Send a laser beam through a photorefractive crystal of barium titanate and something remarkable happens: the beam spreads out in a complicated and curving fan, then collapses into a corner of the crystal, often forming back on itself into a loop.
When the light inside the crystal has settled into a stationary pattern, lo and behold, the laser beam is traveling back exactly along the way it came! The crystal acts as a mirror not an ordinary mirror, but a phase-conjugating one. You perhaps already know about phase conjugation, though in case you don't, we will return to it in a moment. What makes this phenomenon so stunning is the fact that the crystal wants to phase conjugate the incoming light.