One-step fabrication of refractive and diffractive microoptical elements in fused silica with laser etching

High quality etching of transparent materials for applications in micro- and nano-structuring as well as in precision engineering is still a challenge for current laser processing techniques. The recently developed LIBWE and LESAL processes (laser-induced backside wet etching, laser-etching at a surface adsorbed layer, respectively) allow precise etching of transparent materials, e.g., fused silica, at low laser fluences with small etch rates. The etching is initiated by laser beam absorption at the interface to the absorbing medium that will be a liquid (LIBWE) or adsorbate from vapour (LESAL) causing thermal processes that finally results in etching with etch rates per pulse in the nanometer range. At well chosen etch conditions a surface roughness of 0.23 nm (rms) has been measured. In combination with specific laser processing techniques, LIBWE and LESAL processing allow the direct fabrication of, e.g., multilevel elements, phase gratings with variable depth, micro lenses, 3D free-form surface topographies, as well as periodically sub-micron structures with nanometer depth accuracy and a low roughness into fused silica.