Efficient second-harmonic generation (SHG) approaches often rely on periodic poling structures, which can be complex to fabricate. We present a novel quasi-symmetric GaN waveguide optimized for green light SHG, utilizing modal phase matching. Our design incorporates a complex GaN-based multilayered planar waveguide structure with GaN polarity inversion in the core fabricated via metal-organic chemical vapor deposition (MOCVD) [1].
The core of this waveguide is structured to enhance optical mode overlap by using Ga-polar and N-polar GaN layers. The resulting waveguide comprises a 507 nm Ga-polar and 91 nm N-polar GaN sandwich, separated by a 20 nm atomic layer deposition (ALD) of Al\(_2\)O\(_3\). From the sapphire substrate, it is separated by a 420 nm AlGaN cladding layer. Additionally, a TiO\(_2\) layer is deposited on the waveguide core to improve mode confinement.
For SHG experiments, a picosecond laser and the end-fire coupling method were utilized. The observed SHG spectrum exhibited a peak at 1108 nm, accompanied by periodic peaks separated by 20 nm. These peaks are attributed to modal interference involving both transverse magnetic (TM) and transverse electric (TE) modes. Currently, we are advancing in detecting SHG directed perpendicular to the waveguide surface, as theoretically predicted.
This waveguide structure shows promise for tunable SHG applications. However, further research and fine-tuning are necessary to optimize its performance. This research has been carried out in the framework of the „Universities’ Excellence Initiative” programme by the Ministry of Education, Science and Sports of the Republic of Lithuania under the agreement with the Research Council of Lithuania (project No. S-A-UEI-23-6).