University of Rome "Roma Tre", Department of Engineering, INFN, CNISM
The University "Roma Tre", established in 1992, has developed with NooEL a
strong commitment in Optoelectronics and Photonics, carrying out advanced
research in several areas of nonlinear optics, nanophotonics and near infrared detection.
V. Sorianello, G. De Angelis, A. De Iacovo, L. Colace, S. Faralli, and M. Romagnoli, High responsivity SiGe heterojunction phototransistor on silicon photonics platform, Opt. Express 23(22), 28163-28169 (2015)
We report on a novel near infrared SiGe phototransistor fabricated by a standard silicon photonics foundry. The device is first investigated by simulations. The fabricated devices are characterized in terms of current-voltage characteristics at different optical power. Typical phototransistors exhibit 1.55um record responsivity at low optical power exceeding 232A/W and 42A/W at 5V and 1V bias, respectively. A differential detection scheme is also proposed for the dark current cancellation to significantly increase the device sensitivity.
A. Alberucci, A. Piccardi, N. Kravets, O. Buchnev, and G. Assanto, Soliton enhancement of sponaneous symmetry breaking, Optica 2 (9), 783-9 (2015)
Spontaneous symmetry breaking (SSB) occurs when noise triggers an initially symmetric system to evolve toward one of its nonsymmetric states. Topological and optical SSB involve material reconfiguration/transition and light propagation/distribution in time or space, respectively. In anisotropic optical media, light beam propagation and distribution of the optic axis can be linked, thereby connecting topological and optical SSB. Using nonlinear soft matter, namely uniaxial liquid crystals, we report on simultaneous topological and optical SSB, showing that spatial solitons enhance the noise-driven transition of the medium from a symmetric to an asymmetric configuration, while acquiring a power-dependent transverse velocity in either of two specular directions with respect to the initial wavevector. Solitons enhance SSB by further distorting the optic axis distribution through nonlinear reorientation, resulting in power-tunable walk-off as well as hysteresis in beam refraction versus angle of incidence.
Armando Piccardi, Alessandro Alberucci, Nina Kravets, Oleksandr Buchnev, Gaetano Assanto, Power-controlled transition from standard to negative refraction in reorientational soft matter, Nat. Commun. 5:5533 doi: 10.1038/ ncomms6533 (2014)
Refraction at a dielectric interface can take an anomalous character in anisotropic crystals, when light is negatively refracted with incident and refracted beams emerging on the same side of the interface normal. In soft matter subject to reorientation, such as nematic liquid crystals, the nonlinear interaction with light allows tuning of the optical properties. We demonstrate that in such material a beam of light can experience either positive or negative refraction depending on input power, as it can alter the spatial distribution of the optic axis and, in turn, the direction of the energy flow when traveling across an interface. Moreover, the nonlinear optical response yields beam self-focusing and spatial localization into a self-confined solitary wave through the formation of a graded-index waveguide, linking the refractive transition to power-driven readdressing of copolarized guided-wave signals, with a number of output ports not limited by diffraction.
Nina Kravets, Armando Piccardi, Alessandro Alberucci, Oleksandr Buchnev, Malgosia Kaczmarek, and Gaetano Assanto, Bistability with Optical Beams Propagating in a Reorientational Medium, Phys. Rev. Lett. 113, 023901 (2014)
We investigated bistability with light beams in reorientational nematic liquid crystals. For a range of input powers, beams can propagate as either diffracting or self-trapped, the latter corresponding to spatial solitons. The first-order transition in samples exhibiting abrupt self-focusing with a threshold is in agreement with a simple model.
R. Barboza, U. Bortolozzo, G. Assanto, E. Vidal-Henriquez, M. G. Clerc, and S. Residori, Harnessing optical vortices in soft matter lattices, Phys. Rev. Lett. 111, 093902 (2013)
By creating self-induced vortexlike defects in the nematic liquid crystal layer of a light valve, we demonstrate the realization of programable lattices of optical vortices with arbitrary distribution in space. On each lattice site, every matter vortex acts as a photonic spin-to-orbital momentum coupler and an array of circularly polarized input beams is converted into an output array of vortex beams with topological charges consistent with the matter lattice. The vortex arrangements are explained on the basis of light-induced matter defects of both signs and consistent topological rules.
A. Alberucci, and G. Assanto, Light Self-Localization and Power-Dependent Steering in Anisotropic Dielectrics: Spatial Solitons in Uniaxial Nematic Liquid Crystals, Progress in Optical Science and Photonics, Ed. B. Malomed, Springer (Berlin), (2013)
We discuss nonlinear propagation of light beams in anisotropic media, addressing the role of nonlocality and nonlinearity in power-dependent beam self-steering. With specific reference to spatial solitons in positive uniaxial nematic liquid crystals (i.e. nematicons), we describe soliton self-acceleration through reorientational response and nonlinear walk-off.
G. Assanto, A. Alberucci and A. Piccardi, Nematicons: Spatial Optical Solitons in Nematic Liquid Crystals, Nematicons, Ed. G. Assanto, Wiley, ISBN: 978-0-470-90724-5 (2012)
This book introduces the fundamentals of nematicons, their basic features and models, potential applications and novel phenomena.
R. Barboza, U. Bortolozzo, G. Assanto, E. Vidal, M. G. Clerc and S. Residori, Vortex induction via anisotropy self-stabilized light-matter interaction, Phys. Rev. Lett. 109 (14), 143901 (2012)
By sending circularly polarized light beams onto a homeotropic nematic liquid crystal cell with a photosensitive wall, we are able to locally induce spontaneous matter vortices that remain, each, stable and trapped at the chosen location. We discuss the dual light-matter nature of the created vortices and demonstrate the ability of the system to create optical vortices with opposite topological charges that, consistent with angular momentum conservation, both derive from the same defect created in the liquid crystal texture. Theoretically, we identify a self-stabilizing mechanism for the matter vortex, which is provided by the concurrency of light-induced gradients and anisotropy of the elastic constants that characterize the deformation of the liquid crystal medium.
M. Peccianti and G. Assanto, Nematicons, Phys. Rep. 516, 147-208 (2012)
We review the recent developments of spatial optical solitons in nematic liquid crystals, also known as Nematicons. We outline their properties, linked to a reorientational nonlinearity in the presence of significant optical anisotropy and spatial nonlocality. The molecular response is described in conjunction with the pertinent wave-evolution models and their reduction to known equations. Various features and experimental results are reported in to highlight the main concepts and point out potential applications.
A. Piccardi, A. Alberucci, N. Tabiryan and G. Assanto, Dark Nematicons, Opt. Lett. 36 (8), 1456-8 (2011)
We experimentally demonstrate and model dark spatial solitons in azo-doped liquid crystals, in the presence of saturation and nonlocality of the effective nonlinearity due to changes in molecular order. The guiding properties of dark solitons are probed with a weak input of different wavelength
O. Descalzi, M. Clerc, S. Residori, G. Assanto (Editors), Localized States in Physics: Solitons and Patterns, 1st Edition, Springer (Berlin) ISBN: 978-3-642-16548-1 (2011)
Systems driven far from thermodynamic equilibrium can create dissipative structures through the spontaneous breaking of symmetries. A particularly fascinating feature of these pattern-forming systems is their tendency to produce spatially confined states. These localized wave packets can exist as propagating entities through space and/or time. Various examples of such systems will be dealt with in this book, including localized states in fluids, chemical reactions on surfaces, neural networks, optical systems, granular systems, population models, and Bose-Einstein condensates.This book should appeal to all physicists, mathematicians and electrical engineers interested in localization in far-from-equilibrium systems. The authors - all recognized experts in their fields - strive to achieve a balance between theoretical and experimental considerations thereby giving an overview of fascinating physical principles, their manifestations in diverse systems, and the novel technical applications on the horizon.
SPIE Photonics Europe 2016
Group IV Photonics 2015
GE 2015 Annual Meeting
5th Workshop on Liquid Crystals for Photonics
Nonlinear Photonics 2014
Group IV Photonics 2014