The Role of Multilevel Landau-Zener Interference in Extreme Harmonic Generation

Motivated by the observation of multiphoton electric dipole spin resonance processes in InAs nanowires, we theoretically study the transport dynamics of a periodically driven five-level system, modeling the level structure of a two-electron double quantum dot. We show that the observed multiphoton resonances, which are dominant near interdot charge transitions, are due to multilevel Landau-Zener-Stuckelberg-Majorana interference. Here a third energy level serves as a shuttle that transfers population between the two resonant spin states. By numerically integrating the master equation we replicate the main features observed in the experiments: multiphoton resonances (as large as 8 photons), a robust odd-even dependence, and oscillations in the electric dipole spin resonance signal as a function of energy level detuning

Resonant spin-dependent electron coupling in a III-V/II-VI heterovalent double quantum well

We report on design, fabrication, and magnetooptical studies of a III-V/II-VI hybrid structure containing a GaAs/AlGaAs/ZnSe/ZnCdMnSe double quantum well (QW). The structure design allows one to tune the QW levels into the resonance, thus facilitating penetration of the electron wave function from the diluted magnetic semiconductor ZnCdMnSe QW into the nonmagnetic GaAs QW and vice versa. Magneto-photoluminescence studies demonstrate level anticrossing and strong intermixing resulting in a drastic renormalization of the electron effective g factor, in perfect agreement with the energy level calculations.Comment: 4 pages, 5 Postscript figures, uses revtex

Optical orientation of electron spins in GaAs quantum wells

We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 Ang and 198 Ang. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling. We find in wide quantum wells that the measured initial degree of polarization of the luminescence follows closely the spin polarization of the optically excited electrons calculated as a function of energy. This implies that the orientation of the electron spins is essentially preserved when the electrons relax from the optically excited high-energy states to quasi-thermal equilibrium of their momenta. Due to initial spin relaxation, the measured polarization in narrow quantum wells is reduced by a constant factor that does not depend on the excitation energy.Comment: 12 pages, 9 figure

Extreme Harmonic Generation in Electrically Driven Spin Resonance

We report the observation of multiple harmonic generation in electric dipole spin resonance in an InAs nanowire double quantum dot. The harmonics display a remarkable detuning dependence: near the interdot charge transition as many as eight harmonics are observed, while at large detunings we only observe the fundamental spin resonance condition. The detuning dependence indicates that the observed harmonics may be due to Landau-Zener transition dynamics at anticrossings in the energy level spectrum.Comment: Related papers at http://pettagroup.princeton.ed

Spin Injection and Relaxation in Ferromagnet-Semiconductor Heterostructures

We present a complete description of spin injection and detection in Fe/Al_xGa_{1-x}As/GaAs heterostructures for temperatures from 2 to 295 K. Measurements of the steady-state spin polarization in the semiconductor indicate three temperature regimes for spin transport and relaxation. At temperatures below 70 K, spin-polarized electrons injected into quantum well structures form excitons, and the spin polarization in the quantum well depends strongly on the electrical bias conditions. At intermediate temperatures, the spin polarization is determined primarily by the spin relaxation rate for free electrons in the quantum well. This process is slow relative to the excitonic spin relaxation rate at lower temperatures and is responsible for a broad maximum in the spin polarization between 100 and 200 K. The spin injection efficiency of the Fe/Al_xGa_{1-x}As Schottky barrier decreases at higher temperatures, although a steady-state spin polarization of at least 6 % is observed at 295 K.Comment: 3 Figures Submitted to Phys. Rev. Let

Spin relaxation of conduction electrons in bulk III-V semiconductors

Spin relaxation time of conduction electrons through the Elliot-Yafet, D'yakonov-Perel and Bir-Aronov-Pikus mechanisms is calculated theoretically for bulk GaAs, GaSb, InAs and InSb of both $n$- and $p$-type. Relative importance of each spin relaxation mechanism is compared and the diagrams showing the dominant mechanism are constructed as a function of temperature and impurity concentrations. Our approach is based upon theoretical calculation of the momentum relaxation rate and allows understanding of the interplay between various factors affecting the spin relaxation over a broad range of temperature and impurity concentration.Comment: an error in earlier version correcte

Polarization Control of the Non-linear Emission on Semiconductor Microcavities

The degree of circular polarization ($\wp$) of the non-linear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards \textbf{K} $\sim 0$ cavity-like states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the $\pm 1$ spin levels at \textbf{K} $\sim 0$. At short times after a pulsed excitation $\wp$ reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy ($+1/-1$ spin for positive/negative detuning).Comment: 8 pages, 3 eps figures, RevTeX, Physical Review Letters (accepted

Spin dynamics of low-dimensional excitons due to acoustic phonons

We investigate the spin dynamics of excitons interacting with acoustic phonons in quantum wells, quantum wires and quantum disks by employing a multiband model based on the $4\times4$ Luttinger Hamiltonian. We also use the Bir-Pikus Hamiltonian to model the coupling of excitons to both longitudinal acoustic phonons and transverse acoustic phonons, thereby providing us with a realistic framework in which to determine details of the spin dynamics of excitons. We use a fractional dimensional formulation to model the excitonic wavefunctions and we demonstrate explicitly the decrease of spin relaxation time with dimensionality. Our numerical results are consistent with experimental results of spin relaxation times for various configurations of the GaAs/Al$_{0.3}$Ga$_{0.7}$As material system. We find that longitudinal and transverse acoustic phonons are equally significant in processes of exciton spin relaxations involving acoustic phonons.Comment: 24 pages, 3 figure

Collective oscillations driven by correlation in the nonlinear optical regime

We present an analytical and numerical study of the coherent exciton polarization including exciton-exciton correlation. The time evolution after excitation with ultrashort optical pulses can be divided into a slowly varying polarization component and novel ultrafast collective modes. The frequency and damping of the collective modes are determined by the high-frequency properties of the retarded two-exciton correlation function, which includes Coulomb effects beyond the mean-field approximation. The overall time evolution depends on the low-frequency spectral behavior. The collective mode, well separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter