1,721,025 research outputs found
Dynamics and optical manipulation of nuclear spin in self assembled (InGa)As/GaAs quantum dots
Full text linkNuclei and electrons trapped in quantum dots create a strongly interacting spin system due to the strong localization of electrons in quantum dots. Due to strain in self-assembled (In,Ga,As)GaAs quantum dots quadrupole interactions appear which have a significant influence on the nuclear spin system. Therefore it is no longer possible to describe the nuclear spin system as one system with certain characteristics like a buildup time and decay time. Instead, in the case of strain the nuclear spin system can be separated into two subsystems, one with a nuclear spin state |±1/2⟩ which is weakly influenced by quadrupole interactions and the other with nuclear spin states |±3/2⟩, |±5/2⟩,… which is strongly influenced by quadrupole interactions. Each subsystem possesses different characteristic buildup and decay times of dynamic nuclear polarization generated by circular polarized light excitation of the quantum dots. A fast decay mechanism for polarized nuclei occurred during an interruption of the excitation beam for quantum dots with a single resident electron on a nanosecond scale. In relation to this phenomenon strong differences for the dynamic nuclear polarization are detected when either pulsed- or CW-lasers are used for excitation due to the short excitation time in relation to the repetition rate of the pulsed-laser. Further a new effect of optical pumping of nuclear spins in quantum dots is observed. The new effect, called resonant nuclear spin pumping, is observed in a magnetic field oriented perpendicularly to the excitation light beam (Voigt geometry). It creates a nuclear polarization perpendicular to the externally applied magnetic field
Magnetic-field-induced second harmonic generation in semiconductors and insulators
Full text linkIn the first part, it is shown that application of a magnetic field induces optical second harmonic generation (SHG) in GaAs. This phenomenon arises from field-induced symmetry breaking causing new optical nonlinearities. A series of narrow SHG lines is observed in the spectral range from 1.52 to 1.77 eV that is attributed to Landau-level quantization of the band energy spectrum. The rotational anisotropy of the SHG signal distinctly differs from that of the electric-dipole approximation. Model calculations reveal that nonlinear magneto-optical spatial-dispersion that comes together with the electric-dipole term is the dominant mechanism for this nonlinearity.
In the second part, basically different mechanisms of optical second harmonic generation (SHG) in semiconductors, induced by an external magnetic field H, are identified experimentally by studying the diluted magnetic semiconductor (Cd,Mn)Te. For paramagnetic (Cd,Mn)Te the SHG response is governed by spin quantization of electronic states, in contrast with diamagnetic CdTe with its dominating orbital quantization. The mechanisms can be identified by the distinct magnetic field dependence of the SHG intensity which scales with the spin splitting in the paramagnetic case as compared to the H2 dependence observed for the diamagnetic case.
In the third part, three types of optical magnetic-field induced second harmonic (MFISH) generation are discussed in CuB2O4. Unusually sharp and intense electronic transitions in MFISH and linear absorption spectra provide selective access to the two nonequivalent Cu2+ sublattices. The magnetic phase diagram for both sublattices is determined by MFISH. Magnetic structure is dominated by antiferromagnetic order at the 4b site. Sublattice interactions transfer it to the 8d site where it coexists with a discoupled paramagnetic component
New mechanisms of optical harmonic generation in semiconductors
Full text linkIn this work two novel mechanisms of optical harmonic generation in semiconductors are presented and analysed on the basis of symmetry restrictions and the specific electronic structures.
The first mechanism is observed in the hexagonal semiconductor ZnO. Since its valence and conduction band have different parity, electric-dipole second harmonic generation (SHG) processes are symmetry forbidden for light incidence perpendicular to the c-plane. Nevertheless, the nonlinear spectroscopy of ZnO reveals narrow peaks in the spectral range from 3.4 to 3.5 eV, which are induced by an external magnetic field.
Microscopic model calculations establish the magneto-optical Stark effect as the main mechanism for the detected SHG involving 2S and 2P exciton states.
The effect results from the center-of-mass motion of excitons in the magnetic field. This motion leads to a perturbation equivalent of an electric field and causes an admixture of states with different parities.
The second mechanism is found in europium chalcogenides magnetic semiconductors. EuTe and EuSe are investigated by the spectroscopy of second harmonic generation and third harmonic generation (THG) in the vicinity of the optical band gap formed by transitions involving the 4f and 5d electronic orbitals of the magnetic Eu2+ ions. In these materials with a centrosymmetric crystal lattice, the electric-dipole SHG process is symmetry forbidden so that no signal is observed in zero magnetic field. Signal appears, however, in applied magnetic fields with the SHG intensity being proportional to the square of magnetization. The magnetic field and temperature dependencies of the induced SHG allow us to introduce a novel type of nonlinear optical susceptibility determined by the magnetic-dipole contribution in combination with a spontaneous or induced magnetization. The experimental results can be described qualitatively by a phenomenological model based on a symmetry analysis and are in good quantitative agreement with microscopic model calculations accounting for details of the electronic energy and spin structure.
Contrary to the SHG, electric dipole THG contributions are allowed for centrosymmetric materials. Consequently, a crystallographic third harmonic generation signal, showing different resonances, is found in europium chalcogenides over wide spectral ranges. In an external magnetic field a resonant induced magnetic contribution appears additively. Enhanced by a double resonance of the electric dipole THG process, the detected signal is of higher magnitude than the spin induced SHG signal
Optically detected cyclotron resonance in a single GaAs/AlGaAs heterojunction
Full text linkOptically detected far-infrared cyclotron resonance (FIR-ODCR) in GaAs/AlGaAs HJs is interpreted in the frame of an exciton-dissociation mechanism. It is possible to explain the ODR mechanism by an exciton drag, mediated by ballistically
propagating phonons. Furthermore, very narrow resonances are presented and realistic electron mobility values can be calculated. The exceptionally narrow ODCRs allow to measure conduction-band nonparabolicity effects and resolve satellite resonances, close to the main cyclotron resonance line
Optically detected resonances induced by far infrared radiation in quantum wells and quantum dots
Full text linkPhotoluminescence (PL) and optically detected resonances (ODR) where studied on semiconductor quantum wells and quantum dots. Magnetic fields of up to 33 T where applied to samples at temperatures between 0.25 K and 10 K.
In nonmagnetic quantum wells optically detected cyclotron resonance was used to determine basic properties such as effective mass and mobility of GaAs/AlGaAs quantum wells. In CdTe/CdMgTe quantum wells evidence for the singlet and triplet state of the negatively and positively charged exciton was found at high magnetic fields. In a highly n-type doped GaAs/AlGaAs quantum well, signatures of the fractional quantum hall effect were observed in PL and ODR data. Also shake up processes in a variety of quantum wells are discussed.
In magnetic quantum wells, cusps in the exciton shift are present at moderate magnetic fields which could be assigned to next nearest neighbor interactions between Mn2+ ion pairs and single ions. Resonances in InGaAs/GaAs quantum dots induced by far-infrared radiation have been observed optically. They were studied in quantum dots with different confinement potential and under a series of tilting angles between sample normal and magnetic field direction. The resonances could be assigned to trion formation due to cyclotron resonance in the wetting layer and transitions in the internal energy structure of the dots.
Also magnetic CdMnTe/ZnCdTe quantum dots with different Mn content were measured at magnetic fields up to 17 T. At low Mn concentrations a competition between the giant and intrinsic Zeeman splitting leads to a reduction of the polarization of the sample at high magnetic field which makes it possible to determine the Mn content by photoluminescence measurements
Magneto-optical properties of semiconductor nanocrystals in glass
Full text linkUnderstanding of the spin and exciton properties in colloidal nanostructures paves the way for their applications in a variety of fields, such as spintronics and quantum science and technology. Using magneto-optical experimental techniques, the fundamental properties such as polarization, g-factor, spin dynamics in three kind of semiconductor nanocrystals (NCs), i.e. CdSe, CuCl and all-inorganic perovskite (CsPbBr3, CsPbI3) are studied, which help to understand the exciton fine structures and related interac- tions in each material.
By combining both the experimental and theoretical efforts, the puzzling behavior of the polarized emission of dark excitons in the ensemble of CdSe NCs is explained by considering the nanocrystal size dispersion and phonon effect. The spin coherent dynamics in the pump-probe measurements show two components, i.e. one oscillating and one nonoscillating. The Larmor frequency from the oscillating component is unambiguously assigned to the electron based on the theoretical analysis. While the nonoscillating component is clarified to be contributed by the frozen exciton spin polarization created by the pump pulse in NCs with heavy-light hole splitting determined by the crystal field.
The polarization properties and g-factors in CuCl NCs are investigated comprehensively, where the degree of circular polarization of the Z3 exciton emission is found to increase linearly with the magnetic field at low temperatures up to 8 T. In the spin-flip Raman scattering measurements, a g-factor about 2 is resolved which can be assigned to the electron.
The magnetic field and temperature dependent recombination dynamics reveal a dark ground state in both CsPbBr3 and CsPbI3 NCs with corresponding bright-dark exciton splitting being 4.2 meV and 6.5 meV, respectively. In CsPbI3 NCs, very interesting anomalous polarization and spin dynamics are observed, which is related to the interaction between the dark exciton state and the bright exciton fine structure. The spin-flip Raman scattering measurements on CsPbI3 NCs also reveal two g-factors, i.e. one is about 2.5 and another one about 1.5
Recombination and spin dynamics of excitons in indirect (In,Al)As/AlAs quantum dots
Full text linkIn recent years semiconductor QDs have succeeded to perform the step from sample structures in fundamental research, with the aim to implement the spin state as a controllable size, to real device related applications, especially in the role of efficient light emitters. As the original aim has been proven to be difficult to fulfill and control reliably, new concepts for the further development of these structures had to be introduced. In the scope of the last years, the idea of a combining concept has been developed. QDs on the one hand have naturally an increased exciton recombination time, and further to that significantly longer spin relaxation times. On the other hand, classical indirect semiconductors show additionally increased exciton lifetimes, while their spin-orbit interaction is greatly reduced, making spin states more stable and longer-living. The combining concept therefore aims toward an indirect semiconductor with the additional limitations of a zero-dimensional structure. Based on this concept, the indirect (In,Al)As/AlAs QDs have been constructed, possessing a type I band alignment but having an indirect band gap in the momentum space.Main issue of the dissertation at hand is to give a detailed insight into the physical properties of these novel structures. The approaches used for the analysis and determination of the structure characteristics are based on optical measurements of the exciton states. By use of various measuring techniques like photoluminescence (time-integrated and time-resolved) as well as spin-flip Raman scattering spectroscopy different aspects of the sample characteristics have been determined. In this context the first analytical chapter concentrates on the basic optical properties of the indirect excitons in (In,Al)As/AlAs QDs and allows to assign the observed signals to specific energy levels. Geometrical QD characteristics are here shown to have fundamental influence on both exciton energy and the excitonic recombination times, which is shown to be elevated to the micro- and even millisecond timescales. The spin states defining the fine structure of the excitons are spotlighted in the second analytical chapter which is devoted to the spin-flip Raman scattering spectroscopy. This analytical method is shown to be a powerful tool for the determination of single particle and their complex g-factors. The dynamical behavior of the spin states, i.e. the longitudinal spin relaxation times are investigated in the subsequent chapter in which the spin state population of negatively charged excitons are measured by the degree of circular polarization emission. The population differences leading to the polarization degrees are achieved by both high magnetic fields as well as optical orientation of specific spin states. The mechanism of optical orientation is in the finalizing chapter more systematically studied for specific exciton levels that have been revealed and addressed in the scope of the thesis. In conclusion, it has been shown that indirect excitons in QDs possess spin states that are largely undisturbed by their environment enabling long spin relaxation times and large spin state splitting. The extended exciton recombination times furthermore allow the direct observation of these states by optical means making them highly rewarding structures for scientific research
Spinkohärenz und g-Faktor-Anisotropien in Halbleiter-Quantenstrukturen
Full text linkDer Spin von Ladungsträgern, Elektronen oder Löchern in Halbleiter-Quantenstrukturen hat sich in den letzten Jahrzehnten, als ein guter Kandidat zur Realisierung von Quanten-
Bits gezeigt. Eine rein optische Umsetzung der Spinkontrolle ist von besonderem Interesse, da sie unter Ausnutzung moderner Lasertechnologie zur ultraschnellen Quanteninformationsverarbeitung führen kann. Es gibt zahlreiche Bemühungen, durch Initialisierung und Manipulation des Spins Quanteninformationsverarbeitung zu betrieben. In den meisten Fällen wird dies durch ein Zwei-Niveau-System in einem externen Magnetfeld realisiert. Die Größe der resultierenden Spinaufspaltung wird durch den g-Faktor entlang der Feldrichtung ausgedrückt. In dieser Arbeit wird zum einen die Erzeugung von Spinkohärenz, zum anderen die g-Faktor-Anisotropie in Quantenpunkten und -schichten untersucht, um damit die entscheidenden Rückschlüsse auf die Bandstruktur der Quantenstrukturen und ihrer Materialzusammensetzung zu erhalten
- …
