Broadband and highly efficient grating couplers for silicon-based horizontal slot waveguides

Abstract

Optical nonlinear effects have been widely studied in III-V semiconductor photonics. However, nonlinear performance in silicon photonics is still inefficient. An alternative silicon-based waveguide configuration, which is known as slot waveguide, has been recently proposed to improve the nonlinear performance in a very efficient way. In the slot waveguide, the fundamental mode light is highly confined in a very small region, which is called slot, of a low index contrast material between two silicon high index contrast layers. This enables the introduction of new silicon photonic devices in which the characteristics of active optical materials can be efficiently exploited for modulation, switching, sensing, and other applications. Horizontal and vertical slot waveguides for optimum nonlinear performance have been recently proposed. However, the horizontal slot waveguide is more feasible for nonlinear applications. To increase nonlinear performance in the horizontal slot region, silicon nanocrystals (Si-nc) embedded in silica (SiO₂) have been proposed to fill the slot region between the two silicon layers. It is achievable nonlinear performance in the horizontal slot region for down to 50nm thick slots. However, the lower the slot thickness is, the more difficult the coupling to fiber results. One of the most developed silicon photonics efficient vertical coupling techniques is the grating coupler. We demonstrate grating couplers for efficient coupling between horizontal slot waveguides and standard single mode fibers. Broadband and highly efficient horizontal slot waveguide grating couplers have been obtained by means of simulations. These grating couplers configuration are suitable for nonlinear performance in silicon photonics. It is achieved 61% maximum coupling efficiency for λ=1550 nm and TM polarization. Furthermore, a 35 nm 1dB-bandwidth is achievable for the designed grating couplers.