419 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
Large quantum dots with small oscillator strength
We have measured the oscillator strength and quantum efficiency of excitons
confined in large InGaAs quantum dots by recording the spontaneous emission
decay rate while systematically varying the distance between the quantum dots
and a semiconductor-air interface. The size of the quantum dots is measured by
in-plane transmission electron microscopy and we find average in-plane
diameters of 40 nm. We have calculated the oscillator strength of excitons of
that size and predict a very large oscillator strength due to Coulomb effects.
This is in stark contrast to the measured oscillator strength, which turns out
to be much below the upper limit imposed by the strong confinement model. We
attribute these findings to exciton localization in local potential minima
arising from alloy intermixing inside the quantum dots.Comment: 4 pages, 3 figures, submitte
Efficient out-coupling of high-purity single photons from a coherent quantum dot in a photonic-crystal cavity
We demonstrate a single-photon collection efficiency of from
a quantum dot in a low-Q mode of a photonic-crystal cavity with a single-photon
purity of recorded above the saturation power. The high
efficiency is directly confirmed by detecting up to kilocounts per
second on a single-photon detector on another quantum dot coupled to the cavity
mode. The high collection efficiency is found to be broadband, as is explained
by detailed numerical simulations. Cavity-enhanced efficient excitation of
quantum dots is obtained through phonon-mediated excitation and under these
conditions, single-photon indistinguishability measurements reveal long
coherence times reaching ns in a weak-excitation regime. Our work
demonstrates that photonic crystals provide a very promising platform for
highly integrated generation of coherent single photons including the efficient
out-coupling of the photons from the photonic chip.Comment: 13 pages, 8 figures, submitte
Optoelectronic cooling of mechanical modes in a semiconductor nanomembrane
Optical cavity cooling of mechanical resonators has recently become a
research frontier. The cooling has been realized with a metal-coated silicon
microlever via photo-thermal force and subsequently with dielectric objects via
radiation pressure. Here we report cavity cooling with a crystalline
semiconductor membrane via a new mechanism, in which the cooling force arises
from the interaction between the photo-induced electron-hole pairs and the
mechanical modes through the deformation potential coupling. The optoelectronic
mechanism is so efficient as to cool a mode down to 4 K from room temperature
with just 50 uW of light and a cavity with a finesse of 10 consisting of a
standard mirror and the sub-wavelength-thick semiconductor membrane itself. The
laser-cooled narrow-band phonon bath realized with semiconductor mechanical
resonators may open up a new avenue for photonics and spintronics devices.Comment: 5 pages, 4 figure
Measuring the effective phonon density of states of a quantum dot
We employ detuning-dependent decay-rate measurements of a quantum dot in a
photonic-crystal cavity to study the influence of phonon dephasing in a
solid-state quantum-electrodynamics experiment. The experimental data agree
with a microscopic non-Markovian model accounting for dephasing from
longitudinal acoustic phonons, and identifies the reason for the hitherto
unexplained difference between non-resonant cavity feeding in different
nanocavities. From the comparison between experiment and theory we extract the
effective phonon density of states experienced by the quantum dot. This
quantity determines all phonon dephasing properties of the system and is found
to be described well by a theory of bulk phonons.Comment: 5 pages, 3 figures, submitte
Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes
We have performed time-resolved spectroscopy on InAs quantum dot ensembles in
photonic crystal membranes. The influence of the photonic crystal is
investigated by varying the lattice constant systematically. We observe a
strong slow down of the quantum dots' spontaneous emission rates as the
two-dimensional bandgap is tuned through their emission frequencies. The
measured band edges are in full agreement with theoretical predictions. We
characterize the multi-exponential decay curves by their mean decay time and
find enhancement of the spontaneous emission at the bandgap edges and strong
inhibition inside the bandgap in good agreement with local density of states
calculations.Comment: 9 pages (preprint), 3 figure
Near-unity coupling efficiency of a quantum emitter to a photonic-crystal waveguide
A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes
a promising system for the realization of single-photon transistors,
quantum-logic gates based on giant single-photon nonlinearities, and high
bit-rate deterministic single-photon sources. The key figure of merit for such
devices is the -factor, which is the probability for an emitted single
photon to be channeled into a desired waveguide mode. We report on the
experimental achievement of for a quantum dot
coupled to a photonic-crystal waveguide, corresponding to a single-emitter
cooperativity of . This constitutes a nearly ideal
photon-matter interface where the quantum dot acts effectively as a 1D
"artificial" atom, since it interacts almost exclusively with just a single
propagating optical mode. The -factor is found to be remarkably robust
to variations in position and emission wavelength of the quantum dots. Our work
demonstrates the extraordinary potential of photonic-crystal waveguides for
highly efficient single-photon generation and on-chip photon-photon
interaction
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