Thesis defense of Stefan Grisard

Photon echoes emerge from the delayed optical response of inhomogeneous ensembles of emitters upon resonant laser excitation. In semiconductors, they allow to uncover internal scattering and interaction dynamics on the picosecond timescale, while also holding promise as the realization of quantum optical memories in future quantum networks. Based on these two fields of application, this work uses photon echoes to investigate two material systems: Organic-inorganic perovskites and (In,Ga)As quantum dots. Organic-inorganic perovskites have attracted significant attention for their exceptional performance in photovoltaics and light-emitting applications. However, a comprehensive understanding of coherent light-matter interactions in this material class and in particular the role of excitons close to the band gap remained elusive. This work reveals that excitons dominate the nonlinear optical response of MAPbI and FAPbI single crystals and are subject to strong inhomogeneous broadening even at cryogenic temperatures. Compositional substitution is found to induce spatial band gap fluctuations on the nanometer scale that localize excitons accompanied by an extension of their coherence time by two orders of magnitude. Furthermore, exciton interactions are studied through polarization- dependent photon echoes, uncovering the formation of a biexciton state and the contribution of spin-dependent many-body interactions to nonlinear optical spectra. Subsequently, the focus shifts to confined excitons in (In,Ga)As semiconductor quantum dot ensembles, that represent an ideal platform to explore new approaches on how to coherently transfer, manipulate, and retrieve optical information to a solid state on picosecond timescales. First, it is demonstrated that collective Rabi rotations of the photon echoes from a quantum dots ensemble can be observed when a spatially uniform excitation profile is used. In this way, internal mechanisms of decoherence under strong laser excitation are identified. Thereafter, the photon echo sequence is expanded by two control pulses, providing all-optical control over the  emission time, spectral response, and polarization state of photon echoes from quantum dots. Here, the interplay of temporally sorted multi-wave-mixing signals is exploited.

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