EDITORIAL Frontiers of Plasmonics

Plasmonics, an important branch of nanooptics, has seen its prosperous development and exciting applications during the past years [1–3]. Surface plasmons (SPs), the light-driven collective oscillation of free electrons in metals, include the localized type (localized surface plasmons, LSPs) and propagating type (surface plasmon polaritons, SPPs). The most charming characteristics of SPs are the strong confinement of electromagnetic (EM) field (and thus EM enhancement) and long range propagation of EM energy (in case of SPPs). Based on these characters, intriguing applications in many fields have been found, for example, single molecule spectroscopy using surface-enhanced Raman scattering (SERS) [4–7], ultrasensitive detection of chemical and biological species using localized surface plasmon resonance (LSPR) sensor [8, 9], spaser that stems from the amplification of resonant SPs in the cavity of metal nanostructures [10–12], superlens that utilizes the sub-wavelength concentration of SPs [13, 14], and plasmonic circuits that are based on the propagation modulation of SPPs [15–17]. Although the properties and applications of plasmonics have been extensively explored, new concepts have arisen in recent years, for example, quantum plasmonics and graphene plasmonics [18–22]. Quantum plasmonics deal with the electrons tunneling effect in coupled systems, such as nanoparticles with sub-nanometer gaps, and greatly complement the fundamentals of plasmon behaviors [18, 19]. The tunneling effect weakens the classical EM coupling and leads to instead the blueshift of the plasmon resonance when the gap distance decreases to a few angstroms