Monolithic High-Contrast Gratings: Why Do They Not Scatter Light?

Abstract

Monolithic high-contrast gratings (MHCGs) are able to reflect light as well as can classical subwavelength high-contrast gratings (HCGs), without requiring, as do classical HCGs, to be sandwiched between low-refractive-index layers. Although the replacement of low-index cladding on the input side with high-index material should cause strong scattering of the incident wave into higher diffractive orders, this effect can be suppressed so that MHCGs reflect only the zero-order plane wave. This paper describes a numerical investigation of the reflectivity maxima for both classical and monolithic gratings, and conducts a formal mathematical analysis of the plane wave reflection. It shows that the absence of scattering in MHCGs is attributed to a particular structure of the impedance/admittance matrix, which makes the reflectivity of the grating independent of the substrate refractive index. By carefully choosing the grating parameters, this structure of the impedance/admittance matrix can be found for any value of the grating refractive index, allowing MHCGs to be designed in any material wavelength.