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 are out of optical experimental reach. Here, I will talk about the generation of high repetition rate higher order harmonics (HRR-HHG) and its coupling with momentum microscope, to establish HRR time- and angle -resolved photoemission spectroscopy (TR-µARPES), demonstrated on a micron-scale monolayer WSe2 flake (1). This 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 energymomentum 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 visualize the evolution of the conduction band electrons in a 1LWS2-gated device captured through nano-ARPES (4). ILXs-electron-hole pairs bound across two layered semiconductors - have emerged as 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. I will establish the use of TR-µARPES to track the electron-transfer pathways, which leads to the Interlayer excitons (ILX) formation in a MoS2/WSe2 hetero-bilayer, 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, and ILX confinement in the moir\'e unit-cell (5).

References: (+ equal authors)
Chakradhar Sahoo Ph.D. thesis: “Development of efficient ultrafast pump probe detection and generation of XUV pulses towards photoemission spectroscopy”, January 2020.
J. Madéo+, M. K. L. Man+, Chakradhar. Sahoo et. al, Science, 370, 1199-1204,(2020).
M. K. L. Man+, J. Madéo+, Chakradhar. Sahoo et. al, Sci. Adv.,7:eabg0192, (2021).
Chakradhar Sahoo* et. al, “Imaging doping induced screening across graphene/1L-TMDC interface” (Manuscript under preparation).
O. Karani+, E. Barre+, V. Pareek+, J. Georgaras+, MKL Man+, Chakradhar. Sahoo+ et. al, Nature, 603,247-252 (2022).