arXiv subjects

Iver Brevik

Publications and source records attributed to Iver Brevik.

At least 91 records · Page 5Linked to original sources

arXiv2013

What Do Experiments in Optics tell us about Photon Momentum in Media?

In order to get some insight in the intricacies of the Abraham-Minkowski energy-momentum problem in phenomenological electrodynamics, we briefly analyze eight different experimental situations in radiation optics. Six of the experiments are already existing, while the remaining two are suggestions for future endeavors. Among the first six, we distinguish between three which are incapable of informing about photon momentum in a medium, and the remaining three which are able to give us useful information. Our general conclusion is that the Abraham-Minkowski "problem" is essentially a matter of convenience. In actual physical situations it is the Minkowski expression which is most natural alternative to employ. Also, in a canonical context, it is the Minkowski energy-momentum tensor which is the most convenient alternative to work with, as this tensor is divergence-free causing the total radiation momentum and energy to make up a four-vector, although notably a spacelike one.

quant-ph↗

arXiv2013

Casimir Friction Between Dense Polarizable Media

The present paper - a continuation of our recent series of papers on Casimir friction for a pair of particles at low relative particle velocity - extends the analysis so as to include dense media. The situation becomes in this case more complex due to induced dipolar correlations, both within planes, and between planes. We show that the structure of the problem can be simplified by regarding the two half-planes as a generalized version of a pair of particles. It turns out that macroscopic parameters such as permittivity suffice to describe the friction also in the finite density case. The expression for the friction force per unit surface area becomes mathematically well-defined and finite at finite temperature. We give numerical estimates, and compare them with those obtained earlier by Pendry (1997) and by Volokitin and Persson (2007). We also show in an appendix how the statistical methods that we are using, correspond to the field theoretical methods more commonly in use.

quant-ph↗

arXiv2013

Universe Models with Negative Bulk Viscosity

The concept of negative temperatures has occasionally been used in connection with quantum systems. A recent example of this sort is reported in the paper of S. Braun et al. [Science 339,52 (2013)], where an attractively interacting ensemble of ultracold atoms is investigated experimentally and found to correspond to a negative-temperature system since the entropy decreases with increasing energy at the high end of the energy spectrum. As the authors suggest, it would be of interest to investigate whether a suitable generalization of standard cosmological theory could be helpful, in order to elucidate the observed accelerated expansion of the universe usually explained in terms of a positive tensile stress (negative pressure). In the present note we take up this basic idea and investigate a generalization of the standard viscous cosmological theory, not by admitting negative temperatures but instead by letting the bulk viscosity take negative values. Evidently, such an approach breaks standard thermodynamics, but may actually be regarded to lead to the same kind of bizarre consequences as the standard approach of admitting the equation-of-state parameter w to be less than -1. In universe models dominated by negative viscosity we find that the fluid's entropy decreases with time, as one would expect. Moreover, we find that the fluid transition from the quintessence region into the phantom region (thus passing the phantom divide w=-1) can actually be reversed. Also in generalizations of the LCDM-universe models with a fluid having negative bulk viscosity we find that the viscosity decreases the expansion of the universe.

gr-qc↗

arXiv2013

Possible sorting mechanism for microparticles in an evanescent field

Mie scattering theory is used to calculate radiation forces on a dielectric microsphere illuminated by evanescent waves, produced by laser light transmitted obliquely through a flat horizontal dielectric surface. The incident field is identified with the evanescent field, and both $p$ and $s$ polarizations are considered. Our investigation consists of three parts. First, after highlighting the basic formalism, we report results for the radiation force published in an earlier paper [J. Opt. Soc. Am. B {\bf 12}, 2429 (1995)], correcting a few trivial calculational errors. Second - the main objective of our paper - is to show how the vertical (lifting) force on microspheres, typically via a proper adjustment of the laser frequency, can be used to separate spheres differing by a slight amount in their refractive index. This is caused by an oscillatory behavior in the force with respect to the nondimensional wave number $α$ in the surrounding medium. Fine-tuning the wave number $α$, relative to the given refractive indices in the system, may lead to particle expulsion. The sorting mechanism turns out to be feasible when $α$ is about 18-20 or larger, which actually is in the region of practical interest. Finally, we investigate how variations in the angle of incidence $θ_1$ for the laser beam influences the resulting radiation force.

physics.optics↗

arXiv2013

Theoretical considerations of laser induced liquid-liquid interface deformation

In the increasingly active field of optofluidics, a series of experiments involving near-critical two-fluid interfaces have shown a number of interesting non-linear effects. We here offer, for the first time to our knowledge, an explanation for one such feature, observed in experiments by Casner and Delville [Phys. Rev. Lett. {\bf 90}, 144503 (2003)], namely the sudden formation of "shoulder"-like shapes in a laser-induced deformation of the liquid-liquid interface at high laser power. Two candidate explanations are the following: firstly, that the shape can be explained by balancing forces of buoyancy, laser pull and surface tension only, and that the observed change of deformation shape is the sudden jump from one solution of the strongly nonlinear governing differential equation to another. Secondly, it might be that the nontrivial shape observed could be the result of temperature gradients due to local absorptive heating of the liquid. We report that a systematic search for solutions of the governing equation in the first case yields no trace of solutions containing such features. By contrast, an investigation of the second option shows that the narrow shape of the tip of the deformation can be explained by a slight {\it heating} of the liquids. The local heating amounts to a few kelvins, with the parameters given, although there are uncertainties here. Our investigations suggest that local temperature variations are the crucial element behind the instability and the shoulder-like deformation.

physics.flu-dyn↗

arXiv2012

Viscosity in modified gravity

A bulk viscosity is introduced in the formalism of modified gravity. It is shown that, on the basis of a natural scaling law for the viscosity, a simple solution can be found for quantities such as the Hubble parameter and the energy density. These solutions may incorporate a viscosity-induced Big Rip singularity. By introducing a phase transition in the cosmic fluid, the future singularity can nevertheless in principle be avoided.

gr-qc↗

arXiv2012

Static and dynamic response of a fluid-fluid interface to electric point and line charge

We consider the behaviour of a dielectric fluid-fluid interface in the presence of a strong electric field from a point charge and line charge, respectively, both statically and, in the latter case, dynamically. The fluid surface is elevated above its undisturbed level until balance is reached between the electromagnetic lifting force, gravity and surface tension. We derive ordinary differential equations for the shape of the fluid-fluid interface which are solved numerically with standard means, demonstrating how the elevation depends on field strength and surface tension coefficient. In the dynamic case of a moving line charge, the surface of an inviscid liquid-liquid interface is left to oscillate behind the moving charge after it has been lifted against the force of gravity. We show how the wavelength of the oscillations depends on the relative strength of the forces of gravity and inertia, whereas the amplitude of the oscillations is a nontrivial function of the velocity at which the line charge moves.

cond-mat.soft↗

arXiv2012

Thermal Issues in Casimir Forces Between Conductors and Semiconductors

The Casimir effect between metal surfaces has now been well-verified at the few-percent level experimentally. However, the temperature dependence has never been observed in the laboratory, since all experiments are conducted at room temperature. The temperature dependence for the related Casimir-Polder force between an atom and a bulk material has, in contrast, been observed between a BEC and a silica substrate, with the environment and the silica held at different temperatures. There is a controversy about the temperature dependence for the force between metals, having to do with the magnitude of the linear temperature term for both low and high temperature, the latter being most prominent at large distances. There are also related anomalies pertaining to semiconductors. The status of this controversy, and of the relevant experiments, are reviewed in this report.

hep-th↗

arXiv2012

Detection of the Abraham force with a succession of short optical pulses

For over a century, two rival descriptions of electromagnetic field momentum in matter have co-existed, due to Abraham and Minkowski, respectively. We propose a set-up for measuring the difference between Abraham's and Minkowski's predictions in optics. To wit, a set-up is proposed in which the transient "Abraham force", a consequence of the Abraham energy-momentum tensor of 1909 may be measured directly. We show that when a train of short laser pulses is sent through a fiber wound up on a cylindrical drum, the Abraham theory predicts a torque which, by inserting realistic parameters, is found to be detectable. Indeed, the same torque when calculated with the Minkowski tensor takes the opposite sign. Numerical estimates show that with a typical torsion pendulum set-up and standard laser parameters, the angular deflection is in the order of $10^{-3}$ rad, which is easily measurable and even visible to the naked eye. Although its prediction is a century old, the Abraham force has proven experimentally elusive, and to our knowledge no macroscopic experimental demonstration of the difference between the predictions of the two mentioned energy-momentum tensors exists at optical frequencies.

physics.optics↗

arXiv2012

Casimir Theory of the Relativistic Composite String Revisited, and a Formally Related Problem in Scalar QFT

The main part of this paper is to present an updated review of the Casimir energy at zero and finite temperature for the transverse oscillations of a piecewise uniform closed string. We make use of three different regularizations: the cutoff method, the complex contour integration method, and the zeta-function method. The string model is relativistic, in the sense that the velocity of sound is for each string piece set equal to the velocity of light. In this sense the theory is analogous to the electromagnetic theory in a dielectric medium in which the product of permittivity and permeability is equal to unity (an isorefractive medium). We demonstrate how the formalism works for a two-piece string, and for a 2N-piece string, and show how in the latter case a compact recursion relation serves to facilitate the formalism considerably. The Casimir energy turns out to be negative, and the more so the larger the number of pieces in the string. The two-piece string is quantized in D-dimensional spacetime, in the limit when the ratio between the two tensions is very small. We calculate the free energy and other thermodynamic quantities, demonstrate scaling properties, and comment on the meaning of the Hagedorn critical temperature for the two-piece string. Thereafter, as a novel development we present a scalar field theory for a real field in three-dimensional space in a potential rising linearly with a longitudinal coordinate z in the interval 0<z<1, and which is thereafter held constant on a horizontal plateau. The potential is taken as a rough model of the two-piece string potential under simplifying conditions, when the length ratio between the pieces is replaced formally with the mentioned length parameter z.

quant-ph↗

arXiv2012

Enlarged Molecules from Excited Atoms in Nanochannels

The resonance interaction that takes place in planar nanochannels between pairs of excited state atoms is explored. We consider interactions in channels of silica, zinc oxide and gold. The nanosized channels induce a dramatically different interaction from that in free space. Illustrative calculations for two lithium and cesium atoms, demonstrate that there is a short range repulsion followed by long range attraction. The binding energy is strongest near the surfaces. The size of the enlarged molecule is biggest at the center of the cavity and increases with channel width. Since the interaction is generic, we predict that enlarged molecules are formed in porous structures, and that the molecule size depends on the size of the nanochannels

physics.atom-ph↗

arXiv2012

Sign of the Casimir-Polder interaction between atoms and oil-water interfaces: Subtle dependence on dielectric properties

We demonstrate that Casimir-Polder energies between noble gas atoms (dissolved in water) and oil-water interfaces are highly surface specific. Both repulsion (e.g. hexane) and attraction (e.g. glycerine and cyclodecane) is found with different oils. For several intermediate oils (e.g. hexadecane, decane, and cyclohexane) both attraction and repulsion can be found in the same system. Near these oil-water interfaces the interaction is repulsive in the non-retarded limit and turns attractive at larger distances as retardation becomes important. These highly surface specific interactions may have a role to play in biological systems where the surface may be more or less accessible to dissolved atoms.

quant-ph↗

arXiv2012

Ultrathin Metallic Coatings Can Induce Quantum Levitation between Nanosurfaces

There is an attractive Casimir-Lifshitz force between two silica surfaces in a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin (5-50Å) metallic nanocoating to one of the surfaces results in repulsive Casimir-Lifshitz forces above a critical separation. The onset of such quantum levitation comes at decreasing separations as the film thickness decreases. Remarkably the effect of retardation can turn attraction into repulsion. From that we explain how an ultrathin metallic coating may prevent nanoelectromechanical systems from crashing together.

quant-ph↗

arXiv2012

Repulsive Casimir and Casimir-Polder Forces

Casimir and Casimir-Polder repulsion have been known for more than 50 years. The general "Lifshitz" configuration of parallel semi-infinite dielectric slabs permits repulsion if they are separated by a dielectric fluid that has a value of permittivity that is intermediate between those of the dielectric slabs. This was indirectly confirmed in the 1970s, and more directly by Capasso's group recently. It has also been known for many years that electrically and magnetically polarizable bodies can experience a repulsive quantum vacuum force. More amenable to practical application are situations where repulsion could be achieved between ordinary conducting and dielectric bodies in vacuum. The status of the field of Casimir repulsion with emphasis on recent developments will be surveyed. Here, stress will be placed on analytic developments, especially of Casimir-Polder (CP) interactions between anisotropically polarizable atoms, and CP interactions between anisotropic atoms and bodies that also exhibit anisotropy, either because of anisotropic constituents, or because of geometry. Repulsion occurs for wedge-shaped and cylindrical conductors, provided the geometry is sufficiently asymmetric, that is, either the wedge is sufficiently sharp or the atom is sufficiently far from the cylinder.

hep-th↗

arXiv2012

Electrostrictive counter-force on fluid microdroplet in short laser pulse

When a micrometer-sized fluid droplet is illuminated by a laser pulse, there is a fundamental distinction between two cases. If the pulse is short in comparison with the transit time for sound across the droplet, the disruptive optical Abraham-Minkowski radiation force is countered by electrostriction and the net stress is compressive. In contrast, if the pulse is long on this scale, electrostriction is cancelled by elastic pressure and the surviving term of the electromagnetic force, the Abraham-Minkowski force, is disruptive and deforms the droplet. Ultrashort laser pulses are routinely used in modern experiments, and impressive progress has moreover been made on laser manipulation of liquid surfaces in recent times, making a theory for combining the two pertinent. We analyze the electrostrictive contribution analytically and numerically for a spherical droplet.

physics.optics↗

arXiv2012

Casimir-Polder repulsion: Polarizable atoms, cylinders, spheres, and ellipsoids

Recently, the topic of Casimir repulsion has received a great deal of attention, largely because of the possibility of technological application. The general subject has a long history, going back to the self-repulsion of a conducting spherical shell and the repulsion between a perfect electric conductor and a perfect magnetic conductor. Recently it has been observed that repulsion can be achieved between ordinary conducting bodies, provided sufficient anisotropy is present. For example, an anisotropic polarizable atom can be repelled near an aperture in a conducting plate. Here we provide new examples of this effect, including the repulsion on such an atom moving on a trajectory nonintersecting a conducting cylinder; in contrast, such repulsion does not occur outside a sphere. Classically, repulsion does occur between a conducting ellipsoid placed in a uniform electric field and an electric dipole. The Casimir-Polder force between an anisotropic atom and an anisotropic dielectric semispace does not exhibit repulsion. The general systematics of repulsion are becoming clear.

hep-th↗

arXiv2012

Casimir Friction Force Between Polarizable Media

This work is a continuation of our recent series of papers on Casimir friction, for a pair of particles of low relative particle velocity. Each particle is modeled as a simple harmonic oscillator. Our basic method, as before, is the use of quantum mechanical statistical mechanics, involving the Kubo formula, at finite temperature. In this work we begin by analyzing the Casimir friction between two particles polarizable in all spatial directions, this being a generalization of our study in EPL 91, 60003 (2010), which was restricted to a pair of particles with longitudinal polarization only. For simplicity the particles are taken to interact via the electrostatic dipole-dipole interaction. Thereafter, we consider the Casimir friction between one particle and a dielectric half-space, and also the friction between two dielectric half-spaces. Finally, we consider general polarizabilities (beyond the simple one-oscillator form), and show how friction occurs at finite temperature when finite frequency regions of the imaginary parts of polarizabilities overlap.

quant-ph↗

arXiv2012

Retardation turns the van der Waals attraction into Casimir repulsion already at 3 nm

Casimir forces between surfaces immersed in bromobenzene have recently been measured by Munday et al. Attractive Casimir forces were found between gold surfaces. The forces were repulsive between gold and silica surfaces. We show the repulsion is due to retardation effects. The van der Waals interaction is attractive at all separations. The retardation driven repulsion sets in already at around 3 nm. To our knowledge retardation effects have never been found at such a small distance before. Retardation effects are usually associated with large distances.

quant-ph↗