697 research outputs found
Polarization dependence of the two-photon Franz-Keldysh effect
The effect of a constant electric field on two-photon absorption in a direct
band gap semiconductor is calculated using an independent-particle theory. Two
band structure models for GaAs are used: a two-band parabolic model and an
eight-band "k dot p" model. Both predict a strong dependence of the two-photon
electroabsorption spectrum on the polarization of the light with respect to the
constant field. We attribute the polarization dependence to the strong effect
of a constant field on intraband dynamics.Comment: 5 pages, 1 figur
Generation of spin currents via Raman scattering
We show theoretically that stimulated spin flip Raman scattering can be used
to inject spin currents in doped semiconductors with spin split bands. A pure
spin current, where oppositely oriented spins move in opposite directions, can
be injected in zincblende crystals and structures. The calculated spin current
should be detectable by pump-probe optical spectroscopy and anomalous Hall
effect measurement
Spatial fragmentation of a Bose-Einstein condensate in a double-well potential
We present a theoretical study of the ground state of a Bose-Einstein
condensate with repulsive inter-particle interactions in a double-well
potential, using a restricted variational principle. Within such an approach,
there is a transition from a single condensate to a fragmented condensate as
the strength of the central barrier of the potential is increased. We determine
the nature of this transition through approximate analytic as well as numerical
solutions of our model in the regime where the inter-particle interactions can
be treated perturbatively. The degree of fragmentation of the condensate is
characterized by the degrees of first-order and second-order spatial coherence
across the barrier.Comment: 10 pages, 2 figures, submitted to Phys. Rev.
Excitonic effects on the two-color coherent control of interband transitions in bulk semiconductors
Quantum interference between one- and two-photon absorption pathways allows
coherent control of interband transitions in unbiased bulk semiconductors;
carrier population, carrier spin polarization, photocurrent injection, and spin
current injection may all be controlled. We extend the theory of these
processes to include the electron-hole interaction. Our focus is on photon
energies that excite carriers above the band edge, but close enough to it so
that transition amplitudes based on low order expansions in are
applicable; both allowed-allowed and allowed-forbidden two-photon transition
amplitudes are included. Analytic solutions are obtained using the effective
mass theory of Wannier excitons; degenerate bands are accounted for, but
envelope-hole coupling is neglected. We find a Coulomb enhancement of two-color
coherent control process, and relate it to the Coulomb enhancements of one- and
two-photon absorption. In addition, we find a frequency dependent phase shift
in the dependence of photocurrent and spin current on the optical phases. The
phase shift decreases monotonically from at the band edge to 0 over an
energy range governed by the exciton binding energy. It is the difference
between the partial wave phase shifts of the electron-hole envelope function
reached by one- and two-photon pathways.Comment: 31 pages, 4 figures, to be published in Phys. Rev.
Two-photon spin injection in semiconductors
A comparison is made between the degree of spin polarization of electrons
excited by one- and two-photon absorption of circularly polarized light in bulk
zincblende semiconductors. Time- and polarization-resolved experiments in
(001)-oriented GaAs reveal an initial degree of spin polarization of 49% for
both one- and two-photon spin injection at wavelengths of 775 and 1550 nm, in
agreement with theory. The macroscopic symmetry and microscopic theory for
two-photon spin injection are reviewed, and the latter is generalized to
account for spin-splitting of the bands. The degree of spin polarization of
one- and two-photon optical orientation need not be equal, as shown by
calculations of spectra for GaAs, InP, GaSb, InSb, and ZnSe using a 14x14 k.p
Hamiltonian including remote band effects. By including the higher conduction
bands in the calculation, cubic anisotropy and the role of allowed-allowed
transitions can be investigated. The allowed-allowed transitions do not
conserve angular momentum and can cause a high degree of spin polarization
close to the band edge; a value of 78% is calculated in GaSb, but by varying
the material parameters it could be as high as 100%. The selection rules for
spin injection from allowed-allowed transitions are presented, and interband
spin-orbit coupling is found to play an important role.Comment: 12 pages including 7 figure
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Maxwell Equations in Complex Form of Majorana - Oppenheimer, Solutions with Cylindric Symmetry in Riemann S_{3} and Lobachevsky H_{3} Spaces
Complex formalism of Riemann - Silberstein - Majorana - Oppenheimer in
Maxwell electrodynamics is extended to the case of arbitrary pseudo-Riemannian
space - time in accordance with the tetrad recipe of Tetrode - Weyl - Fock -
Ivanenko. In this approach, the Maxwell equations are solved exactly on the
background of static cosmological Einstein model, parameterized by special
cylindrical coordinates and realized as a Riemann space of constant positive
curvature. A discrete frequency spectrum for electromagnetic modes depending on
the curvature radius of space and three parameters is found, and corresponding
basis electromagnetic solutions have been constructed explicitly. In the case
of elliptical model a part of the constructed solutions should be rejected by
continuity considerations. Similar treatment is given for Maxwell equations in
hyperbolic Lobachevsky model, the complete basis of electromagnetic solutions
in corresponding cylindrical coordinates has been constructed as well, no
quantization of frequencies of electromagnetic modes arises.Comment: 39 page
Second harmonic generation and birefringence of some ternary pnictide semiconductors
A first-principles study of the birefringence and the frequency dependent
second harmonic generation (SHG) coefficients of the ternary pnictide
semiconductors with formula ABC (A = Zn, Cd; B = Si, Ge; C = As, P) with
the chalcopyrite structures was carried out. We show that a simple empirical
observation that a smaller value of the gap is correlated with larger value of
SHG is qualitatively true. However, simple inverse power scaling laws between
gaps and SHG were not found. Instead, the real value of the nonlinear response
is a result of a very delicate balance between different intraband and
interband terms.Comment: 13 pages, 12 figure
Discrete molecular dynamics simulations of peptide aggregation
We study the aggregation of peptides using the discrete molecular dynamics
simulations. At temperatures above the alpha-helix melting temperature of a
single peptide, the model peptides aggregate into a multi-layer parallel
beta-sheet structure. This structure has an inter-strand distance of 0.48 nm
and an inter-sheet distance of 1.0 nm, which agree with experimental
observations. In this model, the hydrogen bond interactions give rise to the
inter-strand spacing in beta-sheets, while the Go interactions among side
chains make beta-strands parallel to each other and allow beta-sheets to pack
into layers. The aggregates also contain free edges which may allow for further
aggregation of model peptides to form elongated fibrils.Comment: 15 pages, 8 figure
Positive Quantum Brownian Evolution
Using the independent oscillator model with an arbitrary system potential, we
derive a quantum Brownian equation assuming a correlated total initial state.
Although not of Lindblad form, the equation preserves positivity of the density
operator on a restricted set of initial states
- …