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Acta Physica Sinica

THREE-WAVE DIFFERENCE-FREQUENCY GENERATION IN SODIUM VAPOR

Zongju Xia, ZOU YING-HUA, 北京大学物理系

Published Jan 1, 1989• 0 citations
Publisher

Abstract

Electric quadrupole transition enhanced tthree-wave difference-frequency generation in isotropic medium is investigated. Taking account of the contribution of electric quadrupole interaction, we derive the second-order nonlinear susceptibility. We also discuss the possibility of observation of collision induced second-order nonlinear optical processes. The experiments are performed with 3s→4d electric quadrupole transition in sodium vapor. The experimental results agree with theoretical predictions.

DOI
10.7498/aps.38.24

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Electric quadrupole transition enhanced tthree-wave difference-frequency generation in isotropic medium is investigated. Taking account of the contribution of electric quadrupole interaction, we derive the second-order nonlinear susceptibility. We also discuss the possibility of observation of collision induced second-order nonlinear optical processes. The experiments are performed with 3s→4d electric quadrupole transition in sodium vapor. The experimental results agree with theoretical predictions.
We show that second-order nonlinear optical processes such as sum and difference generation can be observed in isotropic media, despite the fact that such processes are forbidden in the electric-dipole approximation. A theoretical treatment of quadrupole sum- and difference-frequency generation is given, which includes the effects of spin-orbit and Zeeman splittings. The quadrupole nonlinear susceptibility is evaluated and expressions for phase matching and output power are found. We have generally confirmed the theoretical predictions by observing quadrupole sum-frequency generation of ultraviolet light in sodium vapor. Deviations of the data from the simple theory can be satisfactorily accounted for by the effects of two-photon saturation, induced index of refraction changes, and single- and multi-photon absorption. Generalization of the dipole forbidden susceptibilities to the case of quadrupole-pumped processes is discussed, with difference-frequency generation in Cs vapor considered as an example.
We have observed a four-wave mixing process in sodium vapor in which two of the four waves are resonantly enhanced by electric quadrupole transitions. The atom is pumped via a quadrupole transition from the 3s to the 4d state with a single photon at 2894 Å. Intense coherent emission at 3427 Å corresponding to the 3d-3s transition can be observed exiting the Na cell only in the direction parallel to the input beam. Also observed coming from the cell is intense coherent light at 2.34 µm corresponding to the 4d-4s transition in Na.
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.
It is theoretically shown that, under conditions of two-photon absorption of radiation by metal vapors, the medium becomes considerably polarized and efficiently doubles the radiation frequency. Two physical mechanisms by which these effects can occur are detected. The first of them results from the ring frequency mixing, while the second one is caused by a polarization nonlinearity that is quadratic in the field amplitude and that is formed as a result of the electric quadrupole absorption due to the forbidden transition. The effects predicted can form the basis for new optical methods for measuring the electric quadrupole and magnetic dipole moments for forbidden atomic transitions.