Kaiqiang Lin1
Xiamen University1
Two-dimensional transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. We present the existence of a new excitonic species, the high-lying exciton (HX), in transition metal dichalcogenide monolayers with almost twice the band-edge A-exciton energy and with a linewidth as narrow as that of band-edge excitons [1]. The HX is populated through momentum-selective optical excitation in the K-valleys, and is identified experimentally in upconverted photoluminescence (UPL) and theoretically in <i>ab initio</i> <i>GW</i>-BSE calculations. These calculations suggest that the HX is comprised of electrons of negative effective mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalize the efficient exciton-exciton annihilation in TMDC monolayers, and enables the excitonic quantum-interference phenomenon revealed in optical second-harmonic generation (SHG) [2]. We probe the excitonic temporal dynamics in such three-level system through time-resolved sum-frequency generation (SFG) and four-wave mixing (FWM) [3]. High-lying excitons in bilayer WSe<sub>2</sub> can be largely tuned by twisting [4], electrical gate [5] and Stark effect, which gives control over the excitonic quantum interference and the corresponding optical nonlinearity.<br/><br/>[1] K. -Q. Lin et al., <i>Nat. </i><i>Commun.</i> <b>12</b>, 5500 (2021).<br/>[2] K. -Q. Lin, S. Bange, & J. M. Lupton, <i>Nat. </i><i>Phys.</i> <b>15</b>, 242-246 (2019).<br/>[3] J. M. Bauer et al., <i>Nat. </i><i>Photon.</i> <b>16</b>, 777-783 (2022).<br/>[4] K. -Q. Lin et al., <i>Nat. </i><i>Commun.</i> <b>12</b>, 1553 (2021).<br/>[5] K. -Q. Lin et al., <i>Nat. </i><i>Commun.</i> <b>13</b>, 6980 (2022).