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Optics Letters

Picosecond time-resolved four-wave mixing experiments in sodium-seeded flames

John T. Fourkas, Timothy R. Brewer, Hack-Jin Kim, M. D. Fayer

Published Feb 1, 1991• 17 citations
Publisher

Abstract

Picosecond four-wave mixing experiments have been used to study collisions in a Na-seeded, premixed, methane-air flame. Population gratings are used to measure Na excited-state quenching collision rates, while polarization gratings are used to measure Na ground-state hyperfine coherence randomizing collision rates and overall Na diffusion rates, even though these processes are slower than the excited-state quenching rate.

DOI
10.1364/ol.16.000177

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Optics Letters•Jan 1, 1989•5 citations
Picosecond four-wave mixing experiments have been used to study collisions in a Na-seeded, premixed, methane-air flame. Population gratings are used to measure Na excited-state quenching collision rates, while polarization gratings are used to measure Na ground-state hyperfine coherence randomizing collision rates and overall Na diffusion rates, even though these processes are slower than the excited-state quenching rate.
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.
Study of a two-photon near resonant four wave difference mixing (FWDM) in Na in a CH4/air flame has been performed to understand the FWDM process reported by Singh and Yueh (1992). The FWDM signal was measured with reduced flame length and with the focal point at the center and also at one end of the flame to pinpoint the region in which the signal is generated. A partial contribution in the third peak was from the nonresonant background. A FWDM signal is generated slightly away from the focal point and nonresonant background, at the focal point in the present experimental conditions. An absorption spectrum of the Na D-lines at different dye laser energy was recorded to study the depletion of Na ground state at higher dye laser intensity. Comparative study of the FWDM signal with a narrow and broad bandwidth dye laser shows that a lower Na concentration detection limit could be achieved with a narrow linewidth probe laser tuned to about 9/cm lower frequency from the center of the D1 line. The two minima in the FWDM spectrum corresponding to Na D lines have been explained, qualitatively, with the saturation effect in the Na D-lines transition. 27 refs.
We have employed dye-laser pulses of 10-nsec duration and 1.5–10-nm bandwidth in a transient four-wave-mixing experiment to study high-perturber-pressure collisional dephasing in dilute atomic sodium vapor. Working at argon perturber pressures of several atmospheres, we are able clearly to resolve dephasing times over a thousand times shorter than our excitation pulse duration. Measured dephasing rates are in excellent agreement with those measured by more traditional methods. When both of the sodium D lines are excited, our signal displays quantum-beat-like oscillations at the 0.52-THz frequency difference between these two states. We provide a simple picture explaining the origin of these beats.
Four-wave mixing is studied in a Na-seeded flame in the regime of strong saturation of the Na D-line transitions. The output frequency differs from that of the input lasers so that spectral discrimination can distinguish the mixing signal. Scans of the D2 resonance with another laser set at the D1-line resonance reveals two resonances corresponding to the D2 resonance and the frequency difference of the D levels. The mixing efficiency at the D2 resonance increases with the detuning of the D1 resonance. The frequency difference resonance is weak because of interference between multiple coherent pathways.