Direct visualization of intra-, inter-layer excitons motion and free carriers in 2D TMDC captured by TR-µARPES and nano-ARPES.

Abstract: Transition metal dichalcogenides (TMDC) are excellent models for the exploration of semiconductor physics at the 2D limit, with potential applications in electronics, optoelectronics, and quantum devices. The strong Coulomb interactions and distinct structural symmetries in these materials give rise to a rich variety of photoexcited states, including excitonic complexes that are tightly bound electron-hole pairs, and valley-spin polarized. However, directly accessing the momentum direct and indirect excitons and their dynamics in its electronic structure are out of experimental reach. Here, I am going to talk about the generation of high repetition rate higher order harmonics (HRR-HHG) and its combination with momentum microscope, to establish time- and angle (momentum)-resolved photoemission spectroscopy (TRuARPES), demonstrated on a micron-scale monolayer flake of WSe2 (1). This directly measures the momentum direct and forbidden excitonic states across entire Brillouin Zone (BZ) and measures their dynamics under different excitation conditions (2). The direct access of excitonic energy-momentum distribution leads to the measurement of excitonic wave function revealing the exciton size in real and k-space, whose electron follows the downward curvature of its partner hole (3). Beyond excitonic states, I will also visualized the evolution of the conduction band in a 1LWS2 gated device captured through nano-ARPES (4). Layered heterostructures of TMDC monolayers are typically characterized with staggered band-alignment, leading to ultrafast charge-transfer between the band edges at the corners of the BZ following photo excitation. ILXs - electron-hole pairs bound across two layered semiconductors - have emerged as an attractive platforms to study exciton condensation, single-photon emission and other quantum-information applications. Yet under investigations, critical information about their size, valley configuration and the influence of the moir\'e potential remains unknown. Here, I will establish the use of TR-uARPES to track the electron-transfer pathways, which leads to the Interlayer excitons (ILX) formation of a MoS2/WSe2 heterobilayer, and resolve a momentum-preserving hoping in parallel to the momentum-indirect process. Also captured images of the time- and momentum-resolved distribution of both the electron and the hole that bind to form the ILX. We thereby obtain a direct measurement of the interlayer exciton diameter, comparable to the moir\'e unit-cell length, leading to the ILX confinement in the moir\'e unit-cell (5). 



References: (+ equal authors)
1. Chakradhar Sahoo (Ph.D. thesis: “Development of efficient ultrafast pump probe detection and generation of  XUV pulses towards photoemission spectroscopy”), July 2019.
2. J. Madéo+ , M. K. L. Man+ , Chakradhar. Sahoo et. al, Science, 370, 1199-1204 (2020). 
3. M. K. L. Man+ , J. Madéo+ , Chakradhar. Sahoo et. al, Sci. Adv.,7:eabg0192, (2021).
4. Chakradhar Sahoo et. al, “Direct visualization of conduction band evolution mechanism and its doping dependence in 1LWS2 gated device” (Manuscript under preparation). 
5. O. Karani+ , E. Barre+ , V. Pareek+ , J. Georgaras+ , MKL Man+ , Chakradhar. Sahoo+ et. al,Nature,    603,247-252 (2022).