M. Lorke

Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems

A microscopic theory is used to study the optical properties of semiconductor quantum dots. The dephasing of a coherent excitation and line shifts of the interband transitions due to carrier-carrier Coulomb interaction and carrier-phonon interaction are determined from a quantum kinetic treatment of correlation processes. We investigate the density dependence of both mechanisms and clarify the importance of various dephasing channels involving the localized and delocalized states of the system.

Excitation dependences of gain and carrier-induced refractive index change in quantum-dot lasers

The excitation-density dependence of optical gain and refractive index changes in quantum-dot active media is investigated on the basis of a microscopic theory. Carrier-carrier Coulomb interaction and carrier-phonon interaction are treated on the level of a quantum-kinetic description. In the range of small optical gain the authors find small values of the α factor, while in the regime of gain saturation ∣α∣ increases drastically.

Coulomb scattering in nitride-based self-assembled quantum dot systems

We study the carrier capture and relaxation due to Coulomb scattering in group-III nitride quantum dots on the basis of population kinetics. For the states involved in the scattering processes the combined influence of the quantum-confined Stark effect and many-body renormalizations is taken into account. The charge separation induced by the built-in field has important consequences on the capture and relaxation rates. It is shown that its main effect comes through the renormalization of the energies of the states involved in the collisions and leads to an increase in the scattering efficency.

Quantum-dot nano-cavity lasers with Purcell-enhanced stimulated emission

We present a rate equation model for quantum-dot light-emitting devices that take into account Purcell enhancement of both spontaneous emission and stimulated emission as well as the spectral profile of the optical and electronic density-of-states. We find that below threshold the β-factor in a quantum-dot nanolaser depends strongly on the pump. For quantum dots with linewidth comparable to that of the cavity, we then show that an otherwise non-lasing device can lase due to Purcell enhancement of the stimulated emission. Finally, we compare the rate equation model to a microscopic model and obtain good agreement.

Modulation response of quantum dot nanolight-emitting-diodes exploiting purcell-enhanced spontaneous emission

The modulation bandwidth for a quantum dot light-emitting device is calculated using a detailed model for the spontaneous emission including the optical and electronic density-of-states. We show that the Purcell enhancement of the spontaneous emission rate depends critically on the degree of inhomogeneous broadening relative to the cavity linewidth and can improve the modulation speed only within certain parameter regimes.

Two-Dimensional Heterojunctions from Nonlocal Manipulations of the Interactions

We propose to create lateral heterojunctions in two-dimensional materials based on nonlocal manipulations of the Coulomb interaction using structured dielectric environments. By means of ab initio calculations for MoS2 as well as generic semiconductor models, we show that the Coulomb interaction-induced self-energy corrections in real space are sufficiently nonlocal to be manipulated externally, but still local enough to induce spatially sharp interfaces within a single homogeneous monolayer to form heterojunctions. We find a type-II heterojunction band scheme promoted by a laterally structured dielectric environment, which exhibits a sharp band gap crossover within less than 5 unit cells.

High beta lasing in micropillar cavities with adiabatic layer design

We report on lasing in optically pumped adiabatic micropillar cavities, based on the AlAs/GaAs material system. A detailed study of the threshold pump power and the spontaneous emission β factor in the lasing regime for different diameters dc is presented. We demonstrate a reduction of the threshold pump power by over 2 orders of magnitude from dc = 2.25 μm down to 0.95 μm. Lasing with β factors exceeding 0.5 shows that adiabatic micropillars are operating deeply in the cavity quantum electrodynamics regime.

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

We report on density functional theory investigations of the electronic properties of monofunctional ligands adsorbed on ZnO-(1010) surfaces and ZnO nanowires using semi-local and hybrid exchange-correlation functionals. We consider three anchor groups, namely thiol, amino, and carboxyl groups. Our results indicate that neither the carboxyl nor the amino group modify the transport and conductivity properties of ZnO. In contrast, the modification of the ZnO surface and nanostructure with thiol leads to insertion of molecular states in the band gap, thus suggesting that functionalization with this moiety may customize the optical properties of ZnO nanomaterials.

Will Quantum Dots Replace Quantum Wells As the Active Medium of Choice in Future Semiconductor Lasers

The lasing capabilities and limitations of quantum dots are assessed using a first-principles theory with a rigorous treatment of relevant physics and without the free parameters plaguing predictive capabilities in usual gain calculations. Our results reveal quantitatively the extent the reduced threshold advantage is confronted with a larger sensitivity to saturation effects. Added to this intrinsic constraint is the present experimental performance limitation arising from inhomogeneous broadening due to growth fluctuations.

Influence of carrier dynamics on the modulation bandwidth of quantum-dot based nanocavity devices

We theoretically investigate the modulation response of quantum-dot based nanocavity light emitting devices. For high Purcell enhancement factors, our theory predicts the possibility of decreasing the modulation bandwidth with increasing scattering rate into the lasing quantum-dot state. This counterintuitive effect is investigated using a microscopic semiconductor model. The resulting guidelines for possible optimizations of quantum-dot based nanocavity laser devices are given.