Density-matrix theory of the optical dynamics and transport in quantum cascade structures: The role of coherence

Abstract

The impact of coherence on the nonlinear optical response and stationary transport is studied in quantum cascade laser structures. Nonequilibrium effects such as pump-probe signals, the spatio-temporally resolved electron density evolution, and the subband population dynamics (Rabi flopping) as well as the stationary current characteristics are investigated within a microscopic density-matrix approach. Focusing on the stationary current and the recently observed gain oscillations, it is found that the inclusion of coherence leads to observable coherent effects in opposite parameter regimes regarding the relation between the level broadening and the tunnel coupling across the main injection barrier. This shows that coherence plays a complementary role in stationary transport and nonlinear optical dynamics in the sense that it leads to measurable effects in opposite regimes. For this reason, a fully coherent consideration of such nonequilibrium structures is necessary to describe the combined optical and transport properties.

Simulating terahertz quantum cascade lasers: Trends from samples from different labs

“…Simultaneously, simulations have been performed for a large variety of samples with different models. These can be based on rate equations for the electron densities, 3,4 Monte-Carlo simulations of the Boltzmann equation for the occupations of the k-states in the individual subbands, [5][6][7][8] density matrix calculations, [9][10][11][12][13] which have been also done k-resolved, 7,14,15 as well as nonequilibrium Green's functions (NEGF). [16][17][18][19][20] While the published results from either scheme typically agree well with experimental data, it is not clear how the choices of parameters (in particular, interface roughness (IFR) distributions and band offsets), specific approximations (such as screening models or various model-specific assumptions as subband temperatures), or model complexity affect the results.…”

Electron transport Effect on Optical Response of Quantum-Cascade Structures

“…Fig. 12 contains results of pump-probe optical experiments reported in [Weber et al (2009)]. The pump pulse have the shape of the Gaussian function.…”

“…The second pulse reads changes in the media undergoing optical absorption or gain. More details about pump-probe techniques can be found in [Weber et al (2009)]. Fig.…”

“…Especially, it concerns stationary operating regime. Recently, advance in modeling of femtosecond optical response of QCS has been reached [Iotti et al (2001)], [Weber et al (2009)]. Most successful approaches to electron transport modeling have been realized applying the density matrix theory [Meier (2007)].…”

“…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.…”