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Chirality, an intrinsic property of certain entities in the universe, is characterized by the absence of mirror symmetry. Understanding chirality is crucial as it influences molecular interactions and properties. Circular dichroism (CD), measured using circularly polarized light, is a standard technique for probing chirality, but its sensitivity is often limited. Here, we explore extrinsic chirality (i.e. a property arising from asymmetric achiral materials when observed from out of normal incidence directions), using photo-acoustic spectroscopy (PAS). PAS allows direct measurement of local absorption, by monitoring the heat produced and transferred to the surrounding air, regardless the transmitted, reflected, and scattered light that flows away from the sample. In conventional techniques, the CD is usually measured by taking into account only the extinction as transmitted (or reflected) light. In this study, we introduce a new PAS setup that employs an oblique-incidence laser to study extrinsic chirality in silver-coated self-assembled metasurfaces. Our experimental results reveal intriguing CD trends dependent on the angle of incidence and wavelength, indicative of extrinsic chirality. This study expands the application of PAS, enabling simultaneous analysis of multiple wavelengths and providing valuable insights into chiral metasurfaces.

Tailoring nonlinear optical properties at the nanoscale is a hot topic in nowadays nanophotonics, promising for applications spanning from sensing to ultrafast optical communications. Here we present a numerical approach of designing a simple semiconductor nanostructure able to tailor second harmonic emission in the near- and far-field. We start from linear simulations of ZnO nanospheres, which reveal multipolar nature of the scattering. Next, we show how the same nanospheres, with radii in 30–130 nm range, excited at 800 nm, manipulate the directivity of the emitted second harmonic. We observe that the nanospheres which exhibit Kerker condition at 400 nm, emit the second harmonic field in the forward direction. We further investigate how the asymmetry (ellipsoid geometry) tailors the second harmonic directivity. We finally introduce geometry with low chiro-optical response, and observe that the second harmonic far-field depends on the handedness of the light exciting the nanostructure at 800 nm.

Plasmonic nanostructures with achiral, but asymmetric shapes can exhibit chiro-optical phenomena at the nanoscale, given that the nanostructure-light interaction symmetry is broken. Such behaviour is defined as extrinsic chirality, and it is induced by properly arranging the experimental set-up. We show measurement techniques for extrinsic chirality in low-cost, asymmetric samples with nanostructures organized in metasurfaces. We employ widely tuneable chiro-optical characterization of transmission and reflection, as well as the circular polarization degree of the transmitted signal; near-infrared range (680-1080nm) and oblique incidence allow for the detection of resonant features in extrinsic chirality. Other, unconventional experiments use photo-thermal consequences of chirality governed absorption in metasurfaces. Photo-acoustic spectroscopy directly gives circular dichroism as a differential absorption of the left and right circular polarizations exciting the sample. Photo-deflection spectroscopy gives additional information of diffraction phenomena governed by the extrinsic chirality. We showed that these techniques can monitor the extrinsic chiral behaviour of the hybrid plasmonic metamaterials. Moreover, they can be used in combination with fluorescence-detected circular dichroism to measure the emission properties of fluorescent materials.

We present narrow-band polarization-sensitive reflectance of GaN/AlGaN heterostructures in the mid-infrared range. Experimental measurements performed at 15° angle of incidence show the excitation of a Berreman mode at the interface between GaN and sapphire substrate. A transfer matrix method for anisotropic layers has been used to analyze the obtained results. The contribution of the two-dimensional electron gas at the interfaces of the heterostructures has been included by proper modelization of an effective thin layer.

We investigated infrared reflectivity of undoped and Tungsten (W) doped Vanadium dioxide (VO2) films at varying temperatures. Undoped VO2 exhibited a clear phase transition at 100°C, achieving near 0% reflectivity, or perfect light absorption. As W doping concentration increased, the phase-transition temperature decreased, maintaining the zero-reflectivity condition. Only a 0.75% W doping enabled room temperature perfect absorption without heating the film.

We report a narrowband polarization-dependent reflectivity from Al-doped ZnO/ZnWO4 self-assembled eutectic composites in the mid-infrared range. Our results show a reflectivity modulation from 0.05 to 0.75 for two orthogonal polarizations of the incident field with a 10% Al concentration. Acting as natural polarizing filters these eutectic composites could open the way to the future development of low-cost photonics components in the IR.

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.

We show that resonant second harmonic generation can be obtained in random dielectric structures. The scheme is based on internal resonances due to the optical counterpart of Anderson localization. By making use of different localization lengths at the fundamental and at the second harmonic frequencies, we predict a conversion efficiency that is four orders of magnitude higher than a bulk material and even one order of magnitude higher than an ideal phase matched slab of the same size. The method is highly insensitive to fabrication tolerances, and provides excellent angle tunability.

The active modulation of optical response of quantum emitters at the nanoscale is of paramount importance to realize tunable light sources for nanophotonic devices. Herein, a thin film of phase‐change material (VO2) is coupled to a 20 nm‐thick silica layer embedding Er3+ ions, and it is demonstrated how the active tuning of the local density of optical states near the erbium emitters provided by the thermally induced semiconductor‐to‐metal transition of VO2 can be used to dynamically control the Er3+ emission lifetime at telecom wavelength (1.54 μm). A decay rate contrast of a factor 2 is obtained between high temperature (90 °C), when VO2 is metallic, and room temperature, when VO2 is semiconductor, in agreement with calculations performed with the classical dipole oscillator analytical model. A hysteretic behavior is observed by measuring the Er3+ lifetime as a function of the temperature, whose parameters are consistent with those of grazing incidence X‐ray diffraction and optical transmittance measurements. The fractions of Er3+ ions that couple with VO2 in each phase at the different temperatures are determined by the analysis of the temporal decays. The results make the investigated system an optimal candidate for the development of tunable photon sources at telecom wavelength. [ABSTRACT FROM AUTHOR]

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters. [ABSTRACT FROM AUTHOR]