Frontiers of Plasmonics

Abstract

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 surfaceenhanced 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, contrary to the classical indication of redshift. Graphene plasmonics are due to the unique atomic layer structure and yet the support of propagating plasmons of graphene [20–22]. The far-infrared operating frequency of plasmons, along with the opto/electro conversion properties of graphene, makes graphene plasmonics highly potential in applications such as ultrasensitive photodetector and information technology. This special section – recent advances on frontiers of plasmonics – covers part of the hot topics presented during the 2 International Conference on Frontiers of Plasmonics (FOP2) held in April 2012, Chengdu, China. The conference was co-organized by the Institute of Physics, Chinese Academy of Sciences, and the College of Physical Science and Technology, Sichuan University, and co-chaired by Profs. Hong-Xing Xu, Peter Nordlander, Naomi Halas and Hong Zhang. It covered all aspects of plasmonics, including near-field optics, quantum plasmonics, Fano resonances, surface-enhanced spectroscopy, chemical and biological sensing, metamaterials, etc. It attracted ∼200 attendees, had more than 70 oral and over 50 poster presentations. The invited talks were contributed by experts in plasmonics from international regions, including USA, UK, Spain, Germany, Sweden, Israel, Japan, Korea, etc. Under this topic, Frontiers of Physics has invited a part of the contributions and some hot topics in plasmonics are addressed. For example, the elastic/inelastic optical images of metal nanoparticles are diffraction limited in the far field, thus the emission centroid or even the location of molecules is irresolvable. However, super-resolution imaging using models of point spread function can solve this problem with spatial resolution on the order of ∼5 nm. This technique is explained by Prof. K. Willets from University of Texas in the Perspective article [23]. Single-molecule tip-enhanced Raman scattering (TERS) has drawn increasing attention due to the fact that the chemical structure and even chemical reaction of a single molecule are obtainable. In high-vacuum (HV), the environment is clean enough to exclude the influence of the ambient, in particular, oxygen and water, so that clean spectroscopic signals from single molecules are possible. Some advances of HV-TERS are summarized by Sun et al. in a review article for this issue [24]. Single molecule SERS (smSERS) is of fundamental interest in revealing the enhancement mechanisms of SERS – the EM hot spots and charge transfer effects. In his article [25], Kim re-examined the single-molecularity of smSERS, and reviewed what has been newly discovered and what still remains uncertain in smSERS. Yamamoto et al. [26] studied the blinking mechanism of SERS, which is believed to result from the single molecule characteristic, and its basic applications in biological sensing. SP in the deep-ultravoilet (deep-UV) has become an attractive topic due to its advantage of higher optical resolution in imaging and photolithography than SPs in the visible regime. Ono et al. [27] showed the deep-UV SPs in a thin film of Aluminum excited by Kretschmann configuration and its coupling to semiconductor quantum dots. Since the properties of SPs are determined by the detailed morphology of