1 Department of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States
2
Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan
3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, United States
4 Department of Physics, California Institute of Technology, Pasadena, California 91125, United States
Monolayer
transition metal dichalcogenides (TMDs) have intrinsic valley degrees
of freedom, making them appealing for exploiting valleytronic
applications in information storage and processing. WS2 monolayer
possesses two inequivalent valleys in the Brillouin zone, each valley
coupling selectively with a circular polarization of light. The degree
of valley polarization (DVP) under the excitation of circularly
polarized light (CPL) is a parameter that determines the purity of
valley polarized photoluminescence (PL) of monolayer WS2. Here
efficient tailoring of valley-polarized PL from monolayer WS2 at room
temperature (RT) through surface plasmon–exciton interactions with
plasmonic Archimedes spiral (PAS) nanostructures is reported. The DVP
of WS2 at RT can be enhanced from <5% to 40% and 50% by using 2
turns (2T) and 4 turns (4T) of PAS, respectively. Further enhancement
and control of excitonic valley polarization is demonstrated by
electrostatically doping monolayer WS2. For CPL on WS2–2TPAS
heterostructures, the 40% valley polarization is enhanced to 70% by
modulating the carrier doping via a backgate, which may be attributed
to the screening of momentum-dependent long-range electron–hole
exchange interactions. The manifestation of electrically tunable
valley-polarized emission from WS2–PAS heterostructures presents a new
strategy toward harnessing valley excitons for application in ultrathin
valleytronic devices.