  Terahertz spectral range (frequencies of 0.1-1 THz) has recently emerged as
the next frontier for non-destructive imaging, industrial sensing and
ultra-fast wireless communications. Here, we review several classes of
materials such as simple metals, semiconductors, high-k dielectrics, polar
materials, zero gap materials, as well as structured materials that can support
strongly localised electromagnetic modes at material interfaces in the
Terahertz spectral range. We present the basic theory of surface waves, detail
the requirement of strong modal confinement and low loss for the surface waves
propagating at material interfaces and discuss challenges for excitation of
such modes at Terahertz frequencies. A large number of examples related to
naturally occurring and artificial materials is then presented. A variety of
practical applications is envisioned for surface waves at Terahertz frequencies
including non-destructive super-resolution imaging and quality control, high
sensitivity sensors capable of operation with small volumes of analytes that
are opaque in the visible and near-infrared, as well as design of compact
optical circuit for the upcoming ultra-high bitrate THz communication devices.
