Wavelength-sized GaAs optomechanical resonators with gigahertz frequency

We report on wavelength-sized GaAs optomechanical disk resonators showing ultrastrong optomechanical interaction. We observe optical transduction of a disk mechanical breathing mode with 1.4 GHz frequency and effective mass of ∼2 pg. The measured vacuum optomechanical coupling rate reaches g0=0.8 MHz, with a related differential optomechanical coupling factor gom=485 GHz/nm. The disk Brownian motion is optically resolved with a sensitivity of 10−17 m/√Hz at room temperature and pressure.