Using three-pulse four-wave-mixing optical spectroscopy, we study the ultrafast dynamics of the quantum Hall system. We observe striking differences as compared to an undoped system, where the 2D electron gas is absent. In particular, we observe a large off-resonant signal with strong oscillations. Using a microscopic theory, we show that these are due to many-particle coherences created by interactions between photoexcited carriers and collective excitations of the 2D electron gas. We extract quantitative information about the dephasing and interference of these coherences.

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We investigate the formation dynamics of self-assembled polyelectrolyte multilayers on glass substrates by in situ and ex situ second harmonic generation (SHG) measurements and atomic force microscopy (AFM). The time dependence of the SHG signal during the adsorption process is attributed to a time dependent surface potential of the polyelectrolyte film. The dynamics can be quantitatively understood using a random sequential adsorption (RSA) model for the buildup of a film consisting of polyelectrolyte disks with polydisperse sizes. Differences between wet and dry films are also investigated.

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