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IEEE Journal of Quantum Electronics

Parametric generation within a strontium plasma created by two-photon ionization

Kenneth A. Menard, R. M. Measures

Published Sep 1, 1987• 0 citations
Publisher

Abstract

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.

DOI
10.1109/jqe.1987.1073532

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jul 10, 1987•0 citations
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.
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 report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710–740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.
Optically inverted barium ions in the 6p(12)6{p}_{(\frac{1}{2})}6p(21​)​ excited state served as the nonlinear medium for doubly resonant four-wave parametric generation of coherent radiation at 167 nm. The excited barium ions were produced via selective autoionization. Stimulated-emission photons at 650 nm corresponding to the $6{p}_{(\frac{1}{2})}\ensuremath{-}5{d}_{(\frac{3}{2})}$ transition were mixed with input 451-nm laser photons turned to the $6{p}_{(\frac{1}{2})}\ensuremath{-}6{f}_{(\frac{5}{2})}$ two-photon resonance to produce the 167-nm output photons.
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.
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.