Ultra-broadband reflectors covering the entire visible regime based on cascaded high-index-contrast gratings.

We report the design of ultra-broadband, highly reflective all-dielectric reflectors covering the entire visible regime based on two cascaded subwavelength high-index-contrast gratings (HCGs). We find that the spectral distance between the two gratings’ reflective bandwidths, which should be appropriately designed in order to extend the overall bandwidth for high reflectivity, is analogous to the well-known Rayleigh, Abbe and Sparrow criteria for resolution limits. Results illustrated with TM-polarized normal incidence show that high reflectivity above 98.5% covering 400–800 nm can be achieved for two cascaded HCGs with an appropriate spectral distance. The effects of key structural parameters on the bandwidth extension are discussed with physical insights. We expect this work will advance the engineering and applications of HCGs as ultra-thin, ultra-broadband and all-dielectric reflectors. [ABSTRACT FROM AUTHOR]