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 µm) and a wide electronic bandgap (2.26 eV), enabling optical cavities with small mode volumes and low twophoton 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 × 10⁵.We optimize their design to couple the optical eigenmode at ~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 (g₀ = 2φ × 400 kHz) permits amplification of the mechanical mode into the so-called mechanical lasing regime with input power as low as ~20 µW. 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.