  Moir\'e superlattices are emerging as a new route for engineering strongly
correlated electronic states in two-dimensional van der Waals heterostructures,
as recently demonstrated in the correlated insulating and superconducting
states in magic-angle twisted bilayer graphene and ABC trilayer graphene/boron
nitride moir\'e superlattices. Transition metal dichalcogenide (TMDC) moir\'e
heterostructures provide another exciting model system to explore correlated
quantum phenomena, with the addition of strong light-matter interactions and
large spin-orbital coupling. Here we report the optical detection of strongly
correlated phases in semiconducting WSe2/WS2 moir\'e superlattices. Our
sensitive optical detection technique reveals a Mott insulator state at one
hole per superlattice site ({\nu} = 1), and surprising insulating phases at
fractional filling factors {\nu} = 1/3 and 2/3, which we assign to generalized
Wigner crystallization on an underlying lattice. Furthermore, the unique
spin-valley optical selection rules of TMDC heterostructures allow us to
optically create and investigate low-energy spin excited states in the Mott
insulator. We reveal an especially slow spin relaxation lifetime of many
microseconds in the Mott insulating state, orders-of-magnitude longer than that
of charge excitations. Our studies highlight novel correlated physics that can
emerge in moir\'e superlattices beyond graphene.
