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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Picosecond Phase Conjugation By Degenerate Four Wave Mixing In Alkali Vapours - A Method For Measuring Perturbation Times Of The Atomic Wavefunctions

A.C. Cefalas, S. M. Spyrou, Cleanthis A. Nicolaides

Published Jun 20, 1989• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.950567

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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 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.
We study phase-conjugate degenerate four-wave mixing (DFWM) in an optically pumped, two-level atomic system (the 3s2S1∕23s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}3s2S1∕2​, F=2F=2F=2, ${m}_{F}=2\ensuremath{\rightarrow}3p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}$, F=3F=3F=3, mF=3{m}_{F}=3mF​=3 transition in a diffuse, collision-free, thermal beam of sodium) using phase-modulated laser beams. In this investigation, we study the detailed line shape of the four-wave mixing signal as a function of the temporal delay between the probe field and the pump fields at different intensities of the cw pump waves and modulation frequencies. We show that phase modulation of the pump and probe beams leads to strong amplitude modulation of the phase-conjugate beam and report excellent agreement between our experimental observations of the DFWM spectra and the results of numerical calculations. Our numerical results suggest an enhancement of the ``local'' DFWM signal when the sidebands of the phase-modulated laser are resonant with dressed-state transitions, but that this resonance is less obvious in the total signal generated. We also show that the complex line shapes depend sensitively upon the delay of the probe beam, with the peak DFWM signal occurring when the probe beam is advanced compared to the pump beam.
We present experimental observations of phase-conjugate degenerate four-wave mixing in a two-level atomic system (the ${3s}^{2}{S}_{1/2}, F=2, {m}_{F}=\stackrel{\ensuremath{\rightarrow}}{2}{3p}^{2}{P}_{3/2}, F=3, {m}_{F}=3$ transition in a diffuse, collision-free, thermal beam of atomic sodium) driven by cw, narrow-band $(\ensuremath{\sim}250 \mathrm{kHz}),$ stabilized pump and probe fields. The primary measurements that we report are of the peak signal strength of the four-wave-mixing interaction and the bandwidth of the four-wave-mixing spectrum (as the laser frequency is tuned through the resonant frequency) as a function of the intensity of the pump field. We compare these measurements with the predictions of a theoretical analysis in which extension to include the effect of the small transverse velocity $(\ensuremath{\sim}3 \mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c})$ of the atomic beam is critical. These measurements provide a direct verification of the theory of this important nonlinear interaction.
In this study, we extended the observation of the effect of the coherence of a laser field to four-wave mixing interactions in atomic media. These measurements will help us understand quantitatively recent techniques employing broad-band nanosecond laser pulses for measurements of sub-picosecond dipole relaxation lifetimes in a variety of media. The coherence of the laser field in this work is controlled using random phase/frequency modulation techniques, creating a field in the laboratory known as the phase diffusion field. The four-wave mixing signal is generated by an ensemble of two-level atoms (Na) interacting with degenerate broad-band, correlated, pump and probe fields. This two-level system is produced by optically pumping the sodium atoms into a single hyperfine component of the ground state. Data sets were taken without and with noise applied to the optical field. For the latter, we measured the dependence of the power of the four-wave mixing signal on the delay times between the pump and probe beams. These data sets were compared with the computer simulation results.