1,411 research outputs found
Nonlinear dynamics of quantum dot nuclear spins
We report manifestly nonlinear dependence of quantum dot nuclear spin
polarization on applied magnetic fields. Resonant absorption and emission of
circularly polarized radiation pumps the resident quantum dot electron spin,
which in turn leads to nuclear spin polarization due to hyperfine interaction.
We observe that the resulting Overhauser field exhibits hysteresis as a
function of the external magnetic field. This hysteresis is a consequence of
the feedback of the Overhauser field on the nuclear spin cooling rate. A
semi-classical model describing the coupled nuclear and electron spin dynamics
successfully explains the observed hysteresis but leaves open questions for the
low field behaviour of the nuclear spin polarization.Comment: 7 pages, 4 figure
A Clash of Cultures: The Expulsion of Soviet Military Advisors from Egypt
The US Government is as heavily involved with the Egyptian armed forces as any other military in the world, including the Israelis. In 1981 the United States agreed to support Egypt\u27s Five-Year Plan to rebuild its armed forces, and we are providing billions of dollars in equipment that includes tanks, antitank weapons, air defense systems, and some of the latest fighters in our inventory. We now have hundreds of US military personnel living and working in Egypt with our military assistance teams;. US military units have conducted joint desert exercises with the Egyptian armed forces
Dynamics of Quantum Dot Nuclear Spin Polarization Controlled by a Single Electron
We present an experimental study of the dynamics underlying the buildup and
decay of dynamical nuclear spin polarization in a single semiconductor quantum
dot. Our experiment shows that the nuclei can be polarized on a time scale of a
few milliseconds, while their decay dynamics depends drastically on external
parameters. We show that a single electron can very efficiently depolarize the
nuclear spins and discuss two processes that can cause this depolarization.
Conversely, in the absence of a quantum dot electron, the lifetime of nuclear
spin polarization is on the time scale of a second, most likely limited by the
non-secular terms of the nuclear dipole-dipole interaction. We can further
suppress this depolarization rate by 1-2 orders of magnitude by applying an
external magnetic field exceeding 1 mT.Comment: 5 pages, 3 figure
Multiple time scale blinking in InAs quantum dot single-photon sources
We use photon correlation measurements to study blinking in single,
epitaxially-grown self-assembled InAs quantum dots situated in circular Bragg
grating and microdisk cavities. The normalized second-order correlation
function g(2)(\tau) is studied across eleven orders of magnitude in time, and
shows signatures of blinking over timescales ranging from tens of nanoseconds
to tens of milliseconds. The g(2)(\tau) data is fit to a multi-level system
rate equation model that includes multiple non-radiating (dark) states, from
which radiative quantum yields significantly less than 1 are obtained. This
behavior is observed even in situations for which a direct histogramming
analysis of the emission time-trace data produces inconclusive results
Bright single photon emission from a quantum dot in a circular Bragg grating microcavity
Bright single photon emission from single quantum dots in suspended circular
Bragg grating microcavities is demonstrated. This geometry has been designed to
achieve efficient (> 50 %) single photon extraction into a near-Gaussian shaped
far-field pattern, modest (~10x) Purcell enhancement of the radiative rate, and
a spectral bandwidth of a few nanometers. Measurements of fabricated devices
show progress towards these goals, with collection efficiencies as high as ~10%
demonstrated with moderate spectral bandwidth and rate enhancement. Photon
correlation measurements are performed under above-bandgap excitation (pump
wavelength = 780 nm to 820 nm) and confirm the single photon character of the
collected emission. While the measured sources are all antibunched and
dominantly composed of single photons, the multi-photon probability varies
significantly. Devices exhibiting tradeoffs between collection efficiency,
Purcell enhancement, and multi-photon probability are explored and the results
are interpreted with the help of finite-difference time-domain simulations.
Below-bandgap excitation resonant with higher states of the quantum dot and/or
cavity (pump wavelength = 860 nm to 900 nm) shows a near-complete suppression
of multi-photon events and may circumvent some of the aforementioned tradeoffs.Comment: 11 pages, 12 figure
A circular dielectric grating for vertical extraction of single quantum dot emission
We demonstrate a nanostructure composed of partially etched annular trenches
in a suspended GaAs membrane, designed for efficient and moderately broadband
(approx. 5 nm) emission extraction from single InAs quantum dots. Simulations
indicate that a dipole embedded in the nanostructure center radiates upwards
into free space with a nearly Gaussian far-field, allowing a collection
efficiency > 80 % with a high numerical aperture (NA=0.7) optic, and with 12X
Purcell radiative rate enhancement. Fabricated devices exhibit an approx. 10 %
photon collection efficiency with a NA=0.42 objective, a 20X improvement over
quantum dots in unpatterned GaAs. A fourfold exciton lifetime reduction
indicates moderate Purcell enhancement.Comment: (3 pages
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