title: Large, valley-exclusive Bloch-Siegert shift in monolayer WS2.
abstract: Coherent interaction with off-resonance light can be used to shift the energy levels of atoms, molecules, and solids. The dominant effect is the optical Stark shift, but there is an additional contribution from the so-called Bloch-Siegert shift that has eluded direct and exclusive observation in solids. We observed an exceptionally large Bloch-Siegert shift in monolayer tungsten disulfide (WS2) under infrared optical driving. By controlling the light helicity, we could confine the Bloch-Siegert shift to occur only at one valley, and the optical Stark shift at the other valley, because the two effects obey opposite selection rules at different valleys. Such a large and valley-exclusive Bloch-Siegert shift allows for enhanced control over the valleytronic properties of two-dimensional materials.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Edbert J
authors.identifier: 0000-0003-3477-6459
authors.initials: EJ
authors.lastname: Sie
authors.affiliation: Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA.
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 Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Liang
authors.initials: L
authors.lastname: Fu
authors.affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Jing
authors.identifier: 0000-0003-0551-1208
authors.initials: J
authors.lastname: Kong
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. gedik@mit.edu.
authors.forename: Nuh
authors.identifier: 0000-0002-6394-4987
authors.initials: N
authors.lastname: Gedik