  Transition metal dichalcogenide semiconductors represent elementary
components of layered heterostructures for emergent technologies beyond
conventional opto-electronics. In their monolayer form they host electrons with
quantized circular motion and associated valley polarization and valley
coherence as key elements of opto-valleytronic functionality. Here, we
introduce two-dimensional polarimetry as means of direct imaging of the valley
pseudospin degree of freedom in monolayer transition metal dichalcogenides.
Using MoS$_2$ as a representative material with valley-selective optical
transitions, we establish quantitative image analysis for polarimetric maps of
extended crystals, and identify valley polarization and valley coherence as
sensitive probes of crystalline disorder. Moreover, we find site-dependent
thermal and non-thermal regimes of valley-polarized excitons in perpendicular
magnetic fields. Finally, we demonstrate the potential of wide-field
polarimetry for rapid inspection of opto-valleytronic devices based on
atomically thin semiconductors and heterostructures.
