Researchers shrink hyperspectral imaging with metasurface optics

September 18, 2019 //By Julien Happich
hyperspectral imaging
Relying on cleverly designed dielectric optical metasurfaces, researchers from the California Institute of Technology have demonstrated an ultra-compact hyperspectral imager (HSI) that could easily be integrated into smartphone and wearables, without the need for bulky optics.

The researchers describe the HSI element as a folded metasurface platform comprising three reflective and one transmissive dielectric metasurfaces all integrated on a gold-mirrored glass substrate about 1mm thick, in one lithographic step. Looking closer, the metasurfaces consist of α-Si nanoposts with rectangular cross sections, resting on the fused silica substrate and capped by a 2μm thick layer of SU-8 photoresist. The metasurfaces are designed to collectively disperse and focus light of different wavelengths and incident angles on a focal plane parallel to the glass substrate. Light to be analysed enters the HSI through an input aperture in the front gold mirror and is deflected into the substrate and vertically dispersed by the first metasurface. The other two reflective metasurfaces together with the transmissive one focus light with different wavelengths and horizontal incident angles to diffraction-limited spots on a detector array plane parallel to the substrate. The last metasurface is transmissive and simultaneously acts as the output aperture to collect the spectrum images.

As the object is scanned in front of the hyperspectral imager, a 1D spatial image is captured along the direction θ. Light enters the device through an input aperture, interacts with the reflective metasurfaces while it is confined inside the substrate by the two gold mirrors, and exits the output aperture that has a transmissive metasurface built into it. Different wavelengths are dispersed in the vertical direction (λ), and various input angles are focused to different horizontal points.

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