The Giant Magellan Telescope will use laser tomography adaptive optics to correct for atmospheric turbulence using artificial guide stars created in the sodium layer of the atmosphere (altitude ≈90 km). The sodium layer has appreciable thickness (≈11 km), which results in the laser guide star being an elongated cylinder shape. Wavefront sensing with a Shack–Hartmann is challenging as subapertures located further away from the laser launch position image an increasingly elongated perspective of the laser guide star. Large detectors can be used to adequately pack and sample the images on the detector; however, this increases readout noise and limits the design space available for the wavefront sensor. To tackle this challenge, we propose an original solution based on nano-engineered meta-optics tailored to produce a spatially varying anamorphic image scale compression. We present meta-lenslet array designs that can deliver ≈100% of the full anamorphic image size reduction required for focal lengths down to 8 mm and a greater than 50% image size reduction for focal lengths down to 2 mm. This will allow for greatly improved sampling of the available information across the whole wavefront sensor while being a viable design within the limits of current-generation fabrication facilities.
Many technologies, including dot projectors and lidar systems, benefit greatly from using polarized illumination. However, conventional polarizers and polarizing beam splitters have a fundamental limit of 50% efficiency when converting unpolarized light into one specific polarization. Here, we overcome this restriction and achieve near-complete conversion of unpolarized light to a spatially uniform polarization state over several output directions with our topology-optimized metasurfaces. Our results provide a path toward greatly improving the efficiency of common unpolarized light sources, such as LEDs, for a variety of applications requiring uniformly polarized illumination. Our fabricated metasurface realizes a 70% conversion efficiency, surpassing the aforementioned limit, and achieves a polarization extinction ratio exceeding 20, when characterized with laboratory measurements. We further demonstrate that arbitrary power splitting can be achieved between three or more polarized outputs, offering flexibility in target illumination.
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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