  Light can be strongly confined in sub-wavelength spatial regions through the
interaction with plasmons, the collective electronic modes appearing in metals
and semiconductors. This confinement, which is particularly important in the
terahertz spectral region, amplifies light-matter interaction and provides a
powerful mechanism for efficiently generating non-linear optical phenomena.
These effects are particularly relevant in Dirac materials like graphene and
Topological Insulators, where massless fermions show a naturally non-linear
optical behavior in the terahertz range. The strong interaction scenario has
been considered so far from the point of view of light. In this paper, we
investigate instead the effect of strong interaction on the plasmon itself. In
particular, we will show that Dirac plasmons in Bi$_2$Se$_3$ Topological
Insulator are strongly renormalized when excited by high-intensity terahertz
radiation by displaying a huge red-shift down to 60% of its characteristic
frequency. This opens the road towards tunable terahertz non-linear optical
devices based on Topological Insulators.
