Demonstration of continuous-wave second and third harmonic generation in high-Q gallium nitride photonic crystal cavities

Wide bandgap semiconductors such as gallium nitride (GaN) are essential constituents of future optical circuits, as their optical response can accommodate a broad wavelength range while suppressing two-photon absorption and free-carrier absorption effects that are encountered with silicon (Si) structures. Their direct wide bandgap is also favourable for the incorporation of active elements. Furthermore, nonlinear optical processes can be harnessed by exploiting the higher-order susceptibility tensors of the crystal structure to achieve advanced light control modalities, enabling all-optical processing and the generation of entangled photon states. We will report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Genetic optimization is applied to sweep parameter space for the highest cavity quality factors, and simultaneously accounting for power in-coupling. While there is a clear trade-off theoretically between coupling efficiency and Q-factor for a given cavity design, the upper limit on the Q-factor that is imposed by loss channels, given the disorder figure of current fabrication technology, makes room for introducing improved far-field coupling to enhance nonlinear processes without sacrificing the experimentally achievable light confinement. Far-field coupling is addressed through various PhC cavity designs, which enable the excitation of the fundamental mode with a Gaussian beam. Optimized two-dimensional PhC cavities with increased far-field coupling have been characterized with quality factors as high as 44′000, approaching the computed theoretical values. The strong enhancement in light confinement has enabled second harmonic generation (SHG) under continuous-wave excitation, with up-conversion from both 1300 nm and 1550 nm wavelength bands, confirmed by spectral and power dependence measurements. At 1550 nm, normalized SHG conversion efficiency as large as 2.4×10 −3 W −1 are measured as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation