A review of silicon subwavelength gratings: building break-through devices with anisotropic metamaterials

Abstract

Silicon photonics is playing a key role in areas as diverse as high-speed optical communications, neural networks, supercomputing, quantum photonics, and sensing, which demand the development of highly efficient and compact light-processing devices. The lithographic segmentation of silicon waveguides at the subwavelength scale enables the synthesis of artificial materials that significantly expand the design space in silicon photonics. The optical properties of these metamaterials can be controlled by a judicious design of the subwavelength grating geometry, enhancing the performance of nanostructured devices without jeopardizing ease of fabrication and dense integration. Recently, the anisotropic nature of subwavelength gratings has begun to be exploited, yielding unprecedented capabilities and performance such as ultrabroadband behavior, engineered modal confinement, and sophisticated polarization management. Here we provide a comprehensive review of the field of subwavelength metamaterials and their applications in silicon photonics. We first provide an in-depth analysis of how the subwavelength geometry synthesizes the metamaterial and give insight into how properties like refractive index or anisotropy can be tailored. The latest applications are then reviewed in detail, with a clear focus on how subwavelength structures improve device performance. Finally, we illustrate the design of two ground-breaking devices in more detail and discuss the prospects of subwavelength gratings as a tool for the advancement of silicon photonics.

Circular Optical Phased Arrays with Radial Nano-Antennas

“…The SWGs are non-resonant nanophotonic structures with pillar segmentation with dimensions much smaller than the wavelength of light, suppressing reflection and diffraction effects [ 31 ]. Over the years of research, SWG metamaterials have captured a strong interest due to their eminent potential of customizing optical and light propagation properties in planar silicon waveguides [ 31 , 32 , 33 ]. The L -shaped grating exploits vertically asymmetric scatters to maximize the light coupled upwards, while minimizing the light radiated towards the Si substrate.…”

Silicon Multimode Waveguide Crossing Based on Anisotropic Subwavelength Gratings

“…According to the effective medium theory (EMT), an SWG structure can be treated approximately as a homogeneous waveguide made of equivalent birefringent materials and the refractive indices are given by [ 27,40–42 ] $$\begin{equation}n_{||}^2 = fn_{{\rm{Si}}}^2 + \left( {1 - f} \right)n_{{\rm{SiO}}2}^2\end{equation}$$ $$\begin{equation}\frac{1}{{n_ \bot ^2}} = \ f\frac{1}{{n_{{\rm{Si}}}^2}} + \left( {1 - f} \right)\frac{1}{{n_{{\rm{SiO}}2}^2}}\end{equation}$$ $$\begin{equation}n_{ \bot ||}^2 = fn_{{\rm{Si}}}^2 + \left( {1 - f} \right)n_ \bot ^2\end{equation}$$where f is the longitudinal duty cycle of the SWGs, n || ( n ⊥ ) is the equivalent index of the 1D‐SWG waveguide for the polarization mode with a dominant electric field parallel (perpendicular) to the grating, and n ⊥|| is the equivalent index of the 2D‐SWG structure in region D. Figure 1c shows the equivalent indices n || , n ⊥ , and n ⊥ || calculated with Equation (1) and one has n ⊥ < n || < n ⊥ || . Note that there are some other formula to estimate the equivalent index of the 2D‐SWG structure in region D [ 20,43–45 ] and the results are slightly different and still very similar.…”

“…According to the effective medium theory (EMT), an SWG structure can be treated approximately as a homogeneous waveguide made of equivalent birefringent materials and the refractive indices are given by [27,[40][41][42]…”

“…Metamaterials have attracted a lot of attention because it provides a flexible approach to control the flow of light [26,27] including one-dimensional (1D) and two-dimensional (2D) subwavelength structures. In particular, subwavelength gratings (SWGs) have been widely used in silicon photonics for some special on-chip mode and polarization manipulation.…”

Metamaterial-Engineered Silicon Beam Splitter Fabricated with Deep UV Immersion Lithography

“…Within this regime, the grating effectively acts as a homogeneous material whose optical properties (e.g., effective index, dispersion, and anisotropy) are determined by the ensemble of the constituent materials and can be varied by properly designing the geometry of the grating unit cells [1,2]. This type of metamaterials have been successfully implemented in particular in silicon photonic waveguides, allowing an unprecedented control over the field distribution and propagation properties of the guided modes, largely increasing design flexibility compared to conventional waveguides [3][4][5]. SWG metamaterials can be directly integrated within established silicon-on-insulator (SOI) platforms since their fabrication uses the same process of conventional waveguides.…”