Spatio-temporal dynamics of localized excitons in semiconductor nanostructures

We have constructed equipment for spatially resolved optical investigation of semiconductor nanostructures, notably a low-temperature microphotoluminescence system and a high resolution imaging spectrometer. The combination of microphotoluminescence and high spectral resolution opens up new possibilites in studying localized carriers in semiconductor nanostructures. The initial focus will be on localized excitonic states in narrow quantum wells and quantum wires. Interface roughness and chemical inhomogeneities in the semiconductor heterostructure introduce fluctuations in the energy landscape, which lead to the localization of low energy excitons. These excitons form quantum-dot-like states with well defined transition energies and extremely sharp photoluminescence lines. Using the microphotoluminescence technique, such lines may be studied individually and statistical information about the constituents of the luminescent system can be obtained.
To gain more information about carrier dynamics and relaxation in these types of nanostructures the high spatial resolution must be combined with spectroscopic techniques having high temporal resolution. One useful technique is based on coherent control that utilizes phase-locked laser pulses. With a newly built actively stabilized Mach-Zender interferometer we can probe coherence times of individual quantum dot states as an example. In this context we can follow the time evolution of a wavepacket composed of quantum dot states reflecting the atomic-like optical properties of this solid state system. Since disorder in nanostructures complicates the physics it is important to find simpler model systems. One avenue for doing this is to produce coupled quantum dots using various electron beam processing and etching techniques. With dots in controllable distances we are pursuing the possibility to study individual dot-dot couplings that are believed to be dominated by dipole-dipole interactions and migration effects.