Unravelling exciton dynamics in 2D semiconductors through time resolved XUV micro ARPES

Exciton – a Coulomb bound electron-hole pair, dominates the optoelectronic properties of monolayer transition metal dichalcogenides (1L TMDC) and their heterostructures. Traditionally, optical spectroscopic techniques have been used to study excitons and their properties but are blind to one of their fundamental properties – their momentum. Access to the momentum coordinate of the excitons is critical in developing a comprehensive insight into various excitonic properties such as their direct or indirect nature, their size, their wavefunction and their impact on the underlying quasiparticle band structure. Moreover, 1L TMDCs host a variety of optically dark exciton states which is important to resolve for developing a holistic view of exciton dynamics in these materials. 
In this talk I will discuss the advancements made during my PhD in resolving exciton dynamics in 1L TMDCs using multidimensional momentum microscopy. Going forward from our previous work [1-3], I will present our recent measurement of exciton dynamics following a valley polarized excitation. By significantly improving the energy resolution of our experiments, we are now able to track the scattering of the bright excitons to various optically dark states including the spin dark states which are only 30-40 meV below the bright excitonic states in W-based TMDCs. Following this, I will also discuss the decades old problem of the impact of a dense excitonic population on the quasiparticle band structure and show its striking momentum resolved features in a 1L TMDC [4]. I will also discuss how our observations relates to the recently predicted non-equilibrium excitonic insulator problem [5] as well as giant exciton driven Floquet effects [6].
References
[1] J. Madéo et al., Directly Visualizing the Momentum-Forbidden Dark Excitons and Their Dynamics in Atomically Thin Semiconductors, Science 370, 1199 (2020). 
[2] M. K. L. Man et al., Experimental Measurement of the Intrinsic Excitonic Wave Function, Science Advances 7, eabg0192 (2021). 
[3] O. Karni et al., Structure of the Moiré Exciton Captured by Imaging Its Electron and Hole, Nature 603, 247 (2022).
[4] V. Pareek et al., Driving Non-Trivial Quantum Phases in Conventional Semiconductors with Intense Excitonic Fields, http://arxiv.org/abs/2403.08725.
[5] E. Perfetto, D. Sangalli, A. Marini, and G. Stefanucci, Pump-Driven Normal-to-Excitonic Insulator Transition: Josephson Oscillations and Signatures of BEC-BCS Crossover in Time-Resolved ARPES, Phys. Rev. Materials 3, 124601 (2019).
[6] Y.-H. Chan, D. Y. Qiu, F. H. da Jornada, and S. G. Louie, Giant Self-Driven Exciton-Floquet Signatures in Time-Resolved Photoemission Spectroscopy of MoS 2 from Time-Dependent GW Approach, Proc. Natl. Acad. Sci. U. S. A. 120, e2301957120 (2023).