Efficient harmonic generation in high-Q gallium nitride photonic crystal cavities on silicon

Summary form only given. Frequency conversion from the telecom range into the visible is demonstrated in suspended GaN slab photonic crystal (PhC) cavities through second harmonic generation (SHG) and third harmonic generation (THG), under continuous-wave resonant excitation. PhC cavities based on L3 and H0 designs are implemented with embedded injectors, the latter shown in Fig. 1a, enabling improved coupling to the fundamental cavity mode using far-field excitation. Q-factors as high as 2.2×10
⁵
near λ=1550 nm were obtained through automated optimization runs for the localized cavity modes [1], based on guided-mode expansion computations. To confirm scalability across the telecom range, two design wavelengths at 1300 nm and 1550 nm were targeted for the devices.Fabricated cavities were characterized by means of resonant scattering, to extract the unloaded Qfactors and the coupling efficiency. Record Q-factors of up to 4.1×104 and 4.4×104 were measured for the L3 and H0 cavities respectively near λ=1550 nm. For cavities that exhibited intermediate-sized injectors (theoretical external coupling efficiency of 55-60 %), experimental Q-factors were lower than the computed theoretical values by less than a factor of two. The strong enhancement in field coupling and localization in cavities with optimal combination of injector size and Q-factor has enabled efficient SHG, which achieved normalized conversion efficiency values of up to 2.4×10-3 W-1, the highest reported in GaN to date across all platforms. The emitted SHG signal was found to be delocalized due to the absence of a photonic bandgap at the second harmonic for quasi transverse-magnetic modes, and is seen in Fig. 1b to traverse the lattice of an optimized H0 cavity.