The optical emission and gain properties of Ga(AsSb) quantum-islands are investigate. These islands form during growth
in a self-organized process in a series of Ga(AsSb)/GaAs/(AlGa)As heterostructures, resulting in an additional in-plane
hole confinement of several hundreds of meV. The shape of the in-plane confinement potential is nearly parabolic and thus
yields almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum islands are 100nm
in diameter and exhibit an in-plane variation of the Sb concentration of more than 30 %. Up to seven bound hole states
are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation
density, lattice temperature, and excitation photon energy and reveal fast carrier capture into and relaxation within the
quantum islands. Furthermore, the optical gain is measured using the variable stripe-length method and the advantages of
such structures as active laser material are discussed.
Various samples from the GaInNAs dilute nitride material system are modeled microscopically and good agreement with experiment is shown for the optical gain, linewidth enhancement factor, photomodulated reflectance and photoluminescence. Even though the differential gain is reduced by the inclusion of nitrogen, the linewidth enhancement factor is shown to stay almost unchanged. Radiative decay times are calculated and show a strong change in their density dependence above threshold.
A dynamical laser model is coupled to a fully microscopic calculation of scattering rates, allowing effcient calculations without phenomenological parameters. The approach is used to analyze nonequilibrium effects in the switch-on of an optically pumped laser structure. Lasing leads to kinetic hole burning in both electron and hole distribution. The gain spectrum, however, does not show spectrally narrow hole burning but a reduction over a wide range of frequencies compared to the equilibrium gain because of the large homogeneous broadening in the high density lasing system.
We report on a novel electro-optic modulator structure based on the two-dimensional Franz- Keldysh effect (2D-FKE) in multiple quantum well (MQW) structures. Due to the increased electron-hole interaction in these quasi-two-dimensional systems, strong excitonic resonances are observed even at room temperature. If an electric field is applied parallel to the layers of a MQW structure, very low electric fields (10 - 30 kV/cm) are sufficient to cause field ionization of the excitons, because of their weak in-plane confinement. Large absorption changes as high as 7000 cm-1 with field changes of only 30 kV/cm have been observed in GaAs/AlGaAs-MQWs. In addition, an increase of the absorption below and oscillations of the absorption coefficient above each subband transition are obtained due to the two-dimensional Franz-Keldysh effect. These features have been applied in our novel electro- optic modulator structure. Using interdigitated metal-semiconductor-metal (MSM) contacts, high in-plane electric fields can be generated with moderate voltages. Furthermore the low capacitance of these MSM structures is particularly favorable for high speed applications. In a MSM-modulator structure, consisting of 75 GaAs/AlGaAs quantum wells with a distributed Bragg-reflector (DBR) below the MQW-layers, a maximum contrast ratio of 5:1 without using any cavity effects has been achieved with a voltage swing of 20 V.