The interaction of short and strong laser pulses with an atomic Bose-Einstein condensate is found to generate patterns of recoiling atoms that are different from those seen in previous light-scattering experiments. This phenomenon can only be explained by optical stimulation, showing that the previous description of superradiance as atomic stimulation is incomplete and that matter-wave amplification in Bose-Einstein condensates is suppressed at short times. Our experiments clarify the nature of bosonic stimulation in the four-wave mixing of light and atoms.
Multiorder bidirectional superradiant Bose-Einstein condensates (BECs) were generated in a straight line by an irradiation of a single unidirectional short laser pulse along the long axis of a cigar-shaped sodium BEC in a magnetic trap. The probabilities of the diffracted BECs as a function of the laser intensity were well explained by the square of the Bessel functions and it was estimated that the intensity of the end-fire beam was 25
of the laser intensity. The backward diffractions disappeared at pulse duration longer than 5 \ensuremath{\mu}s because of energy conservation. The probability for the
+first-order diffraction grew exponentially with pulse duration when the backward diffractions disappeared. We observed the linearly aligned diffracted BECs along the propagation direction of the laser beam, regardless of the aspect ratio of the condensates. This fact indicates that the end-fire beam is triggered by the small backreflection from the vacuum window.