8,866 research outputs found
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
Anomalous Nonlocal Resistance and Spin-charge Conversion Mechanisms in Two-Dimensional Metals
We uncover two anomalous features in the nonlocal transport behavior of
two-dimensional metallic materials with spin-orbit coupling. Firstly, the
nonlocal resistance can have negative values and oscillate with distance, even
in the absence of a magnetic field. Secondly, the oscillations of the nonlocal
resistance under an applied in-plane magnetic field (Hanle effect) can be
asymmetric under field reversal. Both features are produced by direct
magnetoelectric coupling, which is possible in materials with broken inversion
symmetry but was not included in previous spin diffusion theories of nonlocal
transport. These effects can be used to identify the relative contributions of
different spin-charge conversion mechanisms. They should be observable in
adatom-functionalized graphene, and may provide the reason for discrepancies in
recent nonlocal transport experiments on graphene.Comment: 5 pages, 3 figures, and Supplementary Materials, to appear in Phys.
Rev. Let
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
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
General linewidth formula for steady-state multimode lasing in arbitrary cavities
A formula for the laser linewidth of arbitrary cavities in the multimode
non-linear regime is derived from a scattering analysis of the solutions to
semiclassical laser theory. The theory generalizes previous treatments of the
effects of gain and openness described by the Petermann factor. The linewidth
is expressed using quantities based on the non-linear scattering matrix, which
can be computed from steady-state ab initio laser theory; unlike previous
treatments, no passive cavity or phenomenological parameters are involved. We
find that low cavity quality factor, combined with significant dielectric
dispersion, can cause substantial deviations from the Schawlow-Townes-Petermann
theory.Comment: 5 pages, 2 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
Coherent perfect absorption and reflection in slow-light waveguides
We identify a family of unusual slow-light modes occurring in lossy
multi-mode grating waveguides, for which either the forward or backward mode
components, or both, become degenerate. In the fully-degenerate case, by
varying the wave amplitudes in a uniform input waveguide, one can modulate
between coherent perfect absorption (zero reflection) and perfect reflection.
The perfectly-absorbed wave has anomalously short absorption length, scaling as
the inverse 1/3 power of the absorptivity
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