Ultrafast Nanoscopy of High-Density Exciton Phases in WSe₂

Abstract

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 2 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 × 10 12 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 twodimensional materials.

Ultrafast Dynamics of Valley-Polarized Excitons in WSe2 Monolayer Studied by Few-Cycle Laser Pulses

“…From the known fluence of the pump pulse, we can estimate the exciton density corresponding to the highest fluence to be approximately cm . Auger recombination was previously observed in this region of carrier densities [ 41 ]. The high-density excitation regime is further supported by the observed sub-linear dependence of the peak of the transient reflectivity signal on the pump fluence F , which is caused by state filling.…”

Ultrafast pseudospin quantum beats in multilayer WSe$_2$ and MoSe$_2$

“…With this procedure, we get for the WSe 2 monolayer an initial exciton density of n ∼ 1.3 × 10 12 cm −2 and for the MoSe 2 monolayer n ∼ 1.9 × 10 12 cm −2 , when in both cases the A 1s exciton is excited resonantly. Both values are well below the Mott density [36,61,62]. For the multilayer samples we devide the total exciton density by the number of layers to get an estimate of the density per layer.…”

Tunable interlayer excitons and switchable interlayer trions via dynamic near-field cavity

“…However, in order to enable practical applications of TMD heterostructures using IX and IX − , several major challenges must be overcome, one of which is the large degree of spatial heterogeneity. The underlying processes, e.g., competing interactions of coupling, dephasing, and energy transfer of intra-and inter-layer excitons as well as IX− interconversion, arise at the nanoscale and cannot be understood by diffraction-limited optical approaches, calling for the near-field optical probing [14][15][16][17] . Furthermore, beyond probing, it is highly important to achieve nanoscale control of local IX and IX− properties in TMD heterostructures.…”