Two-Photon Cooperative Cascade Superfluorescence
Final rept. 1 May 1989-30 Apr 1992
COLUMBIA UNIV NEW YORK DEPT OF PHYSICS
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We have studied the time evolution of the radiation field spontaneously emitted by an ensemble of many atoms as they proceed from some particular excited atomic state to a lower one initially unpopulated and then on to the round state. A short laser pulse, two-photon resonant with an even parity optical transition, produces the excited atomic population which is forbidden by symmetry to radiate directly back to the ground state and must therefor radiate first to an intermediate state. If the gain is high, laser action should rapidly deplete the excited state and with sufficient population build-up on the intermediate state one can expect laser action to appear on the transition to the ground state. This scenario becomes more interesting when the short laser pulse coherently excites the atoms into a coherent superposition of the ground and uppermost resonant state. Now laser action on the uppermost transition directly induces laser action on the lower transition even though no population inversion is present there. The effect is to modify the laser action on the uppermost transition. We have used a mode locked laser to produce pulses short enough to coherently excite the 6D sub 32-6S sub 12 transition in Cs vapor. Our observations indicate that the induced coherence inhibits superfluorescence and that laser action on the uppermost transition in the backwards direction opposite to the direction of the short excitation pulse strongly perturbs this interference. The theoretical analysis is highly nonlinear and numerical techniques are applied.
- Atomic and Molecular Physics and Spectroscopy