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DESTRUCTIVE INTERFERENCE BETWEEN PARAMETRIC FOUR-WAVE MIXING AND THREE-PHOTON SCATTERING

Yaakov Shevy, M. Rosenbluh

Published Jul 18, 1988• 0 citations

Abstract

The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.

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AIP conference proceedings•Jan 1, 1987•0 citations

The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.

Two types of parametric four-wave mixing are observed when a single intense laser beam propagates through Na vapor near the D(1) and D(2) resonance lines. The first is related to three-photon scattering, while the second results from the coherent parametric interaction of the laser with excited-state Raman scattering. Identification of the two processes was greatly facilitated by the use of an optical multichannel detector system to observe the total emission spectrum for each individual laser pulse. The experimental conditions that determine whether stimulated emission or one or both of the four-wave mixing processes dominates the emission spectrum are described.

If an extended Na vapor column (heat pipe configuration) is subjected to strong laser pumping near two-photon resonance with 3D3/2,5/2 or 4D3/2,5/2 states at pressures above tens of millitorr, strong stimulated emissions are produced by amplified spontaneous emission (ASE) or stimulated electron in hyper-Raman scattering (SEHRS) and by phase matched parametric four-wave mixing (PFWM). These new electromagnetic fields that are generated within the near-resonant nonlinear medium can strongly influence the total atomic response and produce several saturation and total cancellation types of effect.

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 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.