Dispersive heterodyne Brillouin spectroscopy.

Frequency- and time-domain Brillouin spectroscopy are powerful tools to read out the mechanical properties of complex systems in material and life sciences. Indeed, coherent acoustic phonons in the time-domain method offer superior depth resolution and a stronger signal than incoherent acoustic phonons in the frequency-domain method. However, it does not allow multichannel detection and therefore falls short in signal acquisition speed. Here, we present Brillouin spectroscopy that spans time- and frequency-domain to allow multichannel detection of Brillouin scattering light from coherent acoustic phonons. Our technique maps time-evolve Brillouin oscillations in a chromatic-dispersed laser pulse's instantaneous frequency. Spectroscopic heterodyning of the Brillouin oscillations in the frequency domain enhances signal acquisition speed by at least 100-fold over the time-domain method. As a proof of concept, we imaged heterogeneous thin films over a wide bandwidth with nanometer depth resolution. We, therefore, foresee that our approach catalyses future phonon spectroscopy toward real-time mechanical imaging.