Edge-emitting Quantum Dot Research Articles

Quantum dot laser optimization: selectively doped layers

Edge emitting quantum dot (QD) lasers are discussed. It has been recently proposed to use modulation p-doping of the layers that are adjacent to QD layers in order to control QD's charge state. Experimentally it has been proven useful to enhance ground state lasing and suppress the onset of excited state lasing at high injection. These results have been also confirmed with numerical calculations involving solution of drift-diffusion equations. However, deep understanding of physical reasons for such behavior and laser optimization requires analytical approaches to the problem. In this paper, under a set of assumptions we provide an analytical model that explains major effects of selective p-doping. Capture rates of elections and holes can be calculated by solving Poisson equations for electrons and holes around the charged QD layer. The charge itself is ruled by capture rates and selective doping concentration. We analyzed this self-consistent set of equations and showed that it can be used to optimize QD laser performance and to explain underlying physics.

Optical pulse generation in single section InAs/GaAs quantum dot edge emitting lasers under continuous wave operation

Observation of sub-picosecond pulses from a single section Fabry Perot InAs/GaAs edge emitting quantum dot (QD) based laser at 1.3 μm under continuous wave operation is reported. After group delay dispersion compensation, pulse durations as short as 770 fs in a 45 GHz repetition rate device have been measured, with 1.9 W of peak power and a narrow radio frequency spectrum of only a few kHz linewidth. The experiments show evidence of an unexplored mode locking regime in the InAs/GaAs quantum dot material system, which still needs theoretical modelling and further analysis.

Carrier capture delay and modulation bandwidth in an edge-emitting quantum dot laser

We show that the carrier capture from the optical confinement layer into quantum dots (QDs) can strongly limit the modulation bandwidth ω−3 dB of a QD laser. As a function of the cross-section σn of carrier capture into a QD, ω−3 dB asymptotically approaches its highest value when σn→∞ (the case of instantaneous capture). With reducing σn, ω−3 dB decreases and becomes zero at a certain nonvanishing σnmin. The use of multiple-layers with QDs significantly improves the laser modulation response—ω−3 dB is considerably higher in a multilayer structure as compared to a single-layer structure at the same dc current.

Thermally dependent characteristics and spectral hole burning of the double-lasing, edge-emitting quantum-dot laser

Thermally induced behavior of double-lasing edge-emitting quantum dot (QD) laser is investigated by coupling the electron/hole rate equation model with thermal analysis. The increase in substrate temperature due to laser self-heating causes the gradual and continual degradation of ground-state slope efficiency, roll-over, which eventually leads to a complete loss of ground-state light emission. Early excited-state spectral hole burning is observed, which is attributed to carrier leakage from the excited-state to the ground-state induced by the vigorous ground-state stimulated emission. At elevated temperatures, the enhanced carrier transport/communication yields the electron/hole occupation probabilities approaching quasithermal equilibrium, i.e., thermal equilibration. Spectral analysis also shows that self-heating results in recovery of the ground-state spectral hole burning of electron, which can be explained by the thermal equilibration. Homogeneous broadening optically synchronizes all the inhomogeneously broadened QDs by involving all the carriers at the same mode in different QDs, so that QD laser’s performance becomes more thermally sensitive. The strong coupling between thermally-induced emission and the spectral hole burning is demonstrated.

1.43 µm InAs bilayer quantum dot lasers on GaAs substrate

An edge emitting quantum dot (QD) laser at 1430 nm is demonstrated with a structure grown by molecular beam epitaxy on GaAs substrate. The active region is based on three InAs bilayer QDs embedded in a conventional AlGaAs/GaAs waveguide. The threshold current density is 134 A/cm2. Output power of 23 mW per facet is achieved.