Abstract
The coupling of light between free space and thin film semiconductors is an essential requirement of modern optoelectronic technology. For monochromatic and single mode devices, high performance grating couplers have been developed that are well understood. For broadband and multimode devices, however, more complex structures, here referred to as “coupling surfaces”, are required, which are often difficult to realise technologically. We identify general design rules based on the Fourier properties of the coupling surface and show how they can be used to determine the spatial resolution required for the coupler’s fabrication. To our knowledge, this question has not been previously addressed, but it is important for the understanding of diffractive nanostructures and their technological realisation. We exemplify our insights with solar cells and UV photodetectors, where high-performance nanostructures that can be realised cost-effectively are essential.
Highlights
It was found that the highest required spatial frequency is given by the modal dispersion of the waveguide: the highest spatial frequency corresponds to the point for which the entire absorbing region can be accessed
This requirement does not depend on the details of the coupling surface and can be used as the criteria to design and implement coupling surfaces of any kind
We have approached the spatial resolution issue from two different directions, namely a) via the harmonics we include into the design (Fig. 1a,b,d) and b) via the smallest spatial features present in the pattern (Figs 3a,b and 4b)
Summary
We find that the highest resolution significantly depends on the available mode spectrum and that the technologically required resolution is lower than the resolution one may naively assume We illustrate these observations using two absorbing systems: a silicon thin film relevant for solar cells, and a thin layer of gallium nitride (GaN), which features strong absorption and is employed in ultra-violet (UV) photodetectors. We find that a near-optimum coupling performance is already achieved for spatial resolutions higher than 20 μ m−1 and 30 μ m−1 for the silicon and GaN systems, respectively, typically twice as low as one would naively assume. These results are a useful guide for determining the trade-off between performance and fabrication requirements
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