| Abstract [eng] |
Optical elements manufacturing and dielectric coating technology qualities being on the rise, laser induced damage threshold in ultraviolet wavelength range is still not reaching required values. Depending on polishing technology, fused silica elements at 355 nm are usually being damaged at energy densities from 5 to 15 J/cm2, even though damage threshold of the bulk material is >100 J/cm2. Due to nature of polishing process, a Beilby layer forms on the surface of the glass and under it lays various defects that are not visible by the naked eye. Laser beam propagating through such elements will be partly reflected, scattered or because of the impurities at the surface – partly absorbed - and lead to optical element breakdown. To eliminate such defects various methods are being used such as chemical etching using acid or alkali, plasma, or reactive ion etching. During this research combination of plasma ion and chemical KOH etching including HCl leaching were used, to identify methods’ influence on transmission, surface roughness, flatness, quality, and laser induced damage threshold. Results were then compared with just chemical HF/HNO3, KOH and plasma ion etchings. Seeking to find the best procedure and its course, different thicknesses of the glass were removed using KOH solutions and argon, oxygen plasma etching methods. Results showed that combined processes did not have any influence on flatness, on the other hand, better transmission and lower roughness was received when first processed with plasma. Best damage threshold was reached when removing 50 nm with KOH solution, then 0,6*X nm with argon plasma and 0,16*X nm with oxygen plasma. Using this method defect threshold reached 71,5 J/cm2, material threshold – 154,5 J/cm2 values. Comparing different methods, it was determined, that during combined process, defects brought by chemical solutions limited the possibility to reach higher damage threshold results than after only plasma ion etching. Even though neither surface quality, nor roughness was impacted, and combination of processes showed better results than etching only with chemical solutions, additional interaction with chemical solution did not eliminate discrete surface damage precursors seen after only plasma etching. |
