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Dr. Valentin Walther
ITAMP and Department of Physics, Harvard University

Abstract

Rydberg excitons have only recently been discovered. They are highly-excited, bound electron-hole pairs that represent giant atom-like quasiparticles in a semiconductor environment. These new particles defy conventional theories of exciton physics and open fundamentally new regimes for quantum optics with excitons.

Here, I will show how their remarkable properties originate from strong, long-range polarization forces acting between pairs of such states over thousands of crystal cells, suggesting great potential for optical applications. We develop a semi-analytical cluster-expansion theory to describe the enormous optical nonlinearity of the Rydberg interactive many-body system, revealing the effect of a residual thin electron-hole plasma and bringing our theory into quantitative agreement with corresponding experiments.

However, strong phonon coupling and inter-band dynamics can complicate this simple atom-like picture and limit the usability of Rydberg-Rydberg interactions for optical applications. Using experiments with cuprous oxide as an example, I will discuss the origins of phonon coupling and how the formation of an optical dark state can be used to mitigate its undesired effects. Finally, I will describe ongoing efforts to exploit strong Rydberg interactions of excitons in monolayered semiconductors for the creation of nonclassical light. Our results open the stage for physics with strongly interacting polaritons in the solid state, making it possible to induce nonlinear processes at the level of individual photons.

Bio

Dr. Walther is a candidate for the tenure-track position in theoretical condensed matter physics and optics in the Department of Physics, Engineering Physics and Astronomy at Queen’s University. Dr. Walther is an independent postdoctoral fellow in Physics at Harvard.  Dr. Walther completed his PhD in Physics at the Max Planck Institute for the Physics of Complex Systems and the Technical University Dresden Germany in 2019. His work seeks to advance quantum optics and quantum dynamics in systems with strong atomic or excitonic interactions. He has pioneered the theory of Rydberg excitations in semiconductors and laid the groundwork for their nonlinear optical response.