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Dirk-Gunnar Welsch

Publications and source records attributed to Dirk-Gunnar Welsch.

At least 37 records · Page 2Linked to original sources
arXiv2006

The van der Waals energy of atomic systems near absorbing and dispersing bodies

Within the frame of macroscopic quantum electrodynamics in causal media, the van der Waals interaction between an atomic system and an arbitrary arrangement of dispersing and absorbing dielectric bodies including metals is studied. It is shown that the minimal-coupling scheme and the multipolar-coupling scheme lead to essentially the same formula for the van der Waals potential. As an application, the vdW potential of an atom in the presence of a sphere is derived. Closed expressions for the long-distance (retardation) and short-distance (non-retardation) limits are given, and the effect of material absorption is discussed.

quant-ph↗
arXiv2006

Local-field correction to the spontaneous decay rate of atoms embedded in bodies of finite size

The influence of the size and shape of a dispersing and absorbing dielectric body on the local-field corrected spontaneous-decay of an excited atom embedded in the body is studied on the basis of the real-cavity model. By means of a Born expansion of the Green tensor of the system it is shown that to linear order in the susceptibility of the body the decay rate exactly follows Tomas's formula found for the special case of an atom at the center of a homogeneous dielectric sphere [Phys. Rev. A 63, 053811 (2001)]. It is further shown that for an atom situated at the interior of an arbitrary dielectric body this formula remains valid beyond the linear order. The case of an atom embedded in a weakly polarizable sphere is discussed in detail.

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arXiv2006

Body-assisted van der Waals interaction between two atoms

Using fourth-order perturbation theory, a general formula for the van der Waals potential of two neutral, unpolarized, ground-state atoms in the presence of an arbitrary arrangement of dispersing and absorbing magnetodielectric bodies is derived. The theory is applied to two atoms in bulk material and in front of a planar multilayer system, with special emphasis on the cases of a perfectly reflecting plate and a semi-infinite half space. It is demonstrated that the enhancement and reduction of the two-atom interaction due to the presence of a perfectly reflecting plate can be understood, at least in the nonretarded limit, by using the method of image charges. For the semi-infinite half space, both analytical and numerical results are presented.

quant-ph↗
arXiv2006

Dispersive forces on bodies and atoms: a unified approach

A unified approach to the calculation of dispersive forces on ground-state bodies and atoms is given. It is based on the ground-state Lorentz force density acting on the charge and current densities attributed to the polarization and magnetization in linearly, locally, and causally responding media. The theory is applied to dielectric macro- and micro-objects, including single atoms. Existing formulas valid for weakly polarizable matter are generalized to allow also for strongly polarizable matter. In particular when micro-objects can be regarded as single atoms, well-known formulas for the Casimir-Polder force on atoms and the van der Waals interaction between atoms are recovered. It is shown that the force acting on medium atoms--in contrast to isolated atoms--is in general screened by the other medium atoms.

quant-ph↗
arXiv2005

Matter-screened Casimir force and Casimir-Polder force in planar structures

Using a recently developed theory of the Casimir force (Raabe C and Welsch D-G 2005 Phys. Rev. A 71 013814), we calculate the force that acts on a plate in front of a planar wall and the force that acts on the plate in the case where the plate is part of matter that fills the space in front of the wall. We show that in the limit of a dielectric plate whose permittivity is close to unity, the force obtained in the former case reduces to the ordinary, i.e., unscreened Casimir-Polder force acting on isolated atoms. In the latter case, the theory yields the Casimir-Polder force that is screened by the surrounding matter.

quant-ph↗
arXiv2005

Born expansion of the Casimir-Polder interaction of a ground-state atom with dielectric bodies

Within leading-order perturbation theory, the Casimir-Polder potential of a ground-state atom placed within an arbitrary arrangement of dispersing and absorbing linear bodies can be expressed in terms of the polarizability of the atom and the scattering Green tensor of the body-assisted electromagnetic field. Based on a Born series of the Green tensor, a systematic expansion of the Casimir-Polder potential in powers of the susceptibilities of the bodies is presented. The Born expansion is used to show how and under which conditions the Casimir-Polder force can be related to microscopic many-atom van der Waals forces, for which general expressions are presented. As an application, the Casimir-Polder potentials of an atom near a dielectric ring and an inhomogeneous dielectric half space are studied and explicit expressions are presented that are valid up to second order in the susceptibility.

quant-ph↗
arXiv2005

Quantum theory of light and noise polarization in nonlinear optics

We present a consistent quantum theory of the electromagnetic field in nonlinearly responding causal media, with special emphasis on $χ^{(2)}$ media. Starting from QED in linearly responding causal media, we develop a method to construct the nonlinear Hamiltonian expressed in terms of the complex nonlinear susceptibility in a quantum mechanically consistent way. In particular we show that the method yields the nonlinear noise polarization, which together with the linear one is responsible for intrinsic quantum decoherence.

quant-ph↗
arXiv2005

Ground-state van der Waals forces in planar multilayer magnetodielectrics

Within the frame of lowest-order perturbation theory, the van der Waals potential of a ground-state atom placed within an arbitrary dispersing and absorbing magnetodielectric multilayer system is given. Examples of an atom situated in front of a magnetodielectric plate or between two such plates are studied in detail. Special emphasis is placed on the competing attractive and repulsive force components associated with the electric and magnetic matter properties, respectively, and conditions for the formation of repulsive potential walls are given. Both numerical and analytical results are presented.

quant-ph↗
arXiv2005

Casimir-Polder interaction of atoms with magnetodielectric bodies

A general theory of the Casimir-Polder interaction of single atoms with dispersing and absorbing magnetodielectric bodies is presented, which is based on QED in linear, causal media. Both ground-state and excited atoms are considered. Whereas the Casimir-Polder force acting on a ground-state atom can conveniently be derived from a perturbative calculation of the atom-field coupling energy, an atom in an excited state is subject to transient force components that can only be fully understood by a dynamical treatment based on the body-assisted vacuum Lorentz force. The results show that the Casimir-Polder force can be influenced by the body-induced broadening and shifting of atomic transitions - an effect that is not accounted for within lowest-order perturbation theory. The theory is used to study the Casimir-Polder force of a ground-state atom placed within a magnetodielectric multilayer system, with special emphasis on thick and thin plates as well as a planar cavity consisting of two thick plates. It is shown how the competing attractive and repulsive force components related to the electric and magnetic properties of the medium, respectively, can - for sufficiently strong magnetic properties - lead to the formation of potential walls and wells.

quant-ph↗
arXiv2005

Casimir force acting on magnetodielectric bodies embedded in media

Within the framework of macroscopic quantum electrodynamics, general expressions for the Casimir force acting on linearly and causally responding magnetodielectric bodies that can be embedded in another linear and causal magnetodielectric medium are derived. Consistency with microscopic harmonic-oscillator models of the matter is shown. The theory is applied to planar structures and proper generalizations of Casimir's and Lifshitz-type formulas are given.

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arXiv2004

Generation of long-living entanglement between two separate atoms

A scheme for non-conditional generation of long-living maximally entangled states between two spatially well separated atoms is proposed. In the scheme, $Λ$-type atoms pass a resonator-like equipment of dispersing and absorbing macroscopic bodies giving rise to body-assisted electromagnetic field resonances of well-defined heights and widths. Strong atom-field coupling is combined with weak atom-field coupling to realize entanglement transfer from the dipole-allowed transitions to the dipole-forbidden transitions, thereby the entanglement being preserved when the atoms depart from the bodies and from each other. The theory is applied to the case of the atoms passing by a microsphere.

quant-ph↗
arXiv2004

Three-dimensional Casimir force between absorbing multilayer dielectrics

Recently the influence of dielectric and geometrical properties on the Casimir force between dispersing and absorbing multilayered plates in the zero-temperature limit has been studied within a 1D quantization scheme for the electromagnetic field in the presence of causal media [R. Esquivel-Sirvent, C. Villarreal, and G.H. Cocoletzi, Phys. Rev. Lett. 64, 052108 (2001)]. In the present paper a rigorous 3D analysis is given, which shows that for complex heterostructures the 1D theory only roughly reflects the dependence of the Casimir force on the plate separation in general. Further, an extension of the very recently derived formula for the Casimir force at zero temperature [M.S. Tomaš, Phys. Rev. A 66, 052103 (2002)] to finite temperatures is given, and analytical expressions for specific distance laws in the zero-temperature limit are derived. In particular, it is shown that the Casimir force between two single-slab plates behaves asymptotically like $d^{-6}$ in place of $d^{-4}$ ($d$, plate separation).

quant-ph↗
arXiv2004

Efficiency of tunable band-gap structures for single-photon emission

The efficiency of recently proposed single-photon emitting sources based on tunable planar band-gap structures is examined. The analysis is based on the study of the total and ``radiative'' decay rates, the expectation value of emitted radiation energy and its collimating cone. It is shown that the scheme operating in the frequency range near the defect resonance of a defect band-gap structure is more efficient than the one operating near the band edge of a perfect band-gap structure.

quant-ph↗
arXiv2003

Casimir stress on lossy magnetodielectric spheres

An expression for the Casimir stress on arbitrary dispersive and lossy linear magnetodielectric matter at finite temperature, including left-handed material, is derived and applied to spherical systems. To cast the relevant part of the scattering Green tensor for a general magnetodielectric sphere in a convenient form, classical Mie scattering is reformulated.

quant-ph↗
arXiv2003

QED in dispersing and absorbing media

After giving an outline of the quantization scheme based on the microscopic Hopfield model of a dielectric bulk material, we show how the classical phenomenological Maxwell equations of the electromagnetic field in the presence of dielectric matter of given space- and frequency-dependent complex permittivity can be transferred to quantum theory. Including in the theory the interaction of the medium-assisted field with atomic systems, we present both the minimal-coupling Hamiltonian and the multipolar-coupling Hamiltonian in the Coulomb gauge. To illustrate the concept, we discuss the input--output relations of radiation and the transformation of radiation-field quantum states at absorbing four-port devices, and the spontaneous decay of an excited atom near the surface of an absorbing body and in a spherical micro-cavity with intrinsic material losses. Finally, we give an extension of the quantization scheme to other media such as amplifying media, magnetic media, and nonlinear media.

quant-ph↗
arXiv2003

Electromagnetic-field quantization and spontaneous decay in left-handed media

We present a quantization scheme for the electromagnetic field interacting with atomic systems in the presence of dispersing and absorbing magnetodielectric media, including left-handed material having negative real part of the refractive index. The theory is applied to the spontaneous decay of a two-level atom at the center of a spherical free-space cavity surrounded by magnetodielectric matter of overlapping band-gap zones. Results for both big and small cavities are presented, and the problem of local-field corrections within the real-cavity model is addressed.

quant-ph↗
arXiv2003

Input-output relations at dispersing and absorbing planar multilayers for the quantized electromagnetic field containing evanescent components

By using the Green-function concept of quantization of the electromagnetic field in dispersing and absorbing media, the quantized field in the presence of a dispersing and absorbing dielectric multilayer plate is studied. Three-dimensional input-output relations are derived for both amplitude operators in the ${\bf k}$-space and the field operators in the coordinate space. The conditions are discussed, under which the input-output relations can be expressed in terms of bosonic operators. The theory applies to both (effectively) free fields and fields, created by active atomic sources inside and/or outside the plate, including also evanescent-field components.

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