Electron transport Effect on Optical Response of Quantum-Cascade Structures

Abstract

The quantum-cascade laser is an unique source of the THz laser radiation operated in continuous-wave and pulse regimes [Gmachl et al (2001)]. History of these lasers counts more than ten years. However, many aspects of the carrier transport and interaction with light field are still unclear. Very important question concerning physics of the quantum-cascade structures (QCS) is the following: which kind of transport, coherent or incoherent, is prevailed in QCS? There were many discussions about the problem, and several attempts to estimate kind of transport were successful especially [Iotti et al (2001)],[Weber et al (2009)]. The answer on this question depends on conditions of QCS operation. For example, the coherent electron transport is of interest in the non-equilibrium regime at femtosecond and picosecond time intervals. The incoherent transport is prevalent at the high excitation level in the stationary quasi-equilibrium regime. In both cases, the electron transport influence on optical properties of the device. In this connection, the development of the theory for coherent and incoherent electron transport regimes, included many-body effects and light-matter interactions in QCS, is of actual interest. In this chapter, we provide modeling of optical and transport properties of QCS uncovering influence of the electron transport on optical characteristics. Lasing, light absorption and spontaneous emission in QCS are accompanied and affected by many complicated transport processes such as electron diffusion, drift, tunneling, recombination, generation, capture and escape mediated by electron-electron, electron-phonon and electron-photon scattering events [Piprek (2005)]. Most of these effects can be treated within the quasi-equilibrium approximation. However, the approximation is not valid at ultrashort time intervals which are of interest nowadays due to rapid development of the femtosecond spectroscopy for semiconductor nanostructures [Rulliere (2005)]. Other area demanding consideration of ultra-fast non-equilibrium processes is THz emitting of QCS in the pulse regime; that is under rapid development currently due to promising applications in fundamental and applied science [Lee (2009)]. 11