Ultrafast Dynamics of Rydberg Excitons in Monolayer WSe2

The strong quantum confinement experienced by the electrons and holes in two-dimensional monolayers of Transitional Metal Dichalcogenides (TMDs) reduces the Coulomb screening and leads to the appearance of excitons with high binding energies (up to 0.5 eV) and very large oscillator strengths. Cryogenic temperatures and encapsulation in hBN narrow the exciton linewidths and allow the observation of a hydrogen-like Rydberg series of excitonic states below the free particle bandgap.[1] Rydberg excitons in TMDs can be also dressed by a Fermi sea of free charges generated in the material by natural or artificial doping, either forming three particles bound states (trions) in presence of low doping levels, or manifesting emerging many-body phenomena at elevated doping regimes.[2] The non-equilibrium optical response of neutral and charged TMDs excitons in their lowest state (1s) has been largely investigated by ultrafast optical spectroscopy, focusing the attention on the study of the fundamental mechanisms that determine exciton formation, dissociation and decay processes in TMDs. However, an extensive study on the ultrafast dynamics of neutral and charged higher excitonic Rydberg states is still missing.<br/>In our work we use pump-probe optical microscopy to measure the ultrafast dynamics of the 1s- and 2s-excitons and trions in high quality hBN-encapsulated monolayer WSe₂ at low temperature (8K). In order to focus the laser beams on samples with lateral size of few microns, we developed a custom confocal microscopy setup, equipped with a closed loop He cryostat, capable of transient reflectivity measurements with high spatial and temporal resolution as well as static reflectance contrast (RC) and photoluminescence (PL) characterizations. For the transient measurements, we use broadband ultrashort frequency tunable visible pump and probe pulses, that are delivered collinearly on the focusing lens, and have ~ 3 µm diameter spot on the sample. The pump pulses are temporally compressed by chirp mirrors, ensuring an overall temporal resolution of ~ 40 fs. Upon photoexciting above the bandgap, we clearly observe a faster formation of the 2s-exciton over the 1s state and different relaxation dynamics. The difference in the formation timescale is the result of the exciton cascade process following carrier photoexcitation at high-energy [3], while the longer decay dynamics for the 2s excitons can be ascribed to a reduced exciton–phonon scattering due to their larger Bohr radius [4]. The decay processes of 2s-excitons are also remarkably different if the excitation energy is tuned in resonance with the 1s-states, suggesting that the relaxation pathways for higher Rydberg excitons are strongly dependent on the population of the lowest energy states. Moreover, by monitoring the transient spectrum of the higher excitonic states we observe spectral features at lower energy compared to the 2s-excitons, possibly ascribable to the formation of optically-induced 2s-trions. The rich behaviour of Rydberg excitons explored in our work unveils novel insights on the many-body physics of TMDs, opening up interesting opportunities for exploring Rydberg excitons in monolayer TMDs for future opto-electronic applications.<br/>[1] A. Chernikov et al., Phys. Rev. Lett. 113, 076802 (2014).<br/>[2] K. Wagner et al., Phys. Rev. Lett. 125, 267401 (2020).<br/>[3] C. Trovatello et al., Nat. Commun. 11, 5277 (2020).<br/>[4] S. Brem et al. Nanoscale, 11, 12381 (2019).