For fused silica, combined etching and laser polishing leads to high LIDT

Damage to fused silica optics from high-power UV lasers limits the development of high-power laser systems. The traditional processing method for fused silica is grinding and chemical mechanical polishing (CMP), which is often used in fabricating photolithographic masks. This method takes a long time to produce an ultrasmooth surface and often results in surface and subsurface defects that lead to a significant reduction in the surface damage threshold of the fused silica. A research team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (Shanghai, China) has now combined chemical etching and CO₂ laser polishing to process ground fused silica: chemical etching was used to open the subsurface defects of the ground fused silica, and subsequently, CO₂ laser polishing was applied to reduce surface roughness. The researchers say that the combined process not only can efficiently obtain a supersmooth surface with a low surface roughness, but also can improve the damage resistance of fused silica.

In the experiment, fused silica samples 3 mm thick and 30 mm in diameter were first mechanically ground to a surface root-mean-square (RMS) roughness of 24 nm, measured with an atomic-force microscope (AFM). The researchers then subjected groups of these samples to three different processes: CMP, laser polishing (LP), and etching with hydrofluoric acid (HF) followed by LP. The results were characterized by AFM, showing RMS roughness of 0.33, 0.31, and 0.25 nm, respectively. Laser-induced damage threshold (LIDT) testing was then performed using a frequency-tripled Nd:YAG laser emitting 7.6 ns pulses at 355 nm in a single longitudinal mode. The HF-LP resulted in significantly higher LIDTs than CMP or LP alone: for example, the LIDT for 0% damage for the HF-LP sample was 29.5 J/cm², versus that of the CMP sample at 22.2 J/cm²; the LIDT for 100% damage probability in the HF-LP sample was 47.5 J/cm², versus 34.5 J/cm² for the CMP sample.

The combined HF-LP process was shown to avoid the introduction of surface and subsurface defects, including destructive defects, chemical-structure defects, and photoactive mental impurity elements, resulting in fused silica with lower surface defect densities and better damage resistance. The figure (image by SIOM) shows a schematic of a laser polishing system (a) and surface-morphology evolution during the combined process (b). Reference: Z. Cao et al., Opt. Lett. (2020); https://doi.org/10.1364/ol.409857.