Omnidirectional superfluorescence transients

Alexander Lvovsky, Sven Hartmann, Fred Moshary

This work has been done during the last year of my PhD research and became the main subject of my dissertation. Based on our results, we have published two Physical Review Letters.

A sample of atomic rubidium, two-photon excited, in a large Fresnel number geometry, into a coherent superposition between levels 5S and 5D, produces a coherent flash of omnidirectional superfluorescence on the 5D -> 6P transition.  This involves a coherent population transfer to the 6P level and results in a prompt directional UV emission along 6P->5S, a phenomenon known as yoked superfluorescence (YSF).  The direction(s) of the lower transition emission is determined by the wavevector(s) of the coherent superposition state between 5S and 5D in which the system has been prepared.

Two examples were studied. In the first, the sample was excited by two simultaneous non-collinear laser pulses. The coherent superposition between levels 5S and 5D  is established due to absorption of one photon from each pulse. It results in a conical emission pattern whose apex angle is a function of the angle between the excitation beams (Fig. 1). The conical emission reminds that observed in parametric four-wave mixing (PFWM) [1], but the physics in our case is very different. The PFWM emission occurs in the presence of the pump field and is conical on both upper and lower transition. In our case, on the other hand, the pump and the two YSF fields are separated in time. As a result, the upper transition superfluorescence in the phase matched direction is inhibited and not enhanced [2]. The emission is conical on the lower transition but omnidirectional on the upper. [paper 1]

As mentioned above, the lower transition superfluorescence occurs in the directions determined, via phase matching, by the coherent superposition in which the states 5S and 5D have been prepared. If the 5S state which existed before the laser pulse contains some "prerecorded" coherence, it will manifest itself in additional directions of the lower transition  emission. This coherence can be prepared, in the form of transient induced grating, by a pair of laser pulses applied to the system substantially earlier than the interrogating (third) pulse. This leads to the photon-echo type of emission (Fig. 2) which differs from conventional photon echoes [3] by some unique properties. First, it is generated by a spontaneous relaxation process (superfluorescence). Second, it is a result of a two-photon resonant excitation. Third, its frequency is not equal to that of the generating laser pulses, nor their linear combination. Fourth, it is emitted in a cone. Fifth, for it to be observed, excitation pulses do not have to comply with any particular geometry. [paper 2]

1. W. R. Garrett, Laser Phys. 5, 466 (1994); W. R. Garrett, Phys. Rev. Lett. 70, 4059-4062 (1993); M. A. Moore, W. R. Garrett, and M. G. Payne, Opt. Commun. 68, 310 (1988).
2. J. H. Brownell, X. Lu, and S. R. Hartmann, Phys. Rev. Lett. 75, 3265 (1995)
3. N. A. Kurnit and S. R. Hartmann, Interaction of radiation with solids (Plenum Press, New York, 1970); P. Hu, R. Leigh, and S. R. Hartmann, Phys. Lett. 40A, 164 (1972).