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Physical Review A

Light-shift-induced spatial structures: Application to degenerate four-wave mixing

M. Schiffer, E. Cruse, Wulfhard K. Lange

Published May 1, 1994• 7 citations
Publisher

Abstract

Degenerate four-wave mixing in dense sodium vapor under conditions of strong pressure-induced Zeeman pumping is investigated. Thermal diffusion and its interplay with the Zeeman light shift are found to be responsible for the generation of a grating mechanism that is not sensitive to thermal washout. The effect can be observed experimentally after eliminating the influence of radiation trapping by the use of nitrogen as a buffer gas. Under these conditions large nonlinearities are produced which also influence the propagation of the signal wave via collision-aided circular birefringence.

DOI
10.1103/physreva.49.r3178

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Physical Review A•Mar 1, 1983•15 citations
Degenerate four-wave mixing in dense sodium vapor under conditions of strong pressure-induced Zeeman pumping is investigated. Thermal diffusion and its interplay with the Zeeman light shift are found to be responsible for the generation of a grating mechanism that is not sensitive to thermal washout. The effect can be observed experimentally after eliminating the influence of radiation trapping by the use of nitrogen as a buffer gas. Under these conditions large nonlinearities are produced which also influence the propagation of the signal wave via collision-aided circular birefringence.
Phase-matched four-wave mixing in sodium vapor with a helium buffer gas is carried out in a noncoplanar geometry. The directions of the four light beams remain nondegenerate even if all frequencies are equal. A sharp zerodifference frequency resonance in the intensity of the parametrically generated fourth beam by three incident beams is observed that is proportional to the square of the pressure of the buffer gas. A theoretical interpretation of this effect caused by elimination of destructive interference between two coherent pathways is presented.
We have observed a new component in the signals produced by degenerate four-wave mixing of 5-nsec optical pulses by the DDD-line resonances of sodium vapor. This component is comparable in intensity to that previously observed from a grating pattern of excited sodium atoms, but arises rather from a grating of coherently aligned (but not optically pumped) ground-state atoms. This component exists when there are no spatial intensity beats between input waves.
Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe, N2{\mathrm{N}}_{2}N2​) are characterized at high resolution ( 5 MHz) using lasers tuned near the DDD lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the 3P3P3P-to-3S3S3S transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of N2{\mathrm{N}}_{2}N2​ which collisionally quenches the Na 3P3P3P states. Similar results are obtained in Xe where it is argued that excimer formation with the 3P3P3P states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.
We present experimental data showing destruction of degenerate four-wave-mixing signals by collisions at extremely low pressures of added buffer gases (\ensuremath{\sim}100 mtorr). These experiments are performed on the $3s^{2}S_{\frac{1}{2}}(F=2)\ensuremath{-}3p^{2}P_{\frac{3}{2}}(F=3)$ transition of the D2{D}_{2}D2​ line in atomic sodium for which, in the absence of buffer gases, the lower state is not optically pumped to the 3s2S12(F=1)3s^{2}S_{\frac{1}{2}}(F=1)3s2S21​​(F=1) ground state. However, using a simple rate equation model we are able to qualitatively explain the observed behavior by showing that state-changing collisions have the effect of generating a collision-induced optical pumping. This transfers the F=2F=2F=2 ground-state population to the F=1F=1F=1 ground state, thus reducing the density of atoms available for generating a signal.