Fabrication of Customized, Low-Loss Optical Resonators by Combination of FIB-Milling and CO$_2$ Laser Ablation

Fabry-Perot cavities are essential tools for applications like precision metrology, optomechanics and quantum technologies. A major challenge is the creation of microscopic spherical mirror structures which allow the precise matching with the wavefront of a Gaussian beam, while providing high surface quality. We present a novel fabrication technique, enabling the creation of customized microscopic cavity mirror structures over a wide range of geometrical parameters, by combining focused ion beam milling (FIB) and CO$_2$ laser smoothing. While FIB milling allows us to imprint features on the mirror substrate with a resolution on the nanometer scale, the application of defocused CO$_2$ laser pulses consistently reduces remaining surface deformations down to a roughness of $\sigma_\text{RMS}=0.2\,$nm. The average deviation of the profile from a spherical shape is kept below a few nanometres. This technique enables the customized and repeatable fabrication of low loss optics on a wide range of optical substrates, including optical fibres. Thus, Fabry-Perot cavities can be fabricated with pre-defined modal volume, high finesse and tailored ellipticity. Since the structural shape of the mirror is created by FIB milling, its pixel-by-pixel nature of the writing process can be used to create arbitrary structures with a resolution given by the FIB milling. At the same time, the surface quality is given by the CO$_2$ laser smoothing processes, without degrading the FIB milled shape.