  Using polarization-resolved transient reflection spectroscopy, we investigate
the ultrafast modulation of light interacting with a metasurface consisting of
coherently vibrating nanophotonic meta-atoms in the form of U-shaped split-ring
resonators, that exhibit co-localized optical and mechanical resonances. With a
two-dimensional square-lattice array of these resonators formed of gold on a
glass substrate, we monitor the visible-pump-pulse induced gigahertz
oscillations in intensity of reflected linearly-polarized infrared probe light
pulses, modulated by the resonators effectively acting as miniature tuning
forks. A multimodal vibrational response involving the opening and closing
motion of the split rings is detected in this way. Numerical simulations of the
associated transient deformations and strain fields elucidate the complex
nanomechanical dynamics contributing to the ultrafast optical modulation, and
point to the role of acousto-plasmonic interactions through the opening and
closing motion of the SRR gaps as the dominant effect. Applications include
ultrafast acoustooptic modulator design and sensing.
