Standing-Wave Induced Backward Photon Echoes in Gases
Recent interest in phase conjugation has made the process of degenerate-four-wave-mixing (D4WM) quite familiar.1 In standard D4WM experiments, a standing-and travelling-wave pulse are simultaneously applied to a sample and lead to the generation of a new travelling-wave pulse. The generated pulse propagates backward with respect to, and under appropriate conditions is the phase-conjugate of the applied travelling-wave pulse. An interesting variation of this process has been suggested by Shiren.2 He predicted that the fourth wave would be produced in inhomogeneously broadened solid-state absorbers even if the standing-wave was applied after the input travelling-wave. If the travelling-and standing-waves are displaced by the time τ, Shiren predicted that the phase-conjugate fourth wave would appear a time τ after the standing-wave. The effect predicted by Shiren displays characteristics of both D4WM and photon echoes thereby revealing the essential similarities of those effects. The echoes predicted by Shiren occur only in solids and were only recently observed.3 A three-excitation-pulse echo process also predicted by Shiren to produce phase-conjugate output has been observed in gases.4
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