Anderson photon-phonon co-localization in certain random superlattices

Abstract: Fundamental concepts in quantum physics and technological applications ranging from the detection of gravitational waves to the generation of stimulated Brillouin scattering rely on the interaction between the optical and the mechanical degrees of freedom.\ A key parameter to engineer this interaction is the spatial overlap between the two fields, optimized in carefully designed resonators on a case-by-case basis.\ Disorder is an alternative strategy to confine light and sound at the nanoscale.\ However, it lacks an \textit{a priori} mechanism guaranteeing a high degree of co-localization due to the inherently complex nature of the underlying interference process.\ Here, we propose a way to address this challenge by using GaAs/AlAs vertical distributed Bragg reflectors with embedded geometrical disorder.\ Due to a remarkable coincidence in the physical parameters governing light and motion propagation in these two materials, the equations for both longitudinal acoustic waves in the growth direction and normal-incidence light become practically equivalent for excitations of the same wavelength.\ This guarantees spatial overlap between photons and phonons leading to a statistically significant enhancement in the vacuum optomechanical coupling rate, $g_{0}$, and making this system a promising candidate to explore Anderson localization of high frequency ($\sim$ 20 GHz) phonons enabled by cavity optomechanics.\