The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconductor is an emerging platform for studying these coupling effects due to unique spin-valley locking physics for hosting rich excitonic species and reduced screening for strong Coulomb interaction. In this talk, I will present the observation of both symmetry-allowed and -forbidden valley phonons, i.e. phonons with momentum vectors pointing to the corners of Brillouin zone, in a monolayer semiconductor WSe2. From the analysis of Landé g-factors and emission polarization of photoluminescence peaks, we identified the efficient intervalley scattering of quasi particles in both exciton formation and light emission process. These understandings enable us to unravel a series of photoluminescence peaks as valley phonon replicas of neutral and charged dark excitons, as well as deeply bound excitonic states with anomalously long population lifetime (> 5 µs). Our work not only shows monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons, but also opens opportunities to engineer collective quantum optical phenomena using homogenous intrinsic defect-bound excitons in ultraclean two-dimensional materials.
Xiaodong Xu is a Boeing Distinguished Professor in the Department of Physics and the Department of Materials Science and Engineering at the University of Washington. He received his PhD (Physics, 2008) from the University of Michigan and then performed postdoctoral research (2009-2010) at the Center for Nanoscale Systems at Cornell University. His nanoscale quantum optoelectronics group at University of Washington focuses on creation, control, and understanding of novel device physics based on two-dimensional quantum materials.