Cavity-less Brillouin strong coupling in a solid-state continuous system

Abstract Strong coupling between two systems allows them to exchange coherent information before the systems decohere. This important regime in light–matter interactions has predominantly been reached in optical resonator configurations. Here we demonstrate strong coupling between optical and acoustic fields within a continuum of modes in a cavity-less configuration after a single pass through an optical waveguide. The underlying physical effect of anti-Stokes Brillouin–Mandelstam scattering in a highly nonlinear fibre at cryogenic temperature allows us to experimentally demonstrate strong coupling in a waveguide scenario. We show the splitting of the optoacoustic spectral response and introduce a technique to measure the avoided crossing of hybrid optoacoustic modes via forced detuning. This demonstration opens a path towards in-line acoustic-wave-based quantum signal processing in waveguide systems.