1,822 research outputs found
Coherent Population Trapping of Electron Spins in a Semiconductor
In high-purity n-type GaAs under strong magnetic field, we are able to
isolate a lambda system composed of two Zeeman states of neutral-donor bound
electrons and the lowest Zeeman state of bound excitons. When the two-photon
detuning of this system is zero, we observe a pronounced dip in the
excited-state photoluminescence indicating the creation of the coherent
population-trapped state. Our data are consistent with a steady-state
three-level density-matrix model. The observation of coherent population
trapping in GaAs indicates that this and similar semiconductor systems could be
used for various EIT-type experiments.Comment: 5 pages, 4 figures replaced 6/25/2007 with PRL versio
On the indistinguishability of Raman photons
We provide a theoretical framework to study the effect of dephasing on the
quantum indistinguishability of single photons emitted from a coherently driven
cavity QED -system. We show that with a large excited-state detuning,
the photon indistinguishability can be drastically improved provided that the
fluctuation rate of the noise source affecting the excited state is fast
compared with the photon emission rate. In some cases a spectral filter is
required to realize this improvement, but the cost in efficiency can be made
small.Comment: 18 pages, 3 figures, final versio
400%/W second harmonic conversion efficiency in -diameter gallium phosphide-on-oxide resonators
Second harmonic conversion from 1550~nm to 775~nm with an efficiency of 400%
W is demonstrated in a gallium phosphide (GaP) on oxide integrated
photonic platform. The platform consists of doubly-resonant, phase-matched ring
resonators with quality factors , low mode volumes , and high nonlinear mode overlaps. Measurements and simulations
indicate that conversion efficiencies can be increased by a factor of 20 by
improving the waveguide-cavity coupling to achieve critical coupling in current
devices.Comment: 13 pages, 6 figure
Population pulsation resonances of excitons in monolayer MoSe2 with sub 1 {\mu}eV linewidth
Monolayer transition metal dichalcogenides, a new class of atomically thin
semiconductors, possess optically coupled 2D valley excitons. The nature of
exciton relaxation in these systems is currently poorly understood. Here, we
investigate exciton relaxation in monolayer MoSe2 using polarization-resolved
coherent nonlinear optical spectroscopy with high spectral resolution. We
report strikingly narrow population pulsation resonances with two different
characteristic linewidths of 1 {\mu}eV and <0.2 {\mu}eV at low-temperature.
These linewidths are more than three orders of magnitude narrower than the
photoluminescence and absorption linewidth, and indicate that a component of
the exciton relaxation dynamics occurs on timescales longer than 1 ns. The
ultra-narrow resonance (<0.2 {\mu}eV) emerges with increasing excitation
intensity, and implies the existence of a long-lived state whose lifetime
exceeds 6 ns.Comment: (PRL, in press
Ultrafast control of donor-bound electron spins with single detuned optical pulses
The ability to control spins in semiconductors is important in a variety of
fields including spintronics and quantum information processing. Due to the
potentially fast dephasing times of spins in the solid state [1-3], spin
control operating on the picosecond or faster timescale may be necessary. Such
speeds, which are not possible to attain with standard electron spin resonance
(ESR) techniques based on microwave sources, can be attained with broadband
optical pulses. One promising ultrafast technique utilizes single broadband
pulses detuned from resonance in a three-level Lambda system [4]. This
attractive technique is robust against optical pulse imperfections and does not
require a fixed optical reference phase. Here we demonstrate the principle of
coherent manipulation of spins theoretically and experimentally. Using this
technique, donor-bound electron spin rotations with single-pulse areas
exceeding pi/4 and two-pulses areas exceeding pi/2 are demonstrated. We believe
the maximum pulse areas attained do not reflect a fundamental limit of the
technique and larger pulse areas could be achieved in other material systems.
This technique has applications from basic solid-state ESR spectroscopy to
arbitrary single-qubit rotations [4, 5] and bang-bang control[6] for quantum
computation.Comment: 15 pages, 4 figures, submitted 12/2008. Since the submission of this
work we have become aware of related work: J. Berezovsky, M. H. Mikkelsen, N.
G. Stoltz, L. A. Coldren, and D. D. Awschalom, Science 320: 349-352 (2008
Isolation of Single Donors in ZnO
The shallow donor in zinc oxide (ZnO) is a promising semiconductor spin qubit
with optical access. Single indium donors are isolated in a commercial ZnO
substrate using plasma focused ion beam (PFIB) milling. Quantum emitters are
identified optically by spatial and frequency filtering. The indium donor
assignment is based on the optical bound exciton transition energy and magnetic
dependence. The single donor emission is intensity and frequency stable with a
transition linewidth less than twice the lifetime limit. The isolation of
optically stable single donors post-FIB fabrication is promising for optical
device integration required for scalable quantum technologies based on single
donors in direct band gap semiconductors.Comment: E. R. Hansen and V. Niaouris contributed equally to this work. 13
pages, 11 figure
Photon-noise limited sensitivity in titanium nitride kinetic inductance detectors
We demonstrate photon-noise limited performance at sub-millimeter wavelengths
in feedhorn-coupled, microwave kinetic inductance detectors (MKIDs) made of a
TiN/Ti/TiN trilayer superconducting film, tuned to have a transition
temperature of 1.4~K. Micro-machining of the silicon-on-insulator wafer
backside creates a quarter-wavelength backshort optimized for efficient
coupling at 250~\micron. Using frequency read out and when viewing a variable
temperature blackbody source, we measure device noise consistent with photon
noise when the incident optical power is ~0.5~pW, corresponding to noise
equivalent powers ~3 W/. This
sensitivity makes these devices suitable for broadband photometric applications
at these wavelengths
Survival or death: a dual role of autophagy in stress-induced pericyte loss in diabetic retinopathy
AIMS/HYPOTHESIS: Intra-retinal extravasation and modification of LDL have been implicated in diabetic retinopathy: autophagy may mediate these effects. METHODS: Immunohistochemistry was used to detect autophagy marker LC3B in human and murine diabetic and non-diabetic retinas. Cultured human retinal capillary pericytes (HRCPs) were treated with in vitro-modified heavily-oxidised glycated LDL (HOG-LDL) vs native LDL (N-LDL) with or without autophagy modulators: green fluorescent protein–LC3 transfection; small interfering RNAs against Beclin-1, c-Jun NH(2)-terminal kinase (JNK) and C/EBP-homologous protein (CHOP); autophagy inhibitor 3-MA (5 mmol/l) and/or caspase inhibitor Z-VAD-fmk (100 μmol/l). Autophagy, cell viability, oxidative stress, endoplasmic reticulum stress, JNK activation, apoptosis and CHOP expression were assessed by western blots, CCK-8 assay and TUNEL assay. Finally, HOG-LDL vs N-LDL were injected intravitreally to STZ-induced diabetic vs control rats (yielding 50 and 200 mg protein/l intravitreal concentration) and, after 7 days, retinas were analysed for ER stress, autophagy and apoptosis. RESULTS: Intra-retinal autophagy (LC3B staining) was increased in diabetic vs non-diabetic humans and mice. In HRCPs, 50 mg/l HOG-LDL elicited autophagy without altering cell viability, and inhibition of autophagy decreased survival. At 100–200 mg/l, HOG-LDL caused significant cell death, and inhibition of either autophagy or apoptosis improved survival. Further, 25–200 mg/l HOG-LDL dose-dependently induced oxidative and ER stress. JNK activation was implicated in autophagy but not in apoptosis. In diabetic rat retina, 50 mg/l intravitreal HOG-LDL elicited autophagy and ER stress but not apoptosis; 200 mg/l elicited greater ER stress and apoptosis. CONCLUSIONS: Autophagy has a dual role in diabetic retinopathy: under mild stress (50 mg/l HOG-LDL) it is protective; under more severe stress (200 mg/l HOG-LDL) it promotes cell death. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00125-016-4058-5) contains peer-reviewed but unedited supplementary material, which is available to authorised users
Contributions to the optical linewidth of shallow donor - bound excitonic transition in ZnO
We study the donor-bound exciton optical linewidth properties of Al, Ga and
In donor ensembles in single-crystal zinc oxide (ZnO). Neutral shallow donors
(D) in ZnO are spin qubits with optical access via the donor-bound exciton
(DX). This spin-photon interface enables applications in quantum
networking, memories and transduction. Essential optical parameters which
impact the spin-photon interface include radiative lifetime, optical
inhomogeneous and homogeneous linewidth and optical depth. The ensemble
photoluminescence linewidth ranges from 4-11 GHz, less than two orders of
magnitude larger than the expected lifetime-limited linewidth. The ensemble
linewidth remains narrow in absorption measurements through the 300
m-thick sample, which has an estimated optical depth up to several
hundred. Homogeneous broadening of the ensemble line due to phonons is
consistent with thermal population relaxation between DX states. This
thermal relaxation mechanism has negligible contribution to the total linewidth
at 2 K. We find that inhomogeneous broadening due to the disordered isotopic
environment in natural ZnO is significant, ranging from 1.9 GHz - 2.2 GHz.
Two-laser spectral anti-hole burning measurements, which can be used to measure
the homogeneous linewidth in an ensemble, however, reveal spectral anti-hole
linewidths similar to the single laser ensemble linewidth. Despite this
broadening, the high homogeneity, large optical depth and potential for isotope
purification indicate that the optical properties of the ZnO donor-bound
exciton are promising for a wide range of quantum technologies and motivate a
need to improve the isotope and chemical purity of ZnO for quantum
technologies.Comment: 22 pages, 12 figure
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