Strong Departures from Uniform Plane Wave Pulse Propagation as a Result of Coherent Transverse Effects
Analytic and numerical treatments of the coupled Maxwell-Bloch equations, including transverse and time-dependent phase terms, predict a new self-focusing (SF) effect that does not vanish on resonance and requires coherent pulse-matter interaction. This new SF falls in the category of transient SF problems associated with the inertial response of coherent nonlinear active media. The evolution of this new SF effect (for a given pulse shape and beam profile), its strength and location, the sharpness of its threshold, and the spacing between multiple foci depend on the Fresnel number, absorption coefficient, relaxation times, laser-absorber frequency mismatch, and input on-axis electric field “area”. The effect also occurs in the presence of inhomogeneous broadening. Recently two independent experiments demonstrating coherent self-focusing were reported with as much as a factor of 2 increase in fluency on-resonance and 4.5 off-resonance. The first of these was made in inhomogeneously broadened Na; SF and temporal reshaping were also studied as a function of laser detuning. The observation of SF on-resonance for coherent pulses (but not for cw light) clearly illustrates that coherent transient SF is different from previous SF involving resonant interactions and investigated either in the rate-equation or in the adiabatic-following approximations. The second experiment involved the investigation of the on-resonance coherent SF in quasi-nondegenerate inhomogeneously broadened Ne and its dependence on input pulse on-axis “area” and on pulse duration.
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