Inscription of Nano Gratings on GaN Thin Films and Waveguides with a NIR Femtosecond Laser

Gallium nitride (GaN) and its alloys have attracted significant attention as materials for various light sources in the UV-visible range. Recently, GaN has been studied as a candidate material for integrated optics (IO). GaN grating structures are highly attractive for IO, waveguides, distributed-feedback, and distributed-Bragg-reflectors. However, it is difficult to micromachine GaN and its alloys due to their hardness and high chemical stability. Generally, the primary method for fabricating grating structures in GaN is reactive-ion etching. However, time consumption and processing complexity, especially in the nano-scale, are a significant drawback of this method.

Femtosecond micromachining in transparent materials has been under extensive research in the past two decades. It was found that due to the high peak power of femtosecond pulses in transparent materials, temporal or permanent modifications of the material can occur. This can result in a smooth isotropic refractive index modification, nano and micro voids, anisotropic refractive index changes and ablation. In the last decade, inscribing Bragg gratings (BGs) in an optical fiber using NIR femtosecond laser radiation, a phase mask, and a cylindrical lens, has been proven to be a fast and effective technique. Implementing this technique for on-chip IO is very appealing.

In this work, we show inscription of grating structures on low-loss GaN waveguides. The gratings are a result of ablation due to the high peak power of the femtosecond laser. In addition, we show that various nano gratings were inscribed with a period in the range of ~500 nm and down to ~50 nm. Different grating periods were obtained with various conditions including different repetition rates, polarizations of the incident laser beam, pulse energy, and the number of the inscribed pulses.