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Journal of Physics B Atomic and Molecular Physics

Anomalous blue shifted emission near the D1transition from laser-excited sodium vapour

Ilya Golub, R. Shuker, G. Erez

Published Jan 28, 1987• 9 citations
Publisher

Abstract

The observations of anomalous emission near the D 1 transition of a dense (>or=10 14 cm -3 ) sodium vapour illuminated by a laser detuned to the blue with respect to the D 2 transition is studied and reported. A discussion of existing models is presented. The authors propose a type of parametric down-conversion process to explain this emission. This process involves a laser photon splitting into two photons by the three-level system of sodium, assisted by a magnetic dipole transition.

DOI
10.1088/0022-3700/20/2/006

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Journal of Physics B Atomic and Molecular Physics•Feb 28, 1986•48 citations
The observations of anomalous emission near the D 1 transition of a dense (>or=10 14 cm -3 ) sodium vapour illuminated by a laser detuned to the blue with respect to the D 2 transition is studied and reported. A discussion of existing models is presented. The authors propose a type of parametric down-conversion process to explain this emission. This process involves a laser photon splitting into two photons by the three-level system of sodium, assisted by a magnetic dipole transition.
There Is an ongoing interest in the conical emission resulting from the passage of a laser light tuned in the vicinity of a resonant atomic transition(1–2). We also have observed such emission in high density sodium vapor in the range of 1015 cm−3, when an intense laser radiation is blue detuned with respect to any of the D lines. This conical emission shell has half angle of a few degrees around the laser axis, is red detuned from the respective D line, is spectrally broad (3A) and is obtained in laser intensity and detunings and sodium vapor density ranges where self-focusing occurs. No symmetric blue shifted component was observed. Harter and Boyd(1) have obtained symmetrical Rabi side-band emission in the forward direction at sodium vapor density of 1014 cm−3.
The authors have confirmed earlier observations that fluorescence from dense sodium vapour under high power illumination does not saturate with increasing laser intensity. Direct observations of level population using a variant of the interferometric hook technique with a time resolution of ten nanoseconds have been performed, showing that the level populations of the pumped transition were in the ratio of their statistical weights. The apparent contradiction between this result and that of the laser-induced fluorescence observations is considered.
Experimental and theoretical investigations were made of the characteristics of an 'anomalous' component of the scattering of high-intensity laser radiation in a resonant medium (sodium vapour) observed earlier in the frequency range ω L > ω D2 . The appearance of this component had been attributed by other authors to parametric downconversion. Contrary to this interpretation, the component in question was observed also at frequencies of ω L < ω D1 . An analysis showed that the generation of this component and its properties can be explained fully by a four-wave interaction in a gas of three-level atoms.
Laser excitation and ionization processes in dense (1–10 Torr) sodium vapor have been studied for laser wavelengths near the two-photon allowed nd 2D(n=3, 4, 5) and ns 2S(n=4, 5, 6) states. In particular, the 1 3Σ+g→1 3Σ+u excimer emission in Na2, predicted in 1980 by Konowalow and Julienne and observed recently by Dinev et al, was studied here in greater detail. Strong excimer emission (∼830 nm) was observed for two-photon pumping to both sides of the unresolved 4D states, and weak excimer emission was seen when pumping near the 5D levels. The excimer emission exhibits a complicated pump laser profile with a pronounced ‘‘dip’’ at the 4d 2D two-photon resonance. Similarly, [2+1] photon ionization via the 3d 2D and 4d 2D states shows a dramatic decrease as the sodium density increases. These results can be attributed either to depleted 3d 2D or 4d 2D population due to stimulated electronic Raman scattering (SERS) or to the interference effects recently reported by Malcuit et al. and Krasnikov et al. and treated theoretically by Manykin and Afanas’ev and by Agarwal. It is argued that both mechanisms are operative. Strong ionization and SERS signals were observed at the hybrid resonances corresponding to 3p 2P3/2, 1/2 →4d 2D transitions; however, no excimer lasing at 830 nm was detected. No excimer emission was detected upon two-photon pumping near or at the 3d 2D or ns 2S(n=4, 5, 6) states. Based on these and other observations, the 1 3Σ+g→1 3Σ+u excimer emission is attributed to a molecular Raman process involving stimulated emission or six-wave mixing via a pathway of the type 1 3Σ+u→2hν 3Δu→ j 3Πg→(k 3Πu, l 3Σ +u)→1 3Σ+g→1 3Σ +u .