  Frequency- and time-domain Brillouin scattering 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 scattering spectroscopy that spans the time and frequency
domains to allow the multichannel detection of Brillouin scattering light from
coherent acoustic phonons. Our technique maps the time-evolve Brillouin
oscillations at the instantaneous frequency of a chromatic-dispersed laser
pulse. The spectroscopic heterodyning of Brillouin oscillations in the
frequency domain enhances the signal acquisition speed by at least 100-fold
over the time-domain method. As a proof of concept, we imaged heterogeneous
thin films and biological cells over a wide bandwidth with nanometer depth
resolution. We, therefore, foresee that our approach catalyzes future phonon
spectroscopy toward real-time mechanical imaging.
