255 research outputs found
Formation of optimal-order necklace modes in one-dimensional random photonic superlattices
We study the appearance of resonantly coupled optical modes, optical
necklaces, in Anderson localized one-dimensional random superlattices through
numerical calculations of the accumulated phase. The evolution of the optimal
necklace order m* shows a gradual shift towards higher orders with increasing
the sample size. We derive an empirical formula that predicts m* and discuss
the situation when in a sample length L the number of degenerate in energy
resonances exceeds the optimal one. We show how the \emph{extra} resonances are
pushed out to the miniband edges of the necklace, thus reducing the order of
the latter by multiples of two.Comment: 4 pages, 4 figure
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
Pumping of nuclear spins by the optical solid effect in a quantum dot
We demonstrate that efficient optical pumping of nuclear spins in
semiconductor quantum dots (QDs) can be achieved by resonant pumping of
optically "forbidden" transitions. This process corresponds to one-to-one
conversion of a photon absorbed by the dot into a polarized nuclear spin, which
also has potential for initialization of hole spin in QDs. Pumping via the
"forbidden" transition is a manifestation of the "optical solid effect", an
optical analogue of the effect previously observed in electron spin resonance
experiments in the solid state. We find that by employing this effect, nuclear
polarization of 65% can be achieved, the highest reported so far in optical
orientation studies in QDs. The efficiency of the spin pumping exceeds that
employing the allowed transition, which saturates due to the low probability of
electron-nuclear spin flip-flop.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Overhauser effect in individual InP/GaInP dots
Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a
wide range of external magnetic fields B_ext=0-5T by circularly polarized
optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T
and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of
the nuclear spin on the spin pumping efficiency. This feedback, produced by the
Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of
Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when
the Overhauser field cancels the external field. This counter-intuitive result
is shown to arise from the opposite contribution of the electron and hole
Zeeman splittings to the total exciton Zeeman energy
Overhauser effect in individual InP/GaInP dots
Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a
wide range of external magnetic fields B_ext=0-5T by circularly polarized
optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T
and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of
the nuclear spin on the spin pumping efficiency. This feedback, produced by the
Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of
Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when
the Overhauser field cancels the external field. This counter-intuitive result
is shown to arise from the opposite contribution of the electron and hole
Zeeman splittings to the total exciton Zeeman energy
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
Spin diffusion in the Mn2+ ion system of II-VI diluted magnetic semiconductor heterostructures
The magnetization dynamics in diluted magnetic semiconductor heterostructures
based on (Zn,Mn)Se and (Cd,Mn)Te has been studied experimentally by optical
methods and simulated numerically. In the samples with nonhomogeneous magnetic
ion distribution this dynamics is contributed by spin-lattice relaxation and
spin diffusion in the Mn spin system. The spin diffusion coefficient of
7x10^(-8) cm^2/s has been evaluated for Zn(0.99)Mn(0.01)Se from comparison of
experimental and numerical results. Calculations of the giant Zeeman splitting
of the exciton states and the magnetization dynamics in the ordered alloys and
parabolic quantum wells fabricated by the digital growth technique show perfect
agreement with the experimental data. In both structure types the spin
diffusion has an essential contribution to the magnetization dynamics.Comment: 12 pages, 11 figure
Characterizing Short Necklace States in Logarithmic Transmission Spectrum of Strongly Localized Systems
High transmission plateaus exist widely in the logarithmic transmission
spectra of localized systems. Their physical origins are short chains of
coupled-localized-states embedded inside the localized system, which are dubbed
as "short necklace states". In this work, we define the essential quantities
and then, based on these quantities, we investigate the short necklace states'
properties statistically and quantitatively. Two different approaches are
utilized and the results from them agree with each other very well. In the
first approach, the typical plateau-width and the typical order of short
necklace states are obtained from the correlation function of logarithmic
transmission. In the second approach, we investigate statistical distributions
of the peak/plateau-width measured in logarithmic transmission spectra. A novel
distribution is found, which can be exactly fitted by the summation of two
Gaussian distributions. These two distributions are the results of sharp peaks
of localized states and the high plateaus of short necklace states. The center
of the second distribution also tells us the typical plateau-width of short
necklace states. With increasing the system length, the scaling property of
typical plateau-width is very special since it almost does not decrease. The
methods and the quantities defined in this work can be widely used on Anderson
localization studies.Comment: 6 pages, 4 figure
Magnetophonon resonance in photoluminescence excitation spectra of magnetoexcitons in GaAs/Al0.3Ga0.7As superlattice
Strong increase in the intensity of the peaks of excited magneto-exciton (ME)
states in the photoluminescence excitation (PLE) spectra recorded for the
ground heavy-hole magneto-excitons (of the 1sHH type) has been found in a
GaAs/AlGaAs superlattice in strong magnetic field B applied normal to the
sample layers. While varying B the intensities of the PLE peaks have been
measured as functions of energy separation between excited ME peaks
and the ground state of the system. The resonance profiles have been found to
have maxima at close to the energy of the GaAs LO-phonon.
However, the value of depends on quantum numbers of the
excited ME state. The revealed very low quantum efficiency of the investigated
sample allows us to ascribe the observed resonance to the enhancement of the
non-radiative magneto-exciton relaxation rate arising due to LO-phonon
emission. The presented theoretical model, being in a good agreement with
experimental observations, provides a method to extract 1sHH magneto-exciton
``in-plane" dispersion from the dependence of on the
excited ME state quantum numbers.Comment: 9 pages, 6 figure
Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities
A massive redistribution of the polariton occupancy to two specific wave vectors is observed under conditions of continuous wave excitation of a semiconductor microcavity.
The “condensation” of the polaritons to the two specific states arises from stimulated scattering at final
state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of
the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch.
High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in
the vertical cavity structure
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