Optical Quality Laser Polishing of CVD Diamond by UV Pulsed Laser Irradiation

It is a great technical challenge to precisely polish diamond substrates due to their extremely high hardness, especially for micro-optics fabrication. Here, a simple laser polishing process is demonstrated for the optical quality surface finish of chemical vapor deposition diamond using a 355 nm nanosecond laser. Raman spectroscopy and surface profile analyses reveal that the laser polishing is an ablation-based process that consists of laser graphitization and the subsequent laser ablation of the graphitized layer. An optimized strategy is proposed to realize the high-quality polishing by combining the ablation effect and defocusing laser irradiation. The polishing strategy can effectively reduce the peak-to-valley height difference of a rough surface and automatically enables the laser fluence at a low level close to the ablation threshold. Laser polishing at such critical laser fluence can greatly avoid harmful effects caused by high laser fluence and achieve precision materials' removal while maintaining optical surface quality. The approach is capable of delivering an average roughness R-a down to 8.02 nm and a high transmittance up to 80% of mechanically polished diamond in the visible spectrum. High optical performances make it possible to directly fabricate micro-optical components on diamond substrates using this novel laser polishing approach.