  A two-dimensional (2D) quantum electron system is characterized by the
quantized energy levels, or subbands, in the out-of-plane direction. Populating
higher subbands and controlling the inter-subband transitions have wide
technological applications such as optical modulators and quantum cascade
lasers. In conventional materials, however, the tunability of intersubband
spacing is limited. Here we demonstrate electrostatic population and
characterization of the second subband in few-layer InSe quantum wells, with
giant tunability of its energy, population, and spin-orbit coupling strength,
via the control of not only layer thickness but also out-of-plane displacement
field. A modulation of as much as 350% or over 250 meV is achievable,
underscoring the promise of InSe for tunable infrared and THz sources,
detectors and modulators.
