868 research outputs found
Albedo and laser threshold of a diffusive Raman gain medium
The diffuse reflectance (albedo) and transmittance of a Raman random gain
medium are calculated via semi-analytic two-stream equations with
power-dependent coefficients. The results show good agreement with the
experimental data for barium nitrate powder. Both the Raman albedo AR and Raman
transmittance TR diverge at a critical gain gc, interpreted as the threshold
for diffusive Raman laser generation. However, the ratio TR/AR approaches a
finite limiting value dependent on particle scattering albedo v and scattering
asymmetry g. The dependence of the generation threshold on the scattering
parameters is analysed and the feedback effect of Fresnel reflection at the
gain boundaries evaluated. The addition of external mirrors, particularly at
the pumped surface, significantly reduces the threshold gain.Comment: 15 pages, 8 figure
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
Electrostatic theory for designing lossless negative permittivity metamaterials
In this Letter, we develop an electrostatic theory for designing bulk
composites with effective lossless negative permittivities. The theory and
associated design procedure are validated by comparing their predictions with
those of rigorous full-wave simulations. It is demonstrated that the excitation
of the Frohlich mode (the first-order surface mode) of the constitutive
nanoparticles plays a key role in achieving negative permittivities with
compensated losses.Comment: 9 pages, 2 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
Die Einstellungen in Deutschland vor, während und nach dem Zweiten Weltkrieg am Beispiel des Busches "Mein Jahrhundert" von Günter Grass
Käesolevas lõputöös analüüsiti, kuidas muutus inimeste maailmavaade Günter Grassi raamatu „Minu Aastasada“ („Mein Jahrhundert“) näitel. Selle raamatu publitseeris Günter Grass 1999 aastal ning see koosneb sajast jutustusest iga 20. sajandi aasta peale. Igal jutustusel on oma eraldiolev jutustaja, kas läbi Günter Grassi oma hääle omab. Lõputöö koostaja on valinud seitse jutustust, mis tema seisukohast kõige parem analüüsimiseks sobivad.
Töö koosneb kolmest osast. Esimemes osas antakse sissejuhatust Günter Grassi eluloomingusse ning käsitletakse teose „Minu Aastasada“ struktuuri ja jutustuslaadi.http://www.ester.ee/record=b5144047*es
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
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