A plane elastic wave propagating in a piezoelectric crystal may be accompanied by longitudinal electric fields which provide an additional elastic stiffness. When the crystal is also semiconducting, these fields produce currents and space charge resulting in acoustic dispersion and loss. A linear theory of this effect is developed, taking into account drift, diffusion, and trapping of carriers for both extrinsic and intrinsic semiconductors. Conductivity modulation sets an upper limit on strain amplitude for a linear theory. The directional characteristics and the magnitude of the effects are illustrated for CdS and GaAs. The Appendix treats the interaction of an arbitrary acoustic plane wave with the electromagnetic fields in a piezoelectric crystal (based on a treatment by Kyame [J. J. Kyame, J. Acoust. Soc. Am. 21, 159 (1949); 26, 990 (1954).]) and further shows explicitly that only the effects of longitudinal electric fields need be considered.

An ultrasonic wave traveling in certain directions in a piezoelectric semiconductor such as cadmium sulfide can be amplified or attenuated by application of a dc electric field. The direct current flowing through the medium in the presence of an ultrasonic wave creates a traveling ac field which interacts with the ultrasonic wave. Amplification occurs when the drift velocity of the electrons exceeds the velocity of sound. For strongly piezoelectric semiconductors, amplification of as much as several percent per wavelength of path is obtainable. Calculations show that for properly prepared material, significant amplification is expected up to the microwave frequencies. At high frequencies, gain is reduced because electron diffusion smooths out the electron bunching necessary for amplification. The dc power required increases rapidly with frequency, and at frequencies above one or two thousand megacycles only pulsed operation seems feasible. Applications as a tool in ultrasonic studies, and for devices such as delay lines and amplifiers, are also discussed.

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A new reflective dichroic liquidcrystal display device Appl. Phys. Lett. 30, 619 (1977); 10.1063/1.89282 Liquidcrystal thermooptic effects and two new information display devicesA new liquid-crystal alphanumeric display device is described. The device is of the guest-host type and .involves the orientational change of a pleochroic dye (guest) ill a cholesteric liquid-crystal mixure (host) by the application of an external electric field. The cholesteric nature of the host permits the operation of the device in the reflective mode with excellent brightness since no external polarizers are required. Good contrast ratios have been obtained for new pleochroic. dyes (guests) which have higher optical order parameters than those previously employed. The theory of light absorption in a cholesteric liquid' crystal having both homogeneous and homeotropic boundary conditions is developed and aspects of contrast ratio and its relation to human perception are described.