Recent Advances in Silicon Waveguide Devices Using Sub-Wavelength Gratings

As most integrated optics platforms, silicon offers only a fixed set of material refractive indices for designing devices. The ability of sub-wavelength patterned structures to synthesize arbitrary equivalent index values between nSi ~ 3.5 and nair=1 has thus enabled completely new design approaches that have led to silicon waveguide devices with breakthrough performance. This review covers the latest advances in sub-wavelength structured fiber-to-chip grating couplers and multimode interference couplers. We discuss, among others, single-etch grating couplers with high coupling efficiency ( -1.8 dB), and broad bandwidth ( 115 nm at 3 dB), as well as ultra-short, high performance multi-mode interference couplers. We furthermore introduce the concept of dispersion engineering with sub-wavelength structures, showing its potential to increase the bandwidth of waveguide devices, including multi-mode interference couplers covering a 450 nm wavelength span.