Rydberg Exciton Interaction Dynamics

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Rydberg excitons are the solid state equivalent to Rydberg atoms and share many of their fascinating properties, for example huge interactions among each other up to the point of Rydberg blockade. They may also be used as sensitive probes for the presence of freecarriers, which cause similar blockade effects termed Plasma blockade already at densities of less than 0.01 electron-hole-pairs per cubic micrometer. For subsequent experiments aimed at harnessing Rydberg blockade, it is of fundamental importance to be able todistinguish between these two blockade mechanisms and to create them deterministically. In comparison to Rydberg atoms, the semiconductor surrounding causes faster and more complex carrier and population dynamics which need to be understood in detail first.For example, it is expected that free carriers may form bound exciton states quickly, while excitons may be ionized e.g. by interaction with phonons, so for long times, in general a mixture of both Blockade mechanisms is expected. Therefore, the main aim of the presentprojct lies in gaining a deeper understanding of the complex population dynamics via time-resolved and spatially resolved spectroscopy. To this end, this project aims at investigating timeresolved Rydberg blockade with the help of a tailored modulator that creates Fourier-limited pulses with a duration of approximately 500 PS to 1 ns. This will allow us to perform pump-probe experiments, which will shed light on the rate of exciton formation from plasma, theionization rate of Rydberg excitons, their relaxation towards lowerenergy states and the Auger-assisted formation of plasma from paraexcitons. Further, we will focus on propagation effects and spatially resolved measurements of blockade efficiency to gain a deeper understanding of the spatial dependence of the interaction potential between Rydberg excitons and to understand whether dispersion effects resulting from varying propagation velocities ofpolaritons with different principal quantum numbers result in interactions that vary at different positions inside the crystal or at different times. Finally, we will also investigate the influence of dipoleforbidden excitons on their optically active counterparts. Here, we willfocus on S- and D-excitons, which have wavefunctions with a shape that differs significantly from that of P-excitons, which implies that the interaction potential may be rather complicated. Here we will perform two-photon absorption and second-harmonic spectroscopy todeliberately excite the dipole-forbidden states due to the altered selection rules for two-photon absorption. In summary, the main aim of the present project lies in gaining a detailed understanding of the population dynamics of the several exciton and free-carrier states inCu2O and their interactions.