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

TL;DR

This paper presents a detailed study of laser direct writing nanostructuring on dielectric mirrors with an absorptive layer, revealing reversible thermal effects and permanent pore formation as physical mechanisms, enhancing optical microcavity applications.

Contribution

It provides a comprehensive characterization of laser nanostructuring on dielectric mirrors and investigates the physical origins of reversible and permanent modifications.

Findings

Reversible modifications are due to thermal expansion.

Permanent changes involve pore formation and enlargement in tantalum oxide layers.

The method retains high mirror reflectivity while adding nanostructures.

Abstract

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.