  Vibrational spectroscopy is a ubiquitous technology that derives the species,
constituents, and morphology of an object from its natural vibrations. However,
the vibrational spectra of mesoscopic particles - including most biological
cells - have remained hidden from existing technologies. These particles are
expected to vibrate faintly at megahertz to gigahertz rates, imposing
unpractical sensitivity and resolution for current optical and piezoelectric
spectroscopy. Here we demonstrate the real-time measurement of natural
vibrations of single mesoscopic particles using an optical microresonator,
extending the reach of vibrational spectroscopy to a new spectral window.
Conceptually, a spectrum of vibrational modes of the particles is stimulated
photoacoustically, and correlated to a high-quality-factor optical resonance
for the ultrasensitive readout. Experimentally, this scheme is testified by
measuring mesoscopic particles with different constituents, sizes, and internal
structures, showing an unprecedented signal-to-noise ratio of 50 dB and
detection bandwidth over 1 GHz. This new technology is further applied for the
biomechanical fingerprinting of single microbial cells with different species
and living states. The present method opens up new avenues to study
single-particle mechanical properties in vibrational degrees of freedom, and
may find applications in photoacoustic sensing and imaging, cavity
optomechanics and biomechanics.
