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Physical Review Letters

Resonant Enhancement of Two-Photon Absorption in Sodium Vapor

John E. Bjorkholm, P. F. Liao

Published Jul 15, 1974• 205 citations
Publisher

Abstract

Resonant enhancement of over 7 orders of magnitude of the two-photon absorption cross section is observed in sodium vapor. Two cw dye-laser beams of different frequencies and propagating in opposite directions are utilized to generate high-resolution spectra which allow the enhancement as well as destructive-interference effects of nearly resonant intermediate states to be clearly demonstrated.

DOI
10.1103/physrevlett.33.128

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Resonant enhancement of over 7 orders of magnitude of the two-photon absorption cross section is observed in sodium vapor. Two cw dye-laser beams of different frequencies and propagating in opposite directions are utilized to generate high-resolution spectra which allow the enhancement as well as destructive-interference effects of nearly resonant intermediate states to be clearly demonstrated.
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.
A high-resolution study of magnetic-field-induced second-harmonic generation in sodium vapor has been carried out with use of a single-frequency continuous-wave dye laser. Resonant enhancement was obtained by tuning the laser to the frequency of the 3S\ensuremath{\rightarrow}4D two-photon transition of the sodium atom. Experimental and theoretical results are presented showing the variation of the power and polarization properties of the second-harmonic radiation as the laser is tuned across the two-photon resonance, and as the transverse magnetic field is varied.
An experimental and theoretical study of magnetic-field-induced sum-frequency mixing in sodium vapor has been carried out using two single-frequency, continuous-wave dye lasers as the sources of the fundamental radiation. In addition to the usual two-photon resonant enhancement of the three-wave-mixing nonlinear-optical process, the use of near- or on-resonant single-photon transitions for further resonant enhancement was investigated. The macroscopic phase-matching behavior of the sum-frequency mixing was examined with the intermediate states off resonance. Experimental and theoretical results are presented to illustrate the differences that occur between the coherent quadrupole emission and the incoherent cascade fluorescence when the intermediate states in the two-photon absorption are resonant. Control over the single-photon resonant enhancement and the sum-frequency-mixing phase matching was achieved to increase the power conversion efficiency to 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$.
The $3S\ensuremath{-}5S$ transition of Na atoms is induced by absorption of two photons, one each from two circularly polarized beams traveling in opposite directions through the vapor. The beams are derived from a tunable, narrow-band pulsed dye laser. Doppler broadening is absent and the hyperfine structure is resolved. The hyperfine interaction constant A5S{A}_{5S}A5S​ in the 5S5S5S state of the Na23{\mathrm{Na}}^{23}Na23 atom is determined to be 101\ifmmode\pm\else\textpm\fi{}15 MHz.