Wavelength-sized GaAs optomechanical resonators with GHz frequency
  We report on wavelength-sized GaAs optomechanical disk resonators showing ultra-strong 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 0.8 MHz, with a related differential optomechanical coupling factor of 485 GHz/nm. The disk Brownian motion is optically resolved with a sensitivity of 10-17 m/{\sqrt}Hz at room temperature and pressure. 
Lu Ding, Christophe Baker, Pascale Senellart, Aristide Lemaitre, Sara   Ducci, Giuseppe Leo, and Ivan Favero
physics.optics