  Characterizing and manipulating the circular polarization of light is central
to numerous emerging technologies, including spintronics and quantum computing.
Separately, monolayer tungsten disulfide (WS2) is a versatile material that has
demonstrated promise in a variety of applications, including single photon
emitters and valleytronics. Here, we demonstrate a method to tune the
photoluminescence (PL) intensity (factor of x161), peak position (38.4meV
range), circular polarization (39.4% range), and valley polarization of a
Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762uW) in ambient.
Changes are spatially confined to the laser spot, enabling submicron (814nm)
features, and are long-term stable (>334 days). PL and valley polarization
changes can be controllably reversed through laser exposure in vacuum, allowing
the material to be erased and reused. Atmospheric experiments and
first-principles calculations indicate oxygen diffusion modulates the exciton
radiative vs. non-radiative recombination pathways, where oxygen absorption
leads to brightening, and desorption to darkening.
