Coherent interaction of laser pulses in a resonant optically dense extended medium under the regime of strong field-matter coupling

摘要

Nonstationary pump-probe interaction between short laser pulses propagatingin a resonant optically dense coherent medium is considered. A specialattention is paid to the case, where the density of two-level particles is highenough that a considerable part of the energy of relatively weak externallaser-fields can be coherently absorbed and reemitted by the medium. Thus, thefield of medium reaction plays a key role in the interaction processes, whichleads to the collective behavior of an atomic ensemble in the strongly coupledlight-matter system. Such behavior results in the fast excitation interchangesbetween the field and a medium in the form of the optical ringing, which isanalogous to polariton beating in the solid-state optics. This collectiveoscillating response, which can be treated as successive beats between lightwave-packets of different group velocities, is shown to significantly affectpropagation and amplification of the probe field under its nonlinearinteraction with a nearly copropagating pump pulse. Depending on the probe-pumptime delay, the probe transmission spectra show the appearance of eitherspecific doublet or coherent dip. The widths of these features are determinedby the density-dependent field-matter coupling coefficient and increase duringthe propagation. Besides that, the widths of the coherent features, whichappear close to the resonance in the broadband probe-spectrum, exceed theabsorption-line width, since, under the strong-coupling regime, the frequencyof the optical ringing exceeds the rate of incoherent relaxation. Contrary tothe stationary strong-field effects, the density- and coordinate-dependenttransmission spectra of the probe manifest the importance of the collectiveoscillations and cannot be obtained in the framework of the single-atom model.