Polarization echoes in piezoelectric semiconductors

Abstract

In many piezoelectric crystals, application of a se-quence of two or three microwave electric field pulses creates a polarization which radiates subsequent "echo" signals (analogous to spin echoes). Two distinct types of echo phenomena may be delineated. Both involve a nonlinear interaction of a forward propagating acoustic wave (ω,k) (launched by the first microwave pulse (ω,0), with the uniform microwave electric field of the second pulse (ω,0) . The nonlinearities are derivable on the basis of field induced ionization of electrons from shallow impurity states (traps). The first echo phenomenon, which we term "parametric echo", is a backward wave parametric process. The pump being provided by the 2ω component of the conduction band electron density resulting from ionization by an ω electric field. In the second phenomenon, "holographic echo", electrons are transferred among the traps to create a static charge distribution which is a stored hologram representing the interference pattern (with wave vector k) of the acoustic and electric fields. At low temperatures (4.2°K) in CdS, CdSe, and CdTe the pattern is stable in the dark for months. A third (or any subsequent) pulse, (ω,0) or (ω,k), produces an output echo (ω,k) or (ω,0), respectively, through electrostriction. Under illumination by low level white light the variation of echo amplitude with microwave power involves the energy distribution of the trap levels, and the latter may be obtained from the data. Results for a fully compensated CdS:S sample are presented. © 1975 American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.