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Zinc oxide-zinc tungstate (ZnO-ZnWO

The mimicking of chiral effects at the nanoscale usually requires complex nanostructured systems fabricated by using highly sophisticated techniques. Here, we demonstrate that low-cost diffractive metasurfaces, fabricated by nanosphere lithography, can offer a remarkably rich chiral broadband response in the near-infrared range. We measure the spin-dependent response as a function of the incident angle and wavelength. The resulting extinction maps reveal switching of the resonances governed by extrinsic chirality. We further employ numerical modeling to gain an insight into the electromagnetic behavior over the unit cell. The simulation results agree with experiments, and we further investigate the possibility of tailoring chirality by matching the refractive index below and above the metasurface. Finally, we investigate chiral emission of the ensemble of electric dipoles embedded in the refractive-index-matching layer. This approach opens alternative perspectives for chiral surface-lattice resonances and chiral emission in low-cost materials.

Plasmonic nanostructures with broken symmetry have been proposed for the control of the circular polarization at the nanoscale, important for applications spanning from chiral sensing to circularly polarized nanosources. We recently demonstrated that the low-cost self-assembling nanosphere lithography, combined with tilted metal deposition, opens new possibilities for the design and fabrication of chiral nanostructures. Starting from polystyrene nanospheres (PNS) covered by asymmetric metallic shell, we measured chirality in absorption at 633 nm [1] . We discovered that the nanohole array (NHA) formed due to the shadow, can exhibit chiral properties in a broad near-infrared range [2] , [3] .

Characterization of both natural and artificial chiral structures involves measurements of differential absorption of circularly polarized light of opposite handedness, i.e. circular dichroism (CD). Here we use a scattering-free, non-destructive photo-deflection technique [1] to detect optical chirality in diffracted orders of a metasurface.

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.

Optical enantioselectivity of chiral molecules could be enhanced by depositing them on suitable nanostructured substrates. Different kind of chiral substrates can be developed, but chiral features are in general difficult to fabricate or costly. Self-assembled approach allows realizing plasmonic metasurfaces with a low cost reliable procedure. In this case asymmetric fabrication parameters can induce chiral optical response of the realised substrate. Self-organized polystyrene spheres deposited on glass substrate, are utilised to produce asymmetric hole array on a metal thin film. In our case the spheres (518 nm in diameter) where reduced by selective reactive ion etching and then covered by gold (and other metals), that is evaporated at a glancing angle. After the removing of the spheres an elliptical-hole array is produced forming a circular-dichroic substrate. The circular dichroic response of light interacting with the substrate can be tuned by choosing proper incidence angle and excitation wavelength, while the flat nature of the metasurface is very useful for easy molecular deposition processes. Two new enantiomers (right-handed and left-handed molecules) have been synthesized in order to present a good circular dichroism in the visible range and to be tested on the realized metasurfaces. Different tests were carried out on the samples, investigating the spectral optical properties of the structures with and without chiral molecules on top of them. The results are very promising due to the possibility of easily tuning and optimizing the optical response.

In this work we present an approach to induce chiral effects in well-known plasmonic nanohole arrays with triangular unit cell. Arrays with circular nanoholes in metals can exhibit interesting light-matter interaction mechanisms such as surface plasmon polaritons and extraordinary optical transmission. Moving from circular to elliptical shape and tilting the ellipse away from the lattice symmetry lines, a symmetry breaking induces a different absorption of the circularly polarized light of opposite handedness, i.e. circular dichroism. We numerically investigate circular dichroism at normal incidence in elliptic nanoholes in Au in the spectral range 400–1000 nm. We focus on its dependence on the elliptic nanohole tilt, and further proceed with the ellipse radii optimization. The optimized circular dichroism reaches 84%, and we report on the near field absorption distribution at the wavelengths of this maximum value.

2D metasurfaces based on periodic nanoholes in metal have been proposed in various plasmonic platforms. Specifically, their resonant features have led to applications spanning in biosensing. Here we investigate additional degree of freedom in elliptical nanohole arrays with hexagonal geometry: chiro-optical effects. Namely, the in-plane asymmetry and a slightly elliptical shape of nanoholes were previously shown to differently extinct light of opposite handedness, even at normal incidence. We now fully characterize nanoholes in Ag, fabricated by low-cost nanosphere lithography. We first measure the dependence of the transmitted intensity for opposite handedness, in a broad spectral and angle of incidence range. We then resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. Finally, we numerically investigate the origin of the chiro-optical effect at the nanoscale. We believe that circular polarization resolving of the transmitted degree could be further adapted as a highly sensitive tool in chiral sensing.

Addition of asymmetry in plasmonic nanostructures can lead to chiro-optical phenomena, usually monitored as different absorption of left and right polarization, i.e. circular dichroism. Moreover, interesting features arise when the nanostructure changes the polarization state of the input beam. In this work, we perform extrinsic chirality characterization in a widely tuneable near-infrared range, by monitoring both polarization of the input and of the transmitted beam. We characterize low-cost metasurfaces based on polystyrene nanospheres asymmetrically covered by Ag, by exciting them at different angle of incidence with left, right and linear polarization. We then resolve the circular polarization degree of the transmitted beam, demonstrating resonance-governed circular polarization degree in the output, showing the interplay of both intrinsic and extrinsic chirality.

Functionalized gold and silver nanoparticles have been prepared and characterized by means of spectroscopic and morphological techniques together focused on electrical measurements on cast deposited films. The Au and Ag nanoparticles have been functionalized with a on purpose prepared conjugated dithiol, 9,9-didodecyl-2,7-bis-thiofluorene (FL), giving rise to organic solvents soluble AuNPs-FL and AgNPs-FL samples, respectively. In the case of AuNPs-FL, well separated nanoparticles, with an average size of about 4 nm, assembled into bidimensionally monolayer network and with regular spatial distributions have been observed by TEM. Synchrotron radiation-XPS data support the observed network formation, showing that all bifunctional ligands end-groups are covalently bonded to metal noble atoms at the nanoparticles surfaces. In order to investigate their interesting conduction properties, electrical measurements evidenced a non-ohmic behavior in the case of the AuNPs-FL thin film, with a conduction mechanism that strongly depends on polarons and bipolarons along the carbon active chain belonging to the fluorene bridge. By increasing the stacked layers of the AuNPs-FL thin film, the conductivity behavior changed following approximately the ohmic law. On the contrary, AgNPs-FL shows higher conductivity, and upon an aging process, a diode behavior was observed that opens perspectives as flexible optoelectronics devices.

The light interacting on asymmetric nanostructures can give rise to the phenomenon called extrinsic chirality [1–3], even if the nanostructures are not chiral by themselves. In this cases, light can be manipulated in order to obtain interesting behaviour such as anomalous refraction [4] or circular dichroism [5].

Photoacoustic Spectroscopy (PAS) and Photothermal deflection Spectroscopy (PDS) are excellent techniques for studying the optical absorption spectra of opaque and highly light-scattering substances such as nanomaterials. In this paper we review recent advances in the methodology of PAS and its novel applications. In particular we discuss the performance of PAS to detect circular dichroism of intrinsic chiral materials [1] as well as extrinsic pseudo-chiral metasurfaces [2], showing how the selective absorption of circularly polarized light depend on the orientation of the metasurface. We also applied PAS in the VIS/IR range to measure the resonant absorption peaks related to the guided modes of GaAs-based NW on Si [3–5].

Asymmetric nanostructures can mimic a chiral response when circular polarized light interacts with the structures under particular angle of incidence [1]. This phenomenon is called ‘extrinsic chirality’ and usually is present under linear optical investigation with low visibility. Due to the fact that optical second harmonic generation is possible only in samples with some degree of asymmetry, this can be used in order to investigate the extrinsic chirality with a background free technique, thus inducing a high visibility of the artificial circular dichroism [2,3]. Here we present the second harmonic generation (SHG) measurements obtained on samples composed by GaAs nanowires grown on silicon. The wires present resonant leaky modes around 800nm and at 400nm due to the high refractive index contrast ratio between wires and air, even if these wavelengths lie on the absorption band of GaAs [4]. The measurements performed on this sample present good SHG signal due to the second order nonlinear term of GaAs, but did not present any circular dichroism (SHG-CD). By coating the sample with a 20nm thin layer of gold deposited asymmetrically, by evaporating the metal only from one side of the nanowires, the symmetry of the structure is broken, thus induced high SHG-CD. The SHG-CD is measured by shining the sample with circular polarized pump light at the fundamental wavelength of 800nm and by revealing the second harmonic signal at 400nm in s or p polarization, as a function of sample rotation. Four samples were measured with GaAs wires of about 5 micron in length with different diameters ranging from 140nm to 200nm. In this case it is possible to explore different resonance conditions and different SHG-CD is revealed. For each sample, the measured were carried out before and after the asymmetric gold layer deposition, thus allowing direct comparison of the results. References [1] A. Belardini, M. C. Larciprete, M. Centini, E. Fazio, C. Sibilia, D. Chiappe, C. Martella, A. Toma, M. Giordano, and F. Buatier de Mongeot, Phys. Rev. Lett. 107, 257401 (2011). [2] A. Belardini, M. Centini, G. Leahu, D. C. Hooper, R. Li Voti, E. Fazio, J. W. Haus, A. Sarangan, V. K. Valev, C. Sibilia, Sci. Rep. 2016, 6, 31796. [3] G. Leahu, E. Petronijevic, A. Belardini et al., Adv. Optical Mater. 2017, 1601063 (2017). [4] G. Leahu, E. Petronijevic, A. Belardini, M. Centini, R. Li Voti, T. Hakkarainen, E. Koivusalo, M. Guina, C. Sibilia. Sci. Rep. 7, 2833 (2017).

A great deal of interest over the years has been directed to the optical space solitons for the possibility of realizing 3D waveguides with very low propagation losses. A great limitation in their use for writing complex circuits is represented by the impossibility of making curved structures. In the past, solitons in nematic liquid crystals, called nematicons, were reflected on electrical interfaces, and more recently photorefractive spatial solitons have been, as well. In the present work, we investigate refraction and total reflection of spatial solitons with saturable electro-optic nonlinearity, such as the photorefractive ones, on an electric wall acting as a reflector. Using a custom FDTD code, the propagation of a self-confined beam was analyzed as a function of the applied electric bias. The electrical reflector was simulated by applying different biases in two adjacent volumes. We observed both smaller and larger angles of refraction, up to the critical &pi, /2-refraction condition, and then the total reflection. The radii of curvature of the associated guides can be varied from centimeters down to hundreds of microns. The straight guides showed losses as low as 0.07 dB/cm as previously observed, while the losses associated with single curves were estimated to be as low as 0.2 dB.

Arrays of nanoholes in metal are important plasmonic devices, proposed for applications spanning from biosensing to communications. In this work, we show that in such arrays the symmetry can be broken by means of the elliptical shape of the nanoholes, combined with the in-plane tilt of the ellipse axes away from the array symmetry lines. The array then differently interacts with circular polarizations of opposite handedness at normal incidence, i.e., it becomes intrinsically chiral. The measure of this difference is called circular dichroism (CD). The nanosphere lithography combined with tilted silver evaporation was employed as a low-cost fabrication technique. In this paper, we demonstrate intrinsic chirality and CD by measuring the extinction in the near-infrared range. We further employ numerical analysis to visualize the circular polarization coupling with the nanostructure. We find a good agreement between simulations and the experiment, meaning that the optimization can be used to further increase CD.

Photoacoustic spectroscopy investigation was carried out on ZnO nanoparticles grown at 80 °C on porous surface of diatomite (DE) by sol–gel technique, using zinc acetate dihydrate as ceramic precursor and triethanolamine to mediate the surface growth of the nanoparticles. Absorption and scattering characteristics of hybrid ZnO/DE powder in the UV–Vis range were inferred by photoacoustic spectroscopy, and results were analyzed based on Helander’s theory. In particular, we discussed in detail the procedure to calculate the absorption and scattering spectra of the hybrid powder showing how from the scattering coefficient in the 300–450 nm range it is possible to obtain information on the size distribution of the ZnO nanospheres. By applying photoacoustic spectroscopy for different modulation frequencies, we showed that is also possible to perform a size distribution depth profile of the ZnO aggregates, opening the way to interesting developments in this research field. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.

The possibility to control the infrared (IR) absorption and thermal emission on subwavelength scales has attracted large interest in the recent years thanks to the opportunities granted by nanostructured metamaterials. For example highly frequency selective and directional thermal emitters/absorbers have been proposed for a large variety of applications ranging from sensing and security to cooling and energy harvesting [1]. In order to introduce a control of the emissivity as a function of the temperature thermochromic and phase change materials have been considered. In particular Vanadium dioxide (VO2) has become a widely-studied material for applications such as metamaterials, smart windows and supercapacitors, thanks to its strong optical transmittance changes at the IR and THz regions and huge resistance jump [2]. The control mechanism is achieved by taking advantage of the metal-insulator phase transition of VO2 at its critical temperature (~68 °C). We numerically show the control of spatial and spectral features of the far field thermal emission pattern of nanoantenna arrays, composed of alternating Gold and VO2 rods, as a function of the temperature. In this work we performed a numerical study by modifying a previously developed model [3] based on the fluctuational electrodynamics approach and on the discretization of the resulting volume integral equation to calculate relative emissivity and spatial emission pattern of nanoparticle ensembles smaller than the thermal wavelength lam=hc/kBT [4]. The drastic changes of the VO2 refractive index across its metal-insulator phase transition produce strong differences in the behavior of the overall system by creating or destroying evanescent wave coupling between different elements of the nanoantenna. The study of IR thermal nano-emitters is crucial for the realization of coherent thermal nano-sources in the mid and far IR for sensing applications and thermal management as well as thermal logic gates on the nanoscale. References: [1] I. E. Khodasevych, L. Wang, A. Mitchell, and G. Rosengarten, Micro- and nano- structured surfaces for selective solar absorption, Adv. Opt. Mat. 3, 852 (2015). [2] Liu, M. et al. Phase transition in bulk single crystals and thin films of VO2 by nanoscale infrared spectroscopy and imaging. Phys. Rev. B 91, 245155 (2015). [3] M. Centini, A. Benedetti, M. C. Larciprete, A. Belardini, R. Li Voti, M. Bertolotti, C. Sibila, “Midinfrared thermal emission properties of finite arrays of gold dipole nanoantennas” Phys. Rev. B 92(20) 205411 (2015) [4] C. Wuttke and A. Rauschenbeutel, “Thermalization via Heat Radiation of an Individual Object Thinner than the Thermal Wavelength”, Phys. Rev. Lett. 111, 024301 (2013)

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.

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. acceptedVersion

We report on the extrinsic chirality behavior of GaAs-based NWs asymmetrically hybridized with Au. The samples are fabricated by a recently developed, lithography-free self-organized GaAs growth, with the addition of AlGaAs shell and GaAs supershell. The angled Au flux is then used to cover three-out-of-six sidewalls with a thin layer of Au. Oblique incidence and proper sample orientation can lead to circular dichroism. We characterize this chiral behavior at $$ 532\,{\text{nm}} $$ and $$ 980\,{\text{nm}} $$ by means of photo-acoustic spectroscopy, which directly measures the difference in absorption for the circularly polarized light of the opposite headedness. For the first time to our knowledge, circular dichroism is observed in both the amplitude and the phase of the photo-acoustic signal. We strongly believe that such samples can be used for chiral applications, spanning from circularly polarized light emission, to the enantioselectivity applications.

Photothermal and photoacoustic techniques are utilized to explore the thermal properties of metal based metasurfaces. We experimentally evidence the role of the symmetry in the heat transport and its relation with the optical circular dichroism.

Here we present measurements on a metasurface composed by tilted gold nanowires. The metasurface can induce an optical chiral response of the whole sample when the light impinges on the sample out of the normal incidence angle. In order to investigate the symmetry breaking induced by the geometry, we measured the second harmonic generation (SHG) signal generated by circular polarized pulsed light. This technique has demonstrated to be a powerful method in order to investigate the chiral morphology of nanostructures.

Compact samples of nano-helices built by means of a focused ion beam technology with large bandwidth and high dichroism for circular polarization are promising for the construction of built-in-chip sensors, where the ideal transducer must be sufficiently confined without compromising its filtering ability. Direct all-optical measurements revealed the sample’s dichroic character with insufficient details because of scattering and diffraction interference. On the other hand, photoacoustic measurements resulted to be a possible alternative investigation, since they directly deal with absorbed power and allow to get clear evidences of the differential selection for the two opposite polarization states. Multi-level numerical simulations confirmed the experimental results, proving once again the reliability of photoacoustic technique and the versatility of this class of dichroic artificial materials.

Zinc oxide (ZnO) nanoparticles were synthesized on diatomite (DE) surface by a low temperature sol gel technique, starting from zinc acetate dihydrate (Zn(CH3COO)2 • 2H2O) solution in water/ethyl alcohol, in presence of triethanolamine (TEA) with functions of Zn2+ chelating agent, catalyst and mediator of nanoparticle growth on DE surface. Microstructural features were investigated by field emission scanning electron microscopy and x-ray diffraction. ZnO crystalline nanoparticles, well distributed both on the surface and into the porous architecture of diatomite, were obtained just after the synthesis carried out at 80 °C without the need of calcination treatments. The optical properties of ZnO/DE hybrid powders were measured for the first time by means of photoacoustic spectroscopy (PAS). A new method to retrieve both the optical absorption and scattering coefficients from PAS is here discussed for powder aggregates. The fingerprint of the zinc oxide nanoparticles has been highlighted in the Mie scattering resonance in the UV-Vis range, and in the enhancement of the optical absorption with respect to diatomite. © 2017 IOP Publishing Ltd.

We demonstrate the control of enhanced chiral field distribution at the surface of hybrid metallo-dielectric nanostructures composed of self-assembled vertical hexagonal GaAs-based nanowires having three of the six sidewalls covered with Au. We show that weakly-guided modes of vertical GaAs nanowires can generate regions of high optical chirality that are further enhanced by the break of the symmetry introduced by the gold layer. Changing the angle of incidence of a linearly polarized plane wave it is possible to tailor and optimize the maps of the optical chirality in proximity of the gold plated walls. The low cost feasibility of the sample combined to the simple control by using linearly polarized light and the easy positioning of chiral molecules by functionalization of the gold plates make our proposed scheme very promising for enhanced enantioselective spectroscopy applications. publishedVersion

For the development of new generation portable electronic devices, the realization of thin and flexible electrodes have a crucial role. Conductive organic systems can address this issue in different ways. Indeed, conductance in organic molecules were studied in different papers starting from seminal papers in last 70’s [1] up to recent ones [2]. Among organic species, conduction and electronic characteristics of Fluorene derivatives were studied in different configurations [3,4]. Unfortunately, the conductance of organic materials is limited by charge transport mechanism [5]. Hybrid system with organic conductive compounds covalently linked with metal centres can lead to enhanced conductivity [6]. Here we synthesized gold and silver nanoparticles (AuNPs and AgNPs) stabilized with a fluorene thiolate derivative, namely 9,9-Didodecyl-2,7-bis(acetylthio)fluorene (FL). In the synthesis process the metal nanoparticles (MNPs) size results to be around 5 nm in diameter [7]. When deposited on a planar substrate, the hybrid compound form a regular network of MNPs separated each other by fluorene spacers covalently linked by thiol groups [8]. We deposited the network on substrate with two interdigitated electrodes in order to measure conductive properties (I-V characteristics). In I-V measurements it results to be that AgNPs based network is 200 times more conductive than AuNPs one. Selective oxidation of AgNPs network close to positive electrodes gives rise to a Schottky diode behavior in the I-V characteristic that could find potential applications in nano-electronics devices. The fluorescence and extinction spectra of FL-AgNPs and FL-AuNPs where characterised. References [1] C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and Alan G. MacDiarmid, Phys. Rev. Lett. 39, 1098 (1977). [2] Hylke B. Akkerman, Paul W. M. Blom, Dago M. de Leeuw and Bert de Boer, Nature 441, 69 (2006). [3] Rajendra Prasad Kalakodimi, Aletha M. Nowak, and Richard L. McCreery, Chem. Mater. 17, 4939 (2005). [4] J. Wu, K. Mobley, and R. L. Mc Creery, J. Chem. Phys. 126, 024704 (2007). [5] Cristina Hermosa, Jose Vicente Alvarez, Mohammad-Reza Azani, Carlos J. Gomez-Garcia, Michelle Fritz, Jose M. Soler, Julio Gomez-Herrero, Cristina Gomez-Navarro and Felix Zamora, Nature Commun. 4, 1709 (2013). DOI: 10.1038/ncomms2696. [6] Nunzio Tuccitto, Violetta Ferri, Marco Cavazzini, Silvio Quici, Genady Zhavnerko, Antonino Licciardello and Maria Anita Rampi, Nature Mater. 8, 41 (2009). [7] Quintiliani, M., Bassetti, M., Pasquini, C., et al. J. Mater. Chem. C, 2014, (2), pp. 2517-2527. [8] R. Matassa, G. Familiari, E. Battaglione, Concita Sibilia et al., Nanoscale, 2016,8, 18161-18169.

III–V semiconductors nanowires (NW) have recently attracted a significant interest for their potential application in the development of high efficiency, highly-integrated photonic devices and in particular for the possibility to integrate direct bandgap materials with silicon-based devices. Here we report the absorbance properties of GaAs-AlGaAs-GaAs core-shell-supershell NWs using photo-acoustic spectroscopy (PAS) measurements in the spectral range from 300 nm to 1100 nm wavelengths. The NWs were fabricated by self-catalyzed growth on Si substrates and their dimensions (length ~5 μm, diameter ~140–150 nm) allow for the coupling of the incident light to the guided modes in near-infrared (IR) part of the spectrum. This coupling results in resonant absorption peaks in the visible and near IR clearly evidenced by PAS. The analysis reveal broadening of the resonant absorption peaks arising from the NW size distribution and the interaction with other NWs. The results show that the PAS technique, directly providing scattering independent absorption spectra, is a very useful tool for the characterization and investigation of vertical NWs as well as for the design of NW ensembles for photonic applications, such as Si-integrated light sources, solar cells, and wavelength dependent photodetectors.

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.

International audience; Self-confined beams and spatial solitons were always investigated for a purely academic point of view, describing their formation and cross-interaction. We propose a novel paradigm for integrated photonics circuits based on self-confined interconnections. We consider that circuits are not designed since beginning; a network of writing lasers provide the circuit configuration inside which information at a different wavelength travels. we propose new designs for interconnections and both digital and analog switching gates somehow inspired by Nature, following analog decision routes used in biological networks like brain synapsis or animal path finding.

Plasmonic metamaterials and metasurfaces are important for many linear and nonlinear photonic applications. Here, the possibility to control a nanostructured layer spontaneously formed near an interface of a thin silver film is shown, where the interplay between a grain boundary structure and surface segregation of germanium atoms leads to encapsulation of the grains and, as the result, formation of a composite metamaterial near the film surface. This Ag/Ge composite exhibits strong localized surface plasmon resonances at Ge-encapsulated silver grains, leading to extraordinary second harmonic generation for both transverse magnetic and transverse electric polarized fundamental light with up to two orders of magnitude enhancement compared to thin Ag films without Ge atoms. Segregation phenomena open the possibility for fabrication of a new class of composite materials and gives additional degree of freedom in designing optical properties of nanostructured metamaterials.

In this work we report on the resonant absorption properties of GaAs-based nanowires, and on the chiral response when such nanowires are asymmetrically covered by gold. We use photo-acoustic spectroscopy which is a scattering-free, stable and highly sensitive technique that directly measures the absorption of the nanowire ensembles. Highly uniform samples were fabricated by lithography-free process of selfcatalyzed growth on Si substrates. We report on the experimental resonant absorption in the range (300÷1100) nm, which is in great agreement with simulations. We then measure the circular dichroism at 532nm, and discuss on the formation of near chiral fields for enhanced enantioselectivity applications.

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.