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Journal of Physics B Atomic Molecular and Optical Physics

Observation of the suppression of the stimulated hyper-Raman scattering (3S1/2-4P3/2) near the sodium 4D two-photon resonance

Mao-Hong Lu, Yumei Liu

Published Oct 28, 1994• 9 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/27/20/022

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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.
Experimental investigations of two-photon enhanced stimulated hyper-Raman and parametric four-wave-mixing emissions are described for tuning unfocused laser beams near two-photon resonance with 3D and 4D states in sodium vapor. Over wide ranges of vapor pressures, path lengths, and laser power densities, the gains for stimulated hyper-Raman (SHR) scattering and for parametric four-wave mixing (PFWM) can become very greatly reduced from values expected from simple considerations. The suppression of gain can arise from interferences produced at two-photon and three-photon resonances, and from ac Stark shifts produced by internally generated near-resonant fields. Quantitative comparisons are made between observed and theoretically predicted values for SHR and PFWM production. Of 12 predicted consequences of the interferences and Stark shifting on the nonlinear behavior, ten are demonstrated through the experiments.
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 suppression of forward-propagating stimulated hyper-Raman (SHR) emission is well studied in sodium vapor. Theory predicts that SHR emission to a state linked to the gound state by a one-photon transition is suppressed by an interference with a four-wave-mixing (FWM) emission that links that state to the ground state. Since FWM emission only propagates in the forward direction due to phase-matching considerations, only the forward SHR emission is suppressed. This has been previously demonstrated in sodium. An obvious corollary to this theory is that no suppression occurs if the state is not linked to the ground state by a one-photon transition. A serendipitous arrangement of energy levels in cesium allowed this corollary to be tested and confirmed here.
The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.
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).