427 research outputs found
Nonlinearly-PT-symmetric systems: spontaneous symmetry breaking and transmission resonances
We introduce a class of PT-symmetric systems which include mutually matched
nonlinear loss and gain (inother words, a class of PT-invariant Hamiltonians in
which both the harmonic and anharmonic parts are non-Hermitian). For a basic
system in the form of a dimer, symmetric and asymmetric eigenstates, including
multistable ones, are found analytically. We demonstrate that, if coupled to a
linear chain, such a nonlinear PT-symmetric dimer generates new types of
nonlinear resonances, with the completely suppressed or greatly amplified
transmission, as well as a regime similar to the electromagnetically-induced
transparency (EIT). The implementation of the systems is possible in various
media admitting controllable linear and nonlinear amplification of waves.Comment: 4 pages, 4 figure
Active metamaterials: sign of refraction index and gain-assisted dispersion management
We derive an approach to define the causal direction of the wavevector of
modes in optical metamaterials, which in turn, determines signs of refractive
index and impedance as a function of {\it real and imaginary} parts of
dielectric permittivity and magnetic permeability. We use the developed
technique to demonstrate that the interplay between resonant response of
constituents of metamaterials can be used to achieve efficient dispersion
management. Finally we demonstrate broadband dispersion-less index and
impedance matching in active nanowire-based negative index materials. Our work
opens new practical applications of negative index composites for broadband
lensing, imaging, and pulse-routing
Bulk photonic metamaterial with hyperbolic dispersion
In this work, we demonstrate a self-standing bulk three-dimensional
metamaterial based on the network of silver nanowires in an alumina membrane.
This constitutes an anisotropic effective medium with hyperbolic dispersion,
which can be used in sub-diffraction imaging or optical cloaks. Highly
anisotropic dielectric constants of the material range from positive to
negative, and the transmitted laser beam shifts both toward the normal to the
surface, as in regular dielectrics, and off the normal, as in anisotropic
dielectrics with the refraction index smaller than one. The designed photonic
metamaterial is the thickest reported in the literature, both in terms of its
physical size 1cm x 1cm x 51 mm, and the number of vacuum wavelengths, N=61 at
l=0.84 mm.Comment: 6 pages, 4 figur
Disorder-induced cavities, resonances, and lasing in randomly-layered media
We study, theoretically and experimentally, disorder-induced resonances in
randomly-layered samples,and develop an algorithm for the detection and
characterization of the effective cavities that give rise to these resonances.
This algorithm enables us to find the eigen-frequencies and pinpoint the
locations of the resonant cavities that appear in individual realizations of
random samples, for arbitrary distributions of the widths and refractive
indices of the layers. Each cavity is formed in a region whose size is a few
localization lengths. Its eigen-frequency is independent of the location inside
the sample, and does not change if the total length of the sample is increased
by, for example, adding more scatterers on the sides. We show that the total
number of cavities, , and resonances, , per
unit frequency interval is uniquely determined by the size of the disordered
system and is independent of the strength of the disorder. In an active,
amplifying medium, part of the cavities may host lasing modes whose number is
less than . The ensemble of lasing cavities behaves as
distributed feedback lasers, provided that the gain of the medium exceeds the
lasing threshold, which is specific for each cavity. We present the results of
experiments carried out with single-mode optical fibers with gain and
randomly-located resonant Bragg reflectors (periodic gratings). When the fiber
was illuminated by a pumping laser with an intensity high enough to overcome
the lasing threshold, the resonances revealed themselves by peaks in the
emission spectrum. Our experimental results are in a good agreement with the
theory presented here.Comment: minor correction
Topological Transitions in Metamaterials
The ideas of mathematical topology play an important role in many aspects of
modern physics - from phase transitions to field theory to nonlinear dynamics
(Nakahara M (2003) in Geometry, Topology and Physics, ed Brewer DF (IOP
Publishing Ltd, Bristol and Philadelphia), Monastryskiy M (1987) in Riemann
Topology and Physics, (Birkhauser Verlag AG)). An important example of this is
the Lifshitz transition (Lifshitz IM (1960) Anomalies of electron
characteristics of a metal in the high-pressure region, Sov Phys JETP 11:
1130-1135), where the transformation of the Fermi surface of a metal from a
closed to an open geometry (due to e.g. external pressure) leads to a dramatic
effect on the electron magneto-transport (Kosevich AM (2004) Topology and
solid-state physics. Low Temp Phys 30: 97-118). Here, we present the optical
equivalent of the Lifshitz transition in strongly anisotropic metamaterials.
When one of the components of the dielectric permittivity tensor of such a
composite changes sign, the corresponding iso-frequency surface transforms from
an ellipsoid to a hyperboloid. Since the photonic density of states can be
related to the volume enclosed by the iso-frequency surface, such a topological
transition in a metamaterial leads to a dramatic change in the photonic density
of states, with a resulting effect on every single physical parameter related
to the metamaterial - from thermodynamic quantities such as its equilibrium
electromagnetic energy to the nonlinear optical response to
quantum-electrodynamic effects such as spontaneous emission. In the present
paper, we demonstrate the modification of spontaneous light emission from
quantum dots placed near the surface of the metamaterial undergoing the
topological Lifshitz transition, and present the theoretical description of the
effect
Effects of Spatially Nonuniform Gain on Lasing Modes in Weakly Scattering Random Systems
A study on the effects of optical gain nonuniformly distributed in
one-dimensional random systems is presented. It is demonstrated numerically
that even without gain saturation and mode competition, the spatial
nonuniformity of gain can cause dramatic and complicated changes to lasing
modes. Lasing modes are decomposed in terms of the quasi modes of the passive
system to monitor the changes. As the gain distribution changes gradually from
uniform to nonuniform, the amount of mode mixing increases. Furthermore, we
investigate new lasing modes created by nonuniform gain distributions. We find
that new lasing modes may disappear together with existing lasing modes,
thereby causing fluctuations in the local density of lasing states.Comment: 26 pages, 10 figures (quality reduced for arXiv
Stimulated emission of surface plasmon polaritons
We have observed laser-like emission of surface plasmon polaritons (SPPs)
decoupled to the glass prism in an attenuated total reflection setup. SPPs were
excited by optically pumped molecules in a polymeric film deposited on the top
of the silver film. Stimulated emission was characterized by a distinct
threshold in the input-output dependence and narrowing of the emission
spectrum. The observed stimulated emission and corresponding to it compensation
of the metallic absorption loss by gain enables many applications of
metamaterials and nanoplasmonic devices.Comment: 8 pages; 3 figure
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