  Hypersonic phononic bandgap structures confine acoustic vibrations whose
wavelength is commensurate with that of light, and have been studied using
either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers
are the preferred instruments for characterizing periodic multilayer stacks
from common vacuum deposition techniques, but the detection mechanism requires
the injected sound wave to maintain coherence during propagation. Beyond
acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry-Perot
interferometer (TFPI) find dispersions of two- and three-dimensional phononic
crystals (PnCs) even for highly disordered samples, but with the caveat that
PnCs must be transparent. Here, we demonstrate a hybrid technique for
overcoming the limitations that time- and frequency-domain approaches exhibit
separately: We inject coherent phonons into a non-transparent PnC using a
pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where
pumped appear alongside spontaneously excited (i.e. incoherent) phonons.
Choosing a metallic Bragg mirror for illustration, we determine the bandgap and
compare with conventional time-domain spectroscopy, finding resolution of the
hybrid approach to match that of a state-of-the-art asynchronous optical
sampling setup. Thus, the hybrid pump-probe technique retains key performance
features of the established one and going forward will likely be preferred for
disordered samples.
