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Annual Meeting Optical Society of America

Change of ions’ spontaneous emission by an intense laser beam

Hui W. Cao, Darrell S. DiCicco, S. Suckewer

Published Jan 1, 1993• 0 citations

Abstract

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.

DOI: 10.1364/oam.1993.ws.6

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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.

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.

New interesting effects arise, when three-level atoms interact with the plasma, laser field, or a heat bath. If the atoms inside the plasma are excited by the polarized laser pulse or an electron beam, the line wings of the radiation are completely polarized, while the line core is only partially polarized. The Einstein coefficients are generalized to the case, when the atomic decay channels are subject to quantum interference. Such atoms may develop nonzero quantum coherence between upper levels, if they are in contact with the heat bath in thermal equilibrium. This coherency changes the emissivity of the medium and hence can be experimentally observed. In the case when three-level atoms interact with the laser field, spontaneous emission can be suppressed over the whole frequency spectrum.

Stimulated absorption of spontaneous emission of a gas-discharge plasma can cause a significant increase in the population of the emitting states, compared to their population determined by inelastic collisions and spontaneous decay from upper levels. By imposing a magnetic field, this self-saturation is reduced and its contribution to the level population can be identified. The magnetic-field-dependent changes in Doppler profiles due to radiative transitions caused by saturating spontaneous emission are analyzed in the case of absorption of a weak monochromatic wave.

The effect of dissipation of plasmas on the spontaneous radiation of an ionized atom is investigated by Langevin equations and the modified Weisskopf-Wigner approximation to consider the quenching effect of the Einstein A coefficient. The dissipation of field in plasmas results from inverse bremsstrahlung of free electrons and absorption of the ions of the same kind as the radiator. Under certain conditions the Einstein A coefficient may be reduced significantly. The lifetime of the excited ion will be changed correspondingly.

The rates of spontaneous emission and stimulated emission and absorption for a two-level atom in a dielectric microsphere are calculated for weak E1 coupling. The A coefficient for spontaneous emission is proportional to the zero-point fluctuation of the electric field ℰij evaluated in the previous paper [ J. Opt. Soc. Am. B4, 1995 ( 1987)], which shows sharp resonances in frequency (Q > 104). It is verified that the ratio of the A and B coefficients is determined by the spectral density of thermal radiation. The level shift tends to repel the transition line away from the cavity resonance.