120 research outputs found
Hyperspherical theory of anisotropic exciton
A new approach to the theory of anisotropic exciton based on Fock
transformation, i.e., on a stereographic projection of the momentum to the unit
4-dimensional (4D) sphere, is developed. Hyperspherical functions are used as a
basis of the perturbation theory. The binding energies, wave functions and
oscillator strengths of elongated as well as flattened excitons are obtained
numerically. It is shown that with an increase of the anisotropy degree the
oscillator strengths are markedly redistributed between optically active and
formerly inactive states, making the latter optically active. An approximate
analytical solution of the anisotropic exciton problem taking into account the
angular momentum conserving terms is obtained. This solution gives the binding
energies of moderately anisotropic exciton with a good accuracy and provides a
useful qualitative description of the energy level evolution.Comment: 23 pages, 8 figure
Bose-Einstein Condensation of Excitons: Reply to Tikhodeev's Criticism
The extended version of our reply to Comment on ``Critical Velocities in
Exciton Superfluidity'' by S. G. Tikhodeev (Phys. Rev. Lett., 84 (2000), 3502
or from http://prl.aps.org/) is presented here. The principal question is
discussed: does the moving exciton-phonon packet contain the coherent
`nucleus', or the exciton-phonon condensate?Comment: 3 pages in LaTe
Nonlinear dynamics of polariton scattering in semiconductor microcavity: bistability vs stimulated scattering
We demonstrate experimentally an unusual behavior of the parametric polariton
scattering in semiconductor microcavity under a strong cw resonant excitation.
The maximum of the scattered signal above the threshold of stimulated
parametric scattering does not shift along the microcavity lower polariton
branch with the change of pump detuning or angle of incidence but is stuck
around the normal direction. We show theoretically that such a behavior can be
modelled numerically by a system of Maxwell and nonlinear Schroedinger
equations for cavity polaritons and explained via the competition between the
bistability of a driven nonlinear MC polariton and the instabilities of
parametric polariton-polariton scattering.Comment: 5 pages, 4 Postscript figures; corrected typo
Controlling circular polarization of light emitted by quantum dots using chiral photonic crystal slab
We study the polarization properties of light emitted by quantum dots that
are embedded in chiral photonic crystal structures made of achiral planar GaAs
waveguides. A modification of the electromagnetic mode structure due to the
chiral grating fabricated by partial etching of the wave\-guide layer has been
shown to result in a high circular polarization degree of the quantum
dot emission in the absence of external magnetic field. The physical nature of
the phenomenon can be understood in terms of the reciprocity principle taking
into account the structural symmetry. At the resonance wavelength, the
magnitude of is predicted to exceed 98%. The experimentally achieved
value of % is smaller, which is due to the contribution of
unpolarized light scattered by grating defects, thus breaking its periodicity.
The achieved polarization degree estimated removing the unpolarized nonresonant
background from the emission spectra can be estimated to be as high as 96%,
close to the theoretical prediction
Optical properties of photonic crystal slabs with asymmetrical unit cell
Using the unitarity and reciprocity properties of the scattering matrix, we
analyse the symmetry and resonant optical properties of the photonic crystal
slabs (PCS) with complicated unit cell. We show that the reflectivity is not
changed upon the 180deg-rotation of the sample around the normal axis, even in
PCS with asymmetrical unit cell. Whereas the transmissivity becomes
asymmetrical if the diffraction or absorption are present. The PCS reflectivity
peaks to unity near the quasiguided mode resonance for normal light incidence
in the absence of diffraction, depolarisation, and absorptive losses. For the
oblique incidence the full reflectivity is reached only in symmetrical PCS.Comment: 5 pages, 2 Postscript figure
Relation between inelastic electron tunneling and vibrational excitation of single adsorbates on metal surfaces
We analyse theoretically a relation between the vibrational generation rate
of a single adsorbate by tunneling electrons and the inelastic tunneling (IET)
current in scanning tunneling microscope, and the influence of the vibrational
excitations on the rate of adsorbate motions. Special attention is paid to the
effects of finite lifetime of the vibrational excitations. We show that in the
vicinity and below the IET threshold the rate of adsorbate motion deviates from
a simple power-law dependence on the bias voltage due to the effects of bath
temperature and adsorbate vibrational lifetime broadenings. The temperature
broadening appears to be confined near the threshold voltage within a narrow
region of several , whereas the lifetime broadening manifests itself in
a much wider region of applied voltages below the IET threshold.Comment: 8 pages including 4 figure
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