Collective effects in the emission from semiconductor microcavities

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The present proposal aims at investigations of collective effects in the optical emission from semiconductor microcavities. Besides applying modern spectroscopic techniques to semiconductor systems, special emphasis will also be placed on the development of novel spectroscopic tools. Photon correlation spectroscopy using a streak camera - as developed in Dortmund - is the most adequate method here. The technique enables us to perform measurements of statistical dependences of photon emission processes with a temporal resolution of two picoseconds. As a complementary technique, a setup for correlation measurements using homodyne detection shall be used. This technique offers improved temporal resolution of about 100 femtoseconds and phase sensitivity, but also requires sophisticated data analysis. The systems to be investigated are quantum dot microcavity lasers and colloidal nanostructures, so called nanoplatelets. Concerning quantum dot lasers, we will investigate the question how coherence builds up during a pulse. At the beginning of a pulse, many quantum dots are still in the excited state, so a collective interaction with the light field inside the cavity seems probable. This should result in a significant short-lived increase of photon number fluctuations during the initial phase of pulse formation. In combination with systematic variation of the detuning between the cavity mode and the quantum dot ensemble, the results will allow for a much deeper understanding of the dynamics of quantum dot lasers. Colloidal nanoplatelets are well known for their anisotropy and the presence of a transition towards giant oscillator strength even without the presence of a cavity. The microscopic origin of this transition is not known yet, but collective effects seem like a reasonable explanation. Time-resolved investigations of the coherence of the emission should clarify, whether stimulated emission, superradiance or other effects are taking place. Another experimental technique that shall be realized is quantum optical excitation spectroscopy. The main idea behind this technique lies in the fact that the response of any non-linear system does not only depend on the mean intensity of that excitation, but also on its quantum optical properties in terms of fluctuations around the mean value. Using systematic variations of intensity fluctuations of the optical excitation, we will realize this technique and test some predictions. For example, the response from a non-linear system is expected to show a significant increase in the noise of the response when excited with a light field of strongly fluctuating intensity. This increase should already take place way below the non-linear threshold. Therefore it should open up an interesting and convenient possibility to identify and characterize non-linearities.