3 . Non-linear effects in nanostrustures with closely spaced energy levels

TDissertation is dedicated to theoretic consideration of the efficiency of far-IR generation in low-dimensional semiconductor heterostructures and ultrathin metallic films due to non-linear effects. The interband, intersubband and spin-flip transitions in semiconductor heterostructures and the intersubband electronic transitions in ultrathin metallic films are considered in the framework of the modern quantum kinetic theory. The efficiency of THz transformation of mid-IR radiation in biased superlattices, tunnel-coupled, step-like and asymmetric quantum wells and in ultrathin metallic films has been considered under two frequencies CO2 laser pump. The difference harmonic generations due to the interband transition for two-colour pump in a biased superlattice, and the THz coherent emission under ultrafast excitation by CO2 laser in an ultrathin metallic film have been considered. In asymmetric heterostructures with the spin-splitted energy spectrum, the availability of efficient THz transformation of mid-IR radiation for CO2 laser pump by difference harmonic generation has been demonstrated and the conversions of millimetre into submillimetre radiation due to third harmonic generation have been considered. The influence of many-body effects, which took into account depolarisation shift and non-local exchange, on linear response under the external harmonic excitation is considered for the cases of the intersubband transitions in biased superlattices and of the asymmetric quantum wells.