We use ultrashort optical pulses to excite and detect vibrations of single silica spheres with a diameter of 5 μm placed at the surface of an acoustically mismatched substrate. In addition to the photoelastic detection of picosecond longitudinal acoustic pulses propagating inside the bulk, we detect gigahertz acoustic resonances of the sphere through probe beam defocusing. The mode frequencies are in close accord with those calculated from the elastic vibrations of a free sphere. We also record a resonant enhancement in the amplitude of specific modes of two touching spheres.
Bibliographical noteErratum can be found in Optics Letters, Volume 35(7), 940,