  In this article we propose a system capable of THz radiation with quantum
yield above unity. The system consists of nanoparticles where the material
composition varies along the radial direction of each nanoparticle in such a
way that a ladder of equidistant energy levels emerges. By then exciting the
highest level of this ladder we produce multiple photons of the same frequency
in the THz range. We demonstrate how we can calculate a continuous material
composition profile that achieves a high quantum yield and then show that a
more experimentally friendly design of a multishell nanoparticle can still
result in a high quantum yield.
