Harnessing Simultaneous Optical and Acoustic Purcell Effects to Control Stimulated Brillouin Scattering

In this work we employ spherical plasmonic nanoparticles to work as optoacoustic antennas, capable of modifying both the optical and GHz elastic emission from localized sources.

One of the fundamental results of cavity electrodynamics is the Purcell effect, which states that the rate of spontaneous emission of photons by an atomic or molecular system is proportional to the density of photonic states. This effect allows one to control the dynamics of the internal degrees of freedom of an atomic emitter solely by modifying its environment, as demonstrated in numerous experiments with quantum dots positioned in various photonic cavities. Furthermore, recent developments in the fabrication of nanostructures have offered ways to enhance the coupling between single molecules and submicron plasmonic and dielectric particles - optical nanoantennas - delivering enhancement factors in the spontaneous emission rate of over 10^6 [1].
An extension of this idea towards acoustics was recently demonstrated in two experimental contributions [2-3], which discussed the modification of kHz sound waves generated by a toy gong positioned near a stiff wall [2], and few Hz emission from a speaker embedded in a centimeter-scale metamaterial cavity [3].
In our contribution [4], we translate these ideas to the regime of GHz excitations, tuning them to the elastic emission from SiV centers in diamond or elastic waves generated through stimulated Brillouin scattering (SBS) in waveguides [5]. To this end, we consider a generic nanoantenna used in nanophotonics - a submicron spherical plasmonic nanoparticle immersed in a dielectric medium - and demonstrate that indeed, such acoustic antennas exhibit strong, spectrally separated Mie-like resonances in the few-GHz range, and can couple efficiently to nearby elastic emitters, amplifying or suppressing their emission.
Furthermore, these systems exhibit strong optical scattering, and an optical Purcell effect in the visible regime, due to the excitation of plasmonic modes in the metal. We can therefore consider them as optoacoustic nanoantennas - nanoscale scatterers capable of strong and resonant modification of both optical and acoustic densities of states.
We examine this type of antenna as a tool to control the dynamics of SBS. We analyze the tradeoff between the local enhancement of the Brillouin coupling, and the introduction of additional decay channels, and discuss the possibility of employing such localized optoacoustic resonators for introducing localized phononic memory systems.

[1] R. Chikkaraddy, et al., Nature 535, 127 (2016).
[2] L. Langguth, et al., Phys. Rev. Lett. 116, 224301 (2016).
[3] M. Landi, et al., Phys. Rev. Lett. 120, 114301 (2018).
[4] M.K. Schmidt, et al., Phys. Rev. Lett. 121, 064301 (2018).
[5] J. Sipe, and M.J. Steel, New J. Phys. 18, 045004 (2016).