Developing a transient grating technique to probe fast acoustic dynamics in liquids

The development of new methods for measuring high frequency mechanical responses is important for answering both applied and theoretical questions. The work presented here extends the impulsive stimulated thermal scattering (ISTS) technique, previously used to measure acoustic responses below 5 GHz, to measurements of faster dynamics in thin multi-layer metal films and liquids. Interference between crossed laser pulses generates a transient grating at the surface of a metal film, and at each maximum in the interference pattern, sudden heating and thermal expansion launches a high-bandwidth wavepacket through the plane of the film. The wavepackets in the transient grating can then be detected through optical diffraction of a probe pulse. Wavepackets generated and detected in this manner may be used to measure acoustic responses at frequencies exceeding 200 GHz.; The ultimate goal of this work is to gain direct access to the density response of liquids in the frequency range between 17 GHz and 1 THz, a dynamical region that is of interest to a theoretical understanding of glass forming liquids but is currently inaccessible to direct measurement. ISTS is first applied to measurements of the depths of different layers in multi-layer film stacks. It is then extended to a measurement potentially capable of probing the high frequency density relaxation of liquids at frequencies up to at least 100 GHz. Transient grating excitation in a thin metal film is used to launch wavepackets into the liquid, where they are broadened due to dispersion and damping. The wavepackets are then partly transmitted into a second metal film, where they are detected through optical diffraction. A comparison of the initial and final shapes of the strain profiles yields the acoustic susceptibility spectrum. The preliminary work presented here probes the acoustic response of glycerol for frequencies in the 1–20 GHz range. The results support previous measurements of relaxation in glycerol in the temperature range 235–291 K, which show a constant loss contribution to the susceptibility. The physical processes contributing to constant loss are not currently well understood, and this calls for further development of the technique to extend the range presented here.