  Control over the electronic spectrum at low energy is at the heart of the
functioning of modern advanced electronics: high electron mobility transistors,
semiconductor and Capasso terahertz lasers, and many others. Most of those
devices rely on the meticulous engineering of the size quantization of
electrons in quantum wells. This avenue, however, hasn't been explored in the
case of 2D materials. Here we transfer this concept onto the van der Waals
heterostructures which utilize few-layers films of InSe as quantum wells. The
precise control over the energy of the subbands and their uniformity guarantees
extremely high quality of the electronic transport in such systems. Using novel
tunnelling and light emitting devices, for the first time we reveal the full
subbands structure by studying resonance features in the tunnelling current,
photoabsorption and light emission. In the future, these systems will allow
development of elementary blocks for atomically thin infrared and THz light
sources based on intersubband optical transitions in few-layer films of van der
Waals materials.
