Reduction of debris and thermal destruction by use of transparent material coating method in femtosecond laser processing

We investigate the morphology of a glass surface processed by tightly focused femtosecond laser pulse. Processing of a cavity with submicrometer-sized diameter is performed with irradiation laser pulse energy near a destructive threshold in air. In many cases, the cavity is surrounded by a ring-shaped protrusion, debris, and small droplets. In order to reduce the debris and the thermal destruction, we propose to process with coating a transparent material on a target material. PMMA (Poly-methyl methacrylate) is used as the transparent material. A thick PMMA film reduces dissolution and vaporization that is caused by an interaction between a high-density hot vapor plume and the target material. Furthermore, the dissolution is reduced because a low energy part in a laser pulse is reduced by sharpening the beam shape with the self-focusing of the laser pulse in the thick PMMA film. As the results, submicrometer-sized cavity that debris and the thermal destructive area are reduced dramatically is produced.