Atomically thin transition metal dichalcogenides (TMDs) and halide perovskites have rapidly grown as promising materials for efficient optoelectronic devices. In both material classes, the dynamics of optical excitations and their interactions on ultrafast timescales are still debated.
Using a newly developed theory, we discuss the interaction of TMD excitons and unbound charge carriers with optical phonons in the dielectric environment of the 2d layer. We find a significant reduction of exciton binding energies as well as linewidth broadening due to the dynamical coupling to environmental phonons.
Moreover, we investigate near-band-edge optical transitions in CsPbBr3 single crystals at room temperature by combining ultrafast two-dimensional electronic spectroscopy and semiconductor Bloch equation calculations. An exciton binding energy of ~30 meV and remarkably short <30-fs carrier relaxation rates are extracted.
Atomically thin transition-metal dichalcogenide (TMD) semiconductors possess strong Coulomb interactions due to reduced dielectric screening, leading to the formation of excitons with exceptionally large binding energies. The enhanced stability of excitons in these materials provides a unique platform to investigate excitonic interactions at room temperature and to examine the role of plasma effects and excitonic interactions over a broad range of excitation densities.
We report an excitation-density dependent crossover between two regimes: Using ultrafast absorption spectroscopy, we observe a pronounced red shift of the exciton resonance followed by an anomalous blue shift with increasing excitation density. Using both material-realistic computation and phenomenological modeling, we attribute this observation to long-range Coulomb interaction in the presence of plasma screening in an attraction-repulsion crossover with the short-ranged exciton-exciton interaction that mimics the Lennard-Jones potential between atoms, suggesting a strong analogy between excitons and atoms in respect of inter-particle interaction.
Our findings underline the important role of many-particle renormalizations and screening due to excited carriers in the device-relevant regime of optically or electrically excited TMDs.
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