Frontiers in terahertz sources and plasmonics

Terahertz (THz) radiation offers researchers many intriguing possibilities, ranging from fundamental science through to applications in communications, noninvasive imaging, and other areas. The THz region (~0.1 THz to ~10 THz) is often described as the last frontier of the electromagnetic spectrum because of the relatively low maturity level of components and systems that operate in this region. Although the well-known ‘THz gap’ has begun to be filled through advances in many areas, much of its exciting potential still remains untapped. As a result, the THz community continues to be active and growing, as manifested by the recordsetting attendances at relevant conferences1 and the special issues of journals devoted to the topic (such as this one). From a spectroscopist’s perspective, the excitement surrounding THz science can be easily understood. This spectral region corresponds to photon energies in the range 0.4–40 meV (roughly 3.3– 330 cm−1), a span that includes the energy scale of many fundamental excitations in condensed matter, including lattice vibrations (phonons), superconducting energy gaps, spin quasiparticles and many other excitations. In disordered materials and conductive media, broadband THz spectroscopy can reveal the microscopic nature of incoherent processes; furthermore, many rotational transitions for molecules in the gas phase fall in the THz range. This spectral range is thus fertile ground for spectroscopy. THz technologies have the potential to realize new capabilities in a variety of applications. Many of these applications rely on a unique combination of factors: the transparency of many common packaging materials such as paper, cardboard and most plastics; diffractionlimited, submillimetre spatial resolutions comparable to that of human vision; and the possibility of identifying unknown materials (especially gases, molecular crystals and polycrystalline powders) based on spectral ‘fingerprints’. Researchers have envisioned a wide variety of uses, ranging from security to non-destructive testing. Other ideas exploit the large bandwidth available for short-range wireless communication or the sensitivity to moisture content (as water is a strong absorber in the THz region) for noncontact inspection of food products. Of course, a great deal of effort is still required to make progress in these areas. The development of innovative tools and techniques continues to be critical for improving research capabilities and opening up new avenues of exploration. Indeed, two of the most active areas in this field are the development of novel highintensity THz sources and plasmonics for providing new routes to control and manipulate THz beams. This Commentary focuses on these two forefront research areas.