  We report polarization-resolved resonant reflection spectroscopy of a
charge-tunable atomically-thin valley semiconductor hosting tightly bound
excitons coupled to a dilute system of fully spin- and valley-polarized holes
in the presence of a strong magnetic field. We find that exciton-hole
interactions manifest themselves in hole-density dependent, Shubnikov-de
Haas-like oscillations in the energy and line broadening of the excitonic
resonances. These oscillations are evidenced to be precisely correlated with
the occupation of Landau levels, thus demonstrating that strong interactions
between the excitons and Landau-quantized itinerant carriers enable optical
investigation of quantum-Hall physics in transition metal dichalcogenides.

Mesoscale and Nanoscale Physics