Sound speed and attenuation in thin polymer films in the frequency range 0.2–1 GHz

Abstract

Pressure pulses or pressure steps with rise times on the order of 0.2 ns can be generated in dielectric films by application of laser pulses to a specially coated surface of the dielectric [G. M. Sessler, J. E. West, and R. Gerhard‐Multhaupt, Phys. Rev. Lett. 48, 563 (1982)] or by excitation with a quartz crystal [W. Eisenmenger and M. Haardt, Solid State Commun. 41, 917 (1982)]. The acoustic phenomena propagate in the thickness direction of the films and are reflected at its surfaces. If the samples are electrically biased or charged, electrode signals are generated upon each reflection. The sound speed may be obtained from the relative delays of these signals while the sound attenuation can be evaluated by comparing their spectra. A modification of the laser method consists in the generation of double (or multiple) pulses by splitting the laser beam into two (or more) components and delaying one component relative to the other by a time T. This results in a boost of the spectrum at the frequency f = 1/T. Measurements on PETP, PI, PVDF, and FEP yield sound speeds close to the values determined for bulk material with other methods and sound attenuation coefficients that rise almost linearly with frequency in the range 0.2–1 GHz. The attenuation data can probably be explained by a hysteresis‐type absorption process.