Full-Stokes polarimetric imaging enhances the information available from satellite remote sensing. But the numerous bulky and heavy optical components required to achieve polarimetric imaging limit its use for small-form satellites. We present the modelling of an ultra-thin nanostructured metasurface as a novel solution to the weight and volume constraints faced by small satellites. Positioned in a telescope’s pupil plane, the metasurface diffracts light into five polarization measurements that are imaged onto a detector, restoring the full field of view as the satellite moves over the Earth’s surface. Designed to have effective redundancy, any four out of the five orders enable the reconstruction of the full polarization state, allowing error monitoring. The metasurface material is 1 μm thick silicon repeating patterns on a 460 μm thick sapphire substrate, utilising free-form topology to optimise for throughput efficiency and equal light distribution between the polarization diffraction orders.
The Giant Magellan Telescope will employ laser tomography adaptive optics, using laser guide stars to measure and correct wavefront distortions with a high sky coverage compared to natural guide stars. A laser guide star is the resonance fluorescence induced by a launch laser propagating through a column of the atmospheric sodium layer, with narrowband emission at a 589nm wavelength. The column shape results in the laser guide star having observable elongation depending on perspective. Shack-Hartmann wavefront sensing remains challenging as the elongated axis of a subaperture focal spot can be as large as 10-14''. Currently, detectors with a large number of pixels are used to compromise between sensitivity and accuracy. We propose a novel approach based on a metasurface lenslet array, where each subaperture has a custom anamorphic ratio and orientation. Two metasurfaces with sub-wavelength-thick nanopatterned layers of TiO2 separated by a 6.5mm air gap accommodate a fixed focal length of 8mm and anamorphic ratios up to 1:10, as confirmed by Optics Studio simulations. We identify the experimentally feasible metasurface design suitable for the established nanofabrication approaches.
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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