REFLECTION OF ELECTROMAGNETIC-WAVES FROM NONSTATIONARY MEDIA (EXACTLYSOLVABLE MODELS)

An analysis is made of propagation of electromagnetic waves in media w hich are nonstationary because of relaxation of the refractive index. A series of models of oscillatory and transient regimes of such relaxa tion is developed. Several characteristic times are used in these mode ls and exact analytic solutions of the Maxwell equations can be obtain ed for these regimes. In contrast to the traditional approaches, the e xact solutions are obtained without assuming smallness or slow ness of temporal variations of the parameters of the medium and these solutio ns are valid even when the characteristic relaxation time is comparabl e with the period of oscillations of the wave field. A nonstationary g eneralisation of the Fresnel formulae is derived. It is shown that wav es reflected from a nonstationary surface experience amplitude and fre quency modulation, and the modulation effect is localised in an interv al of the order of one relaxation time. It is shown that a short broad band perturbation pulse forms in the reflected wave and that this puls e contains one or several oscillations of the field. It should be poss ible to use nonstationary broadening of the spectrum of a probe wave r eflected from a surface perturbed by a powerful laser pulse in estimat ing the relaxation times of fast optical processes.