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Optics Letters

Observation of a 19-psec polarization beat

Joshua E. Rothenberg, D. Grischkowsky

Published Jan 1, 1985• 28 citations
Publisher

Abstract

We have observed polarization beats by simultaneously exciting both the D(1) (589.6-nm) and the D(2) (589-nm) lines of sodium in a vapor with a 0.4-psec resonant laser pulse. This impact excitation of both D lines produces two oscillating macroscopic polarizations, which interfere with each other and procedure a beat note at their difference frequency in the intensity of the light emitted from the cell. The beat period of 1.9 psec is the fastest ever observed optically.

DOI
10.1364/ol.10.000022

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We have observed polarization beats by simultaneously exciting both the D(1) (589.6-nm) and the D(2) (589-nm) lines of sodium in a vapor with a 0.4-psec resonant laser pulse. This impact excitation of both D lines produces two oscillating macroscopic polarizations, which interfere with each other and procedure a beat note at their difference frequency in the intensity of the light emitted from the cell. The beat period of 1.9 psec is the fastest ever observed optically.
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.
levels of atomic sodium using ultrashort laser pulses in a stepwise excitation scheme. Two broadband femtosecond laser pulses are used to excite the 6S level from the 3S ground level in sodium vapor. The first one is simultaneously resonant with the D1 and D2 lines of the 3S→3P transition, and the second one with the 3P→6S transition. The delay between the two pulses is scanned in order to probe the dephasing between the two excited intermediate levels. Quantum beating is observed by monitoring the fluorescence signals at 330 nm and 515 nm, namely transition 4P→3S and 6S→3P, respectively. Signals are compared to a semi-classical model that reproduces the oscillations with a very good agreement.
Photon-echo modulation spectroscopy is extended into a new regime to study coherences between states that are widely separated in frequency compared with the bandwidth of the exciting laser pulses. Time-delayed four-wave mixing (TDFWM) is observed on the Na DDD doublet by use of 7-nsec pulses of light spectrally peaked at each of the DDD-line transitions. As the two double-frequency, 7-nsec light pulses are delayed with respect to each other a 1.9-psec beating of the integrated TDFWM signal is observed, even for delays greater than the excitation pulse width.
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.