Metasurfaces with angular sensitivity have been shown to provide a platform for developing an ultra-compact phase imaging system. Their performance, however, is often limited to a narrow range of spatial frequencies. Here, we apply inverse design to design and fabricate a metasurface an asymmetric optical transfer function across a numerical aperture (NA) of 0.6. The engineered response of this device enables phase imaging of microscopic transparent objects.
Sum frequency generation is the process in which two incoming photons are converted into an outgoing photon of higher energy. This process is highly inefficient, and therefore requires either large interaction distances in bulky crystals, or large field concentrations in the non-linear materials. Metasurfaces are one such platform to generate extreme field enhancements with resonant processes. In this work, we use topology optimisation to design metasurfaces that exhibit increase high efficiency sum frequency generation, as well as the ability to tailor the generated polarisation.
We develop and experimentally realise a single-layer metasurface that converts unpolarised light into fully polarised light surpassing the efficiency limit of 50% for conventional polarisers. We achieve this by using an inverse-designed metasurface that splits incoming light into multiple outputs with the same polarisation. We predict a greater than 80% conversion efficiency when combining the powers of two or three outputs. We fabricate the freeform silicon metasurface and experimentally measure a combined efficiency of over 60% in converting unpolarised light to polarised light at 1550 nm with an overall extinction ratio 20.
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