285 research outputs found
Electromagnetic Energy, Absorption, and Casimir Forces. Inhomogeneous Dielectric Media
A general, exact formula is derived for the expectation value of the
electromagnetic energy density of an inhomogeneous absorbing and dispersive
dielectric medium in thermal equilibrium, assuming that the medium is well
approximated as a continuum. From this formula we obtain the formal expression
for the Casimir force density. Unlike most previous approaches to Casimir
effects in which absorption is either ignored or admitted implicitly through
the required analytic properties of the permittivity, we include dissipation
explicitly via the coupling of each dipole oscillator of the medium to a
reservoir of harmonic oscillators. We obtain the energy density and the Casimir
force density as a consequence of the van der Waals interactions of the
oscillators and also from Poynting's theorem.Comment: 13 pages, no figures. Updated version with generalization to finite
temperature and added example
Structural anisotropy and orientation-induced Casimir repulsion in fluids
In this work we theoretically consider the Casimir force between two periodic
arrays of nanowires (both in vacuum, and on a substrate separated by a fluid)
at separations comparable to the period. Specifically, we compute the
dependence of the exact Casimir force between the arrays under both lateral
translations and rotations. Although typically the force between such
structures is well-characterized by the Proximity Force Approximation (PFA), we
find that in the present case the microstructure modulates the force in a way
qualitatively inconsistent with PFA. We find instead that effective-medium
theory, in which the slabs are treated as homogeneous, anisotropic dielectrics,
gives a surprisingly accurate picture of the force, down to separations of half
the period. This includes a situation for identical, fluid-separated slabs in
which the exact force changes sign with the orientation of the wire arrays,
whereas PFA predicts attraction. We discuss the possibility of detecting these
effects in experiments, concluding that this effect is strong enough to make
detection possible in the near future.Comment: 12 pages, 9, figure. Published version with expanded discussio
Structural anisotropy and orientation-induced Casimir repulsion in fluids
In this work we theoretically consider the Casimir force between two periodic
arrays of nanowires (both in vacuum, and on a substrate separated by a fluid)
at separations comparable to the period. Specifically, we compute the
dependence of the exact Casimir force between the arrays under both lateral
translations and rotations. Although typically the force between such
structures is well-characterized by the Proximity Force Approximation (PFA), we
find that in the present case the microstructure modulates the force in a way
qualitatively inconsistent with PFA. We find instead that effective-medium
theory, in which the slabs are treated as homogeneous, anisotropic dielectrics,
gives a surprisingly accurate picture of the force, down to separations of half
the period. This includes a situation for identical, fluid-separated slabs in
which the exact force changes sign with the orientation of the wire arrays,
whereas PFA predicts attraction. We discuss the possibility of detecting these
effects in experiments, concluding that this effect is strong enough to make
detection possible in the near future.Comment: 12 pages, 9, figure. Published version with expanded discussio
Hertz potentials approach to the dynamical Casimir effect in cylindrical cavities of arbitrary section
We study the creation of photons in resonant cylindrical cavities with time
dependent length. The physical degrees of freedom of the electromagnetic field
are described using Hertz potentials. We describe the general formalism for
cavities with arbitrary section. Then we compute explicitly the number of TE
and TM motion-induced photons for cylindrical cavities with rectangular and
circular sections. We also discuss the creation of TEM photons in non-simply
connected cylindrical cavities.Comment: 13 pages, 3 figures, revtex
Disorder in quantum vacuum: Casimir-induced localization of matter waves
Disordered geometrical boundaries such as rough surfaces induce important
modifications to the mode spectrum of the electromagnetic quantum vacuum. In
analogy to Anderson localization of waves induced by a random potential, here
we show that the Casimir-Polder interaction between a cold atomic sample and a
rough surface also produces localization phenomena. These effects, that
represent a macroscopic manifestation of disorder in quantum vacuum, should be
observable with Bose-Einstein condensates expanding in proximity of rough
surfaces
Lateral Casimir-Polder force with corrugated surfaces
We derive the lateral Casimir-Polder force on a ground state atom on top of a
corrugated surface, up to first order in the corrugation amplitude. Our
calculation is based on the scattering approach, which takes into account
nonspecular reflections and polarization mixing for electromagnetic quantum
fluctuations impinging on real materials. We compare our first order exact
result with two commonly used approximation methods. We show that the proximity
force approximation (large corrugation wavelengths) overestimates the lateral
force, while the pairwise summation approach underestimates it due to the
non-additivity of dispersion forces. We argue that a frequency shift
measurement for the dipolar lateral oscillations of cold atoms could provide a
striking demonstration of nontrivial geometrical effects on the quantum vacuum.Comment: 12 pages, 6 figures, contribution to QFEXT07 proceeding
Measurement-induced Squeezing of a Bose-Einstein Condensate
We discuss the dynamics of a Bose-Einstein condensate during its
nondestructive imaging. A generalized Lindblad superoperator in the condensate
master equation is used to include the effect of the measurement. A continuous
imaging with a sufficiently high laser intensity progressively drives the
quantum state of the condensate into number squeezed states. Observable
consequences of such a measurement-induced squeezing are discussed.Comment: 4 pages, 2 figures, submitted to PR
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