Infrared Nano-Imaging of Dirac Magnetoexcitons in Graphene

Magnetic fields can have profound effects on the motion of electrons in quantum materials. Two-dimensional (2D) electron systems subject to strong magnetic fields are expected to exhibit quantized Hall conductivity, chiral edge currents, and distinctive collective modes referred to as magnetoplasmons and magnetoexcitons. Generating these propagating collective modes in charge-neutral samples and imaging them at their native nanometer length scales have thus far been experimentally elusive tasks. In this study, we visualize propagating magenetoexction polaritons at their native length scales and report their magnetic-field-tunable dispersions in near-charge-neutral graphene. Imaging of these collective modes and their associated opto-electrical responses at the sample edges is enabled by innovations to our cryogenic near-field optical microscope that allows us to nano-image the optical responses of 2D materials in magnetic fields up to 7 Tesla. This novel nano-magneto-optics approach represents a new paradigm for exploring and manipulating magnetopolaritons in specimens with low carrier doping via harnessing high magnetic fields.