We investigated the mid-infrared optical properties of GaN/Al(x)Ga(1-x)N-type heterostructures on sapphire substrate grown by the Metal-Organic Chemical Vapor Deposition. We show how the polarization-dependent reflection spectrum is affected by the presence of the strain-induced 2D electron gas at the interfaces between Al(x)Ga(1-x)N and GaN layers. In particular, we show that the 2D electron gas contribution, modeled from its density and transport properties, can play a relevant role at the Berreman mode excitation condition. In this framework, our results offer an advanced approach for the optimization and design of GaN-based broadband optoelectronic and energy management devices.

Phase-transition materials provide exciting opportunities for controlling optical properties of photonic devices dynamically. Here, we systematically investigate the infrared emission from a thin film of vanadium dioxide (VO2). We experimentally demonstrate that such thin films are promising candidates to tune and control the thermal radiation of an underlying hot body with different emissivity features. In particular, we studied two different heat sources with completely different emissivity features, i.e. a black body-like and a mirror-like heated body. The infrared emission characteristics were investigated in the 3.5–5.1 μm spectral range using the infrared thermography technique which included heating the sample, and then cooling back. Experimental results were theoretically analyzed by modelling the VO2 film as a metamaterial for a temperature range close to its critical temperature. Our systematic study reveals that VO2 thin films with just one layer 80 nm thick has the potential to develop completely different dynamic tuning of infrared radiation, enabling both black-body emission suppression and as well as mirror emissivity boosting, in the same single layer device. Understanding the dynamics and effects of thermal tuning on infrared emission will benefit wide range of infrared technologies including thermal emitters, sensors, active IR filters and detectors.

The infrared (IR) emissivity of a set of polymeric fibers composed of randomly oriented carbon nanotubes dispersed into a poly(ethylene terephthalate) matrix host was investigated. Samples containing different amounts of carbon nanotubes in the range between 1 and 10 wt% were analyzed. The effects of the included carbon nanotubes on electrical and morphological properties were studied by means of electric conductance, scanning electron micrography and white light interferometry investigations, respectively. The emissivity was characterized in the 3.5-5.1 mu m spectral range by using the infrared thermography technique under heating regime. To analyze the obtained results, a theoretical model based on the Maxwell Garnett theory has been proposed. A percolation effect in the electrical conductance and a gradual increase of the emissivity has been observed, respectively with increasing carbon nanotubes content. In particular, morphological investigations showed the emerging surface roughness when carbon nanotubes are added to the polymeric matrix which may further contribute to the increase of the emissivity value. On the basis of the obtained results, it can be concluded that the investigated composite fibers have the potential to develop new materials enabling both high IR emission and large electrical conduction.

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].

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 nm and 980 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.

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.

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.

Even non-chiral objects can exhibit effective optical chiral response due to particular symmetry breaking of the investigating light and the sample morphology. Here we show linear and nonlinear optical measurements performed on a metasurface composed by self-assembled tilted golden nanowires on silicon substrate. The measurements are performed in three different schemes: optical reflectance, photoacoustic absorbance and second harmonic generation. In all these schemes circular polarized light was used in order to evidence the optical chiral behavior in different reciprocal disposition of the wires and light direction. The circular dichroism results to be present in all schemes when the three directions formed by i) the wires orientation, ii) the impinging light wave vector and iii) the normal to the metasurface forms a non-planar triad. Indeed non-planar triad of vectors represents a system that cannot be superposed to its mirror image, thus it is chiral system. We measured a sample obtained by vacuum evaporation of gold at glancing angle on a silicon substrate maintained at the temperature of 300K. The gold nanowires form a forest homogeneously distributed on 1 square inch substrate. Even if the chirality was detected both in linear and nonlinear optical measurements, the second harmonic generation process results to be more sensitive.

Here we discuss the second harmonic generation (SHG) signal raised by a sample surface manifesting circular dichroism. The measurement were performed by studying the SHG efficiency in different polarization states of the light. In particular measurement performed with circular polarized light showing a very high sensitivity to the response of the studied metasurfaces.

We studied the far-field thermal emission properties of finite arrays of resonant gold dipole nanoantennas at equilibrium temperature. We numerically investigated the transition from the super-Planckian emission of the single resonant antenna to the sub-Planckian emission inherent to infinite periodic arrays. Increasing the number of unit cells of the array, the overall size of the system increases, and the relative emissivity quickly converges to values lower than the unity. Nevertheless, if the separation between nanoantennas in the array is small compared to the wavelength, the near-field interaction makes the emission of each unit cell multipolar. This opens the doors for additional tailoring of the emitted power and directionality of thermal radiation.

Here we present both an overview of different nonlinear optical phenomena occurring in nanopatterned materials and new results on the symmetry induced second harmonic generation (SHG) signal from metallic nanowires. A discussion about symmetry breaking in artificial chiral metamaterials is presented, while the experimental evidence was given by second order nonlinear optical measurements on different samples.Here, new SHG measurements on regular array of tilted nanowires (NWs) produced by grazing evaporating gold on a silicon substrate were presented and discussed.The surface composed by tilted wires can induce an optical chiral response of the whole sample when the light impinges on the sample on an out-of-normal incidence angle (extrinsic chirality). The measurements were performed by using circular polarised laser excitation at the wavelength of 800nm and by observing the second harmonic response at the wavelength of 400nm in different polarization states.The second harmonic generation process results to be very sensitive to the symmetry breaking at the interfaces of investigated samples.

Here we present the measurements of the second harmonic generation (SHG) signal raised by self ordered dielectric nanospheres partially covered by thin (10nm) Au layer. The measurement were performed by studying the SHG efficiency in different polarization states of the light. In particular measurement performed with circular polarized light show the presence of chiral response of the nanospheres that is induced by the particular geometry of the metasurface.

Regular array of plasmonic nanoantennas (nanocrescents) can be easily produced by grazing evaporating gold on a self-ordered surface formed by hexagonal arrangements of polystyrene nanospheres, thus realising a hybrid plasmonic-photonics nanostructures (HPPN). By using second harmonic generation (SHG) technique we experimentally demonstrated that asymmetry in the shape of the nanoantennas induces an optical chiral response of the whole sample.

The control and tailoring of infrared absorbance/emittance is a crucial task for all those applications involving thermal radiation management and detection.We theoretically investigated the peculiar absorbing/emitting behaviour of pre-fractal Cantor multilayers, in order to design a polarization-insensitive multilayer stack absorbing over a wide angular lobe in the mid wavelength infrared range (8-10 μm).Using transfer matrix method, we explored the spectral properties arising from both the material and the geometrical dispersion.We considered several combinations of the constituent materials: SiO 2 was combined with TiO 2 and Si, respectively.

Gold nanowires in general demonstrate very interesting plasmonic properties. Here, by applying the generalized Snell's law introduced by F. Capasso in 2011, we study how the resonant behavior of the nanowires and their geometrical feature such as the radius of curvature can produce a bent in the propagation direction of a transmitted light beam. The measurements that were performed at a wavelength larger than the nanopatterned features reveal information on the meatusurface morphology.

Nonlinear effects can be enhanced and tailored on a subwavelength scale by taking advantage of high field confinement by selective coupling among resonant plasmonic nanostructures, called plasmonic nanoresonators and nanoantennas. In particular, we focus our attention on the process of second-harmonic (SH) generation. Excitation of localized surface plasmon polaritons at the fundamental frequency in these structures can remarkably modify the response of the system by enhancing surface and/or bulk nonlinear contributions to the overall SH field. Moreover, the radiation pattern of the generated field can be controlled and directed. Indeed, the overall structure behaves as a nonlinear nanoantenna for the SH harmonic field, whose emission pattern can be tailored.

The generalized Snell's law of refraction was introduced by F. Capasso in 2011. One consequence of the law is that resonant metallic nanostructures at the interface between two dielectrics can bend the light in a controlled way. Alternatively, by measuring the bent of a beam of light induced by a nanopatterned metallic surface it is possible to retrieve information on the surface morphology. Here we show that deviation measurements of the light passing through curved metallic nanowires deposited on a glass slide gives information on the morphology only when resonance is excited.

Artificial circular dichroism [1] is investigated for developing novel devices for active polarization controllers, like rotators and modulators and high efficient molecular sensors. Here we show the chiral behavior raised by metal (Au) nanostructures on dielectric (polystyrene, PS) nanosphere substrate produced by a self assembled procedure that guarantees large area fabrication with low time consuming procedure (figure 1). In this material the artificial chirality [2,3] arises from the curvature of the gold layer covering the nanospheres and their reciprocal orientation [4].

Resonant metallic nanostructures, located at the interface between two dielectrics, can produce abrupt phase discontinuities on propagating light that will be anomalously refracted by following the generalized Snell’s law. In this work, we show evidence of anomalous refraction arising when such an interface is nano-patterned with self-assembled bent gold nano-wires having sub-wavelength periodicity.

Here we investigated the asymmetric transmission and the anomalous refraction introduced by a metasurface of bent gold nanowires.The refraction follows the generalized Snell's law that takes into account the resonant behavior of metallic nanostructures located at the interface between two dielectrics.Measurements performed in the linear optical regime reveal a large sensitivity to the subwavelength features of the gold nanostructures.