# 01- Tailoring optical properties of randomly nanotextured layers via Anderson localization

**Source**: https://www.cs.tum.de/spp1839/projects/2nd-period-2018-2021/01.html
**Parent**: https://www.cs.tum.de/spp1839/projects.html

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 the application 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.

## Contributors

Prof. Martin Aeschlimann

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Benjamin Frisch

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Prof. Tobias Brixner

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Dr. Matthias Hensen

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Sebastian Pres

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Prof. Walter Pfeiffer

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Felix Becker

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