Fabrication of Multilevel Fused Silica Diffractive Phase Elements by Laser Processing of Silicon Suboxide

UV-absorbing silicon suboxide films (SiOx, x ≈ 1) can be patterned by excimer laser ablation and subsequently oxidized to fused silica (SiO₂) by thermal annealing. This two-step process allows for the fabrication of microstructured components made entirely of fused silica. For example, diffractive optical elements with two phase-quantized levels (binary DOEs) can be produced via structured ablation of a thin film of silicon suboxide on a fused silica substrate followed by oxidation. In addition to these binary patterns, multilevel phase structures are fabricated by repeating the ablation step after recoating the structured surface with additional layers of silicon suboxide. A diffractive phase element in form of a four-level blazed grating exhibits a diffraction efficiency of 73% in the +1st order in the vicinity of the design wavelength. This efficiency can be enhanced by increasing the number of recoating and ablation steps and, thus, the number of phase levels.