Extraction-Controlled Quantum Cascade Lasers

A simple two-well design for terahertz quantum cascade lasers is proposed which is based on scattering injection and the efficient extraction of electrons from the lower laser level by resonant tunneling. In contrast to existing designs this extraction also controls the positive differential conductivity. The device is analyzed by calculations based on nonequilibrium Green functions, which predict lasing operation well above 200 K at a frequency of 2.8 THz.Comment: 3 pages, 3 figures included in tex

Electron Transport through Nanosystems Driven by Coulomb Scattering

Electron transmission through nanosystems is blocked if there are no states connecting the left and the right reservoir. Electron-electron scattering can lift this blockade and we show that this feature can be conveniently implemented by considering a transport model based on many-particle states. We discuss typical signatures of this phenomena, such as the presence of a current signal for a finite bias window.Comment: final version, to appear in Physical Beview B (6 pages and 6 figures included in text, simulation details added and discussion clarified in comparison to first version

Quantum transport: The link between standard approaches in superlattices

Theories describing electrical transport in semiconductor superlattices can essentially be divided in three disjoint categories: i) transport in a miniband; ii) hopping between Wannier-Stark ladders; and iii) sequential tunneling. We present a quantum transport model, based on nonequilibrium Green functions, which, in the appropriate limits, reproduces the three conventional theories, and describes the transport in the previously unaccessible region of the parameter space.Comment: 4 Page

Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots

We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton $E_{SX}$ in resonance to a double exciton $E_{DX}$. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a $4$ nm PbS quantum dot.Comment: 10 pages, 6 figures. Submitted to the conference "Progress in Nonequilibrium Green's Functions VI Proceedings" at Lund University, Sweden, August 17th - 21st, 2015. To be published in the Journal of Physics: Conference Serie

Zero-phonon linewidth and phonon satellites in the optical absorption of nanowire-based quantum dots

The optical properties of quantum dots embedded in a catalytically grown semiconductor nanowire are studied theoretically. In comparison to dots in a bulk environment, the excitonic absorption is strongly modified by the one-dimensional character of the nanowire phonon spectrum. In addition to pronounced satellite peaks due to phonon-assisted absorption, we find a finite width of the zero-phonon line already in the lowest-order calculation.Comment: final version, to appear in Physical Review Letters (4 pages with 4 figures included, minor changes with respect to first version

A phenomenological position and energy resolving Lindblad approach to quantum kinetics

A general theoretical approach to study the quantum kinetics in a system coupled to a bath is proposed. Starting with the microscopic interaction, a Lindblad master equation is established, which goes beyond the common secular approximation. This allows for the treatment of systems, where coherences are generated by the bath couplings while avoiding the negative occupations occurring in the Bloch-Wangsness-Redfield kinetic equations. The versatility and accuracy of the approach is verified by its application to three entirely different physical systems: (i) electric transport through a double-dot system coupled to electronic reservoirs, (ii) exciton kinetics in coupled chromophores in the presence of a heat bath, and (iii) the simulation of quantum cascade lasers, where the coherent electron transport is established by scattering with phonons and impurities.Comment: accepted version (minor changes with respect to version 1), to appear in Physical Review

Superlattice gain in positive differential conductivity region

We analyze theoretically a superlattice structure proposed by A. Andronov et al. [JETP Lett 102, 207 (2015)] to give Terahertz gain for an operation point with positive differential conductivity. Here we confirm the existence of gain and show that an optimized structure displays gain above 20 cm$^{-1}$ at low temperatures, so that lasing may be observable. Comparing a variety of simulations, this gain is found to be strongly affected by elastic scattering. It is shown that the dephasing modifies the nature of the relevant states, so that the common analysis based on Wannier-Stark states is not reliable for a quantitative description of the gain in structures with extremely diagonal transitions.Comment: 4 pages, 5 figure

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

We present a systematic comparison of the results from our non-equilibrium Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum cascade lasers previously published in the literature. Employing identical material and simulation parameters for all samples, we observe that discrepancies between measured and calculated peak currents are similar for samples from a given group. This suggests that the differences between experiment and theory are partly due to a lacking reproducibility for devices fabricated at different laboratories. Varying the interface roughness height for different devices, we find that the peak current under lasing operation hardly changes, so that differences in interface quality appear not to be the sole reason for the lacking reproducibility.Comment: 9 pages, 6 figures; section VI with 2 figures added in v2; accepted for publication in J. Appl. Phy