Time resolved ARPES study of excitons in 2D semiconductors

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 bandstructure. 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 about the recent developments in the time and angle resolved photoemission spectroscopy which allowed us to access the momentum coordinate of excitons in 1L TMDC and their heterostructures. Using this technique, we were able to observe the momentum dark Q excitons and their dynamics[1], measure the momentum resolved excitonic wavefunction in the 1L TMDC[2] and resolve the interlayer exciton bound electrons and holes in a TMDC heterostructures[3]. Following this, I will discuss the decades old problem of the impact of a dense excitonic population on the quasiparticle bandstructure and show its striking momentum resolved features in a 1L TMDC. I will also discuss how our observations relates to the recently predicted non-equilibrium excitonic insulator problem[4] as well as giant exciton driven Floquet effects[5]. Finally, I will briefly discuss some of our recent efforts in developing a comprehensive understanding of the exciton dynamics in 1L TMDC. 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.





References

[1] Science 370, 1199 (2020).

[2] Science Advances 7, eabg0192 (2021).

[3] Nature 603, 247 (2022).

[4] Phys. Rev. Materials 3, 124601 (2019).

[5] Proc. Natl. Acad. Sci. U. S. A. 120, e2301957120 (2023).