565 research outputs found
Vertical cavity surface emitting laser action of an all monolithic ZnO-based microcavity
We report on room temperature laser action of an all monolithic ZnO-based
vertical cavity surface emitting laser (VCSEL) under optical pumping. The VCSEL
structure consists of a 2{\lambda} microcavity containing 8
ZnO/Zn(0.92)Mg(0.08)O quantum wells embedded in epitaxially grown
Zn(0.92)Mg(0.08)O/Zn(0.65)Mg(0.35)O distributed Bragg reflectors (DBRs). As a
prerequisite, design and growth of high reflectivity DBRs based on ZnO and
(Zn,Mg)O for optical devices operating in the ultraviolet and blue-green
spectral range are discussed.Comment: Copyright (2011) American Institute of Physics. This article may be
downloaded for personal use only. Any other use requires prior permission of
the author and the American Institute of Physics. The following article
appeared in Appl. Phys. Lett. 98, 011101 (2011) and may be found at
http://apl.aip.org/resource/1/applab/v98/i1/p011101_s
Poynting's theorem and energy conservation in the propagation of light in bounded media
Starting from the Maxwell-Lorentz equations, Poynting's theorem is
reconsidered. The energy flux vector is introduced as S_e=(E x B)/mu_0 instead
of E x H, because only by this choice the energy dissipation can be related to
the balance of the kinetic energy of the matter subsystem. Conservation of the
total energy as the sum of kinetic and electromagnetic energy follows. In our
discussion, media and their microscopic nature are represented exactly by their
susceptibility functions, which do not necessarily have to be known. On this
footing, it can be shown that energy conservation in the propagation of light
through bounded media is ensured by Maxwell's boundary conditions alone, even
for some frequently used approximations. This is demonstrated for approaches
using additional boundary conditions and the dielectric approximation in
detail, the latter of which suspected to violate energy conservation for
decades.Comment: 5 pages, RevTeX4, changes: complete rewrit
A nanometer-scale optical electrometer
Self-assembled semiconductor quantum dots show remarkable optical and spin
coherence properties, which have lead to a concerted research effort examining
their potential as a quantum bit for quantum information science1-6. Here, we
present an alternative application for such devices, exploiting recent
achievements of charge occupation control and the spectral tunability of the
optical emission of quantum dots by electric fields7 to demonstrate
high-sensitivity electric field measurement. In contrast to existing
nanometer-scale electric field sensors, such as single electron transistors8-11
and mechanical resonators12,13, our approach relies on homodyning light
resonantly Rayleigh scattered from a quantum dot transition with the excitation
laser and phase sensitive lock-in detection. This offers both static and
transient field detection ability with high bandwidth operation and near unity
quantum efficiency. Our theoretical estimation of the static field sensitivity
for typical parameters, 0.5 V/m/ \surd Hz, compares favorably to the
theoretical limit for single electron transistor-based electrometers. The
sensitivity level of 5 V/m/ \surd Hz we report in this work, which corresponds
to 6.4 * 10-6 e/ \surd Hz at a distance of 12 nm, is worse than this
theoretical estimate, yet higher than any other result attained at 4.2 K or
higher operation temperature
Nonequilibrium nuclear-electron spin dynamics in semiconductor quantum dots
We study the spin dynamics in charged quantum dots in the situation where the
resident electron is coupled to only about 200 nuclear spins and where the
electron spin splitting induced by the Overhauser field does not exceed
markedly the spectral broadening. The formation of a dynamical nuclear
polarization as well as its subsequent decay by the dipole-dipole interaction
is directly resolved in time. Because not limited by intrinsic nonlinearities,
almost complete nuclear polarization is achieved, even at elevated
temperatures. The data suggest a nonequilibrium mode of nuclear polarization,
distinctly different from the spin temperature concept exploited on bulk
semiconductorsComment: 5 pages, 4 figure
A random laser as a dynamical network
The mode dynamics of a random laser is investigated in experiment and theory. The laser consists of a ZnCdO/ZnO multiple quantum well with air-holes that provide the necessary feedback. Time-resolved measurements reveal multi-mode spectra with individually developing features but no variation from shot to shot. These findings are qualitatively reproduced with a model that exploits the specifics of a dilute system of weak scatterers and can be interpreted in terms of a lasing network. Introducing the phase-sensitive node coherence reveals new aspects of the self-organization of the laser field. Lasing is carried by connected links between a subset of scatterers, the fields on which are oscillating coherently in phase. In addition, perturbing feedback with possibly unfitting phases from frustrated other scatterers is suppressed by destructive superposition. We believe that our findings are representative at least for weakly scattering random lasers. A generalization to random laser with dense and strong scatterers seems to be possible when using a more complex scattering theory for this case.Peer Reviewe
Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots
We report strong heavy hole-light mixing in GaAs quantum dots grown by
droplet epitaxy. Using the neutral and charged exciton emission as a monitor we
observe the direct consequence of quantum dot symmetry reduction in this strain
free system. By fitting the polar diagram of the emission with simple
analytical expressions obtained from kp theory we are able to extract
the mixing that arises from the heavy-light hole coupling due to the
geometrical asymmetry of the quantum dot.Comment: 4 pages, 2 figure
КОНЦЕПЦІЯ ЕКОНОМІЧНОЇ БЕЗПЕКИ НА ПІДСТАВІ ВИКОРИСТАННЯ ПРИНЦИПІВ МАТЕМАТИКИ ГАРМОНІЇ
Розглядаються теоретичні питання економічної безпеки держави. Досліджується новий концепту-альний підхід, що пропонує використовувати фун-даментальні математичні константи при дослідженні макроекономічних процесів на прикладі податку на додану вартість.; The article is dedicated to theoretical questions of economical security of state. A new conceptual approach based on usage of basic mathematical constants in investigation of macroeconomics processes (on the example of VAT) is proposed
Coherently tunable third-order nonlinearity in a nanojunction
A possibility of tuning the phase of the third-order Kerr-type nonlinear
susceptibility in a system consisting of two interacting metal nanospheres and
a nonlinearly polarizable molecule is investigated theoretically and
numerically. It is shown that by varying the relative inter-sphere separation,
it is possible to tune the phase of the effective nonlinear susceptibility
\chi^{(3)}(\omega;\omega,\omega,-\omega)2\pi$.Comment: 10 pages 5 figure
Cross-sectional TEM preparation of hybrid inorganic/organic materials systems by ultramicrotomy
Preparation of hybrid inorganic-organic systems (HIOS) for transmission electron microscopy (TEM) in cross sectional view is the key for understanding the interfacial structure. Strikingly different materials properties like hardness, cleavability and heat sensitivity limit the number of applicable preparation strategies. Successful preparation of a HIOS system combining ZnO and para-sexiphenyl (6P) is realized by ultramicrotomy. It is shown that the alignment of the cutting plane with respect to the (0001) cleavage plane of ZnO plays a decisive role for successful preparation of extended TEM lamellae and the preservation of the HIOS structure. In particular, for (0001) oriented ZnO substrates the optimum cut direction is parallel to the HIOS interface. In cross-sectional high-resolution TEM images (100) lattice planes of 6P are observed proving the appropriate preparation strategy.Peer Reviewe
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