In this report, the growth of zinc oxide (ZnO) nanocrystals with various morphologies, nanoflower, nanosheet, and nanorod, on flexible stainless steel (SS) foils to be utilized as photoanodes in photoelectrochemical (PEC) solar cells has been presented. It has been aimed to provide flexibility and adaptability for the next generation systems with the incorporation of SS foils as electrode into PEC cells. Therefore, physical deformation tests have been applied to the prepared ZnO thin film photoanodes. These thin films have been thoroughly characterized before and after straining for better understanding the relationship between the morphology, straining effect and photoelectrochemical efficiency. We observed a notable increase in the maximum incident photon-to-current efficiency (IPCE) and durability of all ZnO photoelectrodes after straining process. The increase in IPCE values by 1.5 and 2.5 folds at 370 nm has been observed for nanoflower and nanorod morphologies, respectively after being strained. The maximum IPCE of 69% has been calculated for the ZnO nanorod structures after straining. Bending of the SS electrodes resulted in the more oriented nanorod arrays compared to its flat counterpart, which improved both the light absorption and also the photo-conversion efficiency drastically. The finite-difference time-domain simulations have also been carried out to examine the optical properties of flat and bent ZnO electrodes. Finally, it has been concluded that SS photoanodes bearing ZnO semiconducting material with nanoflower and nanorod morphologies are very promising candidates for the solar hydrogen generator systems in terms of efficiency, durability, flexibility, and lightness in weight.

Beam splitters play important roles in several optical applications, such as interferometers, spectroscopy, and optical communications. In this study, we propose and numerically examine polarization-insensitive beam splitters utilizing two-step phase gradient all-dielectric metasurfaces in the visible spectrum. The metasurface is made of periodically arranged binary unit cells, and phase difference between neighboring unit cells on the surface is 180 deg. The metasurface is shown to have a special phase gradient whose sign changes periodically. The angle of the split beams on both sides and the corresponding total transmission value at 532 nm wavelength are found to be ±46.8° and 0.90, respectively.

Metasurfaces with superior capabilities to tailor wave fronts of light have become one of the most widely investigated optical elements. Incorporating tunable features into the metasurface design is required. In this study, we have proposed and designed highly efficient rotationally tunable metasurface lens structures inspired by Moiré lenses operating at a 532 nm wavelength. The proposed structures consist of two cascaded all-dielectric metasurfaces, which have reverse phase profiles with respect to each other. The metasurfaces are designed with periodically arranged TiO2 nano-rods on a SiO2 substrate in a square lattice. We demonstrated that the focal length changes nonlinearly according to the mutual rotation of metasurfaces from 30° to 150° with a focusing efficiency as high as 55% and a wide focal length variation between 11.4 μm and 4.2 μm. Moreover, we have designed and proposed a tunable polarization-insensitive multifunctional focal system. Using the proposed multifunctional focal system, focal planes can be formed in consecutive planes and the number of focal planes can be made single or multiple.

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The commonly accepted approach for metasurface design utilizes nanopillars with varying diameters. In this study, contrary to usual design approach, we propose and design highly efficient, broadband and polarization-independent nanohole all-dielectric metasurfaces operating in the visible spectrum. High focusing efficiency above 70% is achieved between 450 and 700 nm wavelength region with a numerical aperture (NA) value of 0.60. Moreover, focusing efficiency is succeeded higher than 47% with NA = 0.85 for a design wavelength of 532 nm. Nanohole metasurfaces exhibit less chromatic aberration (<18%) compared to nanopillar based metasurfaces. The nanohole array metasurfaces is investigated under the oblique illumination condition and its performance is found to be satisfactory in a wide range of incidence angles. Furthermore, nanohole and nanopillar metasurfaces are analyzed and their performances are compared for different incidence angles, NAs and operating wavelengths. It is shown that contrary to dielectric pillars commonly deployed in the design of metasurfaces, nanoholes with varying diameters allow phase changes with better performances.