  We report the design and characterization of a self-assembled, locally
resonant acoustic metamaterial for Lamb waves, composed of a monolayer of
$1.02$ $\mu$m polystyrene microspheres adhered to a $1.3$ $\mu$m thick
free-standing silicon membrane. A laser-induced transient grating technique is
used to generate Lamb waves in the metamaterial and measure its acoustic
response. The measurements reveal a microsphere contact resonance and the
lowest frequency spheroidal microsphere resonance. The measured dispersion
curves show hybridization of flexural Lamb waves with the microsphere contact
resonance. We compare the measured dispersion with an analytical model using
the contact resonance frequency as a single fitting parameter, and find that it
well describes the observed hybridization. Results from this study can lead to
an improved understanding of microscale contact mechanics and to the design of
new types of acoustic metamaterials.
