357 research outputs found
Optical refrigeration with coupled quantum wells
Refrigeration of a solid-state system with light has potential applications
for cooling small-scale electronics and photonics. We show theoretically that
two coupled semiconductor quantum wells are efficient cooling media for optical
refrigeration because they support long-lived indirect electron-hole pairs.
Thermal excitation of these pairs to distinct higher-energy states with faster
radiative recombination allows an efficient escape channel to remove thermal
energy from the system. This allows reaching much higher cooling efficiencies
than with single quantum wells. From band-diagram calculations along with an
experimentally realistic level scheme we calculate the cooling efficiency and
cooling yield of different devices with coupled quantum wells embedded in a
suspended nanomembrane. The dimension and composition of the quantum wells
allow optimizing either of these quantities, which cannot, however, be
maximized simultaneously. Quantum-well structures with electrical control allow
tunability of carrier lifetimes and energy levels so that the cooling
efficiency can be optimized over time as the thermal population decreases due
to the cooling.Comment: 10 pages, 5 figure
Nonuniversal intensity correlations in 2D Anderson localizing random medium
Complex dielectric media often appear opaque because light traveling through
them is scattered multiple times. Although the light scattering is a random
process, different paths through the medium can be correlated encoding
information about the medium. Here, we present spectroscopic measurements of
nonuniversal intensity correlations that emerge when embedding quantum emitters
inside a disordered photonic crystal that is found to Anderson-localize light.
The emitters probe in-situ the microscopic details of the medium, and imprint
such near-field properties onto the far-field correlations. Our findings
provide new ways of enhancing light-matter interaction for quantum
electrodynamics and energy harvesting, and may find applications in
subwavelength diffuse-wave spectroscopy for biophotonics
Action at a distance: Dependency sensitivity in a New World primate
Sensitivity to dependencies (correspondences between distant items) in sensory stimuli plays a crucial role in human music and language. Here, we show that squirrel monkeys (Saimiri sciureus) can detect abstract, non-adjacent dependencies in auditory stimuli. Monkeys discriminated between tone sequences containing a dependency and those lacking it, and generalized to previously unheard pitch classes and novel dependency distances. This constitutes the first pattern learning study where artificial stimuli were designed with the species' communication system in mind. These results suggest that the ability to recognize dependencies represents a capability that had already evolved in humans' last common ancestor with squirrel monkeys, and perhaps before. © 2013 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0/, which permits unrestricted use, provided the original author and source are credited
Size-Dependence of the Wavefunction of Self-Assembled Quantum Dots
The radiative and non-radiative decay rates of InAs quantum dots are measured
by controlling the local density of optical states near an interface. From
time-resolved measurements we extract the oscillator strength and the quantum
efficiency and their dependence on emission energy. From our results and a
theoretical model we determine the striking dependence of the overlap of the
electron and hole wavefunctions on the quantum dot size. We conclude that the
optical quality is best for large quantum dots, which is important in order to
optimally tailor quantum dot emitters for, e.g., quantum electrodynamics
experiments.Comment: 5 pages, 3 figure
Quantitative analysis of quantum dot dynamics and emission spectra in cavity quantum electrodynamics:Paper
We present detuning-dependent spectral and decay-rate measurements to study
the difference between spectral and dynamical properties of single quantum dots
embedded in micropillar and photonic-crystal cavities. For the micropillar
cavity, the dynamics is well described by the dissipative Jaynes-Cummings
model, while systematic deviations are observed for the emission spectra. The
discrepancy for the spectra is attributed to coupling of other exciton lines to
the cavity and interference of different propagation paths towards the detector
of the fields emitted by the quantum dot. In contrast, quantitative information
about the system can readily be extracted from the dynamical measurements. In
the case of photonic crystal cavities we observe an anti crossing in the
spectra when detuning a single quantum dot through resonance, which is the
spectral signature of strong coupling. However, time-resolved measurements
reveal that the actual coupling strength is significantly smaller than
anticipated from the spectral measurements and that the quantum dot is rather
weakly coupled to the cavity. We suggest that the observed Rabi splitting is
due to cavity feeding by other quantum dots and/or multiexcition complexes
giving rise to collective emission effects.Comment: 14 pages, 5 figures, submitte
Impact ionization in GaAs: a screened exchange density functional approach
Results are presented of a fully ab-initio calculation of impact ionization
rates in GaAs within the density functional theory framework, using a
screened-exchange formalism and the highly precise all-electron full-potential
linearized augmented plane wave (FLAPW) method. The calculated impact
ionization rates show a marked orientation dependence in {\bf k} space,
indicating the strong restrictions imposed by the conservation of energy and
momentum. This anisotropy diminishes as the impacting electron energy
increases. A Keldysh type fit performed on the energy-dependent rate shows a
rather soft edge and a threshold energy greater than the direct band gap. The
consistency with available Monte Carlo and empirical pseudopotential
calculations shows the reliability of our approach and paves the way to
ab-initio calculations of pair production rates in new and more complex
materials.Comment: 11 pages, 4 figures, Submitted to Phys. Rev.
Nonlocal Electrodynamics of Rotating Systems
The nonlocal electrodynamics of uniformly rotating systems is presented and
its predictions are discussed. In this case, due to paucity of experimental
data, the nonlocal theory cannot be directly confronted with observation at
present. The approach adopted here is therefore based on the correspondence
principle: the nonrelativistic quantum physics of electrons in circular
"orbits" is studied. The helicity dependence of the photoeffect from the
circular states of atomic hydrogen is explored as well as the resonant
absorption of a photon by an electron in a circular "orbit" about a uniform
magnetic field. Qualitative agreement of the predictions of the classical
nonlocal electrodynamics with quantum-mechanical results is demonstrated in the
correspondence regime.Comment: 23 pages, no figures, submitted for publicatio
Engineering nanoscale hypersonic phonon transport
Controlling the vibrations in solids is crucial to tailor their mechanical
properties and their interaction with light. Thermal vibrations represent a
source of noise and dephasing for many physical processes at the quantum level.
One strategy to avoid these vibrations is to structure a solid such that it
possesses a phononic stop band, i.e., a frequency range over which there are no
available mechanical modes. Here, we demonstrate the complete absence of
mechanical vibrations at room temperature over a broad spectral window, with a
5.3 GHz wide band gap centered at 8.4 GHz in a patterned silicon nanostructure
membrane measured using Brillouin light scattering spectroscopy. By
constructing a line-defect waveguide, we directly measure GHz localized modes
at room temperature. Our experimental results of thermally excited guided
mechanical modes at GHz frequencies provides an eficient platform for
photon-phonon integration with applications in optomechanics and signal
processing transduction
Extraction of optical Bloch modes in a photonic-crystal waveguide
We perform phase-sensitive near-field scanning optical microscopy on
photonic-crystal waveguides. The observed intricate field patterns are analyzed
by spatial Fourier transformations, revealing several guided TE- and TM-like
modes. Using the reconstruction algorithm proposed by Ha, et al. (Opt. Lett. 34
(2009)), we decompose the measured two-dimensional field pattern in a
superposition of propagating Bloch modes. This opens new possibilities to study
specific modes in near-field measurements. We apply the method to study the
transverse behavior of a guided TE-like mode, where the mode extends deeper in
the surrounding photonic crystal when the band edge is approached
Spontaneous emission from large quantum dots in nanostructures: exciton-photon interaction beyond the dipole approximation
We derive a rigorous theory of the interaction between photons and spatially
extended excitons confined in quantum dots in inhomogeneous photonic materials.
We show that, beyond the dipole approximation, the radiative decay rate is
proportional to a non-local interaction function, which describes the
interaction between light and spatially extended excitons. In this regime,
light and matter degrees of freedom cannot be separated and a complex interplay
between the nanostructured optical environment and the exciton envelope
function emerges. We illustrate this by specific examples and derive a series
of important analytical relations, which are useful for applying the formalism
to practical problems. In the dipole limit, the decay rate is proportional to
the projected local density of optical states and we obtain the strong and weak
confinement regimes as special cases.Comment: 14 pages, 4 figure
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