  The ability to monitor and control distinct states is at the heart of
emerging quantum technologies. The valley pseudospin in transition metal
dichalcogenide (TMDC) monolayers is a promising degree of freedom for such
control, with the optical Stark effect allowing for valley-selective
manipulation of energy levels in WS$_2$ and WSe$_2$ using ultrafast optical
pulses. Despite these advances, understanding of valley-sensitive optical Stark
shifts in TMDCs has been limited by reflectance-based detection methods where
the signal is small and prone to background effects. More sensitive
polarization-based spectroscopy is required to better probe ultrafast Stark
shifts for all-optical manipulation of valley energy levels. Here, we show
time-resolved Kerr rotation to be a more sensitive probe of the
valley-selective optical Stark effect in monolayer TMDCs. Compared to the
established time-resolved reflectance methods, Kerr rotation is less sensitive
to background effects. Kerr rotation provides a five-fold improvement in the
signal-to-noise ratio of the Stark effect optical signal and a more precise
estimate of the energy shift. This increased sensitivity allows for observation
of an optical Stark shift in monolayer MoS$_2$ that exhibits both valley- and
energy-selectivity, demonstrating the promise of this method for investigating
this effect in other layered materials and heterostructures.
