GENERATION AND DETECTION OF ULTRASONIC WAVES BEYOND 100 GHz

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Abstract
The phenomenon of piezoelectricity should permit the generation of sound waves over virtually the entire elastic vibrational spectrum of a solid which covers frequencies up to some 1013 Hz. A great advantage of the piezoelectric method is that its dynamical behavior is almost totally independent of the local environment as regards temperature, pressure, and ambient D. C. fields. A disadvantage presents itself in the detection of elastic waves above roughly 100 GHz owing to the fact that the wave front must be parallel to the piezoelectric surface to within an angle of approximately (λ/D), where λ is the sonic wavelength and D is the cross-sectional dimension of the transducer surface. This condition becomes increasingly difficult to meet at high frequencies where λ may range from ten to hundreds of Angstroms, and D will be typically about 10-1 cm. Even if perfect geometric alignment were possible, small imperfections in the sample would distort the wave front to a significant degree. These conditions do not, however, prevent efficient generation of high frequency elastic waves by means of the piezoelectric effect. Possible ways to detect high frequency sound are to : 1) employ incoherent detectors (e. g. spins) which respond to fields over atomic dimensions or 2) use macroscopic detection systems (e. g. Josephson junctions) which, though larger than an atom, are much smaller than typical transverse dimensions of the sample. These methods are currently under study in our laboratory and will be discussed at greater length

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