60 research outputs found
Spin light emitting diode based on exciton fine structure tuning in quantum dots
We propose a concept of quantum dot based light emitting diode that produces
circularly polarized light due to the tuning of the exciton fine structure by
magnetic field and electron nuclear hyperfine interaction. The device operates
under injection of electrons and holes from nonmagnetic contacts in a small
field of the order of milliteslas. Its size can be parametrically smaller than
the light wavelength, and circular polarization degree of electroluminescence
can reach 100%. The proposed concept is compatible with the micropillar
cavities, which allows for the deterministic electrical generation of single
circularly polarized photons.Comment: 6+4 pages, 3+1 figure
Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
The spin structure and spin dynamics of excitons in an ensemble of
(In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both
direct and indirect band gap dots, are studied. Time-resolved and spectral
selective techniques are used to distinguish between the direct and indirect
QDs. The exciton fine structure is studied by means of optical alignment and
optical orientation techniques in magnetic fields applied in the Faraday or
Voigt geometries. A drastic difference in emission polarization is found for
the excitons in the direct QDs involving a -valley electron and the
excitons in the indirect QDs contributed by an -valley electron. We show
that in the direct QDs the exciton spin dynamics is controlled by the
anisotropic exchange splitting, while in the indirect QDs it is determined by
the hyperfine interaction with nuclear field fluctuations. The anisotropic
exchange splitting is determined for the direct QD excitons and compared with
model calculations
Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
The spin structure and spin dynamics of excitons in an ensemble of
(In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both
direct and indirect band gap dots, are studied. Time-resolved and spectral
selective techniques are used to distinguish between the direct and indirect
QDs. The exciton fine structure is studied by means of optical alignment and
optical orientation techniques in magnetic fields applied in the Faraday or
Voigt geometries. A drastic difference in emission polarization is found for
the excitons in the direct QDs involving a -valley electron and the
excitons in the indirect QDs contributed by an -valley electron. We show
that in the direct QDs the exciton spin dynamics is controlled by the
anisotropic exchange splitting, while in the indirect QDs it is determined by
the hyperfine interaction with nuclear field fluctuations. The anisotropic
exchange splitting is determined for the direct QD excitons and compared with
model calculations
Dynamic polarization of electron spins interacting with nuclei in semiconductor nanostructures
We suggest a new spin orientation mechanism for localized electrons:
. The
angular momentum for the electrons is gained from the nuclear spin system via
the hyperfine interaction in a weak magnetic field. For this the sample is
illuminated by an unpolarized light, which directly polarizes neither the
electrons nor the nuclei. We predict, that for the electrons bound in localized
excitons 100% spin polarization can be reached in longitudinal magnetic fields
of a few millitesla. The proof of principle experiment is performed on
momentum-indirect excitons in (In,Al)As/AlAs quantum dots, where in a magnetic
field of 17 mT the electron spin polarization of 30% is measured.Comment: 6+9 pages, 3+9 figure
Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment
The exciton spin dynamics are investigated both experimentally and
theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect
band gap and a type-II band alignment. The magnetic-field-induced circular
polarization of photoluminescence, , is studied as function of the
magnetic field strength and direction as well as sample temperature. The
observed nonmonotonic behaviour of these functions is provided by the interplay
of bright and dark exciton states contributing to the emission. To interpret
the experiment, we have developed a kinetic master equation model which
accounts for the dynamics of the spin states in this exciton quartet, radiative
and nonradiative recombination processes, and redistribution of excitons
between these states as result of spin relaxation. The model offers
quantitative agreement with experiment and allows us to evaluate, for the
studied structure, the heavy-hole factor, , and the spin
relaxation times of electron, s, and hole, s, bound in the exciton.Comment: 17 pages, 16 figure
Influence of the heterointerface sharpness on exciton recombination dynamics in an ensemble of (In,Al)As/AlAs quantum dots with indirect band-gap
The dynamics of exciton recombination in an ensemble of indirect band-gap
(In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime
of confined excitons which are indirect in momentum-space is mainly influenced
by the sharpness of the heterointerface between the (In,Al)As quantum dot and
the AlAs barrier matrix. Time-resolved photoluminescence experiments and
theoretical model calculations reveal a strong dependence of the exciton
lifetime on the thickness of the interface diffusion layer. The lifetime of
excitons with a particular optical transition energy varies because this energy
is obtained for quantum dots differing in size, shape and composition. The
different exciton lifetimes, which result in photoluminescence with
non-exponential decay obeying a power-law function, can be described by a
phenomenological distribution function, which allows one to explain the
photoluminescence decay with one fitting parameter only.Comment: 10 pages, 7 figure
Critical Strain Region Evaluation of Self-Assembled Semiconductor Quantum Dots
A novel peak finding method to map the strain from high resolution transmission electron micrographs, known as the Peak Pairs method, has been applied to In(Ga) As/AlGaAs quantum dot (QD) samples, which present stacking faults emerging from the QD edges. Moreover, strain distribution has been simulated by the finite element method applying the elastic theory on a 3D QD model. The agreement existing between determined and simulated strain values reveals that these techniques are consistent enough to qualitatively characterize the strain distribution of nanostructured materials. The correct application of both methods allows the localization of critical strain zones in semiconductor QDs, predicting the nucleation of defects, and being a very useful tool for the design of semiconductor device
Dynamic Polarization of Electron Spins in Indirect Band Gap (In,Al)As/AlAs Quantum Dots in a Weak Magnetic Field: Experiment and Theory
A novel spin orientation mechanism - dynamic electron spin polarization - has been recently suggested in Phys. Rev. Lett. 125, 156801 (2020)PRLTAO0031-900710.1103/PhysRevLett.125.156801. It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field, and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting. © 2021 American Physical Society.We thank M. M. Glazov and M. O. Nestoklon for fruitful discussions. The experimental part of this research has been supported by the Deutsche Forschungsgemeinschaft (Grant No. 409810106) and by the Russian Foundation for Basic Research (Grant Nos. 19-52-12001 and 19-02-00098). M.B. acknowledges the support by the Deutsche Forschungsgemeinschaft (ICRC TRR 160, project A01). The theoretical studies by D.S.S. were supported by the RF President Grant No. MK-5158.2021.1.2, the Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS,” and the Russian Foundation for Basic Research Grant Nos. 19-52-12054 and 20-32-70048. The theoretical studies by A.V.S. were supported by the Russian Foundation for Basic Research Grant No. 19-02-00184
Spin-flip Raman scattering of the -X mixed exciton in indirect band-gap (In,Al)As/AlAs quantum dots
The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy
exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton
states with potential applications in quantum information. Optical access to
these excitons is provided by mixing of the - and X-conduction band
valleys, from which control of their spin states can be gained. This access is
used here for studying the exciton spin-level structure by resonant spin-flip
Raman scattering, allowing us to accurately measure the anisotropic hole and
isotropic electron factors. The spin-flip mechanisms for the indirect
exciton and its constituents as well as the underlying optical selection rules
are determined. The spin-flip intensity is a reliable measure of the strength
of -X-valley mixing, as evidenced by both experiment and theory.Comment: 5 pages, 3 figure
Semiconductor A3B5 nanostructures for infrared femtosecond lasers
Two techniques were suggested and tested for the recovery time shortening of saturable absorbers on a base of A3B5 compounds including quantum wells. The first one, proposed by authors, is the sample post-growth treatment by UV laser radiation; it implied generation of point defects, which, in its turn, led to electron-hole recombination acceleration and to recovery time shortening by an order of magnitude and more. Another technique based on special design of barriers gave promising results for the fast saturable absorbers. Semiconductor mirrors designed for Yb3+:KY(WO4)2 infrared laser mode locking led to 115 fs stable modelocking regime with average power close to CW operation. Results on fast saturable absorbers for spectral region of 1500 nm are also presented
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