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Journal of Physics B Atomic Molecular and Optical Physics

The observation of the competition and suppression for stimulated Raman scattering near the potassium 4S-5P resonance

Hau-wei Wang, Mao-Hong Lu

Published Oct 31, 2000• 2 citations
Publisher

Abstract

The competition of stimulated Raman scattering (SRS) between 4S-(5P)-5S and 4S-(3D)-5S channels has been observed as a laser is tuned near the 4S-5P resonance in potassium vapour. The experimental results show that SRS only occurs in the 4S-(5P)-5S channel. In this Raman scattering, suppression of the amplified spontaneous emission (ASE) (5S-4P) has been observed. The influence of the four-wave mixing (FWM) processes on the SRS is discussed. In the 4S-(5P)-3D channel, in the backward direction no detectable emission corresponding to SRS (4S-(5P)-3D) has been observed, so the forward (5P)-3D emission should be FWM, not SRS. The ASE (3D-4P) arises from the ASE (5P-3D), which can be observed in the backward direction.

DOI
10.1088/0953-4075/33/22/301

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The competition of stimulated Raman scattering (SRS) between 4S-(5P)-5S and 4S-(3D)-5S channels has been observed as a laser is tuned near the 4S-5P resonance in potassium vapour. The experimental results show that SRS only occurs in the 4S-(5P)-5S channel. In this Raman scattering, suppression of the amplified spontaneous emission (ASE) (5S-4P) has been observed. The influence of the four-wave mixing (FWM) processes on the SRS is discussed. In the 4S-(5P)-3D channel, in the backward direction no detectable emission corresponding to SRS (4S-(5P)-3D) has been observed, so the forward (5P)-3D emission should be FWM, not SRS. The ASE (3D-4P) arises from the ASE (5P-3D), which can be observed in the backward direction.
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 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.
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.
An enhanced two-photon emission has been observed between the 6S and the 4S levels of potassium. The priming field is the stimulated electronic Raman emission at 2720 cm-1due to scattering of ruby laser radiation from the4P_{3/2}state in potassium. The intense emission at the complementary frequency ω2is close to the 5P → 4S resonance lines. As a result, the coupling coefficient has a resonance character that is favorable for the enhancement. The observed results are compared with a simple analysis. The relation of the enhanced emission with two-quantum absorption, stimulated Raman effect, and parametric four-photon processes is discussed.
Abstract\nWe investigate the emissions of atomic potassium excited by two photons near the 4S1/2-6S1/2 transition. Radiation near the 6S1/2-4P3/2, 1/2 and 4P3/2, 1/2-4S1/2 transitions are studied for the unidirectional and the bidirectional cases respectively. It is shown that the total output intensity increases considerably for the bidirectional propagation. This is attributed to the effective increase of the Doppler free excitation and the cancellation of the quantum interference effect that is associated with the multiple paths of excitation of certain states. Keywords. Counter-propagating beams, amplified spontaneous emission (ASE), stimulated hyper Raman scattering (SHRS), parametric four-wave mixing (PFWM).