Femtosecond laser inscription of Bragg gratings on a thin GaN film grown on a sapphire substrate

Bragg gratings were inscribed with a NIR femtosecond laser, cylindrical lens and a phase mask on the surface of a thin GaN film grown on a sapphire substrate. The phase mask was used to produce stable interference patterns from the focused femtosecond laser to form the gratings on the samples. The intensity threshold for the Bragg grating inscription was found to be ∼2.5·10¹² W/cm². By optimizing the inscription parameters, we successfully produced grating structures with uniform periods of ∼1.07 µm on the thin GaN film surface. We compared the inscription threshold to that of sapphire and fused silica. It was found that the threshold for Bragg grating inscription on GaN film is nearly an order of magnitude lower than that on sapphire and fused silica. This can be attributed to the lower-order nonlinear absorption process in GaN compared to that in sapphire or fused silica glass, originating from the smaller energy band gap of GaN.