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Optical Engineering

Study of spontaneous emission coefficients of argon ion by means of a quasi-sidelight method

Huaying Wang, Yafei Wang, Yinguan Sun

Published Nov 1, 1993• 0 citations
Publisher

Abstract

We report the measurements of the change of the spontaneous emission coefficients of Ar ion in the high-intensity laser field by means of a quasi-sidelight method and a spatial filter technique. The variances of A coefficients are found depending on the changes of orbital angular momentum ΔL and total angular momentum ΔJ of Ar ion and the intensity of the light field in the laser cavity.

DOI
10.1117/12.147700

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We report the measurements of the change of the spontaneous emission coefficients of Ar ion in the high-intensity laser field by means of a quasi-sidelight method and a spatial filter technique. The variances of A coefficients are found depending on the changes of orbital angular momentum ΔL and total angular momentum ΔJ of Ar ion and the intensity of the light field in the laser cavity.
For atomic systems under strong influence of electromagnetic fields, the values of Einstein A and B coefficients have been observed to change from those of isolated atoms. However in our previous experiments which demonstrated the changes of A-coefficients, the self absorption of spontaneous emission was very strong and might the experimental result. In order to reduce the absorption drastically so that its effect can be neglected, we conducted a new experiment. In our new experiment, we measured the spontaneous emission from a very short discharge tube where the absorption was less than 4%. We also use a multichannel detector to monitor a number of spontaneous emission lines simultaneously. We observed that, as the incident photon flux increased, the intensity ratio of two spontaneous emission lines from the same upper level decreased. Any possible instrumental effect has been checked and eliminated. Therefore we can conclude that such observed decrement can only due to the change of Einstein A-coefficients.
Abstract This letter demonstrates an experimental approach to measuring the angular-momentum-resolved population of excited states in laser-produced argon ions. By measuring the spectra of free induction decay emissions corresponding to the resonant transitions between Rydberg states, the relative population of the Rydberg states is obtained with known Einstein A-coefficients. This study deepens the mechanistic understanding of coherent dynamics in laser-driven ionic excited states, and establishes experimental benchmarks essential for validating and refining advanced quantum kinetic models in strong-field physics.
Experimental evidence is presented for the change of Einstein's A coefficients for spontaneous transitions from the upper laser level of an argon in laser discharge due to the presence of a high-intensity laser flux. To demonstrate that this quenching effect cannot be attributed to a reduction in self-absorption of the strong spontaneous emission line, absorption and line profile measurements have been performed. Computer modelling of the reduction of self-absorption due to Rabi splitting also indicated that this effect is too small to explain the observed quenching of spontaneous line emissions.
The relationship between optical amplification and spontaneous emission noise has been studied in an argon ion travelling wave amplifier by beat spectrum analysis. Different optical arrangements have been examined with both single and double pass amplification. The results establish that when coherent light travels through a gain tube, the amplified beam acquires intensity fluctuation noise which is greater than that of a Poissonian distribution (shot noise). The noise power is shown to be the same as that of an equivalent unamplified beam mixed with the amplified spontaneous emission from the gain tube. It is shown that such measurements provide a practical technique for the otherwise difficult determination of the degree of population inversion in the gain medium. They also give insight into the potential of optical amplifiers for enhancement of lidar and system performance.
Recently, we observed enhancement and inhibition of visible spontaneous emission by atoms in a confocal resonator.1 In these experiments, the partial emission rate of the atom into the resonator modes is modified by a factor of 1/(1 − R), where R is the mirror reflectivity. The change in the total spontaneous emission rate is proportional to the solid angle ΔΩ subtended by the mirrors; the change was small (+1.6%, −0.5%) because ΔΩ ≈ 0.04 sr was small. Currently we are conducting experiments in which atoms are placed in a concentric resonator of large solid angle ΔΩ ≈ 3.6 sr and finesse of ~10. We estimate that a 30% decrease and 300% increase should be observable in the total spontaneous emission rate, when the resonator is tuned off or on-resonance, respectively. Results of this new experiment are presented. In addition to studying spontaneous processes, one can also consider the stimulated emission which occurs when a cavity field builds up which contains more than one photon. We have formulated a model which describes both the spontaneous and stimulated emission by the atom into the cavity and clarifies the relationship between enhanced spontaneous emission and stimulated emission.