Researchers shrink hyperspectral imaging with metasurface 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.
Sharing their findings in the ACS Photonics Journal under the title “Hyperspectral Imager with Folded Metasurface Optics”, the researchers experimentally demonstrated a push-broom HSI, whereby 1D spatial images of an object are captured as it is moved vertically in front of the imager, each scanning slice yielding a full 2D spectrum image on the HSI’s focal plane.
In their experiment, the resulting 3D data cube resolved over 70 spectral points across the 750−850nm infrared range for each pixel acquired along each slice, amounting to a spectral resolution of about 1.5nm. The researchers note a similar hyperspectral imager could be designed to operate in the visible range using silicon nitride or titanium dioxide nanoposts instead of metasurfaces made out of amorphous silicon.
The authors also demonstrated the metasurface HSI to be polarization independent, offering an angular resolution of about 0.075°, able to distinguish about 400 angular directions in the ±15° range. As small as 8.5mm3 in volume and weighing less than 20mg, the device is easy to mass manufacture across large wafers, making it very attractive for integration in consumer electronics such as wearables volume and weight limitations are key design considerations.
California Institute of Technology – www.caltech.edu
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