  We report a giant Rashba-type spin splitting in two-dimensional
heterostructure PtSe$_2$/MoSe$_2$ with first-principles calculations. We obtain
a large value of spin splitting energy 110 meV at the momentum offset
$k_0$=0.23 \AA$^{-1}$ around $\mathrm{\Gamma}$ point, arising from the emerging
strong interfacial spin-orbital coupling induced by the hybridization between
PtSe$_2$ and MoSe$_2$. Moreover, we find that the band dispersion close to
valence band maximum around $\Gamma$ point can be well approximated by the
generalized Rashba Hamiltonian $H(k_{||})=-\frac{\hbar^2 k_{||}^2}{2m}+c
k_{||}+\alpha_R \vec{\sigma}\cdot(\vec{k}_{||} \times \vec{z})$. It is found
that the generalized Rashba constant $\eta_R=c+\alpha_R$ in PtSe$_2$/MoSe$_2$
is as large as 1.3 eV$\cdot\text{\AA}$, and importantly $\eta_R$ can be
effectively tuned by biaxial strain and external out-of-plane electrical field,
presenting a potential application for the spin field-effect transistor. In
addition, with the spin-valley physics at $\mathrm{K}/\mathrm{K}'$ points in
monolayer MoSe$_2$, we propose a promising model for spin field-effect
transistor with opto-valleytronic spin injection based on PtSe$_2$/MoSe$_2$
heterostructure.
