66 research outputs found
An EPTAS for Scheduling on Unrelated Machines of Few Different Types
In the classical problem of scheduling on unrelated parallel machines, a set
of jobs has to be assigned to a set of machines. The jobs have a processing
time depending on the machine and the goal is to minimize the makespan, that is
the maximum machine load. It is well known that this problem is NP-hard and
does not allow polynomial time approximation algorithms with approximation
guarantees smaller than unless PNP. We consider the case that there
are only a constant number of machine types. Two machines have the same
type if all jobs have the same processing time for them. This variant of the
problem is strongly NP-hard already for . We present an efficient
polynomial time approximation scheme (EPTAS) for the problem, that is, for any
an assignment with makespan of length at most
times the optimum can be found in polynomial time in the
input length and the exponent is independent of . In particular
we achieve a running time of , where
denotes the input length. Furthermore, we study three other problem
variants and present an EPTAS for each of them: The Santa Claus problem, where
the minimum machine load has to be maximized; the case of scheduling on
unrelated parallel machines with a constant number of uniform types, where
machines of the same type behave like uniformly related machines; and the
multidimensional vector scheduling variant of the problem where both the
dimension and the number of machine types are constant. For the Santa Claus
problem we achieve the same running time. The results are achieved, using mixed
integer linear programming and rounding techniques
Polarization Control of the Non-linear Emission on Semiconductor Microcavities
The degree of circular polarization () of the non-linear emission in
semiconductor microcavities is controlled by changing the exciton-cavity
detuning. The polariton relaxation towards \textbf{K} cavity-like
states is governed by final-state stimulated scattering. The helicity of the
emission is selected due to the lifting of the degeneracy of the spin
levels at \textbf{K} . At short times after a pulsed excitation
reaches very large values, either positive or negative, as a result of
stimulated scattering to the spin level of lowest energy ( spin for
positive/negative detuning).Comment: 8 pages, 3 eps figures, RevTeX, Physical Review Letters (accepted
CdSe-single-nanoparticle based active tips for near-field optical microscopy
We present a method to realize active optical tips for use in near-field
optics that can operate at room temperature. A metal-coated optical tip is
covered with a thin polymer layer stained with CdSe nanocrystals or nanorods at
low density. The time analysis of the emission rate and emission spectra of the
active tips reveal that a very small number of particles - possibly down to
only one - can be made active at the tip apex. This opens the way to near-field
optics with a single inorganic nanoparticle as a light source
Multifrequency broadband tapered plasmonic nanoantennas
We suggest a novel multifrequency broadband plasmonic Yagi-Uda-type
nanoantenna equipped with an array of tapered directors. Each director can be
used for the excitation of the antenna by nanoemitters matched spectrally with
the director resonant frequency and placed in the director near-field region.
Multifrequency op- eration of nanoantennas provides tremendous opportunities
for broadband emission enhancement, spectroscopy and sensing. By the principle
of reciprocity, the same tapered nanoantenna architecture can be used both as a
transmitter and/or as a receiver, thus being useful for creating a broadband
wireless communication system
Linear optical absorption spectra of mesoscopic structures in intense THz fields: free particle properties
We theoretically study the effect of THz radiation on the linear optical
absorption spectra of semiconductor structures. A general theoretical
framework, based on non-equilibrium Green functions, is formulated, and applied
to the calculation of linear optical absorption spectrum for several
non-equilibrium mesoscopic structures. We show that a blue-shift occurs and
sidebands appear in bulk-like structures, i.e., the dynamical Franz-Keldysh
effect [A.-P. Jauho and K. Johnsen, Phys. Rev. Lett. 76, 4576 (1996)]. An
analytic calculation leads to the prediction that in the case of superlattices
distinct stable steps appear in the absorption spectrum when conditions for
dynamical localization are met.Comment: 13 Pages, RevTex using epsf to include 8 ps figures. Submitted to
Phys. Rev. B (3 April 97
Field-induced delocalization and Zener breakdown in semiconductor superlattices
We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown
Waveguide Coupled Resonance Fluorescence from On-Chip Quantum Emitter
Resonantly driven quantum emitters offer a very promising route to obtain highly coherent sources of single photons required for applications in quantum information processing (QIP). Realizing this for on-chip scalable devices would be important for scientific advances and practical applications in the field of integrated quantum optics. Here we report on-chip quantum dot (QD) resonance fluorescence (RF) efficiently coupled into a single-mode waveguide, a key component of a photonic integrated circuit, with a negligible resonant laser background and show that the QD coherence is enhanced by more than a factor of 4 compared to off-resonant excitation. Single-photon behavior is confirmed under resonant excitation, and fast fluctuating charge dynamics are revealed in autocorrelation g(2) measurements. The potential for triggered operation is verified in pulsed RF. These results pave the way to a novel class of integrated quantum-optical devices for on-chip quantum information processing with embedded resonantly driven quantum emitters
Miniband-related 1.4â1.8 ÎŒm luminescence of Ge/Si quantum dot superlattices
The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4â1.8 ”m range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 ”m
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