A new high frequency ultrasonic transducer

A new high frequency ultrasonic transducer* In a recent article (Proc. IRE International Convention, New York, 1961, by D. L. White) the development of a new technique for the generation of high frequency ultrasonic waves was described. Perhaps the most important aspect of this development is that which suggests the use of the transducer in the study of elastic wave propagation in solids. In the course of internal friction studies, it is often highly desirable to extend measurements to frequencies of vibration well above 100 MC/S. Heretofore, two transducer types have been employed, namely (a) resonant quartz plate operating at some high overtone and (b) non-resonant excitation of a quartz rod inserted in a microwave re-entrant cavity. In application, however, both the above techniques suffer severe limitations for various reasons. The new transducer is simply the depletion layer formed at a p-n junction or non-ohmic contact in an extrinsic piezoelectric semiconductor. The depletion layer is devoid of current carriers (hence its name) and therefore can sustain an electric field of sufficient strength to cause it to undergo a significant elastic displacement. In marked contrast, the resistivity of the bulk material is much lower. When used as a resonant transducer, an alternating electric field will excite the depletion layer into mechanical vibration at frequencies of wavelength given by 2(d/n) where d is the thickness of the depletion zone and n are odd integers. The elastic wave will propagate into the bulk semiconductor and may be transmitted into other media by appropriate mechanical attachment.

of the bulk semiconductor material and therefore can be easily handled and attached to other media. Experiments employing gallium arsenide, which prove that a high frequency elastic wave can be generated and detected in the depletion layer will be described and discussed. Bell Telephone Laboratories, Whippany, New Jersey

J. C. KING D. L. WHITE

Microwave ultrasonics* Recent experiments have shown that it is possible to generate and propagate elastic waves in crystalline solids at microwave frequencies by means of the piezoelectric effect(1-3). The attenuation of such waves proceeds via anharmonic forces between atoms which couple energy irreversibly into other modes of the system. This degradation of energy takes place rapidly at temperatures above roughly 20°K because of the presence of many thermal phonons with which the ultrasonic wave can interact. At temperatures in the helium range (below 4°K) the number of thermal photons is greatly reduced and under these circumstances elastic waves at 10,000 MC/S can be detected easily after having traversed several meters of quartz. Finally, because the Zeeman splitting of electronic energy levels of paramagnetic ions in crystals lies within the microwave frequency range, numerous interactions between electron spin systems and ultrasonic waves are possible. For example, such interactions lead to direct amplification of elastic waves without recourse to electronic amplifiers, and to “double quantum” detection of incoherent phonon beams.(4-8)

Three features of the depletion layer transducer which make it uniquely valuable for internal friction studies are : (1) Possible to generate efficiently extremely high frequency acoustic waves, perhaps beyond 30,000 MC/S, since the thickness of the depletion layer will usually fall between 10V3 and lo+ cm. (2) Depletion layer thickness, and hence its resonant frequency, can be varied over a considerable range by the superposition of a d.c. biasing electric field. (3) The depletion layer transducer is an integral part

1. H. E. BWMEL and K. DRANSFELD, Phys. Rev. 117, 1245 (1960). 2. E. H.. JACOBSEN, Internat. Symp. on Quantum Electronics (Edited by C. H. Tow) Vol. 1, p. 468. (1960). 3. E. H. JACOBSEN, J. Acoust. Sot. Amer. 32, 949 (1960). 4. E. H. JACOBSEN,N. S. SHIREN and E. B. TUCKER, Phys. Rev. Letters 3, 81 (1959). E. B. TUCKER,Phys. Rev. Letters 6, 183 and 547’ (1961). 2: N. S. SHIREN, Phys. Rev. Letters 6, 168 (1961).

* Abstract of Paper 1 presented at the Conference on Internal Friction held on 10 and 11 July, 1961, at Cornell University.

* Abstract of Paper 2 presented at the Conference on Internal Friction held on 10 and 11 July, 1961, at Cornell University.

ACTA METALLURGICA,

General Electric Resecirch Laboratory E. H. JACOBSEN Schenectady, New York References

VOL. 10, APRIL 1962 [4991