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Journal of Physics B Atomic and Molecular Physics

Optically pumped stimulated emission and stimulated electronic Raman scattering from K atoms

James J. Wynne, P. P. Sorokin

Published Mar 11, 1975• 32 citations
Publisher

Abstract

Stimulated emission in nineteen np-ns and np-(n-2)d transitions in K vapour was observed by optically pumping the np states with the second harmonic of a pulsed dye laser. The observed output wavelengths range from about 10 mu m to above 100 mu m. Wavelength measurements of the np-(n-2)d emissions can provide accurate unambiguous determinations of d-level energies. For the cases n=7 and n=8 stimulated Raman emission was observed at small detunings of the pump beam from the np resonance lines.

DOI
10.1088/0022-3700/8/4/002

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 13, 2017•1 citation
Stimulated emission in nineteen np-ns and np-(n-2)d transitions in K vapour was observed by optically pumping the np states with the second harmonic of a pulsed dye laser. The observed output wavelengths range from about 10 mu m to above 100 mu m. Wavelength measurements of the np-(n-2)d emissions can provide accurate unambiguous determinations of d-level energies. For the cases n=7 and n=8 stimulated Raman emission was observed at small detunings of the pump beam from the np resonance lines.
Two-photon resonant or nearly resonant optical processes have been investigated in atomic barium vapour ( approximately 10 17 cm -3 ) with a pulse dye laser (frequency omega L ) covering the range 735-415 nm. These interactions result in strong coherent emissions emitted either forward and backward, or forward only. The optical processes responsible for these coherent emissions have been identified. Seventy five lines (frequency omega ES ) emitted from 1000 to 360 nm are due to optically pumped stimulated emissions or stimulated electronic hyper-Raman scattering. Twenty eight lines (frequency omega sigma ) observed from 795 to 250 nm are generated through four-wave-mixing parametric processes with omega sigma =2 omega L +or- omega ES , and nine- (795-260 nm) through six-wave-mixing parametric processes with omega sigma =2 omega L - omega ES1 - omega ES2 +or- omega ES3 .
Optical second harmonic generation has been observed on or near the 4s 2 S 1/2 -6s 2 S 1/2 , 4s 2 S 1/2 -np 2 P 1/2, 3/2 (n=5, 6) and 4s 2 S 1/2 -nd 2 D 5/2, 3/2 (n=3,4) two-photon resonances in potassium vapour, as laser scanning in the 660-940 nm wavelength range. The second harmonic intensities and ionisation currents have been measured as a function of pumping laser detuning from the two-photon resonances. On or near the two-photon resonances, the intensity and vapour-density dependence of second harmonic generation have also been investigated. The saturation of second harmonic generation has been observed. Some calculations and analyses have been made with the chi (3) model. The results have been compared with the measurements. Based on these analyses, the mechanisms for second harmonic generation in isotropic vapours are discussed.
Stimulated electronic Raman scattering, SERS, is a simple method of efficiently generating widely tunable infrared radiation. This work describes techniques by which the tuning capability of this process may be extended. To this end several schemes in Caesium and Barium vapours have been investigated. In Barium vapour two Raman transitions were studied using fixed frequency lasers. Poor performance of one of these transitions led to the application of biharmonic pumping or 'coherent Raman mixing' to enable generation of infrared light, and was the first observation of this process in an atomic vapour. This technique was subsequently used to generate infrared light from tunable dye laser light, which gave continuous tuning from 1.16µm to 1.5µm. The process was found to be limited by atomic saturation. The effect which saturation has on the generation efficiency is considered theoretically, and predicts quite accurately the observed tuning behaviour. Two methods aimed at reducing molecular absorption effects in SERS in alkali vapours were investigated. The techniques employed either laser excitation and dissociation of the molecules or a reduction in their ground state concentration by superheating of the vapour. Some degree of success was achieved in improving the available tuning from the 6s-7s transition in Caesium. However, the inclusion of the effect of collision broadening of atomic transitions into a calculation of the Raman threshold suggests that little further improvement can be obtained on this transition. A direct measurement of the Raman threshold power on the 6s-5d5/2 transition in Caesium gave excellent agreement with theoretical predictions based on collision broadening of this Raman transition. In addition, this work gave an insight into the role photoionization plays in increasing the threshold under certain circumstances. The analysis suggests that laser pulse duration and vapour column length are important in determining whether Raman threshold can be exceeded.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.