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.
Gaetano Assanto, Noel F. Smyth, Spin-optical solitons in liquid crystals, Physical Review A, 102
In the framework of nonlinear spin optics, we investigate self-confined light beams in reorientational nematic liquid crystals. Using modulation theory and numerical experiments, we analyze spatial solitary waves supported by the geometric phase arising in a uniaxial when subject to a nonlinear modulation of its optic axis distribution. Spin evolution and optical reorientation in an index-homogeneous medium give rise to a longitudinally periodic, transversely inhomogeneous potential able to counteract the diffraction of a polarized bell-shaped beam, generating a spin-optical solitary wave. Spin-optical solitary waves evolve in polarization and have an oscillatory character in amplitude, size, and ellipticity.
Michal Kwasny, Miroslaw A. Karpierz, Gaetano Assanto, Urszula A. Laudyn, Optothermal vortex-solitons in liquid crystals, Optics Letters, 45 (8)
We report on vortex-solitons generated in dye-doped nematic liquid crystals by a purely optothermal nonlocal nonlinearity. This response not only supports stable doughnut-shaped ordinary-wave beams with orbital angular momentum, but also provides self-confined solitary waves with excellent trajectory and profile stability over time. Using an interferometric technique, we also investigate the role of nonlocal nonlinearity in the non-illuminated axial region.
Ula A. Laudyn, Michal Kwasny, Miroslaw A. Karpierz, Gaetano Assanto, Vortex nematicons in planar cells, Optics Express, 28 (6)
We provide experimental evidence that stable vortex-solitons in nematic liquid crystals, termed vortex nematicons, can be generated in planar cells without any external biases, neither electric nor magnetic. We report on nonlinear vortices with extraordinary-wave beams in various undoped samples, pin-pointing how material nonlocality and birefringence aid their stable propagation. Finally, we also demonstrate confinement and waveguiding of an incoherent co-polarized probe signal by the nonlinear vortex.
Gaetano Assanto, Cassandra Khan, Armando Piccardi, Noel F. Smyth, Temperature control of nematicon trajectories, Physical Review E, 100
Using modulation theory, we develop a simple [(2+1)-dimensional] model to describe the synergy between the thermo-optical and reorientational responses of nematic liquid crystals to light beams to describe the routing of spatial optical solitary waves (nematicons) in such a uniaxial environment. Introducing several approximations based on the nonlocal physics of the material, we are able to predict the trajectories of nematicons and their angular steering with temperature, accounting for the energy exchange between the input beam and the medium through one-photon absorption. The theoretical results are then compared to experimental data from previous studies, showing excellent agreement.
E. Talamas Simola, A. De Iacovo, J. Frigerio, A. Ballabio, A. Fabbri, G. Isella, and L. Colace, Voltage-tunable dual-band Ge/Si photodetector operating in VIS and NIR spectral range, Optics Express Vol. 27, Issue 6, pp. 8529-8539 (2019)
Extending and controlling the spectral range of light detectors is very appealing for several sensing and imaging applications. Here we report on a normal incidence dual band photodetector operating in the visible and near infrared with a bias tunable spectral response. The device architecture is a germanium on silicon epitaxial structure made of two back-to-back connected photodiodes. The photodetectors show a broad photoresponse extending from 390nm to 1600nm with the capability to electronically select the shorter (400-1100 nm) or the longer (1000-1600 nm) portion with a relatively low applied voltage. Devices exhibit peak VIS and NIR responsivities of 0.33 and 0.63 A/W, respectively, a low optical crosstalk (<-30dB), a wide dynamic range (>120dB) and, thanks to their low voltage operation, maximum specific detectivities of 7·1011cmHz1/2/W and 2·1010cmHz1/2/W in the VIS and NIR, respectively.
Gaetano Assanto, Nematicons: reorientational solitons from optics to photonics, Liquid Crystals Reviews, 6 (2)
The most recent advances on phenomena and effects which involve spatial optical solitons stemming from the reorientational optical response of nematic liquid crystals, the so called Nematicons, are reviewed hereby. After the initial assessment and basic understanding, in the past few years, significant progress on nematicon optics has included cavity-less beam bistability and hysteresis, spontaneous symmetry breaking, power-adjusted refraction, curved waveguides. In photonics, a novel approach for the bottom-up realization of permanent optical waveguides has been introduced, as well as a soliton-aided random laser with enhanced directionality and pointing control.
Gaetano Assanto, Panayotis Panayotaros, Noel F. Smyth, Mechanical analogies for nonlinear light beams in nonlocal nematic liquid crystals, Journal of Nonlinear Optical Physics & Materials, 27 (4)
The equations governing nonlinear light beam propagation in nematic liquid crystals form a (2+1)-dimensional system consisting of a nonlinear Schrödinger-type equation for the electric field of the wavepacket and an elliptic equation for the reorientational response of the medium. The latter is “nonlocal” in the sense that it is much wider than the size of the beam. Due to these nonlocal, nonlinear features, there are no known general solutions of the nematic equations; hence, approximate methods have been found convenient to analyze nonlinear beam propagation in such media, particularly the approximation of solitary waves as mechanical particles moving in a potential. We review the use of dynamical equations to analyze solitary wave propagation in nematic liquid crystals through a number of examples involving their trajectory control, including comparisons with experimental results from the literature. Finally, we make a few general remarks on the existence and stability of optically self-localized solutions of the nematic equations.
Sreekanth Perumbilavil, Armando Piccardi, Raouf Barboza, Oleksandr Buchnev, Martti Kauranen, Giuseppe Strangi & Gaetano Assanto , Beaming random lasers with soliton control, Nature Communications 9, 3863 (2018)
Random lasers are resonator-less light sources where feedback stems from recurrent scattering at the expense of spatial profile and directionality. Suitably-doped nematic liquid crystals can random lase when optically pumped near resonance(s); moreover, through molecular reorientation within the transparency region, they support self-guided optical spatial solitons, i.e., light-induced waveguides. Here, we synergistically combine solitons and collinear pumping in weakly scattering dye-doped nematic liquid crystals, whereby random lasing and self-confinement concur to beaming the emission, with several improved features: all-optical switching driven by a low-power input, laser directionality and smooth output profile with high-conversion efficiency, externally controlled angular steering. Such effects make soliton-assisted random lasers an outstanding route towards application-oriented random lasers.
A. De Iacovo, C. Venettacci, L. Colace, L. Scopa, and S. Foglia, Noise performance of PbS colloidal quantum dot photodetectors, Appl. Phys. Lett. Vol. 111, Iss. 21, 211104 (2017)
We report on the noise characterization of photoconductors based on PbS colloidal quantum dots. The devices operate in the near infrared region with peak responsivity exceeding 70 A/W at 1.3 μm at low optical intensity and low voltage bias. The large responsivity, combined with the low dark current of high resistance devices, provides a specific detectivity D* as large as 1011 cm Hz1/2 W−1. The noise characteristics are investigated using noise current spectra measured at different biases both in dark and under optical excitation. The analysis revealed that the noise is clearly dominated by the flicker component up to 100 kHz. The noise performance is investigated at different optical intensities and for different device dimensions and voltage biases.
Alessandro Alberucci , Chandroth P. Jisha, Lorenzo Marrucci, and Gaetano Assanto, Electromagnetic Confinement via Spin-Orbit Interaction in Anisotropic Dielectrics, ACS Photonics, 2016, 3 (12), pp 2249-2254
We investigate electromagnetic propagation in uniaxial dielectrics with a transversely varying orientation of the optic axis, the latter staying orthogonal everywhere in the propagation direction. In such a geometry, the field experiences no refractive index gradients, yet it acquires a transversely modulated Pancharatnam-Berry phase, that is, a geometric phase originating from a spin-orbit interaction. We show that the periodic evolution of the geometric phase versus propagation gives rise to a longitudinally invariant effective potential. In certain configurations, this geometric phase can provide transverse confinement and waveguiding. The theoretical findings are tested and validated against numerical simulations of the complete Maxwell\'s equations. Our results introduce and illustrate the role of geometric phases on electromagnetic propagation over distances well exceeding the diffraction length, paving the way to a whole new family of guided waves and waveguides that do not rely on refractive index tailoring.
Urszula A. Laudyn, Paweł S. Jung, Mirosław A. Karpierz & Gaetano Assanto, Quasi two-dimensional astigmatic solitons in soft chiral metastructures, Scientific Reports 6, Article number: 22923 doi:10.1038/srep22923
We investigate a non-homogeneous layered structure encompassing dual spatial dispersion: continuous diffraction in one transverse dimension and discrete diffraction in the orthogonal one. Such dual diffraction can be balanced out by one and the same nonlinear response, giving rise to light self-confinement into astigmatic spatial solitons: self-focusing can compensate for the spreading of a bell-shaped beam, leading to quasi-2D solitary wavepackets which result from 1D transverse self-localization combined with a discrete soliton. We demonstrate such intensity-dependent beam trapping in chiral soft matter, exhibiting one-dimensional discrete diffraction along the helical axis and one-dimensional continuous diffraction in the orthogonal plane. In nematic liquid crystals with suitable birefringence and chiral arrangement, the reorientational nonlinearity is shown to support bell-shaped solitary waves with simple astigmatism dependent on the medium birefringence as well as on the dual diffraction of the input wavepacket. The observations are in agreement with a nonlinear nonlocal model for the all-optical response.
Andrea De Iacovo, Carlo Venettacci, Lorenzo Colace, Leonardo Scopa, Sabrina Foglia, PbS Colloidal Quantum Dot Photodetectors operating in the near infrared, Scientific Reports 6, Article number: 37913 (2016)
Colloidal quantum dots have recently attracted lot of interest in the fabrication of optoelectronic devices due to their unique optical properties and their simple and low cost fabrication. PbS nanocrystals emerged as the most advanced colloidal material for near infrared photodetectors. In this work we report on the fabrication and characterization of PbS colloidal quantum dot photoconductors. In order to make devices suitable for the monolithic integration with silicon electronics, we propose a simple and low cost process for the fabrication of photodetectors and investigate their operation at very low voltage bias. Our photoconductors feature high responsivity and detectivity at 1.3 μm and 1 V bias with maximum values of 30 A/W and 2x10^10 cmHz^(1/2)W^(−1), respectively. Detectivity close to 10^11 cmHz^(1/2)W^(−1) has been obtained resorting to bridge sensor readout.
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.
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