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Conference on Lasers and Electro-Optics

Observation of four-wave mixing involving two quadrupole transitions in Na vapor

R. L. Panock, Richard R. Freeman, John E. Bjorkholm

Published Jan 1, 1981• 0 citations
Publisher

Abstract

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.

DOI
10.1364/cleo.1981.wf2

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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.
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.
We report observations of 4‐wave and 6‐wave mixing processes based upon two‐photon pumping of the potassium 4S→6S and 4S→4D transitions in a sodium‐potassium mixture. Additionally we observe coherent emissions at the potassium 3D→4P transition frequencies for pump wavelengths between 725 and 760 nm. The excitation spectra for these emissions show several broad peaks, each of roughly 10 nm width. A series of sharp dips is superimposed on the most intense peak due to depletion of the 3D population by laser‐induced absorption to Rydberg F levels. Although the 3D→4P emissions can be observed in both the forward and backward directions, they are ∼50 times more intense in the forward direction. We believe the forward emission is primarily due to 6‐wave mixing while the backward emission can most likely be attributed to amplified spontaneous emission following two‐photon molecular photodissociation. However, at present, the exact mechanisms responsible for these 3D→4P emissions are not fully understood.
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.
Based on the interference between two two-photon processes, we reported a four-level ultrafast modulation spectroscopy in Na vapor, where 3S, 3P, 5S, and 4D formed a four-level system. We studied the temporal behavior of the four-wave mixing signal intensity and obtained signal modulation with period 25 fs, corresponding to the splitting between 5S and 4D. This technique can achieve Doppler-free precision in the measurement of the energy level splitting between two excited states which are dipolar forbidden from the ground state.
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.