2,062 research outputs found
Indifferent electromagnetic modes: bound states and topology
At zero energy the Dirac equation has interesting behaviour. The asymmetry in
the number of spin up and spin down modes is determined by the topology of both
space and the gauge field in which the system sits. An analogous phenomenon
also occurs in electromagnetism. Writing Maxwell's equations in a Dirac-like
form, we identify cases where a material parameter plays the role of energy. At
zero energy we thus find electromagnetic modes that are indifferent to local
changes in the material parameters, depending only on their asymptotic values
at infinity. We give several examples, and show that this theory has
implications for non-Hermitian media, where it can be used to construct
permittivity profiles that are either reflectionless, or act as coherent
perfect absorbers, or lasers.Comment: 10 pages, 6 figure
Unidirectional wave propagation in media with complex principal axes
In an anisotropic medium, the refractive index depends on the direction of
propagation. Zero index in a fixed direction implies a stretching of the wave
to uniformity along that axis, reducing the effective number of dimensions by
one. Here we investigate two dimensional gyrotropic media where the refractive
index is zero in a complex valued direction, finding that the wave becomes an
analytic function of a single complex variable z. For simply connected media
this analyticity implies unidirectional propagation of electromagnetic waves,
similar to the edge states that occur in photonic 'topological insulators'. For
a medium containing holes the propagation is no longer unidirectional. We
illustrate the sensitivity of the field to the topology of the space using an
exactly solvable example. To conclude we provide a generalization of
transformation optics where a complex coordinate transformations can be used to
relate ordinary anisotropic media to the recently highlighted gyrotropic ones
supporting one-way edge states.Comment: 21 pages, 4 figure
The cutoff-dependence of the Casimir force within an inhomogeneous medium
We consider the ground state energy of the electromagnetic field in a piston
geometry. In the idealised case, where the piston and the walls of the chamber
are taken as perfect mirrors, the Casimir pressure on the piston is finite and
independent of the small scale physics of the media that compose the mirrors;
the Casimir-energy of the system can be regularised and is cutoff-independent.
Yet we find that, when the body of the piston is filled with an inhomogeneous
dielectric medium, the Casimir energy is cutoff-dependent, and the value of the
pressure is thus inextricably dependent on the detailed behaviour of the mirror
and the medium at large wave-vectors. This result is inconsistent with recent
proposals for regularising Casimir forces in inhomogeneous media.Comment: 6 pages, 2 figure
Scattering of accelerated wave packets
Wave-packet scattering from a stationary potential is significantly modified
when the wave-packet is subject to an external time-dependent force during the
interaction. In the semiclassical limit, wave--packet motion is simply
described by Newtonian equations and the external force can, for example,
cancel the potential force making a potential barrier transparent. Here we
consider wave-packet scattering from reflectionless potentials, where in
general the potential becomes reflective when probed by an accelerated
wave-packet. In the particular case of the recently-introduced class of complex
Kramers-Kronig potentials we show that a broad class of time dependent forces
can be applied without inducing any scattering, while there is a breakdown of
the reflectionless property when there is a broadband distribution of initial
particle momentum, involving both positive and negative components.Comment: 13 pages, 4 figures, to appear in Phys. Rev.
One-loop renormalisation for the second moment of GPDs with Wilson fermions
We calculate the non-forward quark matrix elements for operators with two
covariant derivatives in one-loop lattice perturbation theory using Wilson
fermions. These matrix elements are needed in the renormalisation of the second
moment of generalised parton distributions measured in lattice QCD. For some
commonly used representations of the hypercubic group we determine the sets of
all mixing operators and find the matrices of mixing and renormalisation
factors.Comment: Lattice 2004(improved), 3 pages, LaTeX style file, espcrc2.sty
include
Localized exciton-polariton modes in dye-doped nanospheres: a quantum approach
We model a dye-doped polymeric nanosphere as an ensemble of quantum emitters
and use it to investigate the localized exciton-polaritons supported by such a
nanosphere. By determining the time evolution of the density matrix of the
collective system, we explore how an incident laser field may cause transient
optical field enhancement close to the surface of such nanoparticles. Our
results provide further evidence that excitonic materials can be used to good
effect in nanophotonics.Comment: 16 pages, 4 figure
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