1,447 research outputs found
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
Conservation relations and anisotropic transmission resonances in one-dimensional PT-symmetric photonic heterostructures
We analyze the optical properties of one-dimensional (1D) PT-symmetric
structures of arbitrary complexity. These structures violate normal unitarity
(photon flux conservation) but are shown to satisfy generalized unitarity
relations, which relate the elements of the scattering matrix and lead to a
conservation relation in terms of the transmittance and (left and right)
reflectances. One implication of this relation is that there exist anisotropic
transmission resonances in PT-symmetric systems, frequencies at which there is
unit transmission and zero reflection, but only for waves incident from a
single side. The spatial profile of these transmission resonances is symmetric,
and they can occur even at PT-symmetry breaking points. The general
conservation relations can be utilized as an experimental signature of the
presence of PT-symmetry and of PT-symmetry breaking transitions. The uniqueness
of PT-symmetry breaking transitions of the scattering matrix is briefly
discussed by comparing to the corresponding non-Hermitian Hamiltonians.Comment: 10 pages, 10 figure
Coherent Perfect Absorbers: Time-reversed Lasers
We show that an arbitrary body or aggregate can be made perfectly absorbing
at discrete frequencies if a precise amount of dissipation is added under
specific conditions of coherent monochromatic illumination. This effect arises
from the interaction of optical absorption and wave interference, and
corresponds to moving a zero of the elastic S-matrix onto the real wavevector
axis. It is thus the time-reversed process of lasing at threshold. The effect
is demonstrated in a simple Si slab geometry illuminated in the 500-900 nm
range. Coherent perfect absorbers are novel linear optical elements, absorptive
interferometers, which may be useful for controlled optical energy transfer.Comment: 4 pages, 4 figure
Can winds driven by active galactic nuclei account for the extragalactic gamma-ray and neutrino backgrounds?
Various observations are revealing the widespread occurrence of fast and
powerful winds in active galactic nuclei (AGNs) that are distinct from
relativistic jets, likely launched from accretion disks and interacting
strongly with the gas of their host galaxies. During the interaction, strong
shocks are expected to form that can accelerate non-thermal particles to high
energies. Such winds have been suggested to be responsible for a large fraction
of the observed extragalactic gamma-ray background (EGB) in the GeV-TeV range
and the diffuse neutrino background in the PeV range, via the decay of neutral
and charged pions generated in inelastic collisions between protons
accelerated by the forward shock and the ambient gas. However, previous studies
did not properly account for processes such as adiabatic losses that may reduce
the gamma-ray and neutrino fluxes significantly. We evaluate the production of
gamma-rays and neutrinos by AGN-driven winds in some detail by modeling their
hydrodynamic and thermal evolution, including the effects of their
two-temperature structure. We find that they can only account for less than
% of the EGB flux, as otherwise the model would violate the
independent upper limit derived from the diffuse isotropic gamma-ray
background. If the neutrino spectral index is steep with , a
severe tension with the isotropic gamma-ray background would arise as long as
the winds contribute more than % of the IceCube neutrino flux in the
TeV range. Nevertheless, at energies ~TeV, we find that the
IceCube neutrino flux may still be accountable by AGN-driven winds if the
spectral index is as small as . The detectability of
gamma-ray point sources also provides important constraints on such scenarios.Comment: 17 pages, 10 figures, to appear in Ap
Steady-State Ab Initio Laser Theory for N-level Lasers
We show that Steady-state Ab initio Laser Theory (SALT) can be applied to
find the stationary multimode lasing properties of an N-level laser. This is
achieved by mapping the N-level rate equations to an effective two-level model
of the type solved by the SALT algorithm. This mapping yields excellent
agreement with more computationally demanding N-level time domain solutions for
the steady state
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