Subwavelength Silicon Photonics

Subwavelength gratings refer to periodic structures that have a period less than half the wavelength of light in the material so that no Bragg diffraction mode is supported. Instead, the light will propagate as if it was in a homogeneous material with anisotropic refractive indices. Subwavelength gratings have attracted great interest recently, as they provide a useful degree of freedom for the crafting of the effective refractive index of the material in photonic devices. In this chapter, we will introduce some of the applications of subwavelength structures for silicon photonics devices. We start by introducing the background theory of subwavelength gratings and then discuss their applications for the engineering of waveguide grating couplers, suspended membrane devices for mid-infrared (mid-IR) wavelengths, and their use with numerical optimization techniques for optimizing photonic devices. We shall discuss the classic effective medium theory (EMT) for subwavelength gratings and show how EMT can reduce time-consuming three-dimensional (3D) numerical optimizations to an effective two-dimensional (2D) optimization problem.