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Journal of Beijing Institute of Technology

Mechanism of Laser Induced Fluorescence

Jixiang Yan

Published Jan 1, 2000• 1 citation
OpenAlex

Abstract

To search after for the dependence of the emission fluorescence on the excitation rate and the spontaneous-radiation rate, the rate equation of the upper level population for two-level system was derived and it was solved on certain. conditions. The variation regularities of the aforementioned population and the photon-number of the emission fluorescence were presented. The fluorescence photon-number is proportional to the excitation rate while the spontaneous-radiation rate has an optimum decided by a trigonometric equation.

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Numerical simulations of the excitation spectra of resonant fluorescence bound by the dynamic interaction of two different two-level nanoparticles have been performed. We considered the excitation of one of the particles by monochromatic light with a frequency close to the transition frequency of this particle, at an irradiation intensity leading to a small population of its excited state. The formalism for the description of resonance fluorescence by the Heitler--Ma method was applied to a complete compound system of the marked particle pair and quantized emission field. The resulting expressions for the excitation spectrum reflect consideration of the absorption of the fluorescence photon without attributing it to the emission of any one particle. Expressions for the excitation spectra when taking into account such absorption of the fluorescence photon that allows us to attribute it to the radiation of one or another of the particles were obtained by solving a modified system of equations of the Heitler--Ma method, which reflects the addition of non-interfering amplitudes of the emission field states of each particle. It was found that the shape of the excitation spectra of the stationary resonance fluorescence of the particles in question is determined by the shape of the corresponding intensity spectra of the spontaneous fluorescence of these particles. Keywords: mathematical modeling, interacting two-level nanoparticles, resonant fluorescence, excitation spectrum.
The dynamics of transformation of a light pulse by a five-level model molecule whose secondary emission spectrum can contain two fluorescence bands is simulated. The system of equations that determine the time behavior of the matrix elements of the statistical operator of the molecule interacting with the light pulse is numerically solved. From this solution, the time dependences of the populations of the molecular states are determined for different values of the parameters of the irradiation pulse, which is described in terms of the classical theory, and of the parameters that characterize the rates of radiative and nonradiative spontaneous transitions of the molecule. Based on particular examples of the choice of these parameters, it is demonstrated that the mechanism by which dual fluorescence occurs in molecules with intramolecular hydrogen bonds can be efficiently established from the numerically simulated intramolecular dynamics.
When fluorescence from electronically excited states follows the absorption of radiation, the emission spectrum is often a key to identification of the excited species. It now emerges that passing off-resonant laser light through such an electronically excited system may enhance or suppress the fluorescent emission. This report establishes the mechanism and theory for this optical control of spontaneous fluorescence, derived by quantum electrodynamical analysis. Experimental techniques to detect the enhanced signal are also proposed.
The problem about fluorescence of organic molecules located in micropartical-cavity at two-photon absorption in the condition of laser pulse radiation influence taking into account stimulated transitions was theoretically investigated. The approach based on the metod of open-resonator theory with light fields expanded in series of quasinormal vibrational eigenmodes of a dielectric full sphere was used. For active molecules in microcavity the model of generalized two-level system noted with processes of two-photon absorption and emission, spontaneous and stimulated emissions on stokes frequencies was considered. The kinetic equations solution were obtained.
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