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Journal of the Optical Society of America B

Studies of picosecond collisional dephasing in atomic sodium vapor using broad-bandwidth transient four-wave mixing

J. E. Golub, THOMAS W. MOSSBERG

Published Apr 1, 1986• 63 citations
Publisher

Abstract

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.

DOI
10.1364/josab.3.000554

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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.
Several groups1–4 have recently proposed a simple and powerful optical technique for obtaining ultrahigh time resolution, which is limited only by the coherence time of the excitation light rather than by the overall pulse duration. The present work reports an application of this technique to study the dephasing rates of the Na and Rb resonance lines perturbed by several atmospheres of argon gas. By comparison with traditional measurements, we demonstrate that relaxation processes occurring on time scales much shorter than the excitation pulse duration can indeed be accurately and unambiguously measured using the present technique.
An angled-beam, transient four-wave mixing experiment is described in which nanosecond-duration, broad-bandwidth excitation pulses are employed to measure picosecond collisional dephasing and subpicosecond interference beats in atomic Rb vapor. The 7.2-THz interference beat, which corresponds to a 237 cm(-1) Rb fine-structure splitting, is apparently the fastest material-specific beat yet observed. We discuss practical limitations on the time resolution attainable with this technique and propose means of circumventing them by using broad-bandwidth collinear excitation beams and an angled narrow-bandwidth probe.
Phase conjugated waves via degenerate four wave mixing with picosecond laser pulses having a full width at half maximum (FWHM) of 80 ps have been observed in alcali vapours. The dephasing time of the 2P3/2 state in sodium was found to be 7.4 ns. This is the lifetime of the transient volume grating of the atomic states which are involved in the phase conjugated experiment. The average time of the perturbation of the atomic wave functions due to a "hot" collision (from a frequency modulation of a Markov type) was found to be 5x10-14 sec.
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.
In femtosecond degenerate four-wave mixing experiments in sodium vapor, quantum beats are observed at the wavelength of the third harmonic when the fundamental beams are three-photon resonant. The period of beating, which corresponds to fine structure energy-level splitting, does not change with the atomic number density, buffer gas pressure, or laser intensity. Surprisingly, the third-harmonic signal is observed only at negative time delays for the transient grating pulse sequence. We relate this to the well-known interference effect between excitation pathways involving the fundamental and the third-harmonic fields, which leaves atoms virtually unexcited and prevents formation of population grating.
High resolution quantum beat spectroscopy has been realized to study atomic wave packet dynamics. Wave packets comprising pairs of low quantum number (n = 5 - 8) electronic states were formed in Rb vapor with ultrafast laser pulses, and their dynamics was observed by an all-optical technique of parametric four wave mixing (PFWM). The experimental apparatus designed, constructed and automated for this work provided unprecedented signal intensity that enabled time-frequency-resolved analysis of wave packet dynamics with picosecond temporal resolution over the course of 1100 ps. Both time-resolved and non-time-resolved discrete Fourier methods were applied to measure the concurrent formation of multiple wave packets. Many novel effects and interactions have been measured either for the first time or with greatly improved signal-to-noise ratio. Among them were the formation of the 8S1/2 - 6D5/2 wave packet that was not directly excited by the ultrafast radiation, formation and relative dynamics of quantum beating harmonics up to 73 THz, and quantum beating revivals. The effects of vapor number density and temperature were analyzed experimentally. Interference between the 7S1/2 - 5D5/2 and 8S1/2 - 6D5/2 wave packets (frequencies of 18.225 THz and 10.73 THz, respectively) has been observed for the first time and will be described in detail.