Fig. 1: Brillouin-based optomechanical system.
From: Optomechanical control of long-lived bulk acoustic phonons in the quantum regime
a, Schematic of a μHBAR. b, Optically driven acoustic response of the μHBAR, showing acoustic resonances with a 6.04-MHz FSR. c, High-resolution measurement of the driven acoustic response, showing Hermite–Gaussian modes (L0, L1, L2) with 140 kHz transverse mode spacing. d, Driven response of the fundamental (L0) acoustic mode showing a linewidth of Γ0/2π = 590 Hz at 7 K. e, Schematic of the cavity optomechanical system; the μHBAR was positioned within an optical FP optimized for acousto-optic mode matching and cooled to 13.6 K. Light in the upper apparatus arm was frequency shifted by an acousto-optic-modulator (AOM), creating an LO for heterodyne detection. Light in the lower arm was modulated by an electro-optic modulator (EOM), generating a probe tone for OMIT/OMIA measurements. The transmitted light was mixed with the LO and detected by a high-speed photo detector (PD); detected signals were analysed with a vector network analyser (VNA) during OMIT/OMIA measurements (switch closed) and a radio frequency spectrum analyser (RFSA) during spontaneous measurements (switch open). Inset: phase-matched optomechanical coupling achieved when two adjacent optical modes had a frequency separation matching the phonon frequency and the spatial periodicity of their interference (red) matched that of the phonon mode (green). f–i, Phase matching and spectral filtering used to realize single-mode coupling. Within the optical FP spectrum (f), two modes separated by the Brillouin frequency (ΩB) enable resonantly enhanced optomechanical scattering. g, A uniform phonon-mode spacing of the fundamental μHBAR modes. h, Modulation of the optomechanical single-photon coupling rate (go) produced by phase matching near the Brillouin frequency. i, The single-mode OMIT spectrum resulting from phase matching and filtering in this system. j–l, OMIT traces for different μHBAR alignments within the FP, from misaligned (j) to partially misaligned (k) and optimally aligned (l); optimal alignment (l) yielded single-mode (L0) coupling with >20-dB suppression of higher-order (L1, L2 and so on) modes. m, Broad OMIT scan at optimal alignment (l), showing selective coupling to the fundamental acoustic mode, while suppressing adjacent modes at Δν = ±6 MHz (Supplementary Information Section VIII). norm., normalized.
