APL Photon. 2, 036103 (2017)

Credit: HTTP://CREATIVECOMMONS.ORG/LICENSES/BY/4.0/

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.