  Spin-orbit coupling (SOC) is a relativistic effect, where an electron moving
in an electric field experiences an effective magnetic field in its rest frame.
In crystals without inversion symmetry, it lifts the spin degeneracy and leads
to many magnetic, spintronic and topological phenomena and applications. In
bulk materials, SOC strength is a constant that cannot be modified. Here we
demonstrate SOC and intrinsic spin-splitting in atomically thin InSe, which can
be modified over an unprecedentedly large range. From quantum oscillations, we
establish that the SOC parameter \alpha is thickness-dependent; it can be
continuously modulated over a wide range by an out-of-plane electric field,
achieving intrinsic spin splitting tunable between 0 and 20 meV. Surprisingly,
\alpha could be enhanced by an order of magnitude in some devices, suggesting
that SOC can be further manipulated. Our work highlights the extraordinary
tunability of SOC in 2D materials, which can be harnessed for in operando
spintronic and topological devices and applications.
