Ultrafast nano-imaging of dark excitons

Understanding the impact of spatial heterogeneity on the behaviour of two-dimensional materials represents one of the grand challenges in applying these materials in optoelectronics and quantum information science. For transition metal dichalcogenide heterostructures in particular, direct access to heterogeneities in the dark-exciton landscape with nanometre spatial and ultrafast time resolution is highly desired but remains largely elusive. Here we report how ultrafast dark-field momentum microscopy can spatio-temporally resolve dark-exciton formation dynamics in a twisted WSe2/MoS2 heterostructure with a time resolution of 55 fs and a spatial resolution of 480 nm. This enables us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate this with the dark-exciton formation and relaxation dynamics. The advantage of the simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy reported here is that it enables spatio-temporal imaging of the photoemission spectral function that carries energy- and momentum-resolved information on the single-particle band structure, many-body interactions and correlation phenomena. Dark-field momentum microscopy makes it possible to spatio-temporally and spatio-spectrally resolve the dark-exciton dynamics in a twisted transition metal dichalcogenide heterostructure.