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Physical Review A

Polarization studies of interferences in two-photon excitation of sodium atoms

Dino Zei, R. N. Compton, John A. D. Stockdale, Michael Stuart Pindzola

Published Nov 1, 1989• 12 citations
Publisher

Abstract

Two independently tunable, linearly polarized dye lasers are counterpropagated through a heat pipe containing sodium vapor. The frequency of the first laser ${\ensuremath{\omega}}_{1}$ is incremented through both 3${p}^{2}$${P}_{1/2}$,3/2 fine-structure levels while the second laser ${\ensuremath{\omega}}_{2}$ is scanned through the energy increment necessary to reach ns $^{2}S_{1/2}$ and ${\mathrm{nd}}^{2}$${D}_{3/2}$,5/2 levels (n=5--35). Following excitation, ionization occurs through chemi-ionizing collisions. The polarization (defined as the ionization intensity measured when the two planes of polarization of the laser beams are parallel minus the intensity when perpendicular divided by the sum of the intensities) P=(${I}_{?}$-${I}_{\ensuremath{\perp}}$)/(${I}_{?}$+${I}_{\ensuremath{\perp}}$) goes from +1 to -1 as ${\ensuremath{\omega}}_{1}$ is tuned between the 3${p}^{2}$${P}_{1/2}$,3/2 fine-structure levels. This novel polarization effect is shown to be due to an interference between the two pathways to the ns $^{2}S_{1/2}$ levels. Upon exciting the unresolved nd $^{2}D_{3/2}$,5/2 levels, the polarization is very close to zero when ${\ensuremath{\omega}}_{1}$ is at or near the 3p $^{2}P_{3/2}$,1/2 levels. A simple few-level atom theory reproduces the experimental observations except for the case when both lasers are in resonance.

DOI
10.1103/physreva.40.5044

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Multiphoton ionization (MPI) spectra of sodium and cesium were obtained in a crossed $N_{2}$ pumped dye laser and alkali beam. Hybrid resonance were observed at the cesium $7d^{2}D_{3/2} -6p^{2}P_{1/2}$ and $7p^{2}P_{3/2}- 6p^{2}P_{1/2}$ transitions and the sodium $4d^{2}D_{3/2, 5/2} - 3p^{2}P_{1/2, 3/2}$ transitions together with the more common MPI via intermediate S, P and D states. Power dependences for these processes are presented together with and interesting case of a switch from red to blue Stark shifting in the sodium MPI spectrum.

•Jan 1, 1982•0 citations
Two independently tunable, linearly polarized dye lasers are counterpropagated through a heat pipe containing sodium vapor. The frequency of the first laser \ensuremathω1{\ensuremath{\omega}}_{1}\ensuremathω1​ is incremented through both 3{p}^{2}$${P}_{1/2},3/2 fine-structure levels while the second laser \ensuremathω2{\ensuremath{\omega}}_{2}\ensuremathω2​ is scanned through the energy increment necessary to reach ns 2S1/2^{2}S_{1/2}2S1/2​ and {\mathrm{nd}}^{2}$${D}_{3/2},5/2 levels (n=5--35). Following excitation, ionization occurs through chemi-ionizing collisions. The polarization (defined as the ionization intensity measured when the two planes of polarization of the laser beams are parallel minus the intensity when perpendicular divided by the sum of the intensities) P=(I?{I}_{?}I?​-I\ensuremath⊥{I}_{\ensuremath{\perp}}I\ensuremath⊥​)/(I?{I}_{?}I?​+I\ensuremath⊥{I}_{\ensuremath{\perp}}I\ensuremath⊥​) goes from +1 to -1 as \ensuremathω1{\ensuremath{\omega}}_{1}\ensuremathω1​ is tuned between the 3{p}^{2}$${P}_{1/2},3/2 fine-structure levels. This novel polarization effect is shown to be due to an interference between the two pathways to the ns 2S1/2^{2}S_{1/2}2S1/2​ levels. Upon exciting the unresolved nd 2D3/2^{2}D_{3/2}2D3/2​,5/2 levels, the polarization is very close to zero when \ensuremathω1{\ensuremath{\omega}}_{1}\ensuremathω1​ is at or near the 3p 2P3/2^{2}P_{3/2}2P3/2​,1/2 levels. A simple few-level atom theory reproduces the experimental observations except for the case when both lasers are in resonance.
An experimental investigation of the atomic-Na 3s 2^{2}2${\mathit{S}}_{1/2--}$5s 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​ two-color two-photon polarization spectrum has been made. Measurements of the linear polarization degree of the two-photon transition were made for one laser tuned within a \ifmmode\pm\else\textpm\fi{}100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range of the atomic-Na 3p 2^{2}2Pj{\mathit{P}}_{\mathit{j}}Pj​ levels, with the second adjusted to satisfy the two-photon resonance condition. The relatively short-pulse (\ensuremath{\sim}0.5 ns) lasers used eliminate effects on the spectrum due to atomic-Na hyperfine interactions and radiation trapping, while greatly reducing depolarization due to collisions with Ar gas. In the absence of collisions, the measured polarization spectrum is in excellent agreement with calculations. Measurements of the Ar pressure dependence of the spectrum revealed strong depolarization in the vicinity of resonant excitation through the 3p 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​ level. A collisional depolarization cross section of 200(30) \AA{} for this process was deduced from the data.
Polarization rotation effects in atomic sodium vapor are studied both experimentally and theoretically. Two pulsed dye laser beams propagate collinearly through a cell containing sodium vapor. The direction of polarization of a linearly polarized dye laser beam of frequency ${\ensuremath{\omega}}_{1}$ is found to be rotated by a circularly polarized beam of frequency ${\ensuremath{\omega}}_{2}$. This rotation is due to the difference in refractive index at ${\ensuremath{\omega}}_{1}$ for right and left circularly polarized light which is produced by the circularly polarized beam. When ${\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$ is tuned near the $3S\ensuremath{-}5S$ two-photon transition of atomic sodium, this difference is primarily due to the dispersion associated with the two-photon transition. Other sources of polarization rotation which were identified include (i) real transfers of population to excited atomic levels due to collisionally induced transitions and (ii) optically induced atomic energy-level shifts. The theory includes both the effects of two-photon dispersion and optically induced energy shifts. The application of the two-photon polarization rotation effect as a fast, optically controlled shutter or modulator and as a sensitive detection technique for two-photon spectroscopy are briefly discussed.
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.
We have observed that the polarizations and intensities of counterpropagating laser beams in sodium vapor are temporally unstable. The output from a stabilized dye laser tuned near to the 3S1/2 → 3P1/2 atomic transition was split to form the two counterpropagating beams. These beams were polarized with the same linear polarization and were weakly focused to a spot size of ~750 μm in sodium vapor having a density of ~3 × 1013 cm−3. We monitored the light generated in the polarization orthogonal to that of the pump beams. For a detuning of 1.01 GHz to the high-frequency side of the 3S1/2(F = 2) → 3P1/2(F = 2) transition and a power of 160 mW in one of the beams, we found that the threshold for the instability occurred at a power of 15 mW in the other beam. Slightly above threshold, the instability was found to be periodic in nature. As the intensity in the counterpropagating beam was increased far above the threshold value, wild fluctuations in the polarizations and intensities of the transmitted beams were observed. We have found that these fluctuations are chaotic and that the system evolves on a strange attractor whose dimension increases with increasing laser power. These observations are in qualitative agreement with the theoretical predictions of Gaeta et al.,1 which predict that the polarizations of counterpropagating beams in a Kerr medium are unstable to the growth of temporal fluctuations.
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Multiphoton ionization (MPI) spectra of sodium and cesium were obtained in a crossed N2N_{2}N2​ pumped dye laser and alkali beam. Hybrid resonance were observed at the cesium 7d2D3/2−6p2P1/27d^{2}D_{3/2} -6p^{2}P_{1/2}7d2D3/2​−6p2P1/2​ and 7p2P3/2−6p2P1/27p^{2}P_{3/2}- 6p^{2}P_{1/2}7p2P3/2​−6p2P1/2​ transitions and the sodium 4d2D3/2,5/2−3p2P1/2,3/24d^{2}D_{3/2, 5/2} - 3p^{2}P_{1/2, 3/2}4d2D3/2,5/2​−3p2P1/2,3/2​ transitions together with the more common MPI via intermediate S, P and D states. Power dependences for these processes are presented together with and interesting case of a switch from red to blue Stark shifting in the sodium MPI spectrum.