Optical excitation and detection of picosecond acoustic pulses in liquid mercury

Ultrashort optical pulses are used to excite and interferometrically detect picosecond longitudinal-acoustic pulses in thin films of liquid mercury sandwiched between sapphire plates. We show that the shape of the strain pulses in the mercury can be directly measured through ultrafast changes in optical reflectivity. By analyzing consecutive acoustic echoes, we derive the dispersion of the ultrasonic attenuation and sound velocity for this liquid at frequencies up to 10 GHz. Significant effects of structural relaxation are observed and are compared to a simple model that indicates the presence of picosecond relaxation times in mercury.