  We study the dynamics and the relaxation of bulk plasmons in strongly coupled
and quantum critical systems using the holographic framework. We analyze the
dispersion relation of the plasmonic modes in detail for an illustrative class
of holographic bottom-up models. Comparing to a simple hydrodynamic formula, we
entangle the complicated interplay between the three least damped modes and
shed light on the underlying physical processes. Such as the dependence of the
plasma frequency and the effective relaxation time in terms of the
electromagnetic coupling, the charge and the temperature of the system.
Introducing momentum dissipation, we then identify its additional contribution
to the damping. Finally, we consider the spontaneous symmetry breaking (SSB) of
translational invariance. Upon dialing the strength of the SSB, we observe an
increase of the longitudinal sound speed controlled by the elastic moduli and a
decrease in the plasma frequency of the gapped plasmon. We comment on the
condensed matter interpretation of this mechanism.
