129 research outputs found
Optical orientation of the homogeneous non-equilibrium Bose-Einstein condensate of bright excitons (polaritons)
A simple model, describing the dynamics of the non-equilibrium pseudospin of
a homogeneous Bose-Einstein condensate of exciton polaritons, has been
formulated. It explains the suppression of spin splitting of a non-equilibrium
polariton condensate in an external magnetic field, the optical alignment, and
the conversion of alignment into orientation of polaritons. It has been shown
that inverse effects are possible, to wit, the spontaneous circular
polarization and the enhancement of spin splitting of a non-equilibrium
condensate of polaritons in the absence of external field.Comment: 21 pages, 2 figure
Nuclear spin warm-up in bulk n-GaAs
We show that the spin-lattice relaxation in n-type insulating GaAs is
dramatically accelerated at low magnetic fields. The origin of this effect,
that cannot be explained in terms of well-known diffusion-limited hyperfine
relaxation, is found in the quadrupole relaxation, induced by fluctuating donor
charges. Therefore, quadrupole relaxation, that governs low field nuclear spin
relaxation in semiconductor quantum dots, but was so far supposed to be
harmless to bulk nuclei spins in the absence of optical pumping can be studied
and harnessed in much simpler model environment of n-GaAs bulk crystal.Comment: 5 pages, 4 figure
Electrical control of optical orientation of neutral and negatively charged excitons in n-type semiconductor quantum well
We report a giant electric field induced increase of spin orientation of
excitons in n-type GaAs/AlGaAs quantum well. It correlates strongly with the
formation of negatively charged excitons (trions) in the photoluminescence
spectra. Under resonant excitation of neutral heavy-hole excitons, the
polarization of excitons and trions increases dramatically with electrical
injection of electrons within the narrow exciton-trion bias transition in the
PL spectra, implying a polarization sensitivity of 200 % per Volt. This effect
results from a very efficient trapping of neutral excitons by the quantum well
interfacial fluctuations (so-called "natural" quantum dots) containing resident
electrons.Comment: 18 pages, 4 figure
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
Multitudes of Stable States in a Periodically Driven Electron-Nuclear Spin System in a Quantum Dot
The periodical modulation of circularly polarized light with a frequency
close to the electron spin resonance frequency induces a sharp change of the
single electron spin orientation. Hyperfine interaction provides a feedback,
thus fixing the precession frequency of the electron spin in the external and
the Overhauser field near the modulation frequency. The nuclear polarization is
bidirectional and the electron-nuclear spin system (ENSS) possesses a few
stable states. A similar frequency-locking effect exists for two-color and
mode-locked excitations, too. However, the pulsed excitation with mode locked
laser brings about the multitudes of stable states in ENSS in a quantum dot.
The resulting precession frequencies of the electron spin differ in these
states by the multiple of the modulation frequency. Under such conditions ENSS
represents a digital frequency converter with more than a hundred stable
channels.Comment: 28 pages, 4 Figure
Longitudinal and transversal spin dynamics of donor-bound electrons in fluorine-doped ZnSe: spin inertia versus Hanle effect
The spin dynamics of the strongly localized, donor-bound electrons in
fluorine-doped ZnSe epilayers is studied by pump-probe Kerr rotation
techniques. A method exploiting the spin inertia is developed and used to
measure the longitudinal spin relaxation time, , in a wide range of
magnetic fields, temperatures, and pump densities. The time of the
donor-bound electron spin of about 1.6 s remains nearly constant for
external magnetic fields varied from zero up to 2.5 T (Faraday geometry) and in
a temperature range K. The inhomogeneous spin dephasing time,
ns, is measured using the resonant spin amplification and Hanle
effects under pulsed and steady-state pumping, respectively. These findings
impose severe restrictions on possible spin relaxation mechanisms.Comment: 10 pages, 7 figure
Electrically tunable g-factors in quantum dot molecular spin states
We present a magneto-photoluminescence study of individual vertically stacked
InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied
electric field tunes the relative energies of the two dots, we observe a strong
resonant increase or decrease in the g-factors of different spin states that
have molecular wavefunctions distributed over both quantum dots. We propose a
phenomenological model for the change in g-factor based on resonant changes in
the amplitude of the wavefunction in the barrier due to the formation of
bonding and antibonding orbitals.Comment: 5 pages, 5 figures, Accepted by Phys. Rev. Lett. New version reflects
response to referee comment
- …