  To efficiently integrate cutting-edge terahertz technology into compact
devices, the highly confined terahertz plasmons are attracting intensive
attentions. Compared to plasmons at visible frequencies in metals, terahertz
plasmons, typically in lightly doped semiconductors or graphene, are sensitive
to carrier density (n) and thus have an easy tunability, which, however, leads
to unstable or imprecise terahertz spectra. By deriving a simplified but
universal form of plasmon frequencies, here we reveal a unified mechanism for
generating unusual n-independent plasmons (DIPs) in all topological states with
different dimensions. Remarkably, we predict that terahertz DIPs can be excited
in 2D nodal-line and 1D nodal-point systems, confirmed by the first-principles
calculations on almost all existing topological semimetals with diverse lattice
symmetries. Besides of n independence, the feature of Fermi-velocity and
degeneracy-factor dependences in DIPs can be applied to design topological
superlattice and multi-walled carbon nanotube metamaterials for broadband
terahertz spectroscopy and quantized terahertz plasmons, respectively.
Surprisingly, high spatial confinement and quality factor, also insensitive to
n, can be simultaneously achieved in these terahertz DIPs. Our findings pave
the way to developing topological plasmonic devices for stable terahertz
applications.
