Phononically shielded photonic-crystal mirror membranes for cavity quantum optomechanics

We present a highly reflective, sub-wavelength-thick membrane resonator featuring high mechanical quality factor and discuss its applicability for cavity optomechanics. The $88.5~\text{nm}$ thin stoichiometric silicon-nitride membrane, designed and fabricated to combine 2D-photonic and phononic crystal patterns, reaches reflectivities up to $99.89~\%$ and a mechanical quality factor of $2.9 \times 10^7$ at room temperature. We construct a Fabry-Perot-type optical cavity, with the membrane forming one terminating mirror. The optical beam shape in cavity transmission shows a stark deviation from a simple Gaussian mode-shape, consistent with theoretical predictions. We demonstrate optomechanical sideband cooling to mK-mode temperatures, starting from room temperature. At higher intracavity powers we observe an optomechanically induced optical bistability. The demonstrated device has potential to reach high cooperativities at low light levels desirable for e.g. optomechanical sensing and squeezing applications or fundamental studies in cavity quantum optomechanics, and meets the requirements for cooling to the quantum ground state of mechanical motion from room temperature.