Laser excitation of high-frequency capillary waves

The laser-induced surface deformation (LISD) technique was applied to generate high-frequency capillary waves on liquid surfaces up to several tens of kHz in a noncontact manner. The dynamic response of the fluid near the surface was theoretically derived under the condition of periodical radiation pressure. The result of the numerical calculation predicts the propagation of induced capillary waves out from the excitation region. The efficiency of the wave generation was experimentally examined by changing the width of the excitation laser beam at the surface. The observed LISD spectra were well reproduced by the theory, showing that the effective frequency band can be extended up to over 100 kHz. The propagation of the optically generated wave was measured with a laser probe sweeping the position of the observation. The spatial profile gives the surface tension and the shear viscosity of the sample liquid. The frequency domain measurement was also carried out and the spectrum obtained at a fixed point agrees with the theory, demonstrating the rapid measurement of frequency-dependent phenomena.