Al2O3.H2O-SiO2 Sol-Gel HR-Coatings for High-Power Laser Applications,

Pursuing our official collaborative ICF-laser search and development program at Limeil National Laboratory LNL with the Lawrence Livermore National Laboratory LLNL for high damage threshold optical coatings, we have now formally established that Sol-Gel highly reflective coatings prepared from colloids are superior to those prepared from solutions of precursor materials. Thus, single layer coatings of bohemite A12 O3.H2 0 have been prepared on fused silica and BK-7 substrates from aqueous colloidal suspensions of hydrated alumina at room temperature . Such coatings were porous and therefore revealed a measured refractive index of about 1.44, lower than the relevant dense material. These single coats when laser damage tested, have exhibited thresholds, under S-on-1 irradiation mode S shots at the same fluence onto a selected site of respectively 12-14 Jcm2 with 3-ns pulses and 35-50 Jcm2 with-16 ns pulses at 30-Hz. Multilayer, high reflectivity, dielectric coatings were also prepared at room temperature by laying down quarterwave-thick, alternating coats of this alumina with silica also prepared from colloidal sol, and having a refractive index of 1.22. To achieve 99 reflectivity 32 to 36 total layers were required. Such HR-coatings revealed damage thresholds as good as those of single layers of the constituting oxides in the same laser conditions.