Middle infrared beam-steering using liquid crystals for spatial light modulation
Middle infrared beam-steering is attractive for a number of applications, particularly in the defence sector, where infrared countermeasures are a cornerstone in protecting lives and assets from infrared seeking weapons. Liquid crystal based alternatives to the current opto-mechanical state of the art are investigated, in particular electrically addressed optical phased arrays. Material and design challenges are addressed. CVD graphene is found to be an appropriate transparent conductor, with a sheet resistance $ < 1 $k$ \Omega / \square $ and an optical transmission $ > 97 \% $ in the frequency band of interest. Liquid crystal BL-037 was found to have a birefringence of 0.23 in the middle infrared, where absorption bands exist between 3.3$ \mu $m and 3.6$ \mu $m, and at 4.5$ \mu $m. When incorporated into a single-pixel anti-parallel aligned cell subject to an electric field, the response time for the liquid crystal layer was found to be slower than expected for typical thicknesses required for middle infrared operation (7.5$ \mu $m - 30$ \mu $m). This was due to high angle reorientation which is typically absent in visible operation. The effects of various considerations, such as aperture size and shape, illumination, pixelation, dead-space and fringing effects, on the performance of a spatial light modulator were investigated. Liquid crystal on silicon implementation was simulated. For $ \lambda= 2.3 \mu$m an efficiency $ > 10 \% $ was attained for angles up to $ 7^{\circ} $. For longer wavelengths this was attained for angles up to $ 11^{\circ} $. Multi-interaction and non-diffractive devices were investigated as alternatives for increasing the viewing range while preserving resolution.
mid-infrared, spatial light modulator, non-mechanical beam steering
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