  The plasmon and phonon polaritons of two-dimensional (2d) and van-der-Waals
materials have recently gained substantial interest. Unfortunately, they are
notoriously hard to observe in linear response because of their strong
confinement, low frequency and longitudinal mode symmetry. Here, we propose a
fundamentally new approach of harnessing nonlinear resonant scattering that we
call stimulated plasmon polariton scattering (SPPS) in analogy to the
opto-acoustic stimulated Brillouin scattering (SBS). We show that SPS allows to
excite, amplify and detect 2d plasmon and phonon polaritons all across the
THz-range while requiring only optical components in the near-IR or visible
range. We present a coupled-mode theory framework for SPS and based on this
find that SPS power gains exceed the very top gains observed in on-chip SBS by
at least an order of magnitude. This opens exciting new possibilities to
fundamental studies of 2d materials and will help closing the THz gap in
spectrocopy and information technology.
