title: Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS2.
abstract: Coherent optical driving can effectively modify the properties of electronic valleys in transition metal dichalcogenides. Here, we observe a new type of optical Stark effect in monolayer WS2, one that is mediated by intervalley biexcitons under the blue-detuned driving with circularly polarized light. We find that such helical optical driving not only induces an exciton energy downshift at the excitation valley but also causes an anomalous energy upshift at the opposite valley, which is normally forbidden by the exciton selection rules but now made accessible through the intervalley biexcitons. These findings reveal the critical, but hitherto neglected, role of biexcitons to couple the two seemingly independent valleys, and to enhance the optical control in valleytronics.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Edbert J
authors.initials: EJ
authors.lastname: Sie
authors.affiliation: Department of Physics and Astronomy, University of California , Riverside, California 92521, United States.
authors.forename: Chun Hung
authors.initials: CH
authors.lastname: Lui
authors.affiliation: Materials Science and Engineering, National Tsing-Hua University , Hsinchu 30013, Taiwan.
authors.forename: Yi-Hsien
authors.initials: YH
authors.lastname: Lee
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Kong
authors.affiliation: Department of Physics, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
authors.forename: Nuh
authors.initials: N
authors.lastname: Gedik
keywords: WS2; biexciton; blue detuned; optical Stark effect; ultrafast optics; valley