The research area "Fundamentals" contains projects with a clear focus on basic nonlinear physics: In project A02 we will experimentally and theoretically study the mixing of transient optical signals in wide-gap semiconductor structures under resonant excitation conditions. In order to generate efficient light pulses in the desired direction and time frame, we will evaluate the dephasing and the relaxation of optical excitations and their dependence on the excitation conditions in the nonlinear regime by wave-mixing and pump-probe experiments. Project A03 is focused on two-photon processes in semiconductor microcavities with embedded quantum dots. The project work will be concentrated on the down-conversion from the biexciton state via an optically induced virtual state, which leads to a cavity enhanced single photon emission. The joint theoretical and experimental quantum optical study aims for the demonstration of full optical control of cavity enhanced spontaneous single photon emission by stimulated down-conversion. Project A04 is focused on the nonlinear physics of polariton condensates in planar semiconductor micro-cavities. The collective nonlinear dynamics will be investigated using a combination of advanced spectroscopic techniques and theoretical modelling. Spatially structured optical excitation and specific design of microcavity structures will be used to tailor and control the polariton condensate propagation and orbital angular momentum states, enabling polariton-based functional photonic elements that can be reconfigured on the fly. The goal of project A06 is the optimization of the ultrafast acoustics for the modulation of the light emission from semiconductors. The main line of action is the tailoring of the acoustic pulses by acoustic superlattices or resonators, by which the spectral and temporal profile of the pulses is modified. We also plan to develop novel light emitters such as coupled microcavities containing quantum wells or dots for modulation. Further, we will extend the method to novel wavelength ranges as well as combination of ultrafast strain pulses with surface acoustic waves. In the second funding period there will be a common and pronounced focus on the enhancement of light-matter interaction with Bragg-type microresonators, which are important for the projects A03, A04, A06, and C04. The structures will be engineered in close cooperation among those projects.
In the new project A07 we will focus on experimental and theoretical studies of widely non-degenerate two-photon emission stimulated by femtosecond pulses. Similar to nonlinear optical absorptions, such processes are expected to be massively enhanced whenever the involved fields differ widely in their frequency. Specifically, we want to investigate non-degenerate two-photon emission in various optically pumped semi-conductors (GaAs, GaN, ZnO, Cu2O) as a function of the frequency ratio of the driving signal and idler field.
The new project A08 aims to provide a detailed microscopic understanding of the giant nonlinear response of nonlinear metasurfaces consisting of periodic arrays of metal nanoparticles on top of quantum-well semiconductor heterostructures. The giant nonlinearities are directly related to the coupling between the near field generated by the plasmonic excitations in the nanoparticle arrays and intersubband transitions. The focus will be on the asymmetry of the coupling, resulting from the geometry of the plasmonic resonators in real space and from the momentum-space asymmetry of the radiative decay of plasmonic excitations.