A review of nanoplasmonics is given. This includes fundamentals, nanolocalization of optical energy and hot spots, ultrafast nanoplasmonics and control of the spatiotemporal nanolocalization of optical fields, and quantum nanoplasmonics (spaser and gain-assisted plasmonics). This article reviews both fundamental theoretical ideas in nanoplasmonics and selected experimental developments. It is designed both for specialists in the field and general physics readership.

Nanoplasmonics: past, present, and glimpse into future

Understanding how light interacts with matter at the nanometre scale is a fundamental issue in optoelectronics and nanophotonics. In particular, many applications (such as bio-sensing, cancer therapy and all-optical signal processing) rely on surface-bound optical excitations in metallic nanoparticles. However, so far no experimental technique has been capable of imaging localized optical excitations with sufficient resolution to reveal their dramatic spatial variation over one single nanoparticle. Here, we present a novel method applied on silver nanotriangles, achieving such resolution by recording maps of plasmons in the near-infrared/visible/ultraviolet domain using electron beams instead of photons. This method relies on the detection of plasmons as resonance peaks in the energy-loss spectra of subnanometre electron beams rastered on nanoparticles of well-defined geometrical parameters. This represents a significant improvement in the spatial resolution with which plasmonic modes can be imaged, and provides a powerful tool in the development of nanometre-level optics.

Mapping surface plasmons on a single metallic nanoparticle

This Perspective provides a contemporary understanding of the shape evolution of colloidal metal nanocrystals under thermodynamically and kinetically controlled conditions. It has been extremely challenging to investigate this subject in the setting of one-pot synthesis because both the type and number of seeds involved would be changed whenever the experimental conditions are altered, making it essentially impossible to draw conclusions when comparing the outcomes of two syntheses conducted under different conditions. Because of the uncertainty about seeds, most of the mechanistic insights reported in literature for one-pot syntheses of metal nanocrystals with different shapes are either incomplete or ambiguous, and some of them might be misleading or even wrong. Recently, with the use of well-defined seeds for such syntheses, it became possible to separate growth from nucleation and therefore investigate the explicit role(s) played by a specific thermodynamic or kinetic parameter in directing the evolution of colloidal metal nanocrystals into a specific shape. Starting from single-crystal seeds enclosed by a mix of {100}, {111}, and {110} facets, for example, one can obtain colloidal nanocrystals with diversified shapes by adjusting various thermodynamic or kinetic parameters. The mechanistic insights learnt from these studies can also be extended to account for the products of conventional one-pot syntheses that involve self-nucleation only. The knowledge can be further applied to many other types of seeds with twin defects or stacking faults, making it an exciting time to design and synthesize colloidal metal nanocrystals with the shapes sought for a variety of fundamental studies and technologically important applications.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals: Thermodynamic versus Kinetic Products

Strong coupling between resonantly matched localized surface plasmons and molecular excitons results in the formation of new hybridized energy states called plexcitons. Understanding the nature and tunability of these hybrid nanostructures is important for both fundamental studies and the development of new applications. We investigate the interactions between J-aggregate excitons and single plasmonic dimers and report for the first time a unique strong coupling regime in individual plexcitonic nanostructures. Dark-field scattering measurements and finite-difference time-domain simulations of the hybrid nanostructures show strong plexcitonic coupling mediated by the near-field inside each dimer gap, which can be actively controlled by rotating the polarization of the optical excitation. The plexciton dispersion curves, obtained from coupled harmonic oscillator models, show anticrossing behavior at the exciton transition energy and giant Rabi splitting ranging between 230 and 400 meV. These energies are, t...

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Near-field Mediated Plexcitonic Coupling and Giant Rabi Splitting in Individual Metallic Dimers

Recent advances in molecular organic photovoltaics (OPVs) have shown 10% power conversion efficiency (PCE) for single-junction cells, which put them in direct competition with PVs based on amorphous silicon. Incorporation of plasmonic nanostructures for light trapping in these thin-film devices offers an attractive solution to realize higher-efficiency OPVs with PCE>>10%. This article reviews recent progress on plasmonic-enhanced OPV devices using metallic nanoparticles, and one-dimensional (1D) and two-dimensional (2D) patterned periodic nanostructures. We discuss the benefits of using various plasmonic nanostructures for broad-band, polarization-insensitive and angle-independent absorption enhancement, and their integration with one or two electrode(s) of an OPV device.

Plasmonic-enhanced organic photovoltaics: breaking the 10% efficiency barrier.

Plasmonic nano-antennas constitute a central research topic in current science and engineering with an enormous variety of potential applications. Here we review the recent progress in the niche of plasmonic nano-antennas operating in the near infrared part of the spectrum which is important for a variety of applications. Tuning of the resonance into the near infrared regime is emphasized in the perspectives of fabrication, measurement, modeling, and analytical treatments, concentrating on the vast recent achievements in these areas.

http://iopscience.iop.org/article/10.1088/0953-8984/24/7/073202/pdf

Nano-plasmonic antennas in the near infrared regime

We analyze an inherent nonlinearity of surface plasmon polaritons at the interface of Fermi-Dirac metal plasma, stemming from the depletion of electron density in high-intensity regions. The derived optical nonlinear coefficients are comparable with the experimental values for metals. We calculate the dispersion relations for the nonlinear propagation of high-intensity surface plasmon polaritons, predicting a nonlinearity-induced cutoff and vanishing group velocity.

Nonlocal ponderomotive nonlinearity in plasmonics.

We demonstrate experimentally and theoretically a broad-band enhancement of the spontaneous two-photon emission from AlGaAs at room temperature by plasmonic nanoantenna arrays fabricated on the semiconductor surface. Plasmonic structures with inherently low quality factors but very small effective volumes are shown to be optimal. A 20-fold enhancement was achieved for the entire antenna array, corresponding to an enhancement of nearly 3 orders of magnitude for charge carriers emitting at the near field of a plasmonic antenna.

Plasmonic Nanoantennas for Broad-Band Enhancement of Two-Photon Emission from Semiconductors

A method for designing plasmonic particles with desired resonance spectra is presented. The method is based on repetitive perturbations of an initial particle shape while calculating the eigenvalues of the various quasistatic resonances. The method is rigorously proved, assuring a solution exists for any required spectral resonance location. Resonances spanning the visible and the near-infrared regimes, as designed by our method, are verified using finite-difference time-domain simulations. A novel family of particles with collocated dipole-quadrupole resonances is designed, demonstrating the unique power of the method. Such on-demand engineering enables strict realization of nano-antennas and metamaterials for various applications requiring specific spectral functions.

Resonances On-Demand for Plasmonic Nano-Particles

We show experimentally and theoretically enhancement of external quantum efficiency in the green-NIR spectrum for organic photovoltaic device, by the incorporation of patterned Au nano-disk arrays that extend from the front electrode into the active layer. Enhancement mechanisms and design rules are extracted by comprehensive simulations which match the experimental findings. The enhanced efficiency is shown to stem from field enhancement originating from both localized plasmonic resonances and periodic nano patch antennas configuration.

Nikolai Berkovitch

Papers

Nano-plasmonic antennas in the near infrared regime

Nonlocal ponderomotive nonlinearity in plasmonics.

Plasmonic Nanoantennas for Broad-Band Enhancement of Two-Photon Emission from Semiconductors

Resonances On-Demand for Plasmonic Nano-Particles

Controlling absorption enhancement in organic photovoltaic cells by patterning Au nano disks within the active layer.