Tailoring optical properties of randomly nanotextured layers via Anderson localization

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This project combines experimental, theoretical, and nanoengineering approaches to identify a universal design concept enabling efficient localization of light in disordered thin optical layers. The absorptivity of a material cannot be easily tuned. However, trapping of light in localized states enhances the light-matter interaction in thin layers and thus enhances absorption. Tailoring the coupling efficiency of localized modes to the external field therefore is an interesting mechanism to enhance absorption. Randomly nanotextured absorbers are known to enhance absorption in thin absorber layers. However, controversies about the enhancement mechanism persist and clear design strategies for efficient absorbers are still missing. We propose investigating theapplication of light localization in disordered but suitable tailored twodimensional layers to achieve enhanced or even perfect absorption. In a preliminary study we demonstrated that light localization in a thin nanotextured a-Si:H absorber layer dominates absorption in the long-wavelength tail of the absorptivity. Based on this we will systematically tailor the disorder in such layers to identify the critical nanotexture parameters that support efficient light localization. Initially a-Si:H will serve as a model absorber material for which the applicability of the used experimental methods is already demonstrated. In the course of the project also other materials such as metal-halide perovskites and transparent conductive oxides (TCOs) as wide-band gap dielectrics will be investigated. Top-down (focused ion beam (FIB)) and bottom-up (growth control, etching) nanotexturing methods in combination with interface characterization (TEM, SEM, AFM) are applied to prepare and characterize nanotextured absorber layers that are then investigated by thermionic emission microscopy in combination with coherent 2D nanoscopy and spectromicroscopy of scattered light. The first milestone of the project is the demonstration of efficient light trapping in nanotextured layers custom made by employing top-down nanofabrication methods. The top-down approach allows a systematic variation of tailored disordered layers and thus the establishment of universal design concepts for efficient light trapping via localization. The identification of crucial nanotexture properties is assisted by theoretical modeling (FDTD solver) of the response of nanostructured layers. Based on this we will tune the coupling efficiency between localized modes and external radiation fields towards the regime of critical coupling, i.e. equal coupling and internal loss. For this case perfect absorption of the localized modes is expected. In addition, the prospect of forming high-quality (high-Q) random resonators via localization in disordered dielectric layers is investigated by extending the scheme to materials with a lower absorptivity than a-Si:H.