Paola Borri

Time-resolved four-wave mixing (TR FWM) is measured on InAs-based quantum-dot (QD) active waveguides for edge emitting QD lasers. Three types of structures have been investigated at room temperature, having different QD confinement energies. One type consists of binary/ternary InAs/InGaAs quantum dots [1] with a ground-state transition emitting at ~1.08mm wavelength, and ~110meV energy separation to the wetting layer transition. The second type consists of mixed-crystal InGaAs QDs emitting at ~1.17mm, with ~210meV separation to the wetting layer. The third type consists of InAs quantum dots grown in a 5nm wide InGaAs quantum well, with a ground-state emission at ~1.25mm and ~280meV separation to the wetting layer. Three stacked layers of QDs separated by thick (20-30nm) GaAs spacers were forming the active region of a PIN a ridge waveguide with tilted facets, allowing for electrical injection and suppression of multiple reflections and lasing. Waveguide geometries of 5-8mm ridge width and 0.5-1mm lengths were used. TR FWM was measured in the absorption, transparency and gain regime of the devices, using ~150fs laser pulses resonant to the dot ground-state transition in an heterodyne detection scheme [1]. A dephasing time of ~250fs is inferred at zero bias current and decreases with increasing electrical injection. While the dephasing time drops to below 50fs in the binary/ternary QDs at the maximum applied current [1], the decrease is less pronounced in the structures with stronger confinement energy. Around transparency the FWM shows a composite structure. This is explained by the discrete occupation number in QDs. At transparency, the number of dots occupied with two excitons , i.e. in the gain, compensates the number of dots with zero excitons, i.e. in the absorption. The third-order polarization is thus the overlap of the contributions from the absorptive dots and the inverted dots. These two contributions have different dephasing times and a relative phase shift of p. Numerical simulations of the TR FWM will be discussed, which are obtained by solving the optical Bloch equations for an inhomogeneously broadened ensemble of two-level systems which are initially partly in the excited state and partly in the ground state. Finally, comparison between the dephasing time and the population dynamics of the dot ground state transition, which have been measured with differential transmission spectroscopy, will be presented. We observe that the population dynamics are in general slower than the dephasing, and are strongly modified by the different confinement energies, indicating an elastic nature of the measured dephasing.

[1] P. Borri et al. Appl. Phys. Lett. 76, 1380 (2000).