Abstract

We report theoretical and experimental investigations of the formation of periodic surface ripples (PSR’s) on fused silica under normal incidence of pulsed linearly polarized light of a wavelength-tunable CO₂ laser (λ = 10.6, 10.3, 9.6, and 9.3 μm). For the first time to our knowledge, the dielectric permittivity ℰ = ℰ′ + iℰ″ = (n + im)² of silica under conditions of PSR formation has been measured. PSR’s are generated at all values of λ. It is shown that the inequality ℰ′ < −1 is valid only for λ = 9.6 μm; at λ = 10.6 and λ = 10.3 μm, ℰ′ > 1. A theoretical description of the phenomenon is based on a model, which considers silica under conditions of PSR formation to be a viscous liquid. The fundamental mechanisms of PSR formation are the generation of capillary waves in this liquid owing to vapor recoil pressure and nonuniform removal of the substance from the surface by evaporation. It is shown that good agreement between theoretical and experimental results can be achieved only if a complete theory of PSR is applied, including both electodynamic and thermal processes as well as the calculation of growth rates for capillary waves with allowance made for the values of ℰ′ and ℰ″, which were measured during the PSR formation. We have demonstrated that it is impossible to describe the phenomenon of PSR’s on the fused silica surface by using only the electrodynamic model.

© 1989 Optical Society of America

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