A miniature circular polarizer that operates over the entire visible region and that has a thickness of just 390 nm has been fabricated and tested by researchers from Seoul National University in South Korea. The device is made from a stack of sets of aluminium nanoscale gratings featuring a subwavelength pitch. The orientation of each set of gratings is gradually twisted with respect to the preceding set beneath in a helix-type arrangement. As the approach relies on the non-resonant anisotropic characteristics of the grating stack, it is able to simultaneously deliver a high extinction ratio and broadband operation. The performance of stacks up to seven grating layers is investigated and as the number of gratings increases so does the extinction ratio of the polarizer, up to a value of eight across the entire visible range, but at the cost of overall transmittance due to the intrinsic loss of the aluminium. The approach is thought to be cost effective for mass production and the miniaturization it brings is thought to be potentially useful for compact mobile or wearable imaging systems. By replacing aluminium with other noble metals such as gold or silver it is possible to fabricate a design that operates in the near-infrared region.
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Graydon, O. Ultrathin circular polarizer. Nature Photon 11, 454 (2017). https://doi.org/10.1038/nphoton.2017.134