55,844 research outputs found
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
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
Antisymmetric PT-photonic structures with balanced positive and negative index materials
We propose a new class of synthetic optical materials in which the refractive
index satisfies n(-\bx)=-n^*(\bx). We term such systems antisymmetric
parity-time (APT) structures. Unlike PT-symmetric systems which require
balanced gain and loss, i.e. n(-\bx)=n^*(\bx), APT systems consist of
balanced positive and negative index materials. Despite the seemingly
PT-symmetric optical potential V(\bx)\equiv n(\bx)^2\omega^2/c^2, APT systems
are not invariant under combined PT operations due to the discontinuity of the
spatial derivative of the wavefunction. We show that APT systems can display
intriguing properties such as spontaneous phase transition of the scattering
matrix, bidirectional invisibility, and a continuous lasing spectrum.Comment: 5 pages, 4 figure
Black Holes, Entropy Bound and Causality Violation
The gravity/gauge theory duality has provided us a way of studying QCD at
short distances from straightforward calculations in classical general
relativity. Among numerous results obtained so far, one of the most striking is
the universality of the ratio of the shear viscosity to the entropy density.
For all gauge theories with Einstein gravity dual, this ratio is \eta/s=1/4\pi.
However, in general higher-curvature gravity theories, including two concrete
models under discussion - the Gauss-Bonnet gravity and the (Riemann)^2 gravity
- the ratio \eta/s can be smaller than 1/4\pi (thus violating the conjecture
bound), equal to 1/4\pi or even larger than 1/4\pi. As we probe spacetime at
shorter distances, there arises an internal inconsistency in the theory, such
as a violation of microcausality, which is correlated with a classical limit on
black hole entropy.Comment: 8 pages, no figures; Invited contribution to appear in the
Proceedings of the 75 Years since Solvay, Singapore, Nov 2008, (World
Scientific, Singapore, 2009
Networking, Resource Acquisition, and the Performance of Small and Medium-Sized Enterprises: An Empirical Study of Three Major Cities in China
Previously, resource-based view (RBV) research has focused on the characteristics of resources, paying less attention to the relationship between resource acquisition and the acquisition method. In addition, entrepreneurship research has focused a lot on the firm’s entrepreneurial network to explain performance. This network is critical not only to resource acquisition but also to overall firm performance. The results of a study of small and medium-sized firms in three major Chinese cities support these notions. The results differ when dividing the sample into two groups (young vs. old).resource acquisition, networking, performance, SMEs, entrepreneurship
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