On the Phonon Interactions and Terahertz Excitations among Coulomb-correlated Charge Carriers of Semiconductors

The first part of this thesis deals with the elementary interaction between charge carriers and lattice vibrations in semiconductors. The investigations are separated into two projects presented in Chaps. 3 and 4 where the effects of lattice vibrations on the semiconductor luminescence are discussed. In a quantum-mechanical sense, these delocalized excited states of vibrational modes in the crystal structure may be described by a quasiparticle called phonon. Among others, longitudinal-optical phonons may also participate in electron-hole recombination giving rise to pronounced replicas, so-called phonon sidebands, in the photoluminescence spectrum of a semiconductor. It is well-known that a dielectrically structured environment substantially alters the luminescence spectra and appearing resonances if a semiconductor nanostructure such as a quantum well is placed at an antinode of the intracavity field. Tuning the microcavity resonance to coincide with the 1s-exciton resonance such that the resonance of the resonator and of the active material are degenerate leads to the famous scenario of normal-mode coupling where the resonance splits into two peaks. On the basis of this thoroughly investigated system, the earlier phonon-related studies are extended in Chap. 3 of this thesis. The discussion delves into the question how a dielectric environment modifies and influences the phonon-assisted photoluminescence. Here, not only a cavity whose resonance frequency coincides with the 1s-exciton resonance is explored but, in particular, a cavity which is detuned to be resonant with the first phonon sideband of the semiconductor emission. After a detailed numerical study of the different cavity configurations, the analysis is topped off by a rigorous analytic model. The second phonon-related project is presented in Chap. 4 where the origin of scattering between carriers and phonons is scrutinized in detail. Set by the crystal structure, a semiconductor material may exhibit strong polar behavior. Thus, depending on the structure and system configuration, either polar carrier-phonon interaction or non-polar carrier-phonon scattering prevails in the semiconductor material. Additionally, the Coulomb attraction modifies the interaction behavior between charge carriers and lattice vibrations such that single carriers behave differently than an interacting many-body system. This raises the question under which conditions and on which basis an intrinsically polar material may be dominated by non-polar carrier-phonon scattering. Supported by experimentally measured phonon-assisted photoluminescence spectra, a systematic many-body theory is presented to explain the origin of exciton-phonon interaction in polar semiconductors and identify its role in an interacting many-body system. The Coulomb interaction in many-body systems is not only of particular importance for mediating the coupling among excitons and LO phonons but also plays a crucial role in intra-exciton transitions where diffusive Coulomb scattering gives rise for extensions of the usual dipole-selection rules. The energies of far-infrared (FIR) fields at terahertz frequencies are in the range of milli-electron volts and thus three orders of magnitude smaller than typical band-gap energies. Since the internal energy structure of correlated semiconductor many-body states falls in this meV range, THz spectroscopy is an unambiguous method to probe these systems, i.e., identify exciton populations, and to induce controlled transitions between the excited quasi-particle states. Consequently, the combination of pulsed optical and THz fields allows not only for the creation of many-body excitations but also for the characterization of their dynamic evolution and the controlled manipulation of the involved quantum states. Applying an external magnetic field, both the exciton properties and THz-induced intra-exciton transitions may be modified. Chapter 5 focuses on the control and manipulation of the intra-exciton carrier transfer by magnetic-field effects. Experimental findings are corroborated by a theoretical description, rigorously expanded by fully including linear and nonlinear magnetic-field contributions to the total-system Hamiltonian