Dynamics of optical vortices in Van der Waals materials

Recent advancements in ultrafast electron microscopy have provided direct access to polariton dynamics, visualizing such dynamics in space and time. This work presents new experimental results revealing a myriad of phenomena involving interactions of vortex-anti-vortex pairs, their creation and annihilation. We show new behaviors that became accessible thanks to a new development in electron microscopy - Free-Electron Ramsey Imaging (FERI) - which enabled extracting both the sub-cycle and group dynamics of polariton wavepackets. Our demonstrations involve optical phonon-polaritons in hexagonal boron nitride (hBN) and Molybdenum oxide, renowned for their unique dispersion and novel wavepacket propagations behaviors. These discoveries not only enhance our understanding of vortex phenomena across various systems, but also offer promising avenues for accessing new kinds of light-matter interactions.