Spectrometers have fixed spectral resolution and cannot usually distinguish between different types of circularly polarized light without the aid of additional optical elements. Exploiting metasurface technologies, Alexander Zhu and co-workers from Harvard University in the USA and the University of Waterloo in Canada have now demonstrated a compact spectrometer comprising multiple planar off-axis meta-lenses and a CMOS camera that features helicity-resolving capability. The meta-lenses are essentially meta-gratings made of TiO2 nanofins on a glass substrate acting as birefringent waveguides. The researchers integrate several off-axis meta-lenses on the same substrate to provide spatially separated focal spots. Because of the different numerical apertures used, each meta-lens possesses different spectral resolutions and ranges, and selectively focuses light with opposite circular polarization states. Combining the functions of focusing and dispersive elements in a single planar structure and using dielectric TiO2 as the working material, which is compatible with the existing CMOS processes, the meta-spectrometer can be made within an area less than 2 × 1 cm2 and at large scale for potential applications in healthcare and environmental sensing.
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Won, R. Chiral spectrometer. Nature Photon 11, 208 (2017). https://doi.org/10.1038/nphoton.2017.53
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DOI: https://doi.org/10.1038/nphoton.2017.53