Spontaneous Density Grating Formation in Hot Atomic Vapor

Recently a new gain mechanism has been observed in a nonlinear optical system: The spontaneous formation of a density grating in an atomic vapor through interaction with a strong pump field. A sodium filled cell is pumped by a high intensity (I 104 W/cm2) circularly polarized laser beam detuned from resonance and is probed by a weak field degenerate in frequency with the pump and with the same polarization. The probe beam is introduced into the cell in two different geometrical configurations: Nearly parallel (angle 5°) and nearly antiparallel (same angle, but opposite direction) to the pump. For sufficiently high pump intensity, and for appropriate values of detuning and atomic density, the probe beam displays a gain as large as 30% (pumping only a small fraction of the probe cross section) at the expense of the pump, only in the nearly counterpropagating geometry.

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