High-contrast gratings fabricated in free-standing membranes of silicon nitride are a remarkable new platform for optomechanics, as they combine high reflectivity, low mass, and a high mechanical quality factor in a single device. In an effort to further improve on our earlier designs, we are now fabricating high-contrast gratings from stoichiometric silicon nitride. The new gratings have a diameter of 80 μm, a thickness of 250 μm, and are patterned in square membranes from 100 μm to 500 μm on a side. We find reflectivities R < 0.994 for these devices, and fundamental mechanical resonance frequencies above 1.5 MHz. In addition, we have incorporated HCGs fabricated from low-stress silicon nitride into a “membrane-in-the-middle” setup, and observe that the cavity transmission spectrum is distorted from a constant free spectral range of 3 GHz to one characterized by anticrossings separated by 72 ± 2 MHz.
Subwavelength diffraction gratings patterned into a silicon nitride membrane offer a novel new platform for cavity
optomechanics. The monolithic device combines high reflectivity, high mechanical quality factor, and low mass.
Here we survey results we have obtained using such a device as one mirror of a Fabry-Perot cavity. With a cavity
finesse of F ≈ 2000, we are able to optically cool hundreds of mechanical modes of the membrane. The lowest
effective temperature we achieve, by detuning a laser to the red side of an optical resonance, is approximately
Teff = 1 K. The cooling is accompanied by an optically-induced shift of the mechanical frequency, as expected; both the degree of cooling and frequency shift are proportional to the power of the cooling laser. When we
detune the laser to the blue side of the resonance, the resulting optical “antidamping” causes the dynamics of the
mechanical system to change from thermal to oscillatory, with a well-defined phase. Finally, we computationally
investigate the feasibility of a proposal to realize radiation pressure optomechanics without a cavity, by use of a
subwavelength grating with a rapid variation of reflectivity with wavelength.