Simulations of nanograting-assisted light coupling in GaN planar waveguide

The numerical simulations of nanogratings integrated with gallium nitride (GaN) planar waveguides as well as the experimental in-coupling results are presented. A simulation tool based on the eigenmode expansion method and advanced boundary conditions provided a rigorous model of 400-nm-period grating couplers. A full-vectorial Maxwell solver allowed performing a number of simulations with varying grating parameters, where coupling efficiency, reflection and transmission characteristics of device were calculated. Gratings with different etch depths and arbitrary shapes were simulated using a staircase approximation, with an optimized number of steps per single slope. For the first time, an impact of dry etch processing on GaN coupler efficiency was evaluated, due to the inclusion of the sloped sidewalls, with regard to the technological constrains. Finally, the experimental results in the visible spectrum region (?? = 633 nm), for 400-nm-deep gratings etched in GaN waveguide, were presented together with theoretical data for binary and trapezoidal profiles of a grating, for different optical mode profiles ( ${{\rm TE}_{0}\div {\rm TE}_{3}\,{\rm modes}}$ ).