PHONON ECHOES IN BULK AND POWDERED MATERIALS

Abstract
Experimental and theoretical studies of phonon echoes in bulk and powdered materials are reviewed. Phonon echoes have been observed in many materials such as bulk piezoelectric crystals, paramagnets, glasses, doped semiconductors, and piezoelectric, magnetic, and metallic powders, etc. The echoes arise from a time reversal of the phase, like spin echoes, of a primary pulsed acoustic excitation due to a second acoustic or rf pulse. The phase reversal occurs through the nonlinear interactions of acoustic oscillations with either themselves or electromagnetic fields. The diversing phonon echo phenomena can be classified into two general classes depending on the type of nonlinear mechanism responsible for echo formation : In parametric "field-mode" interaction systems the applied rf field of the second pulse interacts with the modes excited by the primary pulse to cause phase reversal. The echo amplitude of this class decreases monotonously with the time separation of applied two pulses, τ. Backward and forward propagating wave echoes in bulk materials belong to this class. In "anharmonic oscillator" systems different oscillation modes nonlinearly couple to one another to cause subsequent echo formation. The echo amplitude initially increases with τ and takes a maximum before decreasing exponentially for large τ. Only the powder echoes are in this class. In addition to these dynamic echo phenomena there are quasistatic three-pulse echoes in which phases are stored for days and years, phenomena being applicable to mass storage devices. The essential physics of phonon echo phenomena seems to be well understood although precise quantitative descriptions of each phenomenon are still required. The phonon echo studies are now being applied to a tool for physics and to technology.