Phase Conjugation with Photorefractive Materials
At the beginning of the '80s, it was known that one could generate a phase-conjugate wave in just about anything by four-wave mixing (put in three waves and you'll produce a fourth). These phase-conjugate waves would always travel back from where they came, and with their wavefronts reversed, as though they were propagating backwards in time. Four-wave mixing was seen as a kind of "real-time" holography, and it was fashionable to talk about generating "time-reversed" or "phase-conjugate" light beams. However, these wave-mixing experiments required either a powerful laser or material with a strong optial resonance at the laser's frequency.
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