Coverage threshold for laser-induced lithography

Recent experimental observations of laser-induced adsorption at the interface between an alkali vapor and a dielectric surface have demonstrated the possibility of growing metallic films of nanometric thickness on dielectric surfaces, with arbitrary shapes determined by the intensity profile of the light. The mechanisms directly responsible for the accumulation of atoms at the irradiated surface have been shown to involve photo-ionization of atoms very close to the surface. However, the existence of a vapor-pressure threshold for initiating the film growth still raises questions on the processes occurring at the surface. In this letter, we report on the observation that the vapor-pressure threshold corresponds to a minimum adatom coverage necessary for the surface to effectively neutralize the incoming ions and make possible the growth of a multilayer film. We discuss the hypothesis that the coverage threshold is a surface conductivity threshold.