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Journal of the Optical Society of America B

Absorption spectra of pulse-train-excited sodium two-level atoms

E. Krüger

Published Jan 1, 1995• 11 citations
Publisher

Abstract

The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.

DOI
10.1364/josab.12.000015

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Physical Review Letters•Sep 13, 1971•308 citations
The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.
The subject of this thesis is the construction of a stabilized cw dye laser and its use with a sodium atomic beam to gain information about the range of validity of certain atomic radiation theories. By lengthening the laser cavity, cooling the dye solution and adding many other refinements the bandwidth of a single mode cw dye laser was reduced to 3 MHz over a period of ten seconds. The laser was designed so that by the insertion of three uncoated glass etalons single mode tunable operation was achieved. The purpose of stabilizing the dye laser was to be able to perform experiments on the hyperfine structure of sodium at high enough resolution and intensity to see any possible differences in the theoretical predictions of Quantum Electrodynamics, the optical Bloch equations, and Neoclassical Radiation Theory. The traditional method of eliminating Doppler shifting was used, that is a well-collimated thermal atomic beam of sodium was constructed. By crossing the atomic and laser beams at right angles the natural linewidth of 10 MHz was observable with only one or two MHz residual broadening. Two types of experiments were carried out: those studying the absorption spectrum, and those studying the emission spectrum. In the first experiments Lorentzian lineshapes were observed for all applied field strengths. This proved that if the Lamb shift of Neoclassical Theory exists at all it is small compared with 10 MHz for the sodium D lines. Further, the sharply peaked spectrum predicted by Neoclassical Theory for a certain field strength was not observed. The width of the absorption at high field strengths was not in good agreement with either theory. In the second type of experiment the observed emission spectra were in qualitative, but not good quantitative, agreement with Quantum Electrodynamics when the effects of nearby levels were included. The results are in disagreement with a two-level neoclassical treatment, and we have been unable thus far to obtain better agreement in more sophisticated treatments. Undoubtedly a better multi-level calculation is required in all theories since our system is not really two-level. In addition, we have found that a factor of 2 increase in the critical intensity calibration would bring QED into reasonable agreement with most of the experiments. However, we have been unable to establish the origin of such a factor. While these results were not sufficient to finally and absolutely settle the controversy regarding Quantum Electrodynamics and Neoclassical Theory they did provide a great deal of new information. For the first time the dynamic Stark effect was observed near resonance in the optical region with data that could be compared to theory. Also, the measurements of the absorption spectrum were the most complete and among the highest resolution measurements yet obtained by direct optical techniques. And finally, a number of of optical pumping phenomena were observed.
We have measured the deflection of a thermal sodium beam by a force existing in a field of two counterpropagating short laser pulses. This field was created by a Π-pulse train of a mode-locked (mode) (spacing 82 MHz) dye laser beam, perpendicularly crossing a thermal sodium beam and retroreflected from a mirror behind it. Unlike the spontaneous force, this Π-pulse force is not limited by saturation effects and does not heat up the sodium beam transversally. Calculations yield a maximum Π-pulse force about three times larger than the spontaneous force, being in good agreement with our experimental results.
When sodium vapor is irradiated with a single-frequency dye laser (pump) tuned in the vicinity of either of the Doppler-broadened D lines and enclosed in an optical cavity, Raman shifted oscillation builds up, which is blue shifted for a blue-detuned pump and red shifted for a red-detuned pump by 1.772 GHz (sodium ground-state hyperfine splitting).1 The gain mechanism responsible for this Raman-laser action can be understood by modeling sodium as a three-level atom. The Raman laser frequency is highly correlated with that of the pump as demonstrated earlier by observing a beat frequency bandwidth between the two lasers of as low as 17 kHz.2 (The pump had rms frequency fluctuation of the order of 10 MHz.) For spectroscopic applications, for example, frequency correlated laser beams are commonly generated using acoustooptic techniques which are difficult to implement beyond the gigahertz region. The Raman laser technique is extendable, in principle, even to the millimeter range. Here we present measurements of the sodium-density and pump-intensity dependence of the Raman-laser output and compare the results with those predicted by the three-level-atom model.
Two frequency-stabilized cw dye lasers and an atomic beam were used to prepare sodium atoms in the aligned F=2F=2F=2, MF=2{M}_{F}=2MF​=2 ground state, and to study power broadening in the transition from this ground state to the 3P323{P}_{\frac{3}{2}}3P23​​ F=3F=3F=3, MF=3{M}_{F}=3MF​=3 excited state. This system is, to extremely good accuracy, a two-level atom. The line shape was determined as a function of the incident field strength and found to be accurately a Lorentzian over a large range of incident field strengths. The power broadening and saturation at resonance of the two-level system also agree with theory.
We have used two-photon absorption from counterpropagating laser beams of different frequencies to measure Stark shifts and splittings of the 62S6^{2}S62S, 72S7^{2}S72S, 82S8^{2}S82S, 52D5^{2}D52D, and 62D6^{2}D62D states in sodium. Beams from a fixed-frequency, single-mode Ar+{\mathrm{Ar}}^{+}Ar+ laser and a tunable, single-frequency cw dye laser were focussed into a coarse sodium atomic beam and the fluorescence was monitored. The collimation of the atomic beam reduced the residual Doppler width inherent in using two different frequencies. Using the two lasers allowed two-photon transitions to be observed at frequencies unattainable with the dye laser alone, and enhanced the transition rates due to the near resonance of the dye laser with the strong Na $3^{2}S\ensuremath{\rightarrow}3^{2}P$ transition. Our experimental results for the scalar and tensor polarizabilities, ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{2}$, agree with calculations in the Bates and Damgaard Coulomb approximation within 2% for ${\ensuremath{\alpha}}_{0}$ and 9% for ${\ensuremath{\alpha}}_{2}$. As a by-product the hyperfine-interaction constants aaa for the SSS states and the fine-structure intervals $\ensuremath{\delta}\ensuremath{\nu}=\frac{({E}_{\frac{3}{2}}\ensuremath{-}{E}_{\frac{5}{2}})}{h}$ for the DDD states were obtained.
The sodium DDD resonance lines have been studied in saturated absorption with a repetitively pulsed tunable dye laser. The hyperfine splitting of the 32S123^{2}S_{\frac{1}{2}}32S21​​ and 32P123^{2}P_{\frac{1}{2}}32P21​​ states of Na23{\mathrm{Na}}^{23}Na23 is resolved. Measurements with a delayed probe reveal a remanent hole burning in the velocity distributions of the two ground-state levels, caused by a velocity-selective optical pumping cycle. A time-resolved observation of the collisional-velocity thermalization in the presence of Ar buffer gas is reported.