1,223 research outputs found
Selective spin coupling through a single exciton
We present a novel scheme for performing a conditional phase gate between two
spin qubits in adjacent semiconductor quantum dots through delocalized single
exciton states, formed through the inter-dot Foerster interaction. We consider
two resonant quantum dots, each containing a single excess conduction band
electron whose spin embodies the qubit. We demonstrate that both the two-qubit
gate, and arbitrary single-qubit rotations, may be realized to a high fidelity
with current semiconductor and laser technology.Comment: 5 pages, 3 figures; published version, equation formatting improved,
references adde
Plasmon dispersion in metal nanoparticle chains from angle-resolved scattering
We present angle and frequency resolved optical extinction measurements to
determine the dispersion relation of plasmon modes on Ag and Au nanoparticle
chains with pitches down to 75 nm. The large splitting between transverse and
longitudinal modes and the band curvature are inconsistent with reported
electrostatic near-field models, and confirm that far-field retarded
interactions are important, even for -sized structures. The data
imply that lower propagation losses, larger signal bandwidth and larger maximum
group velocity then expected can be achieved for wave vectors below the light
line. We conclude that for the design of optical nanocircuits coherent
far-field couplings across the entire circuit need to be considered, even at
subwavelength feature sizes.Comment: 4 pages, 4 figures, colo
Mechanisms of arsenic clustering in silicon
A model of arsenic clustering in silicon is proposed and analyzed. The main
feature of the proposed model is the assumption that negatively charged arsenic
complexes play a dominant role in the clustering process. To confirm this
assumption, electron density and concentration of impurity atoms incorporated
into the clusters are calculated as functions of the total arsenic
concentration. A number of the negatively charged clusters incorporating a
point defect and one or more arsenic atoms are investigated. It is shown that
for the doubly negatively charged clusters or for clusters incorporating more
than one arsenic atom the electron density reaches a maximum value and then
monotonically and slowly decreases as total arsenic concentration increases. In
the case of doubly negatively charged cluster incorporating two arsenic atoms,
the calculated electron density agrees well with the experimental data.
Agreement with the experiment confirms the conclusion that two arsenic atoms
participate in the cluster formation. Among all present models, the proposed
model of clustering by formation of doubly negatively charged cluster
incorporating two arsenic atoms gives the best fit to the experimental data and
can be used in simulation of high concentration arsenic diffusion.Comment: 13 pages, 4 figures. Revised and shortened version of the paper has
been published in Phys. Rev. B, Vol.74 (3), art. no. 035205 (2006
Carrier relaxation in GaAs v-groove quantum wires and the effects of localization
Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove
quantum wires (QWRs) with a large subband separation (46 meV). Signatures of
inhibited carrier relaxation mechanisms are seen in temperature-dependent
photoluminescence (PL) and photoluminescence-excitation (PLE) measurements; we
observe strong emission from the first excited state of the QWR below ~50 K.
This is attributed to reduced inter-subband relaxation via phonon scattering
between localized states. Theoretical calculations and experimental results
indicate that the pinch-off regions, which provide additional two-dimensional
confinement for the QWR structure, have a blocking effect on relaxation
mechanisms for certain structures within the v-groove. Time-resolved PL
measurements show that efficient carrier relaxation from excited QWR states
into the ground state, occurs only at temperatures > 30 K. Values for the low
temperature radiative lifetimes of the ground- and first excited-state excitons
have been obtained (340 ps and 160 ps respectively), and their corresponding
localization lengths along the wire estimated.Comment: 9 pages, 8 figures, submitted to Phys. Rev. B Attempted to correct
corrupt figure
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
Large-signal coherent control of normal modes in quantum-well semiconductor microcavity
We demonstrate coherent control of the cavity-polariton modes of a quantum-well semiconductor microcavity in a two-color scheme. The cavity enhancement of the excitonic nonlinearity gives rise to a large signal; modulating the relative phase of the excitation pulses between zero and π produces a differential reflectivity (ΔR/R)(ΔR/R) of up to 20%. The maximum nonlinear signal is obtained for cocircular pump and probe polarization. Excitation-induced dephasing is responsible for the incoherent nonlinear response, and limits the contrast ratio of the optical switching. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71163/2/APPLAB-78-25-3941-1.pd
Spin-dynamic field coupling in strongly THz driven semiconductors : local inversion symmetry breaking
We study theoretically the optics in undoped direct gap semiconductors which
are strongly driven in the THz regime. We calculate the optical sideband
generation due to nonlinear mixing of the THz field and the near infrared
probe. Starting with an inversion symmetric microscopic Hamiltonian we include
the THz field nonperturbatively using non-equilibrium Green function
techniques. We find that a self induced relativistic spin-THz field coupling
locally breaks the inversion symmetry, resulting in the formation of odd
sidebands which otherwise are absent.Comment: 8 pages, 6 figure
Reading bits on a CD-ROM without a photodiode
The authors demonstrate that the bits of a Compact Disc – Read Only Memory (CD-ROM) can be read without a photodiode (PD). A commercial CD-ROM drive was used and left unmodified except a beam splitter was removed which would route the light to a PD. This allowed for the reflected light to be incident on the laser within the laser package itself. By monitoring fluctuations in the voltage across the laser diode under constant-current injection, it was possible to read and interpret the bits from a spinning CD-ROM
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