Nano-Optomechanics

In a semiconductor disk optical resonator, photons are confined at the resonator’s periphery by total internal reflection, travelling along the inner side of the semiconductor-air interface. This circulation of light results in “optical whispering gallery modes”, named in analogy with circular acoustic galleries where two persons can whisper to each other at distance, the sound being guided along the bent sidewall [a]. We employ these strongly confining whispering gallery modes (the mode volume can be inferior to the wavelength cube) to study the coupling between light and mechanical vibrations in disk resonators.

The optomechanical coupling in a disk resonator can be understood as follows: the photons circulating in the disk gallery mode exert a centrifugal photonic force that excites the radial breathing motion of the structure, conversely, when the disk “breathes” mechanically, the photons stored in the disk are modulated. This mutual interaction between optics and mechanics is the basis of optomechanical phenomena.

The coupling between light and mechanical motion (“optomechanics”) has experienced a surge of interest since the beginning of years 2000’s, when first experiments reported that laser light can be used to optically
cool the vibration of mechanical systems, bringing analogy with atom and ions laser cooling techniques [b, c]. The optical cooling of a mechanical resonator down to its quantum regime offers the exciting perspective to reveal and study the “quantumness” of macroscopic solid-state mechanical systems [d, e]. Research in optomechanics are indeed starting to experimentally access this regime now.

The Gallium Arsenide (GaAs) disk nano-optomechanical resonators (see picture) employed in our team possess high-Q optical whispering gallery modes, combined with high-frequency (GHz) mechanical modes. They exhibit amongst the strongest optomechanical couplings, thanks to their ability to confine both the optical and mechanical modes in a sub-wavelength interaction volume. Their ultra-low dissipation, both optical and mechanical, enables to protect their coherence for experiments in the quantum regime and to boost their performances in applications in the classical regime.