  The fundamental properties of an exciton are determined by the spin, valley,
energy, and spatial wavefunctions of the Coulomb bound electron and hole. In
van der Waals materials, these attributes can be widely engineered through
layer stacking configuration to create highly tunable interlayer excitons with
static out-of-plane electric dipoles, at the expense of the strength of the
oscillating in-plane dipole responsible for light-matter coupling. Here we show
that interlayer excitons in bi- and tri-layer 2H-MoSe$_2$ crystals exhibit
electric-field-driven coupling with the ground ($1s$) and excited states ($2s$)
of the intralayer A excitons. We demonstrate that the hybrid states of these
distinct exciton species provide strong oscillator strength, large permanent
dipoles (up to $0.73 \pm 0.01$ enm), high energy tunability (up to $\sim$ 200
meV), and full control of the spin and valley characteristics such that the
exciton g-factor can be manipulated over a large range (from -4 to +14).
Further, we observe the bi- and tri-layer excited state ($2s$) interlayer
excitons and their coupling with the intralayer excitons states ($1s$ and
$2s$). Our results, in good agreement with a coupled oscillator model with spin
(layer)-selectivity and beyond standard density functional theory calculations,
promote multilayer 2H-MoSe$_2$ as a highly tunable platform to explore
exciton-exciton interactions with strong light-matter interactions.
