Metasurfaces have been used to create flat lenses, new AR glasses and even to upconvert radio signals for 6G relays. A non-linear metasurface built from a thin film of (110) gallium arsenide nanocrystals can up-convert IR photons to visible light at room temperature without having to use coolers or specialist sensors.
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The researchers at Nottingham Trent University in the UK and the Australian National University fabricated the metasurface and transferred it to a transparent glass, forming a layer of nanocrystals on the glass surface.
The optical process employed in this technique is nonlinear sum-frequency generation (SFG). In SFG two photons, one of them in the IR spectrum, interact within a nonlinear material to generate emission at higher and visible frequencies.
Rather than using bulky and expensive nonlinear crystals, the team used a nanocrystal thin film of allium arsenide as a metasurface. This acts as a planar array of nanoantennas, designed to manipulate various properties of the incident light including its frequency.
Such metasurfaces can exhibit enhanced frequency conversion as a result of the excitation of optical resonances and good coupling to free space. This provides a way to up-convert IR photons to visible light, and a transparent metasurface means the glasses can simultaneously transmit visible and IR light.
For the experiment, an IR image of a Siemens-star target illuminated the metasurfaces. The IR image of the target was mixed with a second beam and, through the SFG process, up-converted to a visible green light at 550nm where the human eye is most sensitive.
The researchers chose the signal beam at 1530 nm, which corresponds to the maximum band of nightglow (1500 to 1700 nm), with a pump beam at 860 nm since commercial high-power laser diodes are readily available at this wavelength and it is invisible to the human eye.
The visible green images, captured with a conventional camera, correspond to different transverse positions of the target, including the case when the target was fully removed from the path of the IR beam and the SFG emission from the metasurface was observed. Despite different parts of the IR signal beam being up-converted by independent nanocrystals composing the metasurface, the images were well reproduced into the visible.
This metasurface imaging approach offers other opportunities, such as ‘multicolour’ IR imaging by an appropriately designed metasurface that can convert particular frequency bands to visible light. This could also be used for lidar sensors to boost the amount of information acquired for autonomous driving for example.
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