Ultrashort optical pulses applied to ferromagnets excite spin polarized hot electrons. Such an ultrashort-pulse excitation is followed by spin-flip scattering due to spin-orbit coupling, "intrinsic" magnetization dynamics and spin-polarized transport. I will present some of our work concerning the interplay of spin-flip scattering due to spin-orbit coupling and exchange scattering in a model system containing itinerant electrons which are exchange-coupled to localized electronic bands. I will also present results on spin-dependent transport of optically excited hot electrons in ferromagnet-metal heterostructures which have gained interest as THz emitters in recent years.
Ultrashort optical pulses applied to ferromagnets excite spin polarized electronic distributions far from equilibrium (which are often referred to as "hot electrons"). Such an ultrashort-pulse excitation can lead to demagnetization, but also to a loss of electronic spin polarization due to hot-electron transport in and out of ferromagnetic layers. I will present some of our work on both topics starting with a microscopic study of the of electron-phonon and exchange scattering dynamics in a two-sublattice model with itinerant electrons. I will also present results on spin-dependent transport [3] of optically excited hot electrons in ferromagnet-metal heterostructures which have gained interest as THz emitters in recent years. The numerical solution is achieved using a Particle-In-Cell approach to treat both transport and scattering effects in a numerically efficient way that is based on ab-initio input and can be easily adapted to different structures. If time permits, I will briefly discuss a microscopic model of the inverse Faraday effect. In the framework of a ferromagnetic Rashba system with a band gap, one can compute the complete switching dynamics including spin-orbit coupling, mean-field ferromagnetism and the effect of off-resonant optical fields/pulses.
We investigate ultrafast magnetization dynamics due to electron-phonon interaction in a ferromagnetic model sys- tem including spin-orbit coupling. By computing the reduced spin-density matrix and Boltzmann-type scattering integrals, we identify the microscopic mechanism with which the electronic spin is dissipated by electron-phonon scattering processes in ferromagnets. We present in some detail the numerical approach used to compute the scattering dynamics and discuss problems that arise in treating these dynamics.
We review a recently introduced simple model that describes magnetization dynamics in a system consisting of localized and itinerant carriers which are coupled by an antiferromagnetic exchange interaction. By also introducing a Stoner-like mean-field splitting in the itinerant carrier system, the model is intended to capture some important aspects of the electronic structure and the magnetization dynamics of ferrimagnets that involve itinerant carriers, such as GdFe(Co). In the framework of this model one can describe microscopically scattering dynamics due to the exchange interaction as well as coupling to phonons. We review some details of this model and show how it explains heat-induced magnetic switching in two-sublattice ferrimagnets. Further, we discuss the inuflence of angular momentum exchange with the environment, and present results for the magnetization dynamics that result from excitation above the compensation temperature.
We review a microscopic laser theory for quantum dots as active material for quantum cascade lasers, in which carrier collisions are treated at the level of quantum kinetic equations. The computed characteristics of such a quantum-dot active material are compared to a state-of-the-art quantum-well quantum cascade laser. We find that the current requirement to achieve a comparable gain-length product is reduced compared to that of the quantum-well quantum cascade laser.
We investigate theoretically the slowdown of optical pulses due to quantum-coherence effects in InGaAs-based quantum dots and quantum dot molecules. Simple models for the electronic structure of quantum dots and, in particular, quantum-dot molecules are described and calibrated using numerical simulations. It is shown how these models can be used to design optimized quantum-dot molecules for quantum coherence applications. The wave functions and energies obtained from the optimizations are used as input for a microscopic calculation of the quantum-dot material dynamics including carrier scattering and polarization dephasing. The achievable group velocity slowdown in quantum-coherence V schemes consisting of quantum-dot molecule states is shown to be substantially higher than what is achievable from similar transitions in typical InGaAs-based single quantum dots.
We present an investigation of electron-spin dynamics in p-doped bulk GaAs due to the electron-hole exchange
interaction, aka the Bir-Aronov-Pikus mechanism. We discuss under which conditions a spin relaxation times for
this mechanism is, in principle, accessible to experimental techniques, in particular to 2-photon photoemission,
but also Faraday/Kerr effect measurements. We give numerical results for the spin relaxation time for a range
of p-doping densities and temperatures. We then go beyond the relaxation time approximation and calculate
numerically the spin-dependent electron dynamics by including the spin-flip electron-hole exchange scattering
and spin-conserving carrier Coulomb scattering at the level of Boltzmann scattering integrals. We show that
the electronic dynamics deviates from the simple spin-relaxation dynamics for electrons excited at high energies
where the thermalization does not take place faster than the spin relaxation time. We also present a derivation
of the influence of screening on the electron-hole exchange scattering and conclude that it can be neglected for
the case of GaAs, but may become important for narrow-gap semiconductors.
We investigate the influence of the coupling between localized and
continuum states on the optical gain and refractive index in
self-organized quantum-dot structures under high-excitation conditions. For wide-bandgap nitride-based quantum-dot structures we show that the presence of strong many-body Coulomb interactions and the quantum-confined Stark effect result in absorption/gain features that depend on the quantum-dot dimensions in a nontrivial way. For
InAs/GaAs based quantum dots, we investigate the refractive index properties and show that negative α or linewidth enhancement factors may occur in these systems, which makes the beam quality (filamentation) properties of quantum-dot lasers very different from quantum-well lasers. This is consistent with measurements which show a reduction in quantum-dot laser filamentation as the injection level is increased.
This paper analyses the gain and carrier-induced refractive index change in group-III nitride quantum wells. An approach based on the semiconductor Bloch equations with carrier-carrier collisions treated at the level of quantum kinetic theory is used. The influences of the strong carrier-carrier Coulomb interaction and the quantum-confined Stark effect on laser threshold and output beam quality are discussed.
The linear light propagation in semiconductors is analyzed using a microscopic theory. Numerical results are compared with various approximation schemes based on additional boundary conditions (ABCs).
Results of microscopic modeling of semiconductor vertical-cavity surface-emitting lasers (VCSELs) are discussed. The treatment of the laser as a nonequilibrium many-body system provides a detailed understanding of the various processes that determine the laser output and the electron-hole-plasma excitation. It is shown that the transient gain dynamics are strongly influenced by nonequilibrium carrier effects. These gain dynamics together with the cavity design determine the delayed onset and the temporal and spectral shape of the laser output. The theory is evaluated to investigate how the laser output properties can be controlled in terms of (1) excitation conditions of the VCSEL, (2) the mirror design, which allows us to change the cavity quality and the resonance frequency, and (3) the number and position of semiconductor quantum wells as active material.
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