5,521 research outputs found
Parity-Time Symmetry Breaking beyond One Dimension: The Role of Degeneracy
We consider the role of degeneracy in Parity-Time (PT) symmetry breaking for
non-hermitian wave equations beyond one dimension. We show that if the spectrum
is degenerate in the absence of T-breaking, and T is broken in a generic manner
(without preserving other discrete symmetries), then the standard PT-symmetry
breaking transition does not occur, meaning that the spectrum is complex even
for infinitesimal strength of gain and loss. However the realness of the entire
spectrum can be preserved over a finite interval if additional discrete
symmetries X are imposed when T is broken, if X decouple all degenerate modes.
When this is true only for a subset of the degenerate spectrum, there can be a
partial PT transition in which this subset remains real over a finite interval
of T-breaking. If the spectrum has odd-degeneracy, a fraction of the degenerate
spectrum can remain in the symmetric phase even without imposing additional
discrete symmetries, and they are analogous to dark states in atomic physics.
These results are illustrated by the example of different T-breaking
perturbations of a uniform dielectric disk and sphere, and a group theoretical
analysis is given in the disk case. Finally, we show that multimode coupling is
capable of restoring the T-symmetric phase at finite T-breaking. We also
analyze these questions when the parity operator is replaced by another spatial
symmetry operator and find that the behavior can be qualitatively different.Comment: 8 pages, 6 figure
Hidden Black: Coherent Enhancement of Absorption in Strongly-scattering Media
We show that a weakly absorbing, strongly scattering (white) medium can be
made very strongly absorbing at any frequency within its strong-scattering
bandwidth by optimizing the input electromagnetic field. For uniform
absorption, results from random matrix theory imply that the reflectivity of
the medium can be suppressed by a factor ~(l_a/lN^2), where N is the number of
incident channels and l,l_a are the elastic and absorption mean free paths
respectively. It is thus possible to increase absorption from a few percent to
> 99%. For a localized weak absorber buried in a non-absorbing scattering
medium, we find a large but bounded enhancement.Comment: 4 pages, 4 figure
PT-symmetry breaking and laser-absorber modes in optical scattering systems
Using a scattering matrix formalism, we derive the general scattering
properties of optical structures that are symmetric under a combination of
parity and time-reversal (PT). We demonstrate the existence of a transition
beween PT-symmetric scattering eigenstates, which are norm-preserving, and
symmetry-broken pairs of eigenstates exhibiting net amplification and loss. The
system proposed by Longhi, which can act simultaneously as a laser and coherent
perfect absorber, occurs at discrete points in the broken symmetry phase, when
a pole and zero of the S-matrix coincide.Comment: 4 pages, 4 figure
Noise Properties of Coherent Perfect Absorbers and Critically-coupled Resonators
The performance of a coherent perfect absorber (time-reversed laser) is
limited by quantum and thermal noise. At zero temperature, the quantum shot
noise dominates the signal for frequencies close to the resonance frequency,
and both vanish exactly at the resonance frequency. We compute the sensitivity
of the absorbing cavity as a background-free detector, limited by finite signal
or detector bandwidth.Comment: 6 pages, 3 figure
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