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International Laser Science Conference

Multiwave mixing and multiphoton ionization in strontium vapor

K. Bohmer, Jürgen Reif, Eckart Matthias

Published Jan 1, 1986• 0 citations
Publisher

Abstract

Atomic strontium vapor in a thermionic diode is irradiated by the light from a tunable dye laser at two-photon resonance on the 5s5d1D2 state. At a laser intensity of ~ 108 W/cm2 and a vapor pressure of several torrs, not only three-photon ionization occurs but also the generation of new frequencies due to a nonlinear polarization is observed. Simultaneously, strong and strictly directed beams are generated which are resonantly enhanced by the 5s5p1P1 and 5s6p1P1 states, respectively. Close to the first resonance, four-wave mixing causes braodband two-photon emission, the bandwidth of which is determined by the region of negative dispersion due to phase matching. On the other hand the process via the 6p state is exactly resonant as can be shown by comparison in every case with the respective fluorescence light. At higher pressures (~ 10Torr), however, a second line appears, which is also shifted to the side of negative dispersion.

DOI
10.1364/ils.1986.we7

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Journal of the Optical Society of America B•Mar 1, 1984•38 citations
Atomic strontium vapor in a thermionic diode is irradiated by the light from a tunable dye laser at two-photon resonance on the 5s5d1D2 state. At a laser intensity of ~ 108 W/cm2 and a vapor pressure of several torrs, not only three-photon ionization occurs but also the generation of new frequencies due to a nonlinear polarization is observed. Simultaneously, strong and strictly directed beams are generated which are resonantly enhanced by the 5s5p1P1 and 5s6p1P1 states, respectively. Close to the first resonance, four-wave mixing causes braodband two-photon emission, the bandwidth of which is determined by the region of negative dispersion due to phase matching. On the other hand the process via the 6p state is exactly resonant as can be shown by comparison in every case with the respective fluorescence light. At higher pressures (~ 10Torr), however, a second line appears, which is also shifted to the side of negative dispersion.
The coherent interaction of a two‐photon‐resonant, pulsed dye laser with strontium vapor leads to both multiphoton ionization and the emission of coherent light simultaneously at several other frequencies. For a laser intensity of about 4×108 W/cm2 and a vapor pressure around 103 Pa, these frequencies are all of the same order of intensity, about three orders weaker than the laser. Interferences of different channels are observed, which render the usual characterization of non‐linear optical processes rather ambiguous.
An experiment has been performed to demonstrate a third-order nonlinear optical effect involving the ground state of the strontium ion. Evidence of strong and slightly tunable parametric generation from a doubly resonant four-wave mixing process is provided when a single-wavelength laser is used to two-photon ionize strontium vapor and simultaneously pump close to the5s rightarrow 6stwo-photon resonance of the ion. This result appears to conform with a semiclassical wave equation description, although there is evidence that a population rate equation analysis will also be necessary if the process is to be modeled more accurately.
Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.
Competitive inhibition of a resonance enhanced multiphoton ionization process by a resonant four-wave mixing has been observed in Xe atoms. When an intense IR (1064 nm) laser was applied to a sample of Xe which was also being irradiated by a UV laser tuned to the two-photon absorption line of Xe, the two-photon-resonant three-photon ionization signals decreased with increasing IR laser power. This phenomenon is dependent on the resonant states of Xe and the polarization of the two laser beams. Three 6sss excited states [5/2]2{}_{2}2​, [3/2]2{}_{2}2​, and [1/2]0{}_{0}0​ were examined. At the [1/2]0{}_{0}0​ resonant state, the ion signals were not decreased but slightly increased with the increase of the IR laser power. No suppression of the ion signal was observed at the [5/2]2{}_{2}2​ resonant state, when the polarization directions of the lasers were perpendicular to each other. The result of the polarization dependence reflects the selection rules of four-wave mixing. A simple rate equation analysis including the contribution of two-photon ionization from the [1/2]0{}_{0}0​ state by the IR laser well represents the IR laser-power dependence of the ion signal.
A pulsed dye-laser beam (ωL) pumped by a frequency-doubled Nd:YAG laser is focused into a heat-pipe oven containing potassium vapor (nk ≈ 1016 cm−3). Two-photon resonant four-wave and six-wave mixing processes using the potassium 8s 2S1/2 state are investigated. Thirty-two coherent UV radiation lines are generated within the range of 300–400 nm through the processes ωUV = 2ωL ± ωIR and ωUV = 2ωL − ω′IR − ω″IR ± ω‴IR, where ωIR are frequencies of laser-induced IR waves generated in the vapor near the transitions 8S–7P, 7P–7S, 7P–5D, 8S–6P, 7S–6P, 5D–6P, etc. The influence of resonant enhancement on the UV line intensities and the phase-matching condition are analyzed.