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A short overview is presented on the research works related to the zero- gap, which appears as the volume-averaged refraction index vanishes in photonic structures containing both positive and negative-index materials. After introducing the basic concept of the zero- gap based on both rigorous mathematics and numerical simulations, the unique properties of such a band gap are discussed, including its robustness against weak disorder, wide-incidence-angle operation and scaling invariance, which do not belong to a conventional Bragg gap. We then describe the simulation and experimental verifications on the zero- gap and its extraordinary properties in different frequency domains. After that, the unusual photonic and physical effects discovered based on the zero- gap and their potential applications are reviewed, including beam manipulations and nonlinear effects. Before concluding this review, several interesting ideas inspired from the zero- gap works will be introduced, including the zero-phase gaps, zero-permittivity and zero-permeability gaps, complete band gaps, and zero-refraction-index materials with Dirac-Cone dispersion.

An essential element in the synthesis of nanomaterials based on gold nanoparticles comprises the control over parameters such as size, shape and composition, due to their strong influence on the properties of the particles. However, it is the coating material which often plays the primary role in tuning the size, morphology, and even plasmon resonance wavelength or mode multiplicity, as well as colloidal stability and functional versatility, ultimately determining the physical, chemical, optical, electronic and catalytic properties of the nanoparticles. Therefore, it is utterly important to select the adequate wet chemistry synthetic approach with the most suitable coating material for the preparation of gold nanoparticles with the desired requirements. Within this context, this review is focused on describing various types of organic and inorganic coating materials for gold nanoparticles that may notably affect their optical properties by either directly influencing the synthesis procedure or by changing their chemical and physical properties upon post-synthetic modifications, such that they exhibit novel and useful optical properties.

In this paper, we review and discuss how nanoantennas may be used to largely enhance the nonlinear response of optical materials. For single nanoantennas, there have been tremendous advancements in understanding how to exploit the local field enhancement to boost the nonlinear susceptibility at the surface or sharp edges of plasmonic metals. After an overview of the work in this area, we discuss the possibility of controlling the optical nonlinear response using nanocircuit concepts and of significantly enhancing various nonlinear optical processes using planar arrays of plasmonic nanoantennas loaded with χ (2) or χ (3) nonlinear optical materials, forming ultrathin, nanometer-scale nonlinear metasurfaces, as optical nanodevices. We describe how this concept may be used to boost the efficiency of nonlinear wave mixing and optical bistability, due to the large local field enhancement at the nonlinear nanoloads associated with the plasmonic features of suitably tailored nanoantenna designs. We finally discuss three exciting applications of the proposed nonlinear metasurface: dramatically-enhanced frequency conversion efficiency, efficient phase-conjugation for super-resolution imaging and large optical bistabilities.

Nanoplasmonics recently has emerged as a new frontier of photovoltaic research. Noble metal nanostructures that can concentrate and guide light have demonstrated great capability for dramatically improving the energy conversion efficiency of both laboratory and industrial solar cells, providing an innovative pathway potentially transforming the solar industry. However, to make the nanoplasmonic technology fully appreciated by the solar industry, key challenges need to be addressed; including the detrimental absorption of metals, broadband light trapping mechanisms, cost of plasmonic nanomaterials, simple and inexpensive fabrication and integration methods of the plasmonic nanostructures, which are scalable for full size manufacture. This article reviews the recent progress of plasmonic solar cells including the fundamental mechanisms, material fabrication, theoretical modelling and emerging directions with a distinct emphasis on solutions tackling the above-mentioned challenges for industrial relevant applications.

Nanoplasmonic sensing has over the last two decades emerged as and diversified into a very promising experimental platform technology for studies of biomolecular interactions and for biomolecule detection (biosensors). Inspired by this success, in more recent years, nanoplasmonic sensing strategies have been adapted and tailored successfully for probing functional nanomaterials and catalysts in situ and in real time. An increasing number of these studies focus on using the localized surface plasmon resonance (LSPR) as an experimental tool to study a process of interest in a nanomaterial, with a materials science focus. The key assets of nanoplasmonic sensing in this area are its remote readout, non-invasive nature, single particle experiment capability, ease of use and, maybe most importantly, unmatched flexibility in terms of compatibility with all material types (particles and thin/thick layers, conductive or insulating) are identified. In a direct nanoplasmonic sensing experiment the plasmonic nanoparticles are active and simultaneously constitute the sensor and the studied nano-entity. In an indirect nanoplasmonic sensing experiment the plasmonic nanoparticles are inert and adjacent to the material of interest to probe a process occurring in/on this material. In this review we define and discuss these two generic experimental strategies and summarize the growing applications of nanoplasmonic sensors as experimental tools to address materials science-related questions.

Optical microcavities that confine light in high-Q resonance promise all of the capabilities required for a successful next-generation microsystem biodetection technology. Label-free detection down to single molecules as well as operation in aqueous environments can be integrated cost-effectively on microchips, together with other photonic components, as well as electronic ones. We provide a comprehensive review of the sensing mechanisms utilized in this emerging field, their physics, engineering and material science aspects, and their application to nanoparticle analysis and biomolecular detection. We survey the most recent developments such as the use of mode splitting for self-referenced measurements, plasmonic nanoantennas for signal enhancements, the use of optical force for nanoparticle manipulation as well as the design of active devices for ultra-sensitive detection. Furthermore, we provide an outlook on the exciting capabilities of functionalized high-Q microcavities in the life sciences.