Mirror Surface Nanostructuring via Laser Direct Writing -- Characterization and Physical Origins

The addition of an optically absorptive layer to otherwise standard dielectric mirrors enables a set of laser direct writing nanostructuring methods that can add functionality to such mirrors while retaining their high reflectivity. These mirrors are particularly suited for use in optical microcavities, where arbitrary potential landscapes for photons may be constructed. Experiments with photon Bose-Einstein condensates, where high cavity finesse is essential, is one area that has greatly benefited from this approach. A thorough characterization of our implementation of this method is given in this paper, and its physical origins are investigated. In particular, our measurements show that laser direct writing of such mirrors has a reversible and a permanent component, where the reversible process originates from the thermal expansion of the surface and allows a simple yet precise way to temporarily modify the shape of the mirror. Scanning electron microscope cross-sectional images suggest that the permanent part of the nanostructuring process is due to thermally induced pore formation and enlargement in the tantalum oxide layers of the used dielectric mirror.