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Acta Physica Sinica

THERMODYNAMIC APPROACH FOR SUPERFLUORESCENCE

Eng Y

Published Jan 1, 1995• 0 citations
OpenAlex

Abstract

The self-organization process of superfluorescence can be explained clearly basing on the thermodynamics. The present paper includes three parts: 1) the equations of superfluorescence radiation and heat transform cetion are derived, and their transient processes are analysed; 2) the possibility of the generation of superfluorescence is theoretically verified by the analysis of the linear stability of the thermo-dynamic Maxwell-Bloch equations, and the coherent process of superfluorescence is analysed by the temporal coherence function of the field of radiation obtained: 3) the effective initial tipping angle θ0 and the delay time τD are derived from the fluctuation in statistical thermodynamics. Numerical results are in good agreement with those accepted up to now.

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The self-organization process of superfluorescence can be explained clearly basing on the thermodynamics. The present paper includes three parts: 1) the equations of superfluorescence radiation and heat transform cetion are derived, and their transient processes are analysed; 2) the possibility of the generation of superfluorescence is theoretically verified by the analysis of the linear stability of the thermo-dynamic Maxwell-Bloch equations, and the coherent process of superfluorescence is analysed by the temporal coherence function of the field of radiation obtained: 3) the effective initial tipping angle θ0 and the delay time τD are derived from the fluctuation in statistical thermodynamics. Numerical results are in good agreement with those accepted up to now.
For a pencil-shaped extended medium with Fresnel number equal to 1, we have quantum mechanically derived a description of the initiation of superfluorescence in terms of Maxwell-Bloch equations with a fluctuating source due to the zero-point fluctuations of the vacuum field. By the introduction of classical behavior, these equations are extended to include nonlinear behavior due to decreasing atomic inversion. The principal assumption in the derivation is that the main features of superfluorescence are governed by the interaction of atoms with field modes inside two small solid angles around the pencil axis. The delay ${\ensuremath{\tau}}_{D}$, defined as the time at which the mean-squared tipping angle of the collective Bloch vector attains the value 1, turns out to be given by ${\ensuremath{\tau}}_{D}=(\frac{{\ensuremath{\tau}}_{R}}{4}){[\mathrm{ln}{(2\ensuremath{\pi}N)}^{\frac{1}{2}}]}^{2}$, where ${\ensuremath{\tau}}_{R}$ is the radiation time for collective decay and NNN is the number of atoms. The corresponding effective initial tipping angle roughly equals 2(N)12\frac{2}{{(N)}^{\frac{1}{2}}}(N)21​2​. A Fokker-Planck equation is derived to describe the statistics of the initial development of the tipping angle. The variance $\ensuremath{\Delta}{\ensuremath{\tau}}_{D}$ of the delay of the superfluorescence pulse satisfies approximately $\ensuremath{\Delta}{\ensuremath{\tau}}_{D}=\frac{2.3}{\mathrm{ln}N}$. A brief comparison with previous treatments of superfluorescence is given.
Conservation laws for Maxwell-Bloch equations applied to describe superfluorescence are studied. Modifications due to the initial boundary conditions are discussed. For initial polarization in the form of a constant tipping angle, conservation laws are integrated to give exact dependence of all field moments on the propagation length. Three regions of propagation are distinguished: the linear region, where the field energy is negligibly small, the region of rapid exponential growth, and the saturation region. The characteristic distance, where maximum radiation occurs, is explicitly evaluated; it defines the threshold length for superfluoresence. A small chirping, proportional to the square of the tipping angle, is found. The change of the pulse width with propagation length is estimated.
The fluctuating properties of superradiant and superfluorescent systems are investigated by supplementing the equations of motion for the expectation values of quantum observables with random force terms. In the case of a small atomic system the time evolution of the radiated intensity as well as the intensity fluctuations are computed. We also find the time dependence of the average dipole moment in the presence of chirping. For a pencil-shaped geometry, a generalized system of Maxwell–Bloch equations is analysed. The inhomogeneous broadening is then included. We compare the semiclassical equation for the probability distribution of the atomic variables with the quantum equations based both on normal and antinormal ordering. With the exception of the region in the vicinity of the poles of the Bloch sphere, a close correspondence between the semiclassical and quantum theories is established. The oscillatory superfluorescence is treated similarly to quantum analysis by employing a new method of Lugiato for open systems.
We have formulated the quantum theory of formation of the superfluorescence pulse from a pencil-like amplifier. The coupled system of the linearized Bloch equations for two-level atoms and the three-dimensional wave equation for the electric field has been solved in the geometrical-optics approximation for Fresnel numbers $F \ensuremath{\ge} 1$. Several properties of the superfluorescence pulse are discussed. The spatial coherence function of the pulse has been found as a function of the Fresnel number. This function provides a precise formulation of the concept of "diffraction modes." The delay-time distribution function for various Fresnel numbers has been found. The results are consistent with experimental data and numerical simulations.
An approach to modeling the dynamics of x-ray amplified spontaneous emission and superfluorescence -- the phenomenon of collective x-ray emission initiated by intense pulses of X-ray Free Electron Lasers -- is developed based on stochastic partial differential equations. The equations are derived from first principles, and the relevant approximations, derivation steps, and extensions specific to stimulated x-ray emission are presented. The resulting equations take the form of three-dimensional generalized Maxwell-Bloch equations augmented with noise terms for both field and atomic variables. The derived noise terms possess specific correlation properties that enable the correct reconstruction of spontaneous emission. Consequently, the developed theoretical formalism is universally suitable for describing all stages of stimulated x-ray emission: spontaneous emission, amplified spontaneous emission, and superfluorescence. We present numerical examples that illustrate various properties of the emitted field, including spatio-temporal coherence, spectral-angular and polarization characteristics. We anticipate that the proposed theoretical framework will establish a robust foundation for interpreting measurements in stimulated x-ray emission spectroscopy, modeling x-ray laser oscillators, and describing other experiments leveraging x-ray superfluorescence.