We observe an anomalous exciton lifetime in columnar InAs/GaAs quantum rods by using the time-resolved differential reflectivity (TRDR) technique. At low excitation density, the decay time of the TRDR signal show a clear excitonic type of behavior. At increased excitation, a resonant radiative lifetime develops, which seems to be dependent on the excited QD density being resonant with the probe laser spectrum. We interpret the resonance behavior as due to electromagnetic coupling between the excited QDs resonant with the probe laser.
We study a set of low temperature (LT, 250°C) Stranski-Krastanow InAs/GaAs quantum dots (QDs) grown using molecular beam epitaxy (MBE). The QDs are studied by Photoluminescence (PL) and Time Resolved Differential Reflectivity (TRDR) for obtaining the carrier dynamics also. The LT-growth is expected to combine an ultrafast response time with a large QD optical nonlinearity, making it a good candidate for ultrafast all-optical switching devices. We observe a QD photoluminescence peak around 1200 nm on top of a background due to the As<sub>Ga</sub>-V<sub>As</sub> center. We observe that the PL-efficiency is quenched above 30K. The PL-efficiency increases by a factor of 45 - 280 as a function of excitation wavelength around the GaAs bandgap, for different samples. This points towards good optical quality QDs, which are embedded in an LT-GaAs barrier with high trapping efficiency. In the TRDR measurements, we observe an initial fast decay (80ps) followed by a much slower decay of about 800ps. The strong temperature dependence of the PL-signal is not observed in the reflectivity signal. This leads us to conclude that the electrons tunnel out of the QD and are subsequently efficiently trapped by As antisite defects while the hole decay dynamics take place at a slower rate, which is monitored in TRDR. Our observations point towards QDs with good optical quality, embedded in a LT-GaAs barrier in which the carriers are efficiently trapped at anti-site defects.