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Soviet Journal of Quantum Electronics

Laser-induced resonances in the continuum observed in optical frequency mixing in sodium vapor

Stefan Dimov, D.I. Metchkov, Lubomir I. Pavlov, K. V. Stamenov

Published May 31, 1982• 3 citations
Publisher

Abstract

Frequency mixing in sodium vapor was observed when the sum frequency was tuned near a laser-induced autoionization-like resonance. The energy characteristics of phase-matched conversion by a two-photon transition were optimized. The efficiencies of the various processes were determined under different experimental conditions. Saturation of the mixing signal on increase in the incident power was observed experimentally for the first time.

DOI
10.1070/qe1982v012n05abeh012503

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Frequency mixing in sodium vapor was observed when the sum frequency was tuned near a laser-induced autoionization-like resonance. The energy characteristics of phase-matched conversion by a two-photon transition were optimized. The efficiencies of the various processes were determined under different experimental conditions. Saturation of the mixing signal on increase in the incident power was observed experimentally for the first time.
Efficient harmonic generation at laser-induced autoionizing state in continuum, were observed for the first time by our laboratory [1,2,3]. In this paper, nonlinear frequency mixing in alkali metal vapors at induced continuum structure in Na, are studied in detail. Besides, measurement of nonlinear optical susceptibility at induced autoionizing resonance, is performed. Tunable ultraviolet radiations by four-photon processes near induced resonances are obtained. Estimations of the real and imaginary parts of the nonlinearities, are estimated. We will specially note, that the resonant photoabsorption of ions is experimentally investigated since it gives information about the autoionizing states, which is impossible to obtain on the base only of the emission spectra. We use also laser inducing for obtaining of powerful radiation source in VUV. Thus, many actual problems as the selective impact on matter, laser photochemistry and laser isotope separation are connected directly with the induced autoionizing nonlinear resonances. The main interest is to control the spectral characteristics of continuum. The discrete level from the continuum structure leads to appearance of asymmetrical autoionizing resonance onto the background of the broad-band absorption line.
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 nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.
An experimental investigation was made of resonant nonlinear four-wave frequency mixing processes involving cw laser radiation in sodium vapor. It was found that, in spite of an increase in absorption, the use of resonant atomic nonlinearities can substantially improve the frequency conversion efficiency in the cw regime. The results of the measurements were compared with the theoretical conclusions. Ultraviolet radiation was generated at the sum frequency ( λ s = 268 nm) with the proportionality factor C = W s / W 1 W 2 W 3 ≈2W −2