostatic self-assembly of gold nanoparticles onto a gold/silica-coated silicon substrate, producing a high yield of uniformly distributed hotspots on the surface. The hotspots were isolated with a monolayer of a thermoresponsive polymer (poly(N-isopropylacrylamide)), which act as gates for molecular trapping at the hotspots. The sensor shows not only a good SERS reproducibility but also a capability to repetitively trap and release molecules for single-molecular sensing. The single-molecule sensitivity is experimentally verified using SERS spectral blinking and bianalyte methods.


2015 - Room temperature Single Photon Sources in Silicon Carbide [Relazione in Atti di Convegno]
Castelletto, Stefania; Lohrmann, Alex; Rosa, Lorenzo; Johnson, Brett C.; Ohshima, Takeshi; Gali, Adam
abstract

Defects are common in many materials and some were regarded for years as detrimental. Recently with the advent of ultra-sensitive detectors, quantum optical single spin magnetic resonance protocols and advanced material synthesis and doping, diamond intra-band gap defects and their nanostructured counterpart are revealed to be a disruptive discovery for the future of nanoscale sensing and quantum technologies[1]. We will report on recently studied optical centres in another wide-band gap semiconductor, such as silicon carbide (SiC). SiC harbors paramagnetic defects whose quantum properties were recently unraveled [2-7]. As occurred for similar diamond point defects[1], we have recently identified a bright single photon emission in 4H-SiC. We will show more recent results on single defects SiC nanoparticles [8], nanotetrapods [9] and other SiC quantum emission [10] providing novel information on their physics and atomistic structure. The fundamental understanding of these defects is essential for their engineering and deployment in next generation multifunctional sensors and in quantum nano- photonics. We will discuss the potential designs of photonics cavities to further enhance some of these single photon sources.


2015 - Silicon Carbide for Novel Quantum Technology Devices [Capitolo/Saggio]
Castelletto, Stefania; Rosa, Lorenzo; Johnson, Brett C.
abstract

Silicon carbide (SiC) has recently been investigated as an alternative material to host deep optically active defects suitable for optical and spin quantum bits. This material presents a unique opportunity to realise more advanced quantum-based devices and sensors than currently possible. We will summarise key results revealing the role that defects have played in enabling optical and spin quantum measurements in this material such as single photon emission and optical spin control. The great advantage of SiC lies in its existing and well-developed device processing protocols and the possibilities to integrate these defects in a straightforward manner. There is particular current interest in nanomaterials and nanophotonics in SiC that could, once realised, introduce a new platform for quantum nanophotonics and in general for photonics. We will summarise SiC nanostructures exhibiting optical emission due to multiple polytypic bandgap engineering and deep defects. The combination of nanostructures and in-built paramagnetic defects in SiC could pave the way for future single-particle and single-defect quantum devices and related biomedical sensors with single-molecule sensitivity. We will review relevant classical devices in SiC (photonics crystal cavities, microdiscs) integrated with intrinsic defects. Finally, we will provide an outlook on future sensors that could arise from the integration of paramagnetic defects in SiC nanostructures and devices.


2015 - Thermal modeling of gain competition in Yb-doped large-mode-area photonic-crystal fiber amplifier [Articolo su rivista]
Rosa, Lorenzo; Coscelli, Enrico; Poli, Federica; Cucinotta, Annamaria; Selleri, Stefano
abstract

We propose a new model for gain competition effects in high-power fiber amplifiers, which accounts for the thermal effects of heat load on the doped core overlap of the propagating light field. The full-vectorial nature of the fiber modes is modeled by an embedded finite-element method modal solver, and the temperature profile is calculated by a simple and efficient radial heat propagation solver. The model is applied to a Yb3+-doped LPF45 air-clad photonic-crystal fiber amplifier for coand counter-propagating pumping setups, showing gain competition in conditions of severe heat load.


2015 - Towards Single Biomolecule Imaging via Optical Nanoscale Magnetic Resonance Imaging [Articolo su rivista]
Boretti, A.; Rosa, Lorenzo; Castelletto, S.
abstract

Nuclear magnetic resonance (NMR) spectroscopy is a physical marvel in which electromagnetic radiation is charged and discharged by nuclei in a magnetic field. In conventional NMR, the specific nuclei resonance frequency depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms. NMR is routinely utilized in clinical tests by converting nuclear spectroscopy in magnetic resonance imaging (MRI) and providing 3D, noninvasive biological imaging. While this technique has revolutionized biomedical science, measuring the magnetic resonance spectrum of single biomolecules is still an intangible aspiration, due to MRI resolution being limited to tens of micrometers. MRI and NMR have, however, recently greatly advanced, with many breakthroughs in nano-NMR and nano-MRI spurred by using spin sensors based on an atomic impurities in diamond. These techniques rely on magnetic dipole-dipole interactions rather than inductive detection. Here, novel nano-MRI methods based on nitrogen vacancy centers in diamond are highlighted, that provide a solution to the imaging of single biomolecules with nanoscale resolution in-vivo and in ambient conditions. Recent measurements with near surface NV centers in diamonds have allowed high resolution nano-MRI in ambient conditions. This brings us closer to the visualization of the full 3D morphology of biomolecules. The image shows a confocal modality to sense a small ensemble of nuclear spins at nanometric distance from an NV center sensor.


2014 - Engineering gold alloys for plasmonics [Articolo su rivista]
Nishijima, Yoshiaki; Hashimoto, Yoshikazu; Seniutinas, Gediminas; Rosa, Lorenzo; Juodkazis, Saulius
abstract

We demonstrate the formation of metal alloys in Au-Ag, Au-Cu and Au-Pd systems and the experimental determination of their optical properties using optical transmission and reflection spectroscopy. The optical constants define the plasmon resonance wavelength and electromagnetic field local intensity. However, the optical constants behavior cannot be precisely modeled based on the data of pure metals due to unknown morphology and composition of the alloy. It has to be determined experimentally. We demonstrate the surface-enhanced Raman scattering using alloy metals. Depending on the metal to which molecules are adsorbed, we observe enhancement of different Raman modes. It is mainly due to the chemical enhancement effect between metal and molecules.


2014 - Long-range interaction of localized surface plasmons in periodic and random patterns of Au nanoparticles [Articolo su rivista]
Nishijima, Yoshiaki; Rosa, Lorenzo; Juodkazis, Saulius
abstract

The plasmon resonance of periodic and random arrays of Au-nanodiscs and nano-squares was investigated experimentally and numerically. By randomization, plasmon resonance spectra become broadened, however, the electro-magnetic field enhancement is augmented by a factor of 10-10 times at some specific wavelength as shown by the finite-element time-domain calculations. The spatial localization takes place at a fewer hot-spots. The randomized structures are promising for the applications in opto-electronics, sensing and light harvesting.


2014 - Mono- and bi-metallic plasmonic photocatalysts for degradation of organic compounds under UV and visible light irradiation [Articolo su rivista]
Kowalska, E.; Janczarek, M.; Rosa, Lorenzo; Juodkazis, S.; Ohtani, B.
abstract

Titania powders were surface modified with gold and/or silver nanoparticles (NPs) by photodeposition method. Gold modified titania exhibited much higher photocatalytic activity during methanol dehydrogenation under UV irradiation than titania modified with monometallic silver and bimetallic Au-Ag NPs. Bimetallic photocatalysts exhibited either enhanced or reduced visible light activity, depending on properties of noble metal NPs, sequence of their deposition on titania, and properties of host titania. Though numerical simulation (3D-FDTD) showed strong plasmonic field enhancement at the interface between titania and bimetallic core(Au)-shell(Ag) NP, it is thought that co-deposition caused an enhancement of charge carriers recombination (by electron sinking in nearby second metal) inhibiting photoactivity under visible light irradiation. The highest level of photoactivity showed large rutile NPs with successively deposited metals, mainly in the form of individual monometallic NPs, due to size/shape polydispersity of deposited NPs, and thus with ability of light absorption in a broad wavelength range.


2014 - Optical constants of gold-silver-copper alloy system [Relazione in Atti di Convegno]
Hashimoto, Y.; Nishijima, Y.; Seniutinas, G.; Rosa, Lorenzo; Juodkazis, S.
abstract

Plasmon resonance using gold – silver – copper alloy system is promising materials in the field of enhanced fluorescence, surface enhanced Raman scattering (SERS) and so on. The resonance wavelength and electromagnetic field enhancement of plasmon resonance depends on the permittivity of the metal. We have demonstrated that the optical constants of gold – silver alloy system become different features from the linear combination of each optical constant [1], and revealed the importance of experimentally determination of optical constants. In this study, we determined the optical constants of gold-silver, gold-copper, silver-copper alloy system and explained the relationship between the optical constants and the Drude parameters (ωp; plasma frequency and τ relaxation time).


2014 - Scaling rules for surface enhanced Raman scattering [Relazione in Atti di Convegno]
Nishijima, Y.; Hashimoto, Y.; Khurgin, J. B.; Rosa, Lorenzo; Juodkazis, S.
abstract

An intricate relationship between the intensity of surface-enhanced Raman scattering (SERS) and the optical extinction are revealed. The observed unusual trend of SERS intensity decrease with the increase of extinction is explained analytically and numerically.


2014 - Scaling Rules of SERS Intensity [Articolo su rivista]
Nishijima, Yoshiaki; Hashimoto, Yoshikazu; Rosa, Lorenzo; Khurgin Jacob, B.; Juodkazis, Saulius
abstract

An intricate relationship is revealed between the intensity of surface-enhanced Raman scattering (SERS) and the optical extinction. The unusual trends of SERS intensity decrease while the extinction increases observed in experiments is fully explained for the first time. The SERS intensity is well known to be strongly correlated with the extinction at excitation and Stokes wavelengths, but the dependence is more involved than the simple product of the two extinction enhancements. The radiative and nonradiative losses, different for different Stokes wavelengths, strongly affect the relationship between SERS and extinction. Dye molecules are shown experimentally and numerically to experience different local field enhancements for the different Raman modes, and a simple qualitative analytical description of this universal phenomenon is provided here.


2014 - SERS scaling rules [Articolo su rivista]
Nishijima, Yoshiaki; Hashimoto, Yoshikazu; Rosa, Lorenzo; Khurgin Jacob, B.; Juodkazis, Saulius
abstract

We demonstrate the qualitative analysis of surface-enhanced Raman scattering (SERS) intensity and optical extinction by experimentally and numerically. This analytical methods are well matched not only the simple square lattice array of nanostructures, but also the rectangular lattices. We also demonstrate SERS selectivity of modes controlling the optical extinction of excitation and scattering wavelength. Both square lattice and rectangular lattice have similar tendency, but the rectangular lattice structures have much higher selectivity of SERS modes.


2014 - Simulation and Measurement of Solar Harvesting Enhancement of Silver Plasmonic Nanoparticles on GaSb Nanodots [Articolo su rivista]
Rosa, Lorenzo; Ranjan, Mukesh; Bhatnagar, Mukul; Mortazavi, Daryoush; Mukherjee, Subroto; Juodkazis, Saulius
abstract

The performance of a plasmonic antireflection layer which can be utilized for deep-space radiationresistant GaSb solar cells is investigated numerically and experimentally. The layer consists of nanodots made by plasma etching of a GaSb substrate and subsequent physical vapor deposition of Ag nanoparticles on the nanodot tips, in a partially ordered configuration determined by the plasma energy level. This technique is readily applicable to patterning of silicon. We measure the substrate reflectivity and model the reflection and absorption of the substrates using the 3D finite difference time domain (FDTD) method, which are realistically imported as 3D layers from the scanning electron microscopy (SEM) images. The variation of the height of the Ag nanoparticles on top of the GaSb pillars shows that the plasmonic effect remarkably enhances the absorption. The presence of GaSb pillars enhances absorption and tunes the maximum absorption wavelength peak.


2014 - Tunable Raman Selectivity via Randomization of a Rectangular Pattern of Nanodisks [Articolo su rivista]
Nishijima, Yoshiaki; Khurgin Jacob, B.; Rosa, Lorenzo; Fujiwara, Hideki; Juodkazis, Saulius
abstract

We show that randomization of a nanodisk array on a rectangular lattice can be used to spectrally tune the surface-enhanced Raman scattering (SERS) intensity and to control the field enhancement. The spectral selectivity of the Raman modes exhibits a nonlinear dependence on the extinction. Randomization of the rectangular pattern brings a favorable increase of SERS intensity via a reduced selectivity and extinction changes at the Stokes band. The extinction dependence for two polarizations on the spacing in the rectangular array is demonstrated. For a given polarizations on the extinction strength depends primarily on the spacing between the nanodisks in the orthogonal direction. Such randomization and polarization control of SERS increases the versatility, e.g., all-optical control, of this important sensing tool.


2013 - Additional Enhancement of Electric Field in Surface-Enhanced Raman Scattering due to Fresnel Mechanism [Articolo su rivista]
Jayawardhana, Sasani; Rosa, Lorenzo; Juodkazis, Saulius; Stoddart, Paul R.
abstract

Surface-enhanced Raman scattering (SERS) is attracting increasing interest for chemical sensing, surface science research and as an intriguing challenge in nanoscale plasmonic engineering. Several studies have shown that SERS intensities are increased when metal island film substrates are excited through a transparent base material, rather than directly through air. However, to our knowledge, the origin of this additional enhancement has never been satisfactorily explained. In this paper, finite difference time domain modeling is presented to show that the electric field intensity at the dielectric interface between metal particles is higher for "far-side'' excitation than "near-side''. This is reasonably consistent with the observed enhancement for silver islands on SiO2. The modeling results are supported by a simple analytical model based on Fresnel reflection at the interface, which suggests that the additional SERS signal is caused by near-field enhancement of the electric field due to the phase shift at the dielectric interface.


2013 - Arrays of Arbitrarily Shaped Nanoparticles: Overlay-Errorless Direct Ion Write [Articolo su rivista]
Gervinskas, Gediminas; Seniutinas, Gediminas; Rosa, Lorenzo; Juodkazis, Saulius
abstract

A simple solution to lithographically write down to 20-30 nm features over micrometer-sized nanoparticle arrays with high fidelity of pattern transfer from the designed to fabricated architectures is shown. It is achieved via a two-step approach: (i) fabrication of basic shape nanoparticles by electron beam lithography, gold deposition via sputtering and lift-off, then (ii) nano-patterning by focused ion beam lithography. Application potential of 3D tailored nanoparticles for nanotweezers is discussed on the basis of numerical modeling and experimental measurements of extinction.


2013 - Augmented sensitivity of an IR-absorption gas sensor employing a metal hole array [Articolo su rivista]
Nishijima, Yoshiaki; Adachi, Yuta; Rosa, Lorenzo; Juodkazis, Saulius
abstract

We demonstrate the use of plasmonic extraordinary transmission at IR wavelengths for surface-enhanced infrared absorption (SEIRA) spectroscopy in gas sensing. Gas detection was performed through non-dispersive infrared (NDIR) absorption. The sensitivity of SF6 gas detection is increased around similar to 27 times with metal hole array (MHA) microstructures placed on the gas cell mirrors, as compared with non-structured mirrors; an absorption change of 2% per 100 ppm was obtained on a standard commercial pyroelectric detector. Down-sizing of IR-sensors to a sub-1 mm gas cell width, delivering similar to 40 nM (or 1 ppm) of SF6 sensitivity, can be foreseen with a simple source-detector setup. (C) 2013 Optical Society of America


2013 - Chiral plasmonic nano structures : experimental and numerical tools [Relazione in Atti di Convegno]
Gervinskas, Gediminas; Rosa, Lorenzo; Brasselet, Etienne; Juodkazis, Saulius
abstract

A combination of electron and ion beam lithographies has been applied to fabricate patterns of plasmonic nanoparticles having tailored optical functions: they create hot-spots at predefined locations on the nanoparticle at specific wavelengths and polarizations of the incident light field. Direct inscribing of complex chiral patterns into uniform nano-disks of sub-wavelength dimensions, over extensive 20 by 20 mu m2 areas, is achieved with high fidelity and efficiency; typical groove widths are in 10-30 nm range. Such patterns can perform optical manipulation functions like nano-tweezing and chiral sorting. Fabrication procedures can be optimized to pattern thin 0.1-2.5 mu m-thick membranes with chiral nanoparticles having sub15 nm grooves. Peculiarities of optical force and torque calculations using finite difference time domain method are presented.


2013 - Fabrication of nanoparticles for generation of force and torque at nanoscale [Relazione in Atti di Convegno]
Gervinskas, Gediminas; Seniutinas, Gediminas; Rosa, Lorenzo; Brasselet, Etienne; Juodkazis, Saulius
abstract

Chiral patterns are created by focused ion-beam milling nano-grooves with sub-15 nm resolution on thin metal films and arrays of nanoparticles, scattering and absorbing light selectively for left and right circularly polarized light, with high fidelity over fields up to 100 x 100 mu m(2) without positioning errors. This allows to carry out numerical simulations to estimate light enhancement and circular dichroism both on ideal and realistic particles taken from SEM images, showing doubling of scattering cross-section and enhancement changes up to 5 times controlled by dichroism, with localized field enhancements up to 20000. 3D plasmonic structures extending out of a gold film plane are created by dry etching of the film in the openings of a resist mask defined by electron beam lithography. Conical vertical protrusions (nano-wells) are left, and their optical properties are numerically simulated, showing easily reachable out-of-plane trapping of both dielectric and metal plasmonic nano-spheres, with trapping forces up to 20 pN/W/mu m(2). Wideband refractive index spectra in 3D-FDTD are correctly represented by overcoming the polynomial approximations to give accurate field and force/torque results for generalized artificial materials.


2013 - Ion-beam and plasma etching of a conical-pores photonic crystal for thin-film solar cell [Relazione in Atti di Convegno]
Gervinskas, Gediminas; Rosa, Lorenzo; Juodkazis, Saulius
abstract

Conical holes bored in the active layer of a thin-film silicon solar cell by ion-beam lithography (IBL) show increase of effective optical absorption in the underlying silicon active layer. The optical properties are numerically simulated by the 3D finite-difference time-domain method (3D-FDTD), showing wideband increase of the UV, visible, and IR quantum efficiency. An experimental fabrication procedure is developed using IBL for high wide-area repeatability. A further optimization on the cone shapes is performed in order to make fabrication feasible with plasma etching techniques.


2013 - Optical properties of periodic/random pattern of Au nanodiscs [Relazione in Atti di Convegno]
Nishijima, Y.; Rosa, Lorenzo; Juodkazis, S.
abstract

The plasmon resonance of periodic/random arrays of Au-nanodiscs has been investigated experimentally and numerically. We found the electro-magnetic field enhancement 10-102 times in the random patterns.


2013 - Optoplasmonics: Hybridization in 3D [Relazione in Atti di Convegno]
Rosa, Lorenzo; Gervinskas, G.; Žukauskas, A.; Malinauskas, M.; Brasselet, E.; Juodkazis, S.
abstract

Femtosecond laser fabrication has been used to make hybrid refractive and di ractive micro-optical elements in photo-polymer SZ2080. For applications in micro- uidics, axicon lenses were fabricated (both single and arrays), for generation of light intensity patterns extending through the entire depth of a typically tens-of-micrometers deep channel. Further hybridisation of an axicon with a plasmonic slot is fabricated and demonstrated nu- merically. Spiralling chiral grooves were inscribed into a 100-nm-thick gold coating sputtered over polymerized micro-axicon lenses, using a focused ion beam. This demonstrates possibility of hybridisation between optical and plasmonic 3D micro-optical elements. Numerical modelling of optical performance by 3D-FDTD method is presented.


2013 - Plasmonic gas sensor [Relazione in Atti di Convegno]
Nishijima, Yoshiaki; Adachi, Yuta; Rosa, Lorenzo; Juodkazis, Saulius
abstract

Gas detection is an important field of medical gas sensing from exhalation, bio-markers of disease can be detected without any medical intervention. In case of environmental sensing in landfills, detection of greenhouse effect gases, exhaust fumes from cars, monitoring of industrial processes, the gases concentration should be monitored continuously in real time with 1 ppm precision or better.


2013 - Plasmonic solutions for light harvesting in solar and sensing applications [Capitolo/Saggio]
Juodkazis, S; Rosa, Lorenzo; Nishijima, Y.
abstract

Plasmonics has become an extensive eld of research since last decade due to its unique possibilities to engineer material’s optical and electrical properties, which can be harnessed for a new generation of high-speed all-optical processing of information and delivering light with nanoscale precision, and intensity enhancement can reach 108 when single-molecular detection becomes possible. It is no surprise that modern science and technology ventured to explore all these fascinating possibilities.


2013 - Randomization of gold nano-brick arrays: a tool for SERS enhancement [Articolo su rivista]
Nishijima, Yoshiaki; Khurgin Jacob, B.; Rosa, Lorenzo; Fujiwara, Hideki; Juodkazis, Saulius
abstract

Surface enhanced Raman scattering (SERS) was measured on periodic and randomly arranged patterns of Au nano-bricks (rectangular parallelepipeds). Resonant SERS conditions were investigated of a near-IR dye deposited on nanoparticles. Random mixtures of Au nano-bricks with different aspect ratio R showed stronger SERS enhancement as compared to periodic patterns with constant aspect ratio (R varies from 1 to 4). SERS mapping revealed up to similar to 4 times signal increase at the hot-spots. Experimental observation is verified by numerical modeling and is qualitatively consistent with generic scaling arguments of interaction between plasmonic nanoparticles. The effect of randomization on the polarization selectivity for the transverse and longitudinal modes of nano-bricks is shown. (C) 2013 Optical Society of America


2013 - Silver-gold nano-composites for photo-catalytic and sensing applications [Capitolo/Saggio]
Kowalska, E; Nishijima, Y; Rosa, Lorenzo; Juodkazis, S.
abstract

xperimental results of photo-catalytic activity of Ag-Au nano-composites on different type of titania are compared with numerical simulations. The action spectra of photo-catalyzed oxidation and the extinction (absorption together with scattering) spectra are compared for similar shape and size nanoparticles. Different shell-core composites using Ag and Au are numerically modeled for qualitative understanding of the light enhancement and its localization on the nanoparticle-titania substrate. We show that the electrical field component perpendicular to the interface between nanoparticle and substrate contributes the major field enhancement portion. Light enhancement at hot-spots and its scaling with gap size are revealed by surface enhanced Raman spectroscopy (SERS). Alloying of Ag and Au on nanoscale is demonstrated. The sp-to-d band transition of electrons in the plasmonic Ag and Au is proposed as a model of photo-catalysis at the wavelengths longer that the fundamental optical transition in titania (approximately 3.1 eV or 400 nm). Surface-enhanced infrared absorption (SEIRA) spectroscopy, is applied to highlyselective metal hole arrays (MHA), enhancing specific molecular vibrations for molecule fingerprinting, with up to 10 times enhancement of the absorption peaks within the 200-2000 cm−1 band. Selective IR band enhancement can be used for identification of specific molecules within complex mixtures and can be extended to longer wavelengths to probe the THz molecular bands. Discussed systems can bemodeled with a high fidelity using finite-difference timedomain (FDTD) methods. Sensing applications with microparticles can also be modeled providing insight in applications in far-IR (10-50 μm) and towards THz spectral range. The very same principles are applicable for broadband light energy harvesting.


2013 - Surface enhanced infrared absorption measurements with micro metal hole array [Relazione in Atti di Convegno]
Nishijima, Y.; Rosa, Lorenzo; Juodkazis, S.
abstract

SEIRA spectroscopic measurement using the metal hole array (MHA) is demonstrated with high spectral selectivity. The molecular IR absorption peaks are enhanced up to 10 times at the transmission peak of MHA structure.


2013 - 3D nano-structures for laser nano-manipulation [Articolo su rivista]
Seniutinas, Gediminas; Rosa, Lorenzo; Gervinskas, Gediminas; Brasselet, Etienne; Juodkazis, Saulius
abstract

The resputtering of gold films from nano-holes defined in a sacrificial PMMA mask, which was made by electron beam lithography, was carried out with a dry plasma etching tool in order to form well-like structures with a high aspect ratio (height/width approximate to 3-4) at the rims of the nano-holes. The extraordinary transmission through the patterns of such nano-wells was investigated experimentally and numerically. By doing numerical simulations of 50-nm and 100-nm diameter polystyrene beads in water and air, we show the potential of such patterns for self-induced back-action (SIBA) trapping. The best trapping conditions were found to be a trapping force of 2 pN/W/mu m(2) (numerical result) exerted on a 50-nm diameter bead in water. The simulations were based on the analytical Lorentz force model.


2012 - Additional enhancement in surface-enhanced Raman scattering due to excitation geometry [Relazione in Atti di Convegno]
Rosa, Lorenzo; Jayawardhana, Sasani; Juodkazis, Saulius; Stoddart Paul, R.
abstract

It is well known that surface-enhanced Raman scattering (SERS) substrates based on metal island films exhibit higher levels of enhancement when excited through a transparent base material than when excited directly through air. However, to our knowledge, the origin of this enhancement has never been satisfactorily explained. An initial suggestion that the additional enhancement was due to a "nearest layer effect" cannot account for the observation of additional enhancement for monolayer adsorbates. In this paper, finite difference time domain (FDTD) modelling is presented to show that the electric field intensity in between metal particles at the interface is higher for "far-side" excitation. This is reasonably consistent with the observed enhancement for silver islands on SiO2. The modelling results are in agreement with a simple physical model based on Fresnel reflection at the interface. This suggests that the additional enhancement is due to a near-field enhancement of the electric field due to the phase shift at the dielectric interface, when the light passes from the higher to the lower region of refractive index.


2012 - Design and fabrication of LMA low-bending loss leakage channel fibers [Relazione in Atti di Convegno]
Pal, M.; Paul, M. C.; Ghosh, D.; Bhadra, S. K.; Saitoh, K.; Rosa, Lorenzo
abstract

We report design and fabrication of both all-glass fluorine doped and air-clad leakage-channel fibers (LCF) to achieve large-mode-area (LMA) and effectively single-mode operation with a minimum bending loss (BL) for compact Yb-doped fiber laser delivery applications. The BL and effective area for the fabricated LCF were numerically calculated by using full-vector FEM for both A-A′ plane and B-B′ plane. A high enough differential BL between the fundamental mode (∼ 0.1dB/m) and the higher order mode (100dB/m) can be found in A-A′ plane. The measured BL of the air-clad LCF was 0.09 dB/m and effective area of 350 μm2 for 5 cm bending radius which is the best performance for minimum bendable LCF. The dispersion characteristic of the air-clad LCF is calculated and it has zero dispersion at 1.264μm. Single-mode nature of 1064nm light was measured at bent LCF. All-glass F-doped LCF was fabricated for easy splicing with standard SMF.


2012 - Light enhancement in surface-enhanced raman scattering at oblique incidence [Articolo su rivista]
Jayawardhana, S.; Rosa, Lorenzo; Buividas, R.; Stoddart, P. R.; Juodkazis, S.
abstract

Surface enhanced Raman scattering (SERS) measurements have been carried out at different focusing conditions using objective lenses of different numerical apertures. The experimentally observed dependence of SERS intensity of thiophenol-coated Ag nano-islands shows a close-to-linear scaling with the collection aperture. The linear relationship breaks down for large numerical apertures, which suggests that the scattering is anisotropic. Numerical simulations of realistically shaped Ag nano-islands were carried out, and the spatial distribution of hot-spots has been revealed at different heights near the nano-islands. Local field enhancements of up to 100 times were estimated. The simulation also suggests an explanation for the anisotropy in the scattering observed for larger numerical aperture objectives. This appears to be due to a reduction in the local field enhancement as the electric field vector component in the plane of the shallow metal islands reduces at larger angles of incidence.


2012 - Optical response simulation and measurement of silver plasmonic nano-particles in hexagonal patterns for high-efficiency solar harvesting [Relazione in Atti di Convegno]
Rosa, Lorenzo; Ranjan, M.; Zhou, J.; Fritzsche, M.; Facsko, S.; Mukherjee, S.; Juodkazis, S.
abstract

We investigate numerically and experimentally the performance of an antireflection layer for solar cell applications, in an ordered nano-dots configuration realized by plasma etching of a GaSb substrate and subsequent PVD deposition of Ag nano- particles on the nano-dot tips, and in a non-ordered configuration of Ag islands deposited on flat GaSb, leading to a technique that can be readily applied to patterning of silicon. We measure the substrate reflectivity and model the reflection, absorption, and transmission of the substrates using the 3D-FDTD method, which are realistically represented from the microphotographs. Favourable conditions for broadband resonances and multiple absorption peaks by varying parameters such as nano-dots height and order/disorder are reported.


2012 - Plasmonic nano-structures for opto-mechanical and sensing applications [Relazione in Atti di Convegno]
Rosa, Lorenzo; Gervinskas, G.; Brasselet, E.; Juodkazis, S.
abstract

We simulate the spectral properties of simple plasmonic nanoparticles (NPs), which we have fabricated with electron beam lithography (EBL) and lift-off, with or without nano-grooves milled by ion-beam lithography (IBL). We estimate the NP behaviour under laser illumination using 3D finite difference time domain (FDTD) simulations, showing that simple non-chiral NPs can generate torque from circularly polarized beams and emit optical vortices via spin-orbit coupling. We demonstrate numerically the position and intensity control of field enhancement hot-spots in chiral NPs illuminated by optical vortex, by combining spin/orbit momentum and NP chirality.


2012 - Selective enhancement of infrared absorption with metal hole arrays [Articolo su rivista]
Nishijima, Yoshiaki; Nigorinuma, Hiroki; Rosa, Lorenzo; Juodkazis, Saulius
abstract

We use a surface-enhanced infrared absorption (SEIRA) spectroscopy, a useful sensing and surface analysis method complimentary to the Raman scattering spectroscopy, for the individual enhancement of specific molecular vibration bands and fingerprinting of molecular vibrations. SEIRA spectroscopic measurement using the metal hole array (MHA) is demonstrated with high spectral selectivity. The molecular IR absorption peaks are enhanced up to 10 times at the transmission peak of MHA structure when electromagnetic field enhancement is localized on the walls inside the holes. Experimental and numerical simulations results are in a good qualitative agreement. Selective IR band enhancement can be used for identification of specific molecules within complex mixtures and it can be extended to the longer wavelengths at THz molecular bands. (C) 2012 Optical Society of America


2012 - Surface plasmon resonances in periodic and random patterns of gold nano-disks for broadband light harvesting [Articolo su rivista]
Nishijima, Yoshiaki; Rosa, Lorenzo; Juodkazis, Saulius
abstract

We analyze the localized surface plasmon resonance spectra of periodic square lattice arrays of gold nano-disks, and we describe numerically and experimentally the effect of disorder on resonance width, spectrum, and EM field enhancement in increasingly randomized patterns. The periodic structure shows a narrower and stronger extinction peak, conversely we observe an increase of up to (1-2) x 10(2) times enhancement as the disorder is gradually introduced. This allows for simpler, lower resolution fabrication, cost-effective in light harvesting for solar cell and sensing applications. We show that dipole-dipole interactions contribute to diffract light parallel to the surface as a mean of long-range coupling between the nano-disks. (C) 2012 Optical Society of America


2012 - Tailoring plasmonic field enhancement in spatial and spectral domains [Relazione in Atti di Convegno]
Juodkazis, S.; Rosa, Lorenzo; Gervinskas, G.; Brasselet, E.
abstract

Three-dimensional patterning of gold nano-particles on SiO2 and Si substrates is performed by ion-beam lithography with 15-20 nm resolution. Realization of an on-demand achiral and chiral modifications of nano-structures are demonstrated. Simulations of optical properties of the obtained nano-architectures reveal the possibility to tailor the field enhancement both in the spatial and spectral domains, which are foreseen to be applicable for light harvesting and opto-mechanics at nanoscale.


2011 - Alumina-embedded Au nanowires for SERS sensing [Relazione in Atti di Convegno]
Rosa, Lorenzo; Buividas, R.; Juodkazis, S.; Kondo, T.; Masuda, H.
abstract

We propose a novel substrate for SERS bio-sensing composed of an hexagonal array of gold nanorods embedded in an anodically-oxidized alumina substrate, reaching a surface field enhancement up to 220 times.


2011 - FDTD modeling to enhance the performance of an organic solar cell embedded with gold nanoparticles [Articolo su rivista]
Poh Chung, How; Rosa, Lorenzo; Juodkazis, Saulius; Dastoor, Paul
abstract

Optical enhancement is demonstrated in a bilayer P3HT-C-60 solar cell by embedding gold nanoparticles directly into the P3HT layer of the photovoltaic device. FDTD simulations are used to model the observed performance gain. A qualitative agreement between the experimental and numerical results is achieved. This validates the numerical model and the simulation is subsequently extended to predict the performance gain of the bilayer device constructed with thinner P3HT layer. The numerical results reveal that the plasmonic structure has even larger effect on such thinner bilayer device. The enhancement is expected to be most significant when the p-n interface is allowed to assume the conformal hemispherical profile of the metal particles. (C) 2011 Optical Society of America


2011 - Limitation on Effective Area of Bent Large-Mode-Area Leakage Channel Fibers [Articolo su rivista]
Saitoh, Kunimasa; Varshney, Shailendra; Sasaki, Kaori; Rosa, Lorenzo; Pal, Mrinmay; Paul Mukul, Chandra; Ghosh, Debashri; Bhadra Shyamal, Kumar; Koshiba, Masanori
abstract

We investigate the bending characteristics of leakage channel fibers (LCFs) to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for compact Yb-doped fiber applications. Through numerical simulations, carried by the full-vectorial finite-element method, we present the limitations on the effective area of LCFs under bent condition and compare their limits with that of conventional step-index LMA fibers. Due to a better controllability of the low numerical aperture and a large value of the differential bending loss (similar to 20 dB/m) between the fundamental and higher order modes in LCFs, the LMA of similar to 500 mu m(2) (core diameter of similar to 36 mu m) at 1064 nm can be achieved when the optimized LCF is bent into a 10 cm bending radius.


2011 - Localized photocatalysis by Au-titania plasmonics [Relazione in Atti di Convegno]
Rosa, Lorenzo; Juodkazis, S.; Kowalska, E.
abstract

We investigate numerically the properties of a novel bio-compatible titania (TiO2) substrate functionalized by deposition of gold and titania nanoparticles, achieving high photocatalytic activity for biological applications.


2011 - Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback [Articolo su rivista]
Buividas, Ricardas; ROSA, Lorenzo; Sliupas, Remigijus; Kudrius, Tadas; Slekys, Gintas; Datsyuk, Vitaly; Juodkazis, Saulius
abstract

The mechanism of the fine ripples, perpendicular to laser polarization, on the surface of (semi) transparent materials with period smaller than the vacuum wavelength, lambda, of the incident radiation is proposed and experimentally validated. The sphere-to-plane transformation of nanoplasma bubbles responsible for the in-bulk ripples accounts for the fine ripples on the surface of dielectrics and semiconductors. The mechanism is demonstrated for 4H:SiC and sapphire surfaces using 800 nm/150 fs and 1030 nm/300 fs laser pulses. The ripples are pinned to the smallest possible standing wave cavity inside material of refractive index n. This defines the corresponding period, Lambda = (lambda/n)/2, of a light standing wave with intensity, E-2, at the maxima of which surface ablation occurs. The mechanism accounts for the fine ripples at the breakdown conditions. Comparison with ripples recorded on different materials and via other mechanisms using femtosecond pulses is presented and application potential is discussed.


2011 - Novel plasmonic applications in physics and chemistry [Relazione in Atti di Convegno]
Rosa, Lorenzo; Sun, K.; Kowalska, E.; Juodkazis, S.
abstract

We present novel plasmonic methodologies combining electron- and ion-beam lithography for fabrication of wideband fractal nano-antennas and slotted gold nano-particles. Performance of such patterns with nanometric-size grooves, and functionalized gold-titania substrates for photo-catalysis, are demonstrated.


2011 - Sculpturing of photonic crystals by ion beam lithography: towards complete photonic bandgap at visible wavelengths [Articolo su rivista]
Juodkazis, Saulius; Rosa, Lorenzo; Bauerdick, Sven; Peto, Lloyd; El Ganainy, Ramy; John, Sajeev
abstract

Three dimensional (3D) ion beam lithography (IBL) is used to directly pattern 3D photonic crystal (PhC) structures in crystalline titania. The process is maskless and direct write. The slanted pore 3D structures with pore diameters of 100 nm having aspect ratio of 8 were formed. It is shown that chemical enhancement of titania removal up to 5.2 times is possible in XeF2 gas for the closest nozzle-to-sample distance; the enhancement was similar to 1.5 times for the actual 3D patterning due to a sample tilt. Tolerances of structural parameters and optimization of IBL processing required for the fabrication of PhCs with full photonic bandgap in visible spectral range in rutile are outlined. Application potential of 3D-IBL is discussed. (c) 2011 Optical Society of America


2011 - Sierpinski fractal plasmonic nanoantennas [Articolo su rivista]
Rosa, Lorenzo; Sun, Kai; Juodkazis, Saulius
abstract

We propose plasmonic Sierpinski gasket, a self-replicating fractal, with structural elements spanning from similar to 100 nm to similar to 5 mu m made by standard electron beam lithography (EBL), metal deposition, and lift-off sequence. Such structures demonstrate light field enhancement from visible to far-IR spectral range and can be scaled up towards THz band. Numerical simulations show that as the fractal order is increased, the optical extinction band broadens from the visible light towards far-IR, achieving a light field enhancement of more than four orders of magnitude in the nano-gap proximity. Such antennas are prospective for IR-THz filter, detection, and emission applications. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


2011 - Tailoring plasmonic nanoparticles and fractal patterns [Relazione in Atti di Convegno]
Rosa, Lorenzo; Juodkazis, Saulius
abstract

We studied new three-dimensional tailoring of nano-particles by ion-beam and electron-beam lithographies, aiming for features and nano-gaps down to 10 nm size. Electron-beam patterning is demonstrated for 2D fabrication in combination with plasmonic metal deposition and lift-off, with full control of spectral features of plasmonic nano-particles and patterns on dielectric substrates. We present wide-angle bow-tie rounded nano-antennas whose plasmonic resonances achieve strong field enhancement at engineered wavelength range, and show how the addition of fractal patterns defined by standard electron beam lithography achieve light field enhancement from visible to far-IR spectral range and scalable up towards THz band. Field enhancement is evaluated by FDTD modeling on full-3D simulation domains using complex material models, showing the modeling method capabilities and the effect of staircase approximations on field enhancement and resonance conditions, especially at metal corners, where a minimum rounding radius of 2 nm is resolved and a five-fold reduction of spurious ringing at sharp corners is obtained by the use of conformal meshing.


2011 - 3D-Tailored Gold Nanoparticles for Light Field Enhancement and Harvesting over Visible-IR Spectral Range [Articolo su rivista]
Rosa, Lorenzo; Sun, Kai; Mizeikis, Vygantas; Bauerdick, Sven; Peto, Lloyd; Juodkazis, Saulius
abstract

A method for practical area upscaling of nanopatterning for light-harvesting and photocatalytic applications is presented. Large area electron beam lithography is used to design patterns of simple-shape nanoparticles. After evaporation of gold, ion beam lithography is used to slice nanoparticles with grooves as narrow as 17 +/- 3 nm in width for the required spectral performance and light field enhancement. It is demonstrated by systematic numerical simulations that cutting grooves into the Si and SiO(2) substrates up to a similar to 10 nm depth augments the volume where the light-field enhancement occurs. The dominant component of the field enhancement in the groove is vertical bar E(z)vertical bar(2), perpendicular to the substrate's surface. The application potential of 3D-tailored nanoparticles in light harvesting applications is discussed.


2010 - Design and fabrication of F-doped large mode area leakage channel fiber [Relazione in Atti di Convegno]
Pal, M.; Paul, M. C.; Mahanty, T.; Ghosh, D.; Bhadra, S. K.; Rosa, Lorenzo; Saitoh, K.
abstract

We report design and fabrication of all-glass LMA leakage-channel fiber constituting fluorine doped rods surrounding silica core. Fabrication of fluorine doped rods and fiber are described. Silica bridge width is engineered for effectively single-mode behaviour.


2010 - Effective area limit of large-mode-area solid-core photonic bandgap fibers for fiber laser applications [Articolo su rivista]
Saitoh, Kunimasa; Murao, Tadashi; Rosa, Lorenzo; Koshiba, Masanori
abstract

We investigate the bending characteristics of solid-core photonic bandgap fibers (SC-PBGFs) aiming to achieve large mode area (LMA) and effectively single-mode operation with a practically allowable bending radius for high-power Yb-doped fiber lasers and amplifiers. Through detailed numerical simulations based on the finite element method (FEM), we evaluate the impacts of the order of photonic bandgap on the bending performance and point out the limits of core size enlargement in the SC-PBGFs with a 1-cell core structure due to the increment of bending loss. In addition, under practical constraints, we find that the SC-PBGFs having a 7-cell core can achieve sufficient differential bending loss between the fundamental mode and the higher-order modes and a much larger effective area limit as compared with previously-reported index-guiding LMA fibers. (C) 2010 Elsevier Inc. All rights reserved.


2010 - Limitation on effective area of large-mode-area leakage channel fibers under bent condition [Relazione in Atti di Convegno]
Saitoh, K.; Varshney, S. K.; Sasaki, K.; Rosa, Lorenzo; Pal, M.; Paul, M.; Bhadra, S.
abstract

The bending characteristics of two-ring leakage-channel-fibers are investigated to achieve large-mode-area and effectively single-mode operation with a practically allowable bending radius for Yb-doped fiber applications, enhancing differential bending loss 10-dB/m between the fundamental and higher-order modes.


2010 - Octagonal Large-Mode-Area Leakage Channel Fiber with Reduced Bending Loss [Relazione in Atti di Convegno]
Rosa, Lorenzo; K., Saitoh; M., Koshiba; F., Poli; A., Cucinotta; S., Selleri; Vincetti, Luca; M., Pal; M., Paul; D., Ghosh; S., Bhadra
abstract

We investigate a novel design for high-power large-modearea leakage channel fibers (LCFs) with a single octagonal dual-diameter air-hole ring encircling a nine-cell pure-silica core, having a tenfold differential mode loss and increased resistance to bending.


2010 - Optimization of large-mode-area tapered-index multi-core fibers with high differential mode bending loss for ytterbium-doped fiber applications [Relazione in Atti di Convegno]
Rosa, Lorenzo; Saitoh, K.
abstract

Large-mode-area (LMA) tapered-index multi-core fibers (MCFs), made by hexagonal stacking of up-doped cores in silica, are optimized through a genetic algorithm (GA) for high fundamental mode (FM) effective area and high differential loss with higher-order modes (HOMs) in bending condition.


2010 - Realistic squared-rods circular F-doped large-mode-area leakage channel fibers with low bending loss [Relazione in Atti di Convegno]
Rosa, Lorenzo; Saitoh, K.; Koshiba, M.; Pal, M.; Paul, M.; Ghosh, D.; Mahanty, T.; Bhadra, S.; Vincetti, Luca; Selleri, S.
abstract

We investigate realistic all-glass large-mode-area leakage channel fibers (LCFs) with a single down-doped-silica circular flattened-rod ring, engineering the silica bridge width for effectively single-mode behavior and reduced bending loss.


2010 - Ripple-patterned substrates for light enhancement applications [Relazione in Atti di Convegno]
Buividas, Ricardas; Kudrius, Tadas; Sliupas, Remigijus; Rosa, Lorenzo; Slekys, Gintas; Bagdonas, Saulius; Rotomskis, Ricardas; Juodkazis, Saulius
abstract

We report on surface structuring of sapphire, silicon carbide, and silicon by femtosecond laser pulses in multipulse irradiation mode. The formed ripples on the flat surface or on the vertical walls with hierarchical structures whose feature sizes are ranging from the irradiation wavelength down to similar to 50 nm are prospective templates for surface enhanced Raman scattering after coating with plasmonic metals. We study complex patterns of fine ripples with periods Lambda(r), as small as lambda/Rp, where Rp similar or equal to 3 - 5. The mechanisms suggested for such Rp values are discussed: temperature and density of breakdown plasma, angle of incidence, mechanism of second harmonic generation (SHG) and absorption. Predictions of the surface and bulk models of ripple formation are compared with experimental values of Rp-factor. We propose a model of ripple formation on the surface, which is based on the known in-bulk sphere-to-plane formation of breakdown plasma in the surface proximity. In semiconductor 4H:SiC normal ripples with periods 190 and 230 nm were recorded with 800 nm and 1030 nm fs-laser pulses respectively. We show that the period of ripples is defined by the dielectric properties of crystalline (solid) phase rather than the molten phase in the case of silicon. Generation of SHG on the surface of sample and plasma nano-bubbles are discussed: surface-SHG is found not important in ripples' formation as revealed by comparative study of periods on Al2O3 and TiO2 at 800 nm wavelength of irradiation. We propose that ripple periodicity is pinned to the smallest possible standing wave cavity (lambda/n)/2 inside material of refractive index n.


2010 - Simple suppression technique for higher-order mode amplification in bent large mode area triple-cladding fibers [Relazione in Atti di Convegno]
Saitoh, K.; Ichii, K.; Takenaga, K.; Tanigawa, S.; Matsuo, S.; Fujimaki, M.; Rosa, Lorenzo; Koshiba, M.
abstract

A novel technique to suppress the higher-order mode in bent LMA triple-cladding fibers is proposed. It is shown that the higher-order mode can be coupled to a cladding mode with only an adjustment of the bending diameter.


2010 - Surface defect mediated electron hopping between nanoparticles separated by a nano-gap [Articolo su rivista]
Juodkazis, Saulius; Rosa, Lorenzo
abstract

The influence of defects induced by electron or ion beam lithography in the pre-surface region of the substrate on optical properties of plasmonic nanoparticles is analyzed. It is demonstrated that electron hopping between the defect sites of closely located nanoparticles is probable. The effect of electron transport via defect states will cause an apparent electrical connectivity between the nanoparticles and can be observed in extinction measurements. It is shown that this type of connectivity is important for gap sizes smaller than similar to 10 nm and can reduce the actual light field enhancement. [GRAPHICS] FDTD simulation scene: two nanoparticles separated by a nano-gap. The defects due to electron/ion lithography are present in the sub-surface region. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


2010 - Tailoring spectral position and width of field enhancement by focused ion-beam patterning of plasmonic nanoparticles [Articolo su rivista]
Rosa, Lorenzo; Sun, Kai; Szymanska, Joanna; Hudson Fay, E.; Dzurak, Andrew; Linden, Andre; Bauerdick, Sven; Peto, Lloyd; Juodkazis, Saulius
abstract

We propose to use ion-beam patterning of plasmonic nanoparticles made by standard electron beam lithography, plasmonic metal deposition, and lift-off sequence. With this approach new three-dimensional (3D) tailoring of nanoparticles becomes possible: cut through the particle and into the substrate, cut-through at an angle, trim or bore nanoparticles or substrate. Here, we numerically simulate the expected optical properties of such 3D patterned nanoparticles. We show a particular case when spectrally broad extinction band can be created with a strong light field enhancement on a Si substrate.Nanoparticles with a 10-20 nm nano-gap cut by ion-beam at the chosen location and to the required depth into the substrate. Scanning electron microscopy (SEM) image of the fabricated Au-nanoparticles.(© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Free three-dimensional (3D) patterning of plasmonic nanoparticles by ion beam lithography can be used to control the light-field enhancement and its spectral position. This direct-write approach does not require resist and 3D tailoring of nanoparticles becomes possible by cut-through at arbitrary angle, boring nanoparticles and substrate. Authors show numerically that optical properties of such 3D patterned gold nanoparticles on silicon substrate are prospective for use in solar cells.


2009 - Design of all-solid leakage channel fibers with large mode area and low bending loss [Articolo su rivista]
Saitoh, Kunimasa; Tsuchida, Yukihiro; Rosa, Lorenzo; Koshiba, Masanori; Poli, Federica; Cucinotta, Annamaria; Selleri, Stefano; Pal, Mrinmay; Paul, Mukul; Ghosh, Debashri; Bhadra, Shyamal
abstract

We investigate a novel design for all-solid large mode area (LMA) leakage channel fibers (LCFs) for high-power Yb-doped fiber lasers and amplifiers, based on a single down-doped-silica rod ring surrounding a seven-cell pure-silica core, aiming for effectively single-mode behavior and low bending loss characteristics. Through detailed numerical simulations based on the finite element method (FEM), we find that the proposed all-solid LMA-LCFs, having a seven-cell core and two different sizes of down-doped rods, can achieve sufficient differential mode loss and much lower bending loss, as compared with a previously-reported LCF with a one-cell core and six large down-doped-silica rods. (C) 2009 Optical Society of America


2009 - Effect of rod inaccuracy on genetic-algorithm-designed C-band photonic-crystal waveguide interleavers [Relazione in Atti di Convegno]
Rosa, Lorenzo; Saitoh, K.; Kakihara, K.; Koshiba, M.
abstract

We investigate the effect of construction inaccuracies on C-band photonic-crystal waveguide (PCW) interleavers with flattened pass-band, designed through the employment of single-and multi-objective genetic algorithm (GA) approaches.


2009 - Genetic-Algorithm Assisted Design of C-Band CROW-Miniaturized PCW Interleaver [Articolo su rivista]
Rosa, Lorenzo; Saitoh, Kunimasa; Kakihara, Kuniaki; Koshiba, Masanori
abstract

We investigate the design of a miniaturized C-band photonic-crystal waveguide (PCW) interleaver, employing coupled-resonator optical waveguide ( CROW) ring resonators. An optimization approach based on a genetic algorithm (GA) has been applied to the filter components, such as tapers and directional couplers, leading to low reflection and high phase linearity. To evaluate the interleaver transmittivity function, an matrix approach is worked out, based on the optimized building blocks. A sensitivity analysis is performed to evaluate the effect of parameter tolerances.


2009 - Genetic-algorithm assisted design of C-band photonic-crystal waveguide interleavers using ring resonators [Relazione in Atti di Convegno]
Rosa, Lorenzo; Saitoh, K.; Kakihara, K.; Koshiba, M.
abstract

We investigate the optimization of C-band photonic-crystal waveguide (PCW) interleavers in different implementations using coupled-resonator optical waveguide (CROW) ring resonators. Genetic algorithms (GAs) are employed to optimize the filters for high transmittivity and flatness.


2008 - Design and optimization of devices for C-band photonic-crystal waveguide interleaver with flattened pass-band [Relazione in Atti di Convegno]
Rosa, Lorenzo; Saitoh, K.; Kakihara, K.; Koshiba, M.
abstract

We investigate the optimization of a C-band photonic-crystal waveguide (PCW) interleaver with flattened pass-band, using coupled-resonator optical waveguides (CROW). A genetic algorithm (GA) is employed for optimizing the filter devices for high transmittivity and flatness.


2008 - Design of single-mode leakage channel fibers with large-mode-area and low bending loss [Relazione in Atti di Convegno]
Tsuchida, Y.; Saitoh, K.; Varshney, S.; Rosa, Lorenzo; Koshiba, M.
abstract

Novel design of large-mode-area leakage channel fibers (LCFs) with a single air-hole ring surrounding a multi-unit-cell solid core is investigated for realizing robust single-mode and low bending loss characteristics, with comparison to reported LCF configurations.


2008 - Fibre large mode area leakage channel poligonali a dispersione modale ridotta [Relazione in Atti di Convegno]
F., Poli; D., Passaro; A., Cucinotta; S., Selleri; Rosa, Lorenzo; Vincetti, Luca
abstract

In the context of high-power fiber laser applications, effectively single-mode large-mode-area leakage channel fibers employing a single air-hole ring are presented according to a novel design with hole polygonal symmetry around a solid core. Octagonal and decagonal symmetries are studied with attention to penalties arising in bending conditions, aiming for low mode distortion characteristics with respect to hexagonal-lattice fibers.


2008 - Fundamental and high-order mode bending loss in leakage channel fibers [Relazione in Atti di Convegno]
F., Poli; Vincetti, Luca; D., Passaro; A., Cucinotta; S., Selleri; Rosa, Lorenzo; K., Saitoh; Y., Tsuchida; S. K., Varshney; M., Koshiba
abstract

Penalties induced by bending are investigated in large mode area polygonal leakage channel fibers for high-power applications. Fundamental and high-order mode losses are reduced considering decagonal symmetry fibers.


2008 - Polygonal Large-Mode-Area Leakage Channel Fibers with Reduced Mode Distortion [Relazione in Atti di Convegno]
Poli, F; Passaro, D; Cucinotta, A; Selleri, S; Vincetti, Luca; Rosa, Lorenzo; Saitoh, K; Tsuchida, Y; VARSHNEY S., K; Koshiba, M.
abstract


2008 - Single-Mode Large-Mode-Area Leakage Channel Fibers with Octagonal Symmetry [Relazione in Atti di Convegno]
Rosa, Lorenzo; Saitoh, K.; Tsuchida, Y.; Varshney, S. K.; Koshiba, M.; Poli, F.; Passaro, D.; Cucinotta, A.; Selleri, S.; Vincetti, Luca
abstract

A novel design for large-mode-area leakage channel fibers with a single ring of air-holes organized with octagonal symmetry has been proposed, obtaining lower bending loss and guided mode distortion with respect to hexagonal symmetry fibers.


2007 - Distortion performance prediction in multi-band radio over fiber systems exploiting direct laser modulation [Relazione in Atti di Convegno]
Rosa, Lorenzo; Selleri, S.; Tartarini, G.; Faccin, P.; Fabbri, E. M.
abstract

An efficient numerical tool has been developed for Radio over Fiber (RoF) link performance evaluation. Successful comparisons have been made between modeled and measured quantities in RoF links intended for wireless signal distribution.


2007 - Patch array antenna for UWB radar applications [Relazione in Atti di Convegno]
Gaboardi, P.; Rosa, Lorenzo; Cucinotta, A.; Selleri, S.
abstract

This paper presents an antenna design for Ultra Wideband (UWB) Radar applications. It consists of a patch array antenna designed to operate from 5.35 to 5.85 GHz. Details of the proposed antenna design, realization and measured results are presented and discussed.


2007 - Simulation and Measurement of Intermodulation Induced by Chirp-Dispersion Interaction in Radio-over-Fiber Systems [Relazione in Atti di Convegno]
ROSA, Lorenzo; D., PASSARO; S., SELLERI; G., TARTARINI; P., FACCIN; E. M., FABBRI
abstract

Intermodulation induced by chirp-dispersion interaction, a major issue in radio over fiber link deployment, is modeled through a simulation code. Comparison with measured intercept point results shows successful matching within 1 dB. The adiabatic chirp coefficient is studied under the influence of the physical parameters of a semiconductor diode laser in order to determinate an optimal working point for a direct optical modulation. Finally the injection-locking technique is proposed in order to reduce the chirp coefficient of the laser.


2007 - Simulazione e Misura dell'Intermodulazione Prodotta dall'Interazione tra Frequency Chirp e Dispersione in Sistemi Radio-over-Fiber a Modulazione Diretta [Relazione in Atti di Convegno]
Rosa, Lorenzo; D., Passaro; S., Selleri; G., Tartarini; P., Faccin; E. M., Fabbri
abstract

Simulazione e Misura dell'Intermodulazione Prodotta dall'Interazione tra Frequency Chirp e Dispersione in Sistemi Radio-over-Fiber a Modulazione Diretta


2006 - Effect of hole diameter inaccuracy on tapers between photonic-crystal and wire waveguides [Relazione in Atti di Convegno]
Rosa, Lorenzo; Foroni, M.; Poli, F.; Cucinotta, A.; Selleri, S.
abstract

The performance of tapered interfaces between photonic-crystal and wire-waveguides can be impaired by as much as 35% by diameter inaccuracies of the taper holes. The effect is studied for 2√3δ-and 3√3δ-size wire-waveguides.


2006 - Fibre a cristallo fotonico Solid-Core e Air-Guiding: analisi e progetto tramite il metodo degli elementi finiti [Relazione in Atti di Convegno]
Rosa, Lorenzo; M., Foroni; F., Poli; A., Cucinotta; S., Selleri; Maini, Moreno; Vincetti, Luca; Borsari, Gian Paolo; Zoboli, Maurizio
abstract

The finite element method has proved to be a powerful and reliable tool for the analysis and the design of photonic crystal fibers, new and exciting waveguides which can be exploited to enhance proper characteristics of the light for a huge range ofapplications, for example in telecommunications, sensing or biophotonics, unthinkable with standard optical fibers.


2006 - Harmonic and intermodulation distortion modeling in IM-DD multi-band radio over fiber links exploiting injection locked lasers [Articolo su rivista]
Tartarini, Giovanni; Lena, Alessandra; Passaro, Davide; Rosa, Lorenzo; Selleri, Stefano; Faccin, Pier; Fabbri Enrico, Maria
abstract

A comprehensive numerical tool has been developed for the evaluation of the performances of Radio over Fiber (RoF) links intended for wireless signal distribution.At the transmitter end an appropriate set of rate equations allows to model the optical source as a solitary laser or as an appropriately injection locked laser. The optical channel is modeled putting into account the combined effect of fiber dispersion, laser source non ideal performances (e.g. non-linear effects, frequency chirp), and quadratic detection of the receiving photodiode. The simulation model developed can be a useful tool at the design stage allowing a preliminary evaluation of the characteristics of real RoF links.


2006 - Intermodulation distortion modelling in IM-DD multi-band radio over fibre links [Relazione in Atti di Convegno]
Rosa, Lorenzo; Selleri, S.; Tartarini, G.; Fabbri, E. M.; Faccin, P.
abstract

Efficient numerical tools have been developed for Radio over Fiber (RoF) link performance evaluation. Successful comparisons have been made between modeled and measured quantities in RoF links intended for wireless signal distribution.


2006 - Modelling leaky photonic wires: A mode solver comparison [Articolo su rivista]
Bienstman, P.; Selleri, S.; Rosa, Lorenzo; Uranus, H. P.; Hopman, W. C. L.; Costa, R.; Melloni, A.; Andreani, L. C.; Hugonin, J. P.; Lalanne, P.; Pinto, D.; Obayya, S. S. A.; Dems, M.; Panajotov, K.
abstract

We present results from a mode solver comparison held within the frameworkof the European COST P11 project. The structure modelled is a high-index contrast photonic wire in silicon-on- insulator subject to substrate leakage. The methods compared are both in-house developed and commercial, and range from effective index and perturbation methods, over finite-element and finite-difference codes, beam propagation methods, to film mode matching methods and plane wave expansion methods.


2006 - Modified Honeycomb Photonic Bandgap Fiber Effectively Single-Mode Regime: A Numerical Analysis [Relazione in Atti di Convegno]
Vincetti, Luca; Maini, Moreno; Rosa, Lorenzo; Poli, F; Foroni, M; Cucinotta, A; Selleri, S.
abstract

An accurate analysis of the single-mode properties of modified honeycomb photonic bandgap fibers with different geometric characteristics has been carried out by calculating the confinement losses of the fundamental and the higher-order modes. Simulation results have shown that confinement loss lower than 0.1 dB/km and an effectively single-mode behaviour over a wavelength range of about 150 nm can be achieved with a 8 air-hole ring fiber.


2006 - Numerical approaches for the analysis of optical devices [Relazione in Atti di Convegno]
Rosa, Lorenzo
abstract

The research activity presented is focused on the study of numerical methods for electromagnetism and the development of simulation codes for integrated and fiber optics devices. The theoretical approach is mainly based on the Finite Element Method (FEM). The developed codes permit both modal and propagative generalpurpose analysis, suitable for the study of structures with arbitrary geometry, distribution or refractive index variation and media features. For modal analysis, mono- and bi-dimensional codes have been developed which are suitable for media that can also be anisotropic and nonlinear, both with and without losses. Both scalar and full-vectorial formulations have been developed. Propagative analysis has been performed with the development of Beam Propagation Method (BPM) semi- and full-vectorial bi-dimensional cross-section codes. Moreover, formulations of propagators for Finite-Element Frequency-Domain (FEFD) and Finite-Element Time-Domain (FETD) analysis have also been implemented.


2006 - Polarization splitter based on a square-lattice photonic-crystal fiber [Articolo su rivista]
ROSA, Lorenzo; Poli, F; Foroni, M; Cucinotta, A; Selleri, S.
abstract

A three-core polarization splitter based on a square-lattice photonic-crystal fiber is presented. The component separates the input field into two orthogonally polarized beams that are coupled to the horizontal and vertical output ports. The splitter has been designed through modal and beam propagation analysis by employing high-performance codes based on the finite-element method. Results obtained for a device length of 20 mm show extinction ratios as low as -23 dB with bandwidths as great as 90 nm. (c) 2006 Optical Society of America.


2006 - Square-lattice photonic crystal fiber cutoff properties [Relazione in Atti di Convegno]
Poli, F.; Foroni, M.; Rosa, Lorenzo; Cucinotta, A.; Selleri, S.
abstract

The cutoff properties of square-lattice photonic crystal fibers have been accurately investigated by analyzing the leaky behaviour of the second-order mode. The cutoff analysis has been carried out at different wavelengths, in order to take into account the silica chromatic dispersion.


2006 - Tunability of the gain spectrum in an erbium-doped fiber with depressed-cladding [Relazione in Atti di Convegno]
Poli, F.; Foroni, M.; Cucinotta, A.; Ruggeri, L.; Rosa, Lorenzo; Selleri, S.; Vavassori, P.
abstract

The bending losses of a depressed-cladding erbium-doped fiber have been exploited to tune the fiber amplifier gain in the wavelength range between the S-band and the C-band.


2005 - Cutoff properties of large-mode-area photonic crystal fibers [Relazione in Atti di Convegno]
Foroni, M.; Poli, F.; Rosa, L.; Cucinotta, A.; Selleri, S.
abstract

The cutoff properties of triangular photonic crystal fibers with different core dimensions have been deeply investigated. In particular, 7-rod core fibers, obtained by removing the air-holes belonging to the first ring, as well as the central one in the fiber cross-section, have been considered. A phase diagram which describes the regions of single-mode and multi-mode operation for these large mode area fibers has been calculated with two different approaches. Simulation results have demonstrated that, for a fixed air-filling fraction, triangular photonic crystal fibers with a wider silica core region are single-mode in a smaller wavelength range. As a consequence, also the endlessly single-mode region of 7-rod core triangular fibers is smaller than that of 1-rod core ones. However, by properly choosing low air-filling fraction and relatively small hole-to-hole distance, 7-rod core photonic crystal fibers can provide high effective area values, as well as single-mode operation, without the problem of leakage losses.


2005 - Design of photonic-crystal and wire waveguide interface [Articolo su rivista]
Rosa, L.; Selleri, S.; Poli, F.
abstract

The coupling of light between wire-dielectric and photonic-crystal waveguides, characterized by a triangular lattice of air holes, is investigated through a finite-element time domain (FE-TD) approach, in order to optimize the transmission spectrum. The displacement of the inner-hole rows of the photonic-crystal waveguide, on both sides of the interface, has been shown to considerably improve the coupling, resulting in transmission values higher than 90% and 95% over bandwidths of hundreds of nanometers around 1550 nm. The new design approach is successfully proposed for input and output waveguides with width equal to √3Λ, as well as for larger widths, like 2√3Λ and 3√3Λ, with Λ being the period of the photonic crystal. © 2005 IEEE.


2005 - In- and out-coupling of light in photonic-crystal and conventional dielectric waveguides of arbitrary width [Relazione in Atti di Convegno]
Rosa, L.; Poli, F.; Foroni, M.; Seller, S.
abstract

A new approach for light coupling between wire dielectric and photonic crystal waveguides, is investigated to optimize the transmission spectrum around 1550 nm. The approach is successfully proposed for input and output coupling with waveguides of arbitrary width, even much larger than the single line defect photonic crystal waveguide. It is shown that the displacement across the interface of the inner hole rows of the photonic crystal waveguide considerably enhances the coupling, resulting in transmission values higher than 90% over bandwidths of hundreds of nanometers. © 2005 IEEE.


2005 - Multipump flattened-gain Raman amplifiers based on photonic-crystal fibers [Articolo su rivista]
Poli, F.; Rosa, L.; Bottacini, M.; Foroni, M.; Cucinotta, A.; Selleri, S.
abstract

Raman amplifiers based on photonic crystal fibers, whose cross section can be designed to enhance the nonlinear properties, have been investigated in this work. The use of multiple pumps is studied in order to flatten the amplifier gain spectrum, by varying their number, power, and wavelength, yielding a flatness as low as 0.5 dB in the wavelength range between 1540 and 1572 nm. © 2005 IEEE.


2005 - Optical parametric amplification in all-silica triangular-core photonic crystal fibers [Articolo su rivista]
Poli, F.; Adami, F.; Foroni, M.; Rosa, L.; Cucinotta, A.; Selleri, S.
abstract

The present analysis demonstrates that photonic crystal fibers can be designed to enhance their properties for optical parametric amplification. In particular, it is possible to achieve high nonlinear coefficient values and flattened dispersion curves around 1550 nm to fully exploit the four-wave-mixing effect. These results have been obtained for triangular-core photonic crystal fibers without the need of modifying the refractive-index profile by adding dopants in the fiber cross section. © Springer-Verlag 2005.


2005 - Optical parametric amplification in dispersion-flattened highly nonlinear photonic crystal fibers [Relazione in Atti di Convegno]
Selleri, S.; Cucinotta, A.; Poli, F.; Foroni, M.; Rosa, L.
abstract

Photonic Crystal Fibers (PCFs) are optical fibers with unique guiding characteristics as well as unusual nonlinear and dispersion properties. Since PCFs offer the possibility to engineer the zero-dispersion wavelength, the dispersion curve and the nonlinear coefficient value, they are very interesting for optical parametric amplification. In the present paper the phase-matching condition has been deeply analyzed in different triangular PCFs configurations. In particular, highly nonlinear PCFs have been designed to achieve flattened dispersion curves around the zero-dispersion wavelength in the C band. Very flat parametric gain, around W dB, on a bandwidth up to 35 nm can be obtained with short PCF and low pump power level.


2005 - Polarization selective coupling in three-core holey fibers [Relazione in Atti di Convegno]
Rosa, L.; Poli, F.; Foroni, M.; Bertolaccini, S.; Cucinotta, A.; Selleri, S.
abstract

Polarization beam splitters based on three-core photonic crystal fibers are presented. The two different polarizations of the input field are separated to the horizontal and vertical output ports. The splitter design has been done through modal and beam propagation analysis with high performance codes based on the finite element method. Device length of few mm and extinction ratios between -14 dB and -23 dB have been obtained for bandwidths up to 90 nm. ©2005 IEEE.


2005 - Single-mode regime of square-lattice photonic crystal fibers [Articolo su rivista]
Poli, F.; Foroni, M.; Bottacini, M.; Fuochi, M.; Burani, N.; Rosa, L.; Cucinotta, A.; Selleri, S.
abstract

The modal cutoff of square-lattice photonic crystal fibers with a finite number of air-hole rings has been accurately investigated to our knowledge for the first time. By analyzing the leaky behavior of the second-order mode, we have obtained a phase diagram that describes the regions of single-mode and multimode operation as well as the endlessly single-mode regime. Furthermore, starting from these results, we have obtained the cut-off normalized frequency according to two different formulations of the V parameter previously adopted for fibers with a triangular lattice. A final comparison of the cutoff properties of fibers characterized by a square lattice and a triangular lattice has been carried out. © 2005 Optical Society of America.


2005 - Test pattern for microwave dielectric properties of SrBi2Ta2O9 [Relazione in Atti di Convegno]
Delmonte, N.; Watts, B. E.; Rosa, L.; Chiorboli, G.; Cova, P.; Menozzi, R.
abstract

A test structure employing a one-step lithography process has been built for measuring the complex impedance of ferroelectric capacitors at microwaves. The measurements are compared to the results of a finite element analysis with the aim of developing an electrical model of the test structure in which parasitic elements appear. These elements can be experimentally measured and partially de-embedded. The purpose of this paper is the characterization of strontium-bismuth tantalate (SBT) capacitors for microwave ICs or SoCs.


2004 - Photonic crystal fiber based polarization splitter [Relazione in Atti di Convegno]
Selleri, S.; Rosa, L.; Poli, F.; Cucinotta, A.
abstract

A new polarization splitter based on a three-core photonic crystal fiber is presented. Device length, and splitting ratio values respectively lower than 7 mm and -10 dB have been obtained for a bandwidth larger than 40 nm.


2004 - Splitter di polarizzazione in fibra ottica a cristallo fotonico [Relazione in Atti di Convegno]
Ghidoni, S.; Rosa, L.; Bouk, A. H.; Selleri, S.; Cucinotta, A.; Poli, F.
abstract

A new polarization splitter based on triangular photonic crystal fiber is presented. The structure consists of a three core PCF that splits the input field of the central core into the two output ones with orthogonal polarizations. Splitting ratios higher than 10 dB and device length of less than 7 mm have been obtained.