The development of metasurfaces has enabled unprecedented portability and functionality in flat optical devices. Spaceplates — devices that can mimic free propagation to replace space in an imaging system — have recently been introduced as a complementary element to reduce the space between individual metalenses. Spaceplates require an angle-dependent optical phase response which depends on the transverse spatial frequency component of a light field. Therefore, it is challenging both to design them and to assess their ultimate performance and potential. Here, we employ inverse-design techniques to explore the behaviour of general thin-film-based spaceplates.
Ultra-thin metalenses promise to miniaturize imaging systems. However, all lenses require an additional propagation length to allow for light to form an image on a detector. We show that by operating directly on the Fourier components of a complex light field, one may develop an optical element that acts to “propagate” light for a distance that is longer than its physically occupied space. We experimentally demonstrate this effect using a 30-mm-long calcite crystal, mimicking an additional 3.5 mm of free-space propagation while preserving the magnification. This work represents an essential step in the miniaturization of all electro-optical systems.
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