  Elusive features in photonic and electronic devices can be detected by means
of advanced, time-domain spectroscopic techniques. In this letter we introduce
a novel kind of modulation spectroscopy, based on the optomechanical
interaction of photonic and mechanical modes. Applying the technique to a Si
metasurface and its drum-like mechanical modes, we detect narrow-band
quasi-Bound-State-in-the-Continuum (q-BIC) modes close to normal incidence,
where their measurement can be hindered by a high symmetry protection and
undesired background modes. Showing a visibility enhancement of more than one
order of magnitude, the optomechanical modulation spectroscopy can be an
innovative tool for precise spectroscopy of a wide set of photonic devices,
including the goal of measuring purely symmetry protected BIC resonances.

Optics