1,363 research outputs found
Small distance expansion for radiative heat transfer between curved objects
We develop a small distance expansion for the radiative heat transfer between
gently curved objects, in terms of the ratio of distance to radius of
curvature. A gradient expansion allows us to go beyond the lowest order
proximity transfer approximation. The range of validity of such expansion
depends on temperature as well as material properties. Generally, the expansion
converges faster for the derivative of the transfer than for the transfer
itself, which we use by introducing a near-field adjusted plot. For the case of
a sphere and a plate, the logarithmic correction to the leading term has a very
small prefactor for all materials investigated.Comment: 5 pages, 3 figure
Casimir forces in the time domain II: Applications
Our preceding paper introduced a method to compute Casimir forces in
arbitrary geometries and for arbitrary materials that was based on a
finite-difference time-domain (FDTD) scheme. In this manuscript, we focus on
the efficient implementation of our method for geometries of practical interest
and extend our previous proof-of-concept algorithm in one dimension to problems
in two and three dimensions, introducing a number of new optimizations. We
consider Casimir piston-like problems with nonmonotonic and monotonic force
dependence on sidewall separation, both for previously solved geometries to
validate our method and also for new geometries involving magnetic sidewalls
and/or cylindrical pistons. We include realistic dielectric materials to
calculate the force between suspended silicon waveguides or on a suspended
membrane with periodic grooves, also demonstrating the application of PML
absorbing boundaries and/or periodic boundaries. In addition we apply this
method to a realizable three-dimensional system in which a silica sphere is
stably suspended in a fluid above an indented metallic substrate. More
generally, the method allows off-the-shelf FDTD software, already supporting a
wide variety of materials (including dielectric, magnetic, and even anisotropic
materials) and boundary conditions, to be exploited for the Casimir problem.Comment: 11 pages, 12 figures. Includes additional examples (dispersive
materials and fully three-dimensional systems
Casimir repulsion between metallic objects in vacuum
We give an example of a geometry in which two metallic objects in vacuum
experience a repulsive Casimir force. The geometry consists of an elongated
metal particle centered above a metal plate with a hole. We prove that this
geometry has a repulsive regime using a symmetry argument and confirm it with
numerical calculations for both perfect and realistic metals. The system does
not support stable levitation, as the particle is unstable to displacements
away from the symmetry axis.Comment: 4 pages, 4 figures; added references, replaced Fig.
Novel applications of Maxwell's equations to quantum and thermal phenomena
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 229-244).This thesis is concerned with the extension of Maxwell's equations to situations far removed from standard electromagnetism, in order to discover novel phenomena. We discuss our contributions to the efforts to describe quantum fluctuations, known as Casimir forces, in terms of classical electromagnetism. We prove that chirality in metamaterials can have no appreciable effect on the Casimir force, and design an alternative metamaterial in which the structure can have a strong effect on the Casimir force. We present a geometry that exhibits a repulsive Casimir force between metallic objects in vacuum, and describe our efforts to enhance this repulsive force using the numerical techniques that we and others developed. We then show how our techniques can be extended to study the physics of near-field radiative heat transfer, computing for the first time the exact heat transfer and power flux profiles between a plate and non-spherical objects. We find in particular that the heat flux profile is non-monotonic in separation from the cone tip. Finally, we demonstrate how techniques to compute photonic bandstructures in periodic systems can be extended to certain types of quasi-periodic structures, termed photonic-quasicrystals (PQCs).by Alexander P. McCauley.Ph.D
Achieving a Strongly Temperature-Dependent Casimir Effect
We propose a method of achieving large temperature sensitivity in the Casimir
force that involves measuring the stable separation between dielectric objects
immersed in fluid. We study the Casimir force between slabs and spheres using
realistic material models, and find large > 2nm/K variations in their stable
separations (hundreds of nanometers) near room temperature. In addition, we
analyze the effects of Brownian motion on suspended objects, and show that the
average separation is also sensitive to changes in temperature . Finally, this
approach also leads to rich qualitative phenomena, such as irreversible
transitions, from suspension to stiction, as the temperature is varied
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
Designing evanescent optical interactions to control the expression of Casimir forces in optomechanical structures
We propose an optomechanical structure consisting of a photonic-crystal
(holey) membrane suspended above a layered silicon-on-insulator substrate in
which resonant bonding/antibonding optical forces created by externally
incident light from above enable all-optical control and actuation of stiction
effects induced by the Casimir force. In this way, one can control how the
Casimir force is expressed in the mechanical dynamics of the membrane, not by
changing the Casimir force directly but by optically modifying the geometry and
counteracting the mechanical spring constant to bring the system in or out of
regimes where Casimir physics dominate. The same optical response (reflection
spectrum) of the membrane to the incident light can be exploited to accurately
measure the effects of the Casimir force on the equilibrium separation of the
membrane
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
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