All-Optical Reinforcement Learning in Solitonic X-Junctions

L'etologia ha dimostrato che gruppi di animali o colonie possono eseguire calcoli complessi distribuendo semplici processi decisionali ai membri del gruppo. Ad esempio, le colonie di formiche possono ottimizzare le traiettorie verso il cibo eseguendo sia un rinforzo (o una cancellazione) delle tracce di feromone sia spostarsi da una traiettoria ad un'altra con feromone più forte. Questa procedura delle formiche possono essere implementati in un hardware fotonico per riprodurre l'elaborazione del segnale stigmergico. Presentiamo qui innovative giunzioni a X completamente integrate realizzate utilizzando guide d'onda solitoniche in grado di fornire entrambi i processi decisionali delle formiche. Le giunzioni a X proposte possono passare da comportamenti simmetrici (50/50) ad asimmetrici (80/20) utilizzando feedback ottici, cancellando i canali di uscita inutilizzati o rinforzando quelli usati.Ethology has shown that animal groups or colonies can perform complex calculation distributing simple decision-making processes to the group members. For example ant colonies can optimize the trajectories towards the food by performing both a reinforcement (or a cancellation) of the pheromone traces and a switch from one path to another with stronger pheromone. Such ant's processes can be implemented in a photonic hardware to reproduce stigmergic signal processing. We present innovative, completely integrated X-junctions realized using solitonic waveguides which can provide both ant's decision-making processes. The proposed X-junctions can switch from symmetric (50/50) to asymmetric behaviors (80/20) using optical feedbacks, vanishing unused output channels or reinforcing the used ones

Photoacoustic detection of circular dichroism in a square array of nano-helices

A novel nano-structured material has been assembled by means of a focused ion beam technique. This artificial material is composed of a square array of nano-helices built upon a multilayered substrate. Optical measurements of circular dichroism of a sample are confirmed by photo-acoustic investigations, which allow to directly study the helix-field interaction apart from the dielectric substrate. The study is consistent with 3D numerical simulations, and demonstrates to be an efficient tool of investigation for the entire class of these novel structured materials

Self-Phase-Matched Second-Harmonic and White-Light Generation in a Biaxial Zinc Tungstate Single Crystal

Second-order nonlinear optical materials are used to generate new frequencies by exploiting second-harmonic generation (SHG), a phenomenon where a nonlinear material generates light at double the optical frequency of the input beam. Maximum SHG is achieved when the pump and the generated waves are in phase, for example through birefringence in uniaxial crystals. However, applying these materials usually requires a complicated cutting procedure to yield a crystal with a particular orientation. Here we demonstrate the first example of phase matching under the normal incidence of SHG in a biaxial monoclinic single crystal of zinc tungstate. The crystal was grown by the micro-pulling-down method with the (102) plane perpendicular to the growth direction. Additionally, at the same time white light was generated as a result of stimulated Raman scattering and multiphoton luminescence induced by higher-order effects such as three-photon luminescence enhanced by cascaded third-harmonic generation. The annealed crystal offers SHG intensities approximately four times larger than the as grown one; optimized growth and annealing conditions may lead to much higher SHG intensities

Resonant Absorption in GaAs-Based Nanowires by Means of Photo-Acoustic Spectroscopy

Semiconductor nanowires made of high refractive index materials can couple the incoming light to specific waveguide modes that offer resonant absorption enhancement under the bandgap wavelength, essential for light harvesting, lasing and detection applications. Moreover, the non-trivial ellipticity of such modes can offer near field interactions with chiral molecules, governed by near chiral field. These modes are therefore very important to detect. Here, we present the photo-acoustic spectroscopy as a low-cost, reliable, sensitive and scattering-free tool to measure the spectral position and absorption efficiency of these modes. The investigated samples are hexagonal nanowires with GaAs core; the fabrication by means of lithography-free molecular beam epitaxy provides controllable and uniform dimensions that allow for the excitation of the fundamental resonant mode around 800 nm. We show that the modulation frequency increase leads to the discrimination of the resonant mode absorption from the overall absorption of the substrate. As the experimental data are in great agreement with numerical simulations, the design can be optimized and followed by photo-acoustic characterization for a specific application

Second harmonic generation on self-assembled GaAs/Au nanowires with thickness gradient

Here we investigated the SH generation at the wavelength of 400 nm (pump laser at 800 nm, 120 fs pulses) of a "metasurface" composed by an alternation of GaAs nano-grooves and Au nanowires capping portions of flat GaAs. The nano-grooves depth and the Au nanowires thickness gradually vary across the sample. The samples are obtained by ion bombardment at glancing angle on a 150 nm Au mask evaporated on a GaAs plane wafer. The irradiation process erodes anisotropically the surface, creating Au nanowires and, at high ion dose, grooves in the underlying GaAs substrate (pattern transfer). The SHG measurements are performed for different pump linear polarization angle at different positions on the "metasurface" in order to explore the regions with optimal conditions for SHG efficiency. The pump polarization angle is scanned by rotating a half-wave retarder plate. While the output SH signal in reflection is analyzed by setting the polarizer in s or p configuration in front of the detector. The best polarization condition for SHG is obtained in the configuration where the pump and second harmonic fields are both p polarized, and the experiments show a SH polarization dependence of the same symmetry of bulk GaAs. Thus, the presence of gold contributes only as field localization effect, but do not contributes directly as SH generator

Nanostructured materials for circular dichroism and chirality at the nanoscale: towards unconventional characterization [Invited]

In this work, we review the last attempts to use nanostructured materials for the enhancement of the chiro-optical effects at the nanoscale. Starting from the numerical design, we review different geometries that exhibit circular dichroic behavior in the far field; we then focus on the new branch of near-field chirality, where numerous nanostructures have been proposed for background-free chiral sensing. The next section reports on nanofabrication methods, with a special focus on self-assembling, cost- and time-efficient techniques. Finally, we review the chiro-optical experiments. Besides conventional extinction-based techniques, we are today able to reveal chiro-optical effects via photothermal behavior and photoluminescence, going down to single nanostructure chirality with sophisticated near-field techniques. We believe that the novel designs, state-of-the-art nanofabrication and modern characterization techniques have come to a stage to provide chiro-optical sensors and light components based on nanostructures

Quantitative evaluation of emission properties and thermal hysteresis in the mid-infrared for a single thin film of vanadium dioxide on a silicon substrate

We present a comparative study of the emission properties of a vanadium dioxide thin film (approximately 200 nm) deposited on a silicon wafer in different sub-spectral-ranges of the mid-infrared, with particular attention to the windows of transparency of the atmosphere to the infrared radiation (i.e., 3–5 μm, 8–12 μm). The infrared emission properties of the structure are closely related to the well-known phase transition of the first order, from semiconductor to metal, of the vanadium dioxide around the temperature of 68 °C. The characterization of the emissivity in the sub-regions of the mid-infrared was carried out both in the front configuration, that is on the VO2 film side, and in the rear configuration on the silicon wafer side, and showed a strong difference in the hysteresis thermal bandwidth, in particular between the short wave region and the long wave region. The bandwidth is equal to 12 °C for the front and 15 °C for the rear. The emissivity behaviors as a function of temperature during the semiconductor-metal transition in the mid-infrared subregions were analyzed and explained using the theories of the effective medium of Maxwell Garnett and Bruggeman, highlighting the greater functionality of one theory with respect to the other depending on the spectral detection band

Bound and free waves in non-collinear second harmonic generation

We analyze the relationship between the bound and the free waves in the noncollinear SHG scheme, along with the vectorial conservation law for the different components arising when there are two pump beams impinging on the sample with two different incidence angles. The generated power is systematically investigated, by varying the polarization state of both fundamental beams, while absorption is included via the Herman and Hayden correction terms. The theoretical simulations, obtained for samples which are some coherence length thick show that the resulting polarization mapping is an useful tool to put in evidence the interference between bound and free waves, as well as the effect of absorption on the interference patternComment: 10 pages, 7 figure. to be published on Optics Expres

Constraints on transmission, dispersion, and density of states in dielectric multilayers and stepwise potential barriers with arbitrary layer arrangement

Normal-incidence transmission and dispersion properties of optical multilayers and one-dimensional stepwise potential barriers in the non-tunneling regime are analytically investigated. The optical paths of every constituent layer in a multilayer structure, as well as the parameters of every step of the stepwise potential barrier, are constrained by a generalized quarter-wave condition. No other restrictions on the structure geometry is imposed, i.e., the layers are arranged arbitrarily. We show that the density of states (DOS) spectra of the multilayer or barrier in question are subject to integral conservation rules similar to the Barnett-Loudon sum rule but ocurring within a finite frequency or energy interval. In the optical case, these frequency intervals are regular. For the potential barriers, only non-periodic energy intervals can be present in the spectrum of any given structure, and only if the parameters of constituent potential steps are properly chosen. Abstract The integral conservation relations derived analytically have also been verified numerically. The relations can be used in dispersion-engineered multilayer-based devices, e.g., ultrashort pulse compressors or ultracompact optical delay lines, as well as to design multiple-quantum-well electronic heterostructures with engineered DOS.Comment: 10 pages, 5 figures, to be submitted to PR

TOPS project: Development of new fast timing plastic scintillators.

In particle physics charged particles are measured exploiting many different detection strategies. The plastic scintillators are cheap, versatile and show good time response, thus are traditionally employed as timing detectors. TOPS (Time Of flight Plastic Scintillators) is an R&D project devoted to the synthesis and characterization of a novel class of plastic scintillators. Liquid and solid sam- ples of tens of new scintillators have been tested and characterized. Some of them (2N, 1N, 2B, P2, T2) have shown a larger light output with respect to antracene, a standard benchmark material, and good timing properties. In order to improve the matching between the scintillators emission and the optimal trasmittive region in the absorption spectra, a doping material has been added as wave-shifter. The use of POPOP as doping improved the performances of a fraction of the scintillator samples. Based on the comparison of the light output values in measurements with cosmic rays, a selection of the most promising scintillators has been investigated also from the timing point of view. The scintillation time characteristics of the TOPS plastic samples have been studied with minimum ionizing particles using a commercial plastic scintillator BC-412 as a reference. The light output and timing properties have been also investigated with proton beams at different energies (70, 120, 170, 220 MeV) and show promising results providing a time of flight measure- ments accuracy of 150–300ps. In this contribution, preliminary results obtained with this new class of scintillators developed in the TOPS project will be presented