Quantum Cascade Lasers in High Magnetic Fields

The frontier of the rapidly emerging field of nano-optoelectronics relies on the understanding and control of intersubband transitions in low-dimensional systems. The continual search for new optoelectronics concepts and materials (including, but not limited to, III-V semiconductors, nitrides, and Si/Ge) has resulted in a rapid expansion of the field of intersubband physics and quantum cascade devices. A quantum cascade (QC) structure is a general concept of an optoelectronic device (laser, LED, frequency mixer, or detector) based on a cascade of radiative transitions between size-quantized energy levels in a multi-quantum-well structure. Today, Quantum Cascade Lasers (QCL), which are the only semiconductor devices operating from the mid-infrared (MIR) to the THz range of frequencies, represent one of the most striking outcomes of intersubband structure engineering, and provide a state-of-the-art model structure to study the basic properties of low-dimensional semiconductor systems. This dissertation concerns the experimental study of MIR and THz QC structures in high magnetic fields. Because of the similar energy and size scales of the spatial and magnetic confinements, the application of an external magnetic field offers a unique experimental tool to control and understand the most basic processes determining the performance of QC nanostructures: quantum confinement and intersubband relaxation. Specific issues addressed in this thesis are (i) mechanisms of intersubband electron relaxation, including electron-phonon, electron-electron, and interface effects; (ii) intersubband transitions in the effective zero-dimensional system ("magnetic" quantum box system); and (iii) intersubband radiative transitions in tilted magnetic fields. First we present detailed studies of GaAs/AlGaAs and GaInAs/AlInAs mid-IR QCLs. By comparing the experimental data and the model of the electron lifetime in the presence of a strong magnetic field, the lifetimes of the elastic and inelastic scattering processes are determined. Ultimately this results in the development (formulation) of a new powerful spectroscopic tool to study the scattering mechanisms in QC structures – intersubband magneto-spectroscopy. Secondly, a study of InAs/AlSb mid-IR QC structures is performed. By applying the method of the intersubband magneto-spectroscopy, we directly measured the quantum efficiency of intersubband processes in a model two-level system, and then obtain electron lifetimes of the upper-state of the radiative transition. Thirdly, GaAs/AlGaAs THz QCLs are studied. Here, a magnetic field was used as a tool to controllably transform a 2D multi-QW structure into effective 0D system with reduced (eventually quenched) non-radiative intersubband scattering. This allowed us to achieve laser emission from a single device in an unprecedented range of frequencies from 0.68 THz to 3.33 THz. Moreover, the device shows 1 THz lasing at temperatures up to 215 K, and 3 THz lasing up to 225 K. This is the longest wavelength, the widest spectral coverage, and the highest operational temperatures of any single THz solid state laser to date. The last chapter discusses QCL angular-resolved magnetospectroscopy. At tilted magnetic fields, additional optical transitions, never observed in QC structures, are allowed as a result of the intersubband-cyclotron coupling. Also, angular field measurements are an effective tool to study the effects related to cyclotron- and spin-splitting phenomena. Here we demonstrated the feasibility of QCL angular measurements at high magnetic fields, and discuss the first results.