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The density-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron-hole pairs within a WSe

The density-driven transition of an exciton gas into an electron–hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron–hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron–hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron–hole plasma above 7 × 1012 cm–2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.

Many-body interactions between excitons in a transition-metal dichalcogenide bilayer drive a transition into an electron-hole liquid at high densities. Using ultrafast polarization nanoscopy, we unveil spatiotemporal dynamics of this continuous Mott transition on the nanoscale.

We employ THz near-field microscopy to investigate ultrafast interlayer charge transfer in van der Waals heterostructures (HS) by tracing the resulting THz emission on the nanoscale. We further employ a novel concept of contact-free tunneling microscopy to monitor the interlayer electron-hole (e-h) pair population through its build up and decay, providing access to the ultrafast carrier dynamics with spatial resolution orders of magnitude better than the diffraction limit.

Tunneling between atomic layers strongly changes the polarizability of electron-hole pairs in van der Waals heterobilayers. By monitoring this dynamics with a near-field microscope, we record subcycle nano-movies of local tunneling processes even in insulators.