  Twisted light carries a well-defined orbital angular momentum (OAM) per
photon. The quantum number l of its OAM can be arbitrarily set, making it an
excellent light source to realize high-dimensional quantum entanglement and
ultra-wide bandwidth optical communication structures. To develop solid-state
optoelectronic systems compatible with such promising light sources, a timely
challenging task is to efficiently and coherently transfer the optical OAM of
light to certain solid-state optoelectronic materials. Among the
state-of-the-art emergent materials, atomically thin monolayer transition metal
dichalcogenide (ML-TMD), featured by ultra-strong light-matter interaction due
to its reduced dimensionality, renders itself a potential material suitable for
novel applications. In this study, we carried out photoluminescence (PL)
spectroscopy studies of ML-MoS2 under photoexcitation of twisted light with
well-defined quantized OAM. We mainly observed pronounced increases in the
spectral peak energy for every increment of l of the incident twisted light.
The observed non-linear l-dependence of the spectral blue shifts evidences the
OAM transfer from the exciting twisted light to the valley excitons in ML-TMDs,
which is well accounted for by our analysis and computational simulation. Even
more excitingly, the twisted light excitation is shown to make excitonic
transitions relative to the transferred OAM, enabling us to infer the exciton
band dispersion from the measured spectral shifts. Consequently, the measured
non-linear l-dependent spectral shifts revealed an unusual lightlike exciton
band dispersion of valley excitons in ML-TMDs that is predicted by previous
theoretical studies and evidenced for the first time via our experimental setup
that utilizes the unique twisted light source.
