Design of etch-through nanohole array waveguide couplers on an SOI platform

We design etch-through nanohole arrays as waveguide couplers on a silicon-on-insulator (SOI) platform around 1550nm. The simulation is based on a Lumerical finite-difference time-domain (FDTD) solver. Compared to traditional grating couplers, this coupler is more straightforward to fabricate because it only requires a single etching step for both the waveguide and the coupler regions. Both periodic and aperiodic arrays are designed for the nanohole structures. An etch-through 2D grating coupler with the same nanohole array size (12×12μm²) is also designed for comparison. The thickness of the SOI wafer is with a bottom oxide of 3μm, a device layer of 220nm, and a 250nm top oxide cover. A 70μm long linear taper connects the coupler and the single-mode waveguide, with a width of 400nm. The optimized coupling efficiency of a periodic nanohole array is 45.4%, with an array period of 690nm and a hole diameter of 350nm. By tuning the array period, an aperiodic nanohole array's optimized coupling efficiency reaches 47.5%. Compared with a conventional 2D grating coupler, the optimized coupling efficiency is 24.8% with a grating period and duty cycle of 800nm and 0.66, respectively. With an extended propagation length of 18μm on the nanohole arrays, the coupling efficiency increases to 49.4% and 53.7% for periodic and aperiodic ones, respectively. The far-field radiation angle of the designed output couplers characterized by a Fourier transform method is 14.66°, 12.03°, and 12° for the periodic nanohole array, aperiodic nanohole array, and 2D grating coupler, respectively, with a footprint of 12×12μm². For the periodic and aperiodic nanohole array with a footprint of 12×18μm², the radiation angle is 12.95° and 8.79°, respectively. The proposed coupler offers an alternative approach for vertical light coupling on the SOI platform.