Enabling materials for subwavelength-size low-loss surface modes in the Terahertz spectral range

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.