998 research outputs found
Fine structure and optical pumping of spins in individual semiconductor quantum dots
We review spin properties of semiconductor quantum dots and their effect on
optical spectra. Photoluminescence and other types of spectroscopy are used to
probe neutral and charged excitons in individual quantum dots with high
spectral and spatial resolution. Spectral fine structure and polarization
reveal how quantum dot spins interact with each other and with their
environment. By taking advantage of the selectivity of optical selection rules
and spin relaxation, optical spin pumping of the ground state electron and
nuclear spins is achieved. Through such mechanisms, light can be used to
process spins for use as a carrier of information
Giant nonlinearity and entanglement of single photons in photonic bandgap structures
Giantly enhanced cross-phase modulation with suppressed spectral broadening
is predicted between optically-induced dark-state polaritons whose propagation
is strongly affected by photonic bandgaps of spatially periodic media with
multilevel dopants. This mechanism is shown to be capable of fully entangling
two single-photon pulses with high fidelity.Comment: 7 pages, 1 figur
Fast spin rotations by optically controlled geometric phases in a quantum dot
We demonstrate optical control of the geometric phase acquired by one of the
spin states of an electron confined in a charge-tunable InAs quantum dot via
cyclic 2pi excitations of an optical transition in the dot. In the presence of
a constant in-plane magnetic field, these optically induced geometric phases
result in the effective rotation of the spin about the magnetic field axis and
manifest as phase shifts in the spin quantum beat signal generated by two
time-delayed circularly polarized optical pulses. The geometric phases
generated in this manner more generally perform the role of a spin phase gate,
proving potentially useful for quantum information applications.Comment: 4 pages, 3 figures, resubmitted to Physical Review Letter
Lactose and benign ovarian tumours in a case–control study
We investigated the relation between benign ovarian tumours and lactose among 746 case women identified at seven New York metropolitan hospitals and 404 community controls, age and hospital frequency matched to the expected case distribution. No increase in risk was found for lactose (highest quartile versus lowest: adjusted odds ratio = 0.82 (95% CI 0.57–1.20) or for any other lactose foods. © 2000 Cancer Research Campaign http://www.bjcancer.co
Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots
We report on the coherent optical excitation of electron spin polarization in
the ground state of charged GaAs quantum dots via an intermediate charged
exciton (trion) state. Coherent optical fields are used for the creation and
detection of the Raman spin coherence between the spin ground states of the
charged quantum dot. The measured spin decoherence time, which is likely
limited by the nature of the spin ensemble, approaches 10 ns at zero field. We
also show that the Raman spin coherence in the quantum beats is caused not only
by the usual stimulated Raman interaction but also by simultaneous spontaneous
radiative decay of either excited trion state to a coherent combination of the
two spin states.Comment: 4 pages, 3 figures. Minor modification
Stimulated Raman spin coherence and spin-flip induced hole burning in charged GaAs quantum dots
High-resolution spectral hole burning (SHB) in coherent nondegenerate
differential transmission spectroscopy discloses spin-trion dynamics in an
ensemble of negatively charged quantum dots. In the Voigt geometry, stimulated
Raman spin coherence gives rise to Stokes and anti-Stokes sidebands on top of
the trion spectral hole. The prominent feature of an extremely narrow spike at
zero detuning arises from spin population pulsation dynamics. These SHB
features confirm coherent electron spin dynamics in charged dots, and the
linewidths reveal spin spectral diffusion processes.Comment: 5 pages, 5 figure
The improvement of Mo/4H-SiC Schottky diodes via a P2O5 surface passivation treatment
Molybdenum (Mo)/4H-silicon carbide (SiC) Schottky barrier diodes have been fabricated with a phosphorus pentoxide (P2O5) surface passivation treatment performed on the SiC surface prior to metallization. Compared to the untreated diodes, the P2O5-treated diodes were found to have a lower Schottky barrier height by 0.11 eV and a lower leakage current by two to three orders of magnitude. Physical characterization of the P2O5-treated Mo/SiC interfaces revealed that there are two primary causes for the improvement in electrical performance. First, transmission electron microscopy imaging showed that nanopits filled with silicon dioxide had formed at the surface after the P2O5 treatment that terminates potential leakage paths. Second, secondary ion mass spectroscopy revealed a high concentration of phosphorus atoms near the interface. While only a fraction of these are active, a small increase in doping at the interface is responsible for the reduction in barrier height. Comparisons were made between the P2O5 pretreatment and oxygen (O2) and nitrous oxide (N2O) pretreatments that do not form the same nanopits and do not reduce leakage current. X-ray photoelectron spectroscopy shows that SiC beneath the deposited P2O5 oxide retains a Si-rich interface unlike the N2O and O2 treatments that consume SiC and trap carbon at the interface. Finally, after annealing, the Mo/SiC interface forms almost no silicide, leaving the enhancement to the subsurface in place, explaining why the P2O5 treatment has had no effect on nickel- or titanium-SiC contacts
Local Optical Spectroscopy in Quantum Confined Systems: A Theoretical Description
A theoretical description of local absorption is proposed in order to
investigate spectral variations on a length scale comparable with the extension
of the relevant quantum states. A general formulation is derived within the
density-matrix formalism including Coulomb correlation, and applied to the
prototypical case of coupled quantum wires. The results show that excitonic
effects may have a crucial impact on the local absorption with implications for
the spatial resolution and the interpretation of near-field optical spectra.Comment: To appear in Phys. Rev. Lett. - 11 pages, 3 PostScript figures (1
figure in colors) embedded. Uses RevTex, and psfig style
Manipulation of the Spin Memory of Electrons in n-GaAs
We report on the optical manipulation of the electron spin relaxation time in
a GaAs based heterostructure. Experimental and theoretical study shows that the
average electron spin relaxes through hyperfine interaction with the lattice
nuclei, and that the rate can be controlled by the electron-electron
interactions. This time has been changed from 300 ns down to 5 ns by variation
of the laser frequency. This modification originates in the optically induced
depletion of n-GaAs layer
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