Fabrication optimization of highreflectivity photonic crystals

Abstract

With cavity optomechanics, one can cool mechanical resonators to their quantum ground state, generate squeezed light, and transduce signals between microwave and optical domains. However, to reach the strong coupling needed for this, a compact cavity with large mechanical zero-point motion is required. High-stress Si3N4 membranes are well suited for this due to their high mechanical quality factors, but a conventional dielectric mirror is too heavy to place on such a membrane without degrading its mechanical performance. Photonic crystals, patterned from a single sub-wavelength-thick layer, can act as light-weight, high-reflectivity mirrors through the interference of overlapping Fano resonances, offering a combination of high-quality mechanical resonators with high-reflectivity mirrors. In this work, a membrane-at-the-end fiber Fabry-Pérot cavity is built in which the far mirror consists of an array of amorphous silicon nano-pillars patterned directly onto a suspended Si3N4 membrane. The dry etching recipe used to fabricate the pillars was optimized through parameter sweeps of temperature, RF platen power, chamber pressure, and gas composition, and the resulting structures were characterized with scanning electron microscopy and atomic force microscopy to quantify sidewall and top-surface roughness. The optical performance of the fabricated photonic crystals was evaluated both through free-space reflectivity measurements and by placing the samples inside a fiber cavity to measure finesse. Samples fabricated from LPCVD-deposited silicon showed lower surface roughness and consistently outperformed those made from sputtered silicon, reaching finesse values on the order of a few thousand. Comparing the measured finesse to calculations of clipping, scattering, and absorption losses identifies absorption as the dominant loss channel, with an absorption-limited finesse close to the values actually measured. These results demonstrate that a photonic crystal-patterned membrane can function as an end mirror in a fiber cavity, and indicate that further improvements in reflectivity are primarily a materials challenge rather than a fabrication-precision or geometry problem.