Subwavelength grating couplers for an ultrathin silicon waveguide

Subwavelength grating structures are widely used in integrated optical devices due to their compact footprints and flexible refractive index tailoring. In this paper, we present a subwavelength grating (SWG) coupler for a 70-nm-thick silicon waveguide without the upper cladding. The device was designed by using a particle swarm optimization algorithm in 3D finite-difference time-domain simulation. Experimental measurements show that the SWG coupler with a minimum feature size of ∼210 nm has a peak coupling efficiency of -7.0 dB with a 1-dB bandwidth of ∼40 nm and a backreflection of ∼-18.7 dB at 1550 nm wavelengths for transverse-electric polarized light. The ultrathin device structure is simple and reproducible, making it well suited for sensing and optoelectronic integration applications at low cost.