Optomechanics with one-dimensional gallium phosphide photonic crystal cavities

Gallium phosphide offers an attractive combination of a high refractive index (n > 3 for vacuum wavelengths up to 4 mu m) and a wide electronic bandgap (2.26 eV), enabling optical cavities with small mode volumes and low two-photon absorption at telecommunication wavelengths. Heating due to strongly confined light fields is therefore greatly reduced. Here, we investigate the benefits of these properties for cavity optomechanics. Utilizing a recently developed fabrication scheme based on direct wafer bonding, we realize integrated one-dimensional photonic crystal cavities made of gallium phosphide with optical quality factors as high as 1.1 x 10(5). We optimize their design to couple the optical eigenmode at similar to 200 THz via radiation pressure to a co-localized mechanical mode with a frequency of 3 GHz, yielding sideband-resolved devices. The high vacuum optomechanical coupling rate (g(0) = 2 pi x 400 kHz) permits amplification of the mechanical mode into the so-called mechanical lasing regime with input power as low as similar to 20 mu W. The observation of mechanical lasing implies a multiphoton cooperativity of C > 1, an important threshold for the realization of quantum state transfer protocols. Because of the reduced thermo-optic resonance shift, optomechanically induced transparency can be detected at room temperature even in non-sideband-resolved devices in addition to the normally observed optomechanically induced absorption. Considering that GaP is also piezoelectric, these results establish GaP as an attractive material for future electro-opto-mechanical systems. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement