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Abstract |
Two dimensional materials provide new avenues for synthesizing compound
quantum systems. Monolayers with vastly different electric, magnetic or
optical properties can be combined in van der Waals heterostructures which
ensure the emergence of new functionalities; arguably, the most spectacular
example to date is the observation of strong correlations and low electron
density superconductivity in moire superlattices obtained by stacking two
monolayers with a finite twist angle. Optically active monolayers such as
molybdenum diselenide provide a different "twist" as they allow for
investigation of nonequilibrium dynamics in van der Waals heterostructures by
means of femtosecond pump-probe measurements. Moreover, interactions between
electrons and the elementary optical excitations such as excitons or
polaritons, provide an ideal platform for investigation of quantum impurity
physics, with possibilities to probe both Fermi- and Bose-polaron physics as
well as mixtures with tunable density of degenerate fermions and bosons.
After introducing the framework we use to describe many-body optical
excitations in van der Waals heterostructures, I will describe two recent
developments in the field. The first experiment uses pump-probe measurements
to demonstrate how exciton-electron interactions lead to strong enhancement of
polariton-polariton interactions, as well as to optical gain by stimulated
cooling of exciton-polaron-polaritons. The second experiment shows that a tri-
layer system, consisting of two semiconducting monolayers separated by an
insulating layer, provides an exciting platform for investigating strongly
correlated electronic states in moire superlattices using optical
spectroscopy.
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