| Electron-electron interactions in semiconductor heterostructures are typically much weaker than the quantized energy spacings between their states. For materials with unusually small level spacings in the terahertz frequency range, however, these Coulomb interaction energies become comparable and can result in unique effects. Here, several systems are investigated in which these interactions play a major role.;InGaAs quantum posts are nanostructures with terahertz conduction level spacings. They are approximately cylindrical in shape, with heights in the range of 10-60 nm and diameters of ∼30 nm, and are embedded in an InGaAs quantum well of the same height. Here, the terahertz absorption of quantum posts is investigated when they contain their maximum allowable number of electrons. At this point, strong Coulomb repulsion in the posts prevents further charging, and the surrounding well is charged. Theoretical calculations show that absorption in this state is due to the "ionization" of charge from electronic states in the quantum posts to states in the surrounding quantum well. This ionizing transition, which otherwise would occur in the mid-IR band, is shifted to terahertz frequencies only because of a strong modification of the effective band structure of the quantum post and quantum well when highly charged. Ionization of these artificial atoms surrounded by an electron gas has no known analogue in atomic physics.;The use of quantum posts for quantum cascade lasers is also investigated. Quantum posts are integrated as the active region in a single period cascade structure. The UCSB Free Electron Laser, an intense quasi-CW source of terahertz radiation, is used to induce a resonant photocurrent as a method of characterization of charge injection and extraction in these structures.;Additionally, ultrafast spectroscopic techniques are used to determine the carrier capture times of quantum posts. Dynamics of charge pumped directly into the posts is also investigated, resulting in time and pump wavelength dependent absorption behavior that show complex charge dynamics.;Finally, the response of asymmetric quantum wells to intense terahertz fields is investigated, with a goal of finding a period-doubling bifurcation in the electromagnetic response. Period doubling is expected to occur in this system due to the strong Coulomb interaction of charge in the asymmetric well, which introduces nonlinearity into the equations of motion. Extensive simulations are performed to model the period doubling emission response, and an optimal well design is found. Experimental investigations of this effect in real samples are performed with the Free Electron Lasers at UCSB and in Dresden. |
