Non-Linear Pulse Propagation in Many-Photon Active Isotropic Media

Abstract

It is an experimental fact that light propagation in a medium is sensitively dependent on the shape and intensity of the optical pulse as well as on the electronic and vibrational structure of the basic molecular units. We review in this paper results of systematic studies of this problem for isotropic media. Our theoretical approach is based on numerical solutions of the density matrix and Maxwell’s equations and a quantum mechanical account of the complexity of the many-level electron-nuclear medium. This allows to accommodate a variety of non-linear effects which accomplish the propagation of strong light pulses. Particular attention is paid to the understanding of the role of coherent and sequential excitations of electron-nuclear degrees of freedoms. We highlight the combination of quantum chemistry with classical pulse propagation which allows to estimate the optical transmission from cross sections of multi-photon absorption processes and from considerations of propagation effects, saturation and pulse effects. It is shown that in the non-linear regime it is often necessary to account simultaneously for coherent one-step and incoherent step-wise multi-photon absorption, as well as for off-resonant excitations even when resonance conditions prevail. The dynamic theory of non-linear propagation of a few interacting intense light pulses has been successfully applied to study, for example, frequency-upconversion cavity-less lasing in a chromophore solution, namely in an organic stilbenechromophore 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulphonyl) dissolved in dimethyl sulphoxide. Furthermore, the theory has been used to explain observed differences between spectral shapes of one- and two-photon absorption in the di-phenyl-amino-nitro-stilbene molecule. The present simulations evidence that the reason for this effect is the competition between two-step and coherent two-photon absorption processes