Abstract

The three-pulse stimulated photon echo has been considered as one of the most fundamental transient four wave mixing processes¹, where each of the three excitation pulses induces a single elementary interaction process between a laser field and an inhomogeneously broadened two-level system., resulting in a burst of the fourth wave, i.e., echo. Now if four or more optical pulses are applied to a medium, a number of photon echoes should be expected, in general. At low excitation intensities where the perturbation theory is applicable, however, the dominant nonlinear process is still the three-pulse stimulated echo and consequently each echo signal is generated only by three of the preceding pulses. It is rather a remarkable result that the echo intensity is unaffected by the presence of all the rest of the pulses other than its three generators. With this in mind, it is theoretically and experimentally shown that by applying a single optical pulse (#1) propagating along k₁ followed by N pairs of counter-propagating pulses (#2_j (k_2j) and #3_j− (k_3j = −k_2j); j=1,…N) in various directions, it is possible to obtain N photon echoes along the direction -k₁ opposite to the first pulse #1. We will call this degenerate photon echoes, degenerate in. the sense that the echoes are spatially overlapping and temporally simultaneous. When N=l, this scheme is reduced to the ordinary backward photon echo^2,3. The doubly degenerate case (N=2) is illustrated in Fig. 1. The j-th echo will be generated by the set of pulses (#1, #2_j, #3_j) and appear at t = t_3j + t_2j − t₁ where t₁, i_2j and t_3j are the arrival times of the pulses #1, #2_j and #3_j, respectively.

© 1988 Optical Society of America