Coherent processes of multiple photons in gases: parametric generation and pulsed excitation

Stephen Garner, Cyrus D. Cantrell

We use a semlclassical, effective-state approach to laser pulse propagation in a gas of polyatomic molecules. Our numerical model of multiple-photon excitation includes quasicontinuum transitions to calculate the effects of coherent propagation on multiple- photon absorption. The molecular model consists of a single ground state and several bands of discrete states, which are successively connected by means of electric-dipole transitions.1 This model correctly simulates some of the major characteristics of multiple- photon absorption, such as power threshold, fluence-dependent absorption, and complete population inversion at large fluences. As the pulse propagates, the Rabi oscillations induced in the lower-level states are encoded onto the pulse and create a series of peaks and troughs similar to those found in pulse propagation in two-level systems.2 Another coherent propagation effect that can change the excitation process is coherent sideband generation. Temporal distortions and the generation of new frequencies result in changes in the population distribution among the states, as well as changes in the total population transferred to the quasicontinuum. The effective increase in total population transfer by propagation effects is offset by the decrease in fluence. The resulting transfer of energy from the pulse to the molecule depends strongly on the initial characteristics of the pulse.

V. I. Barantsov, A. K. Popov, G. Kh. Tartakovskiǐ

Yu. P. Malakyan

A theory is developed for transient hyper-Raman scattering and four-wave parametric processes in a gaseous medium subjected to an ultrashort pump pulse field. Investigations are made of the intensity and spectrum of the infrared and ultraviolet radiation generated in a medium as a result of these processes, bearing in mind the transient properties of the pump (phase modulation, pulse profile). It is shown that the initial nonlinearity of the pump field and the parametric processes result in new effects not observed in transient stimulated Raman scattering. For a large two-photon detuning, the hyper-Raman forward scattering is suppressed because of destructive interference with stimulated parametric generation of ultraviolet radiation. The gain of the infrared and ultraviolet radiation depends on the pump pulse profile. Under exact two-photon resonance conditions in the absence of any phase modulation of the pump, infrared and ultraviolet pulses are coherently split into various subpulses and a time-dependent Autler–Townes doublet is observed in their spectrum.

É. M. Verlan, V. I. Zadorozhnyi

B. A. Veklenko

Ashraf Mohamed El Hassan, Henri Vo Van Qui, Katia Gallo

Two-dimensional lattices of the optical nonlinearity implemented by periodic poling in$\chi^{(2)}$materials afford additional degrees of freedom for enhanced gains in optical parametric generation [1–2] and the generation of multipartite photon entanglement in compact device formats [3]. So far all investigations have considered single-frequency excitation of the crystals in classical or quantum parametric downconversion regimes. However, the effects underpinned by the coherent coupling of multiple nonlinear resonances unique to 2D lattices, may be expected to further enlarge the spectrum of possibilities also in suitably seeded coherent parametric amplification and difference frequency generation (DFG) processes.