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Journal of Physics B Atomic and Molecular Physics

Anomalous behaviour in the saturation of the sodium D lines under high power laser illumination

Jim Salter, D. Burgess, N. A. Ebrahim

Published Dec 28, 1979• 27 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/12/24/001

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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.
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i n Ùnal for m Marc h 1, 2001) W e rep ort results of investigation on the Ûuorescence induced in sodium v ap our by pulses of laser light tuned near the 3 S ! 4 P transition (330 nm ).T he Ûuorescence w as observed in perp endi cular and parallel direction s in resp ect to the laser b eam.A depletion of Ûuorescence intensity corresp ondin g to the 5 D ! 3 P transition (498 nm) was observed along the laser beam.W e explain this e˜ect as a result of reabsorption that is caused by intense p opulati on of the 3 P level due to cascade stimulated pro cesses.T he most intense depletio n occurs ab out 20 ns after the laser pulse and it corresp onds to the moment of maximal populati on of the 3 P level.
We have measured the time-dependent fluorescence of the sodium DDD lines following pulsed excitation of one DDD line, in the presence of radiation trapping with optical depths k0L{k}_{0}Lk0​L of \ensuremath{\sim}10 to 2000. When collisional coupling of the 3P123{P}_{\frac{1}{2}}3P21​​ and 3P323{P}_{\frac{3}{2}}3P23​​ levels and different radiative-escape probabilities for each DDD line are taken into account, we obtain excellent agreement with Holstein's theory for the effective radiative decay rates in the Doppler region (${k}_{0}L\ensuremath{\sim}10\ensuremath{-}300$) and in the redistributed Lorentzian region (k0L>1000{k}_{0}L>1000k0​L>1000). For k0L{k}_{0}Lk0​L between these two regions, we observe an abrupt transition between the two limiting formulas. The buildup rate of the sensitized fluorescence signal also yields a $3{P}_{\frac{3}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ excitation transfer cross section in agreement with our previously reported cw measurement. Additionally, we have measured the dependence of the 3P3P3P fluorescence on laser power and beam diameter, and we explain the observed approach to saturation. We suggest that the previously reported "anomalous" approaches to saturation may be explained in terms of the laser beam burning through the optically thick vapor. Laser-beam spatial intensity variations and self-focusing also contribute to fluorescence signals that deviate from the usual single-atom saturation behavior.