STUDY OF OSCILLATIONS IN PIEZOELECTRIC CRYSTALS USING MULTIPATH INTERFEROMETRY

The development of multipath interferometry, which has made it possible to carry out studies of the microrelief of surfaces with an accuracy of up to 10 Å, has also opened up the possibility of studying the oscillations of the surfaces of piezoelectric crystals and other objects. Among a number of works devoted to this question, of greatest interest is the article by Tolansky and Bardsley*). The authors describe in detail both the measurement technique and the results obtained. The technique is reduced, in essence, to the creation of a multipath interferometer, one of whose plates is the oscillating surface. In this case two types of experiments are possible. In one of them, intended for the study of standing waves, measurements are carried out under continuous illumination.

The clearly observed blurring of the interference fringes in this case reveals the location of the antinodes, and the degree of blurring makes it possible to measure the amplitude of the oscillations. Displacements and rotations of the interference fringes, readily effected by changing the relative positions of the plates of the interferometer, make it possible to study in detail the structure of the standing waves over the entire surface. The authors present numerous photographs convincingly demonstrating the effectiveness of the method and giving an exceptionally clear picture of the oscillations of the surface of piezoquartz. It is interesting that even at frequencies of the order of \(10^6\) Hz and amplitudes on the order of thousands of angstroms, the velocities of the individual elements of the surface are very small (of the order of several cm/sec), and the influence of the Doppler effect is entirely negligible. Equally negligible is the influence of changes in the refractive index of the air layer separating the plates of the interferometer. Therefore the contours of the interference fringes give the actual relief of the oscillating crystal.

Since in reflected light the interference fringes appear as thin dark lines on a light background, when they are blurred the visibility of the interference pattern proves to be considerably worse than in transmitted light. Therefore, measurements in transmitted light should undoubtedly be preferred, especially for large amplitudes of oscillation. In general, the picture of the surface oscillations is obtained the more clearly, the smaller the amplitude of the oscillations and the denser the arrangement of the interference fringes.

*) S. Tolansky and W. Bardsley, Proc. Phys. Soc. B64, 224 (1951).

Another method for studying surface oscillations is based on the use of the stroboscopic effect. For this purpose a light source is used that is modulated at the same frequency as the surface oscillations (up to \(10^6\) cycles). The interference fringes obtained in this way give images of the instantaneous relief of the oscillating surface. By varying the phase relations between the surface oscillations and the oscillations of the light intensity, one can trace the change in the surface relief over time.

A number of such instantaneous photographs of the relief of the surface of an oscillating crystal are also presented by the authors. By changing the position of the interferometer plate, and in particular by using it as applied to the surface of the crystal, it proves possible to study also volume waves in the body of the crystal. Finally, a similar technique can also be applied to the study of the mechanism of destruction of crystals as a result of an increase in the amplitude of their oscillations.

The paper under review, like the other works developing this method, is still in the nature of working out the technique. However, the results achieved make it possible to hope that the method of multipath interferometry will prove very useful in the study of surface oscillations.

V. Yuryev