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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Multichannel Nonlinear Optical Processes In Atomic Vapors

Jürgen Reif, K. Bohmer, Eckart Matthias

Published Jul 10, 1987• 0 citations
Publisher

Abstract

The coherent interaction of a two-photon resonant pulsed dye laser with strontium vapor leads to both multiphoton ionization and the simultaneous emission of coherent light at several other frequencies. For a vapor pressure around 1.2x103 Pa, this light is generated at intensities of about 10-3 times that of the laser. Interferences of different channels are observed.

DOI
10.1117/12.937120

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•Jan 1, 2010•0 citations
The coherent interaction of a two-photon resonant pulsed dye laser with strontium vapor leads to both multiphoton ionization and the simultaneous emission of coherent light at several other frequencies. For a vapor pressure around 1.2x103 Pa, this light is generated at intensities of about 10-3 times that of the laser. Interferences of different channels are observed.
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.
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.
We develop a theoretical model of two-photon amplification in laser-driven potassium atoms and use it to analyze the recent experiments reported by Pfister et al. [Phys. Rev. A 60, R4249 (1999)]. The model takes into account most of the essential factors influencing the amplification process, including the atomic hyperfine structure (which makes multiphoton emission possible) and the simultaneous interaction with intense drive and probe beams with arbitrary detunings. We determine the origin and analyze the properties of different multiphoton gain resonances that appear in the light-matter interaction. In particular, the influence of the drive and probe field amplitudes and detunings on the strength and frequency of the two-photon amplification resonance is studied in detail, showing clearly the differences with respect to the behavior of single-photon or other multiphoton amplification processes. In addition, we investigate interferences between different quantum pathways originating from the hyperfine structure and determine the conditions under which they can enhance or suppress multiphoton resonances. The predictions of the model are in good agreement with the observations, indicating that it can be used to understand recent experiments on two-photon lasing reported by Pfister et al. [Phys. Rev. Lett. 86, 4512 (2001)].
Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.
Atomic potassium is ionized by three channels when it is excited by a pulse dye laser. They are energy pooling induced ionization, 4S→6S and 4S→4D two photon resonant three photon ionization. The ionization current intensity at various vapor temperature and laser intensity were investigated for the former two ionization channels. The atomic radiation trapping and nonlinear ionization were found in ionization current spectra. The qualitative explanations for above phenomena are given.
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.