  We image GHz zero-group-velocity (ZGV) Lamb waves in the time domain by means
of an ultrafast optical technique, revealing their stationary nature and their
acoustic energy localization in two dimensions. The acoustic field is imaged to
micron resolution on a nanoscale bilayer consisting of a silicon-nitride plate
coated with a titanium film. Temporal and spatiotemporal Fourier transforms
combined with a technique involving the intensity modulation of the optical
pump and probe beams gives access to arbitrary acoustic frequencies, allowing
ZGV modes to be isolated. The dispersion curves of the bilayer system are
extracted together with the Q factor and lifetime of the first ZGV mode.
Applications include the testing of bonded nanostructures.
