We demonstrate a broadband second-harmonic generator for the green spectral range based on a periodical polarity GaN waveguide structure. We grew the structure with a periodicity of 4 \(\mu \) m using an MOCVD (metal-organic chemical vapor deposition) reactor. We tested the structure using femtosecond laser pulse pumping and obtained the second harmonic ( \(\lambda _{SH}\) =512 nm) conversion efficiency 0.79 \(\cdot \) 10 \(^{-7}\) % \(\cdot \) W \(^{-1}\) for the pump peak power of 440 kW inside the waveguide. Based on the theoretical modeling of the dispersion curves of the pump and the second harmonic, we found multiple crossings, including an extended crossing. We relate this fact to the experimentally measured broadened second harmonic spectrum.
