COHERENCE INHIBITION IN CASCADE SUPERFLUORESCENCE

The output of a synchronously pumped dye laser is frequency selected a t lambda = 8851 Angstrom and amplified in order to resonantly two-phot on excite a coherent superposition between the 6S(1/2) and 6D(3/2) sta tes (two-photon coherence) of atomic Cs vapor. Superfluorescence at la mbda = 8761 Angstrom, corresponding to the upper 6D(3/2)-6P(1/2) trans ition, is observed in both the forward (parallel to the pump beam dire ction) and the backward directions. Superradiance at lambda = 8944 Ang strom, corresponding to the lower 6P(1/2)-6S(1/2) transition, is also observed, but predominantly in the forward direction. The laser excita tion pulse exits the sample long before the onset of any superfluoresc ence pulse. On the upper transition, superfluorescence in the forward direction is delayed compared to that in the backward direction. This provides experimental evidence that forward superfluorescence is inhib ited by two-photon coherence in a cascade three-level system.