Observation of Slow Velocities and Self-Steepening of Optical Pulses Described by Adiabatic Following

Abstract

The observation and the theoretical explanation of phenomena which accompany near-resonant pulse propagation in atomic vapors are described. These phenomena bridge the gap between linear dispersion theory, which applies in the far wings of lines, and strictly resonant effects characteristic of self-induced transparency. The results indicate that very low propagation velocities and strong pulse reshaping are not unique to transparency situations.

Short pulses of narrow-line, low-intensity, dye laser light nearly resonant with the Zeeman split ²P_1/2 resonance line (7948 Å) of rubidium were observed to propagate through the dilute vapor as slowly as c/14 [1]. Low-intensity pulses were essentially undistorted by passage through the vapor. High-intensity pulses not only propagated slowly but were self-steepened in the process [2]. Their risetimes changed from typically 4 nsec to less than 1 nsec and complicated envelope shapes developed.

All observations are very well accounted for by the adiabatic following model in which the pseudomoments of the atoms remain closely aligned along the effective field of the laser light. At low intensity, the Rabi precession frequency is very small compared to the frequency difference between the light and the resonance line. In this limit, the approximation is equivalent to linear dispersion theory. The low-level pulse velocity is v_g> = δω/δk, in good agreement with experiment. At high intensity, adiabatic following predicts a nonlinear dielectric response demonstrated earlier in self-focusing [3] and self-defocusing [4] experiments. This leads to two self-steepening mechanisms, which have familiar analogs in other nonlinear optics media: i) an intensity dependence of the pulse velocity, ii) self-phase modulation together with strong group velocity dispersion. The numerical integration of the resulting equations gives excellent quantitative agreement with observations. Further, it indicates optical shock formation on the leading edge of the pulses.

Work of this author was partially supported by ONR under Contract No. N00014-70-C-0187.