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Optics and Spectroscopy

Thermal fluctuations of a quantized electromagnetic field in weakly absorbing media

B. A. Veklenko, Yu. B. Sherkunov

Published Sep 1, 2005• 0 citations
Publisher

Abstract

The quantum fluctuations of an electromagnetic field in thermally excited media are calculated using the quantum electrodynamical method of Γ operators and without invoking phenomenological elements. The drawbacks of the standard theory based on the fluctuation-dissipation theorem and on the Matsubara technique of temperature Green’s functions are indicated. The decisive role of the correct consideration of higher order photon-photon correlators and the inadmissibility of their approximation by products of lower order correlators are underlined. It is shown that, contrary to the accepted opinion, the quantum fluctuations of an electromagnetic field cannot be expressed via the refractive indices of media introduced into the theory for the calculation of mean fields. The results obtained are compared with those previously derived using the Matsubara technique for calculation of the Green’s functions of an electromagnetic field in dispersive media under conditions of thermodynamic equilibrium. The previous results are shown to be incorrect at least for media consisting of atoms or molecules with a discrete energy spectrum.

DOI
10.1134/1.2055950

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The quantum fluctuations of an electromagnetic field in thermally excited media are calculated using the quantum electrodynamical method of Γ operators and without invoking phenomenological elements. The drawbacks of the standard theory based on the fluctuation-dissipation theorem and on the Matsubara technique of temperature Green’s functions are indicated. The decisive role of the correct consideration of higher order photon-photon correlators and the inadmissibility of their approximation by products of lower order correlators are underlined. It is shown that, contrary to the accepted opinion, the quantum fluctuations of an electromagnetic field cannot be expressed via the refractive indices of media introduced into the theory for the calculation of mean fields. The results obtained are compared with those previously derived using the Matsubara technique for calculation of the Green’s functions of an electromagnetic field in dispersive media under conditions of thermodynamic equilibrium. The previous results are shown to be incorrect at least for media consisting of atoms or molecules with a discrete energy spectrum.
A theory of thermodynamic fluctuations of electromagnetic field in slightly absorbing media is developed using the quantum electrodynamicsmethod of Γ -operators -without phenomenology.The hypothesis offered by Yury L. Klimontovich is under consideration.The necessity of correct consideration of photon-photon correlation functions is shown.The results are compared with the ones obtained with the help of standard theory based upon fluctuation-dissipation theorem (FDT).The latter results are shown to have no field of application at least for the case of thermally excited media of the atoms described with two-level model.
The electromagnetic field is quantized in dielectric media that show both loss and dispersion. The complex dielectric function of the medium is assumed to be a known function and the loss is modeled by Langevin forces in the forms of noise current operators. The noise current correlation function is related to the assumed dielectric function by the fluctuation-dissipation theorem. Field quantization is carried out for the infinite homogeneous dielectric, the semi-infinite dielectric, and the dielectric slab, where the fields in the second and third cases are restricted to propagation perpendicular to the dielectric surfaces. The forms of the vector potential operator are obtained in the different spatial regions for all three geometries, and in each case the required canonical commutation relation for the vector potential and its conjugate generalized momentum operator is verified. The spatial dependence of the vacuum field fluctuations is calculated for the two dielectric geometries that have surfaces.
We present the theoretical framework of fluctuational quantum electrodynamics in frequency-dispersive and dissipative time-varying media. Our theory accounts for dispersion and losses in the temporal modulation, which is treated in an exact manner, without relying on perturbative methods. Thus, our work constitutes the first consistent quantization of the electromagnetic field in time-modulated material bodies. We derive a Fermi Golden Rule for time-varying media and use it to define the local density of states for these time-dependent systems, which includes both loss and gain contributions. Additionally, we prove the equivalence between the quantum Fermi Golden Rule and the power emitted by a classical harmonic point dipole. Moreover, we show that neglecting the dispersive and dissipative nature of the time modulation leads to erroneous predictions for both slow and fast modulations. Furthermore, we analyze the thermal radiation emitted by a time-varying material body, revealing new features in the enhancement of thermal emission in time-varying media. Finally, we study the dynamical Casimir effect, showing how the time modulation amplifies vacuum fluctuations and generates entangled pairs of polaritons exhibiting non-local spatial correlations.
This work continues the author’s study of the processes of interaction between a quantum electromagnetic field and a spatially non-uniform gaseous medium [1, 5]. Previously unknown optical effects caused by consideration of high-order quantum correlators were discovered. This is caused by the problem of calculation of high-order photonic correlators starting from the fourth order, which appears when the interaction between the quantum electromagnetic field is described using the Feynman diagrams, According to T.A. Matsubara’s technique applied nowadays, these correlators are just neglected. The author’s articles [1, 5] describe the calculation method allowing us to avoid thiscomplication. The method is based on the following. High-order quantum correlators appear when multi-particle problems are studied. If the number of particles is small (hydrogen atom), the problem disappears. Therefore, if we replace a multi-particle problem with a single-particle problem (even if this particle is rather complex), the problem will be solved. Such solution (by using the Γ-operators technique) is described in the said author’s articles. While the standard secondary quantisation technique uses operators of creation and destruction of separate particles, the Γ-operators technique deals with operators of creation and destruction of photon conglomerates (systems). It turns out that high-order correlators just do not appear when the Γ-operators technique is used. This article demonstrates a specific example of application of the Γ-technique.
A three-dimensional electromagnetic field quantization scheme is developed using the Green's-function method in a homogeneous medium that shows loss and dispersion. Special attention is paid to the decomposition of the quantized fields into their longitudinal and transverse parts. The characteristic nature of these two parts is discussed. To demonstrate the generality of this formulation, the field quantization in lossless dispersive media and in free space is obtained by imposing appropriate limits. We also calculate the vacuum fluctuations of the field for the transverse and longitudinal parts separately.
It is demonstrated that presence of stimulated radiation processes in gaseous media makes it possible to generate rays inversely reflected from interfaces of thermally excited gaseous media and the processes of left-hand refraction of the rays having passed through these media. These processes are caused by availability of coupled photonic states and stimulated radiation in media the properties of which differ from those of stimulated radiation in vacuum. Estimation of these processes is based on exclusion of the procedure of forced breaking of quantum correlators and correct consideration of high-order correlators. Along with Schrödinger’s and Heisenberg’s representations, this article uses also the less-known Γ- representation, which allows us to reduce a multi-particle problem of quantum electrodynamics to a single-particle problem within the an infinite-dimensional space. The equations formed in Г- representation are mathematically simpler and may be studied using the standard Wick’s theorem rather than its approximated thermodynamic variant suggested by T.A. Matsubara. Therefore it is possible to keep all possible solutions of the initial system of equations in mind. Of course, approximation methods are used for explicit solution of these equations. Using Feynman diagrams instead of the Dyson equation for quantum correlators, it is possible to find an equation defining the complete density matrix of a photonic sub-system.