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Acta Physica Sinica (Overseas Edition)

Competition effect in atomic-molecular system

Jia Suo-Tang, Qin Lijuan, Qian Zu-liang, Zugeng Wang, Gang Wang, Guosheng Zhou

Published May 1, 1996• 0 citations
Publisher

Abstract

The competition effects among the processes of atomic ionization, optical pumped stimulated radiation (OPSR), four-wave frequency mixing (FWFM) and molecular stimulated diffuse band radiation at the atomic two-photon resonance of 3S approaches 4D in Na2 - Na mixture were observed. The dip at the two-photon resonance in the excitation spectrum for the diffuse-band radiation was interpreted as suppression of population in 4D state.

DOI
10.1088/1004-423x/5/5/002

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Physical Review A•Feb 1, 1987•118 citations
The competition effects among the processes of atomic ionization, optical pumped stimulated radiation (OPSR), four-wave frequency mixing (FWFM) and molecular stimulated diffuse band radiation at the atomic two-photon resonance of 3S approaches 4D in Na2 - Na mixture were observed. The dip at the two-photon resonance in the excitation spectrum for the diffuse-band radiation was interpreted as suppression of population in 4D state.
Competition has been observed between amplified spontaneous emission (ASE) and four-wave mixing (FWM). Previously, competition between an incoherent and coherent process was observed in the form of suppression of multiphoton ionization by third harmonic generation.1,2 In our experiment, an input laser was tuned precisely to the 3s → 3d two-photon allowed transition in atomic sodium vapor and the emission was observed in both the forward and backward directions. In these conditions, either FWM or ASE can occur. Ignoring the possibility of competition between these two processes, the calculated gain for ASE is much larger than that for FWM. However, we have observed experimentally that FWM and not ASE occur. To determine that it is the presence of efficient FWM that suppresses ASE, we have excited the atomic vapor instead with counterpropagating pump waves of different frequency. In this case, FWM, being a phase-matched process, cannot occur and strong ASE is observed. The nature of this competition is explained by solving the coupled Maxwell and density matrix equations for a three-level system. It is found that the suppression is due to the destructive interference between two different pathways of excitation of the 3d level which results in trapping of the population in the ground state.
An experimental observation of the competition between quantum interference and stimulated electronic Raman scattering (SERS) via equal frequency hybrid excitation and collision aided radiative excitation in a Na 2 Na system is reported for the first time. It is found that the SERS for 3P 1/2,3/2 4P are suppressed and the optically pumped stimulated emission (OPSE) for 5S 4P are enhanced dramatically at a particular excitation wavelength where constructive quantum interference between two transitions 3P 1/2 5S and 3P 3/2 5S in atomic sodium takes place. A theoretical analysis based on the dressed atom approach is carried out to explain the experimental results.
Three-photon ionization of Na out of the 4d two-photon state is discussed. Results from an atomic beam and an atomic vapor in a heat pipe are compared. The 4d ionization in the vapor was a factor of 10/sup 4/ smaller than in the beam. Absorption in the vapor was less than concentration arguments might suggest. This reduction is explained by very fast processes which dump the resonant state into lower levels and by a process which suppresses the two-photon excitation at high concentrations. Experimental evidence of these processes was observed as strong infrared and ultraviolet emission in the forward direction corresponding to the 4d ..-->.. 4p and 4p ..-->.. 3s transition, respectively. 11 refs., 5 figs.
Competitive inhibition of a resonance enhanced multiphoton ionization process by a resonant four-wave mixing has been observed in Xe atoms. When an intense IR (1064 nm) laser was applied to a sample of Xe which was also being irradiated by a UV laser tuned to the two-photon absorption line of Xe, the two-photon-resonant three-photon ionization signals decreased with increasing IR laser power. This phenomenon is dependent on the resonant states of Xe and the polarization of the two laser beams. Three 6sss excited states [5/2]2{}_{2}2​, [3/2]2{}_{2}2​, and [1/2]0{}_{0}0​ were examined. At the [1/2]0{}_{0}0​ resonant state, the ion signals were not decreased but slightly increased with the increase of the IR laser power. No suppression of the ion signal was observed at the [5/2]2{}_{2}2​ resonant state, when the polarization directions of the lasers were perpendicular to each other. The result of the polarization dependence reflects the selection rules of four-wave mixing. A simple rate equation analysis including the contribution of two-photon ionization from the [1/2]0{}_{0}0​ state by the IR laser well represents the IR laser-power dependence of the ion signal.
The stimulated hyper-Raman scattering (SHRS) emissions associated with the excitation of the 4P 3/2 state in sodium vapour have been observed in both the forward and backward directions as the pumping laser was tuned near the 3S-4D two-photon resonance. The results show that though the forward SHRS process is greatly reduced by a destructive interference between it and the four-wave mixing (FWM) process induced by itself, the forward SHRS emission has a positive gain and can still be observed under a certain condition.
Competition between amplified spontaneous emission (ASE) and the four-wave-mixing (FWM) process has been observed under conditions of two-photon resonant excitation of the sodium 3d level. The nature of the competition is that the FWM process is able to prevent the occurrence of ASE, even though the gain of the ASE process calculated in the absence of competition effects is much larger than that of FWM. The ASE is suppressed because the fields generated by the FWM process create a new excitation pathway connecting the ground and 3d levels, and under quite general conditions this pathway interferes destructively with that due solely to the applied laser field. These effects are modeled theoretically by solving perturbatively the density-matrix equations of the atomic system, thereby determining the population in the upper level and the nonlinear polarization of the medium. The coupling between the various optical fields due to the nonlinear polarization is described by coupled amplitude equations. The solution to these equations predicts that when the wave-vector mismatch is not too large the fields evolve spatially to reach steady-state values, and that the population excited to the 3d level by the total steady-state optical field is much smaller than that due to the incident laser field alone. We have observed experimentally the suppression of ASE by FWM and have observed that this suppression does not occur when the medium is excited with counterpropagating beams that cannot efficiently excite the FWM process. In addition, we have conducted a series of experiments that shows that the degree of suppression of ASE depends on the intensity and focusing characteristics of the incident laser as expected on the basis of our theoretical model.