Several groups1–4 have recently proposed a simple and powerful optical technique for obtaining ultrahigh time resolution, which is limited only by the coherence time of the excitation light rather than by the overall pulse duration. The present work reports an application of this technique to study the dephasing rates of the Na and Rb resonance lines perturbed by several atmospheres of argon gas. By comparison with traditional measurements, we demonstrate that relaxation processes occurring on time scales much shorter than the excitation pulse duration can indeed be accurately and unambiguously measured using the present technique.
We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the
5D state from the
5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the
6P state and then to the
5S state. The SF emission at 420 nm on the
6P--
5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.