thus paves the way for the important applications spreading from negative refraction, [1][2][3] chiral sensing [4,5] to production of optical field carrying out optical angular momentum for quantum information applications. [6] It has been shown that plasmonic nanostructures forming 1D [7] elements, 2D metasurfaces, [8][9][10] and 3D metamaterials [11] can exhibit linear chiral response due to their own, intrinsic chirality. Also semiconductor nanostructures can exhibit chiral features. [12][13][14][15][16] From the optical point of view, chiral structures possess the ability to rotate the plane of the polarization of electromagnetic waves (optical activity), and give rise to circular dichroism-i.e., the difference in the absorption of right-and lefthanded circular polarized light.Apart from 3D chiral objects, the possibility to obtain optical chirality, i.e., optical activity, with nonchiral elements was studied in the past, [17] but only recently reconsidered. [18,19] This phenomenon is obtained when the experimental configuration composed by both the nonchiral object and the optical incident field is nonsuperimposable on its mirror image. [20] This is called "extrinsic" chirality; in our previous works we have investigated this type of chirality in tilted golden nanowires by means of both linear (reflection and absorption) and nonlinear (second harmonic generation) measurements. [20][21][22][23] III-V semiconductor nanowires (NWs) have been widely investigated since they exhibit good waveguiding properties thus offering a light manipulation at nanoscale. Coupling of the incident light to the discrete leaky waveguide modes above the bandgap in NWs leads to increased resonant absorption, thus paving the way for important applications such as energy harvesting, spectral selectivity, lasing, spin angular momentum generation, etc. [24][25][26][27] Metallic NWs have also been investigated for plasmonic laser applications [28][29][30] and possibility of surface plasmon polaritons excitation. [31] One step further is the partial covering of such NWs with gold: this can induce, along with the proper experiment setup, the symmetry breaking which leads to chiral response.In this paper, for the first time to our knowledge, we report on a circular dichroism behavior from semiconductor hexagonal Semiconductor nanostructures hybridized with metals have been known to offer new opportunities in nonlinear optics, plasmonics, lasing, biosensors; among them GaAs-based nanowires (NWs) hybridized with gold can offer new functionalities, as chiral sensing and light manipulation, as well as circular polarization sources. This study investigates GaAs-AlGaAs-GaAs NWs fabricated by self-catalyzed growth on Si substrates, and partially covered with gold, thus inducing the symmetry breaking and a potential chiral response. Three different samples are investigated, each of them with a different morphology, as the length and the overall diameter ranging from 4.6 to 5.19 µm and from 138 up to 165 nm, respectively. The samples are first char...

Nowadays nanophotonics aims towards low-cost, chip-scale devices that can tailor electromagnetic properties, one of which is the control of the circular polarization at the nanoscale, important for novel optical devices. Here we show that nanosphere lithography, combined with tilted metal deposition, can provide novel metasurfaces with chiral properties.We apply the photo-acoustic technique to characterize the circular dichroism at 633 nm of polystyrene nanospheres covered by three different metals: Au-and Cr-covered samples show extrinsic chiral behavior, while the Ag-covered sample shows circular dichroism at normal incidence, characteristic for intrinsic chirality. As the experimental data are in good agreement with numerical predictions, we believe that such design can be optimized to get efficient circularly polarized detection at the nanoscale. THE MANUSCRIPTChirality, a lack of the mirror symmetry of an object 1 , is an important property of some of the building blocks of our world: many molecules, amino-acids, DNA, sugars, drugs are chiral. Two mirror images of the same object differently interact with circularly polarized light of the opposite handedness, while having other measurable properties equal. In particular, chirality can affect the absorption and/or phase velocity of circularly polarized light, therefore it is possible to measure a difference in absorption directly related to the molecules' chirality. This measurement is known as Circular Dichroism (CD). At the nanoscale, when the nanostructures are comparable or smaller than the light wavelength, and organized periodically, they form a metasurface; generally, if the symmetry of the metasurface is broken, a chiral behavior is expected 2 . Chiral metasurfaces can manipulate electromagnetic fields and enhance the interaction with chiral molecules, important for chiral sensing 3 . On the other hand, they can control the polarization state of the optical field, or emit circularly polarized light, thus leading to applications in optical and quantum communications 4 . Geometric features of intrinsically chiral metasurfaces (the nanostructure in the unit cell is usually helix or gammadion-like) can be complicated to fabricate and implement at the nanoscale. This problem can be solved by a proper experimental set-up following the rule that the impinging light wavevector, the average surface normal, and the sample direction must be nonplanar. Such chiral behavior is called extrinsic chirality as it is governed by both experimental set-up and the a) Electronic mail:

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.

Electromagnetically induced transparency (EIT) is a pump-induced narrowband transparency window within an absorption line of the probe beam spectrum in an atomic system. In this paper we propose a way to bring together the all-dielectric metamaterials to have EIT-like effects and to optically tune the response by hybridizing them with a layer of a phase change material. We propose a design of the metamaterial based on Si nanoresonators that can support an EIT-like resonant response. On the top of the resonators we consider a thin layer of a chalcogenide phase change material, which we will use to tune the optical response. Our choice is Ge₂Sb₂Te₅ (GST), since it has two stable phases at room temperature, namely amorphous and crystalline, between which it can be switched quickly, nonvolatively and reversibly, sustaining a large number of switching cycles. They differ in optical properties, while still having moderately low losses in telecom range. Since such dielectric resonators do not have non-radiative losses of metals around 1550nm, they can lead to a high-Q factor of the EIT-like response in this range. Firstly, we optimize the starting structure so that it gives an EIT-like response at 1550 nm when the GST layer is in the amorphous state. Our starting design uses glass as a substrate, but we also consider implementation in SOI technology. If we then switch the thin layer of GST to its crystalline phase, which has higher losses, the EIT-like response is red shifted, providing around 10:1 contrast at 1550nm. This reversible tuning can be done with an ns visible pulsed laser. We discuss the results of the simulation of the dielectric metasurface for different configurations and the tuning possibility.

In situ XRD and IR optical measurements demonstrate the coexistence of M1 and R phases during the SMT transition of VO2 thin films.

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.

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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.