Casimir experiments showing saturation effects

We address several different Casimir experiments where theory and experiment disagree. First out is the classical Casimir force measurement between two metal half spaces; here both in the form of the torsion pendulum experiment by Lamoreaux and in the form of the Casimir pressure measurement between a gold sphere and a gold plate as performed by Decca et al.; theory predicts a large negative thermal correction, absent in the high precision experiments. The third experiment is the measurement of the Casimir force between a metal plate and a laser irradiated semiconductor membrane as performed by Chen et al.; the change in force with laser intensity is larger than predicted by theory. The fourth experiment is the measurement of the Casimir force between an atom and a wall in the form of the measurement by Obrecht et al. of the change in oscillation frequency of a 87 Rb Bose-Einstein condensate trapped to a fused silica wall; the change is smaller than predicted by theory. We show that saturation effects can explain the discrepancies between theory and experiment observed in all these cases.Comment: 10 pages, 11 figure

Numerical study of the effect of structure and geometry on van der Waals forces

We use multipolar expansions to find the force on a gold coated sphere above a gold substrate; we study both an empty gold shell and a gold coated polystyrene sphere. We find four characteristic separation ranges. In the first region, which for the empty gold shell occurs for distances, d, smaller than the thickness of the coating, the result agrees with that on a solid gold sphere and varies as d^(-2); for larger separations there is a region where the force behaves as if the coating is strictly two dimensional and varies as d^(-5/2); in the third region the dependence is more unspecific; in the forth region when d is larger than the radius, the force varies as d^(-4). For homogeneous objects of more general shapes we introduce a numerical method based on the solution of an integral equation for the electric field over a system of objects with arbitrary shapes. We study the effect of shape and orientation on the van der Waals interaction between an object and a substrate and between two objects.Comment: 8 pages, presented in the QFEXT07 conference, submitted to Journal of Physics

Saturation effects in experiments on the thermal Casimir effect

We address three different problematic Casimir experiments in this work. The first is the classical Casimir force measured between two metal half spaces; here in the form of the Casimir pressure measurement between a gold sphere and a gold plate as performed by Decca et al. [Phys. Rev. D 75, 077101 (2007)]; theory predicts a large negative thermal correction, absent in the high precision experiment. The second experiment is the measurement of the Casimir force between a metal plate and a laser irradiated semiconductor membrane as performed by Chen et al. [Phys. Rev. B 76, 035338 (2007)]; the change in force with laser intensity is larger than predicted by theory. The third experiment is the measurement of the Casimir force between an atom and a wall in the form of the measurement by Obrecht et al. [Phys. Rev. Lett. 98 063201 (2007)] of the change in oscillation frequency of a Rb Bose-Einstein condensate trapped to a fused silica wall; the change is smaller than predicted by theory. We show that saturation effects can explain the discrepancies between theory and experiment observed in all these cases.Comment: 4 pages, 4 figure

Casimir interactions in graphene systems

The non-retarded Casimir interaction (van der Waals interaction) between two free standing graphene sheets as well as between a graphene sheet and a substrate is determined. An exact analytical expression is given for the dielectric function of graphene along the imaginary frequency axis within the random phase approximation for arbitrary frequency, wave vector, and doping.Comment: 4 pages, 4 figure

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

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.Comment: 5 pages, 6 figure

New features of the thermal Casimir force at small separations

The difference of the thermal Casimir forces at different temperatures between real metals is shown to increase with a decrease of the separation distance. This opens new opportunities for the demonstration of the thermal dependence of the Casimir force. Both configurations of two parallel plates and a sphere above a plate are considered. Different approaches to the theoretical description of the thermal Casimir force are shown to lead to different measurable predictions.Comment: 5 pages, 3 figures, to appear in Phys. Rev. Let

Repulsive van der Waals forces due to hydrogen exposure on bilayer Graphene

We consider the effect of atomic hydrogen exposure to a system of two undoped sheets of graphene grown near a silica surface (the first adsorbed to the surface and the second freestanding near the surface). In the absence of atomic hydrogen the van der Waals force between the sheets is attractive at all separations causing the sheets to come closer together. However, with addition of atomic hydrogen between the sheets the long range van der Waals interaction turns repulsive at a critical concentration. The underlying triple layer structure (SiO2 -Atomic Hydrogen Gas -Air) gives rise to a long range repulsion that at large enough separations dominates over the more rapidly decaying attraction between the two-dimensional undoped graphene sheets (and between the outer graphene sheet and SiO2). This may be an avenue to tune the separation between two graphene sheets with the gas concentration. Doping of the graphene layers increases the attractive part of the interaction and hence reduces the net repulsive interaction

Gravitation as a Casimir interaction

Gravitation is considered to be one of the four fundamental interactions in nature. However, one has so far failed to observe the graviton, the quantum particle that is believed to transmit the gravitational force at a distance - the analogue to the photon in electromagnetism. Maybe it is now time to re-evaluate the status of the gravitation as a fundamental interaction. Here, we propose a completely new interpretation of gravitation. In this description the gravitational force is no longer a fundamental force. It is an induced force, a dispersion force, and the analogue to the Casimir force in electromagnetism. The fundamental force is in our description a force between particles with a parabolic interaction potential. In our model the nucleons are made up from these particles. We find the retarded dispersion force between these composite particles has the correct distance dependence, 1/r. If this interpretation is correct it has a broad range of implications. Our view on the fundamental concept mass is altered; our view on the expansion of the Universe may change

Circular Optical Nanoantennas: An Analytical Theory

An entirely analytical theory is provided for describing the resonance properties of optical nanoantennas made of a stack of homogeneous discs, i.e. circular patch nanoantennas. It consists in analytically calculating the phase accumulation of surface plasmon polaritons across the resonator and an additional contribution from the complex reflection coefficient at the antenna termination. This makes the theory self-contained with no need for fitting parameters. The very antenna resonances are then explained by a simple Fabry-Perot resonator model. Predictions are compared to rigorous simulations and show excellent agreement. Using this analytical model, circular antennas can be tuned by varying the composition of the stack