  Semi-holographic models of non-Fermi liquids have been shown to have
generically stable generalised quasi-particles on the Fermi surface. Although
these excitations are broad and exhibit particle-hole asymmetry, they were
argued to be stable from interactions at the Fermi surface. In this work, we
use this observation to compute the density response and collective behaviour
in these systems.
  Compared to the Fermi liquid case, we find that the boundaries of the
particle-hole continuum are blurred by incoherent contributions. However, there
is a region inside this continuum, that we call inner core, within which
salient features of the Fermi liquid case are preserved. A particularly
striking prediction of our work is that these systems support a plasmonic
collective excitation which is well-defined at large momenta, has an
approximately linear dispersion relation and is located in the low-energy tail
of the particle-hole continuum.
  Furthermore, the dynamic screening potential shows deep attractive regions as
a function of the distance at higher frequencies which might lead to long-lived
pair formation depending on the behaviour of the pair susceptibility. We also
find that Friedel oscillations are present in these systems but are highly
suppressed.
