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We theoretically explore the role of mesoscopic fluctuations and noise on the spectral and temporal properties of systems of PT-symmetric coupled gain-loss resonators operating near the exceptional point, where eigenvalues and eigenvectors coalesce. We show that the inevitable detuning in the frequencies of the uncoupled resonators leads to an unavoidable modification of the conditions for reaching the exceptional point, while, as this point is approached in ensembles of resonator pairs, statistical averaging significantly smears the spectral features. We also discuss how these fluctuations affect the sensitivity of sensors based on coupled PT-symmetric resonators. Finally, we show that temporal fluctuations in the detuning and gain of these sensors lead to a quadratic growth of the optical power in time, thus implying that maintaining operation at the exceptional point over a long period can be rather challenging. Our theoretical analysis clarifies issues central to the realization of PT-symmetric devices, and should facilitate future experimental work in the field.

We study the optical response of individual nm-wide plasmonic nanocavities using a nanoparticle-on-mirror design utilised as an electrode in an electrochemical cell. In this geometry Au nanoparticles are separated from a bulk Au film by an ultrathin molecular spacer, giving intense and stable Raman amplification of 100 molecules. Modulation of the plasmonic spectra and the SERS response is observed with an applied voltage under a variety of electrolytes. Different scenarios are discussed to untangle the various mechanisms that can be involved in the electronic interaction between NPs and electrode surfaces.

Molecular spontaneous emission and fluorescence depend strongly on the emitter local environment. Plasmonic nanoparticles provide excellent templates for tailoring fluorophore emission, as they exhibit potential for both fluorescence enhancement and quenching, depending on emitter positioning in the nanoparticle vicinity. Here we explore the influence of hitherto disregarded nonclassical effects on the description of emitter-plasmon hybrids, focusing on the roles of the metal nonlocal response and especially size-dependent plasmon damping. Through extensive modelling of metallic nanospheres and nanoshells coupled to dipole emitters, we show that within a purely classical description a remarkable fluorescence enhancement can be achieved. However, once departing from the local-response approximation, and particularly by implementing the recent generalised nonlocal optical response theory, which provides a more complete physical description combining electron convection and diffusion, we show that not only are fluorescence rates dramatically reduced compared to the predictions of the local description and the common hydrodynamic Drude model, but the optimum emitter-nanoparticle distance is also strongly affected. In this respect, experimental measurements of fluorescence, the theoretical description of which requires a precise concurrent evaluation of far- and near-field properties of the system, constitute a novel, more sensitive probe for assessing the validity of state-of-the-art nonclassical theories. This journal is © The Royal Society of Chemistry 2016.

Gold nanoparticles are separated above a planar gold film by 1.1 nm thick self-assembled molecular monolayers of different conductivities. Incremental replacement of the nonconductive molecules with a chemically equivalent conductive version differing by only one atom produces a strong 50 nm blue-shift of the coupled plasmon. With modeling this gives a conductance of 0.17G(0) per biphenyl-4,4'-dithiol molecule and a total conductance across the plasmonic junction of 30G(0). Our approach provides a reliable tool quantifying the number of molecules in each plasmonic hotspot, here

Resumen del trabajo presentado al 5th International Topical meeting on Nanophotonics and Metamaterials, celebrado en Seefeld (Austria) del 5 al 8 de enero de 2015., Plasmonically-coupled metal nanoparticles with nanometer-sized gaps produce extremely-localised and strongly enhanced optical fields. We tune gaps and use the fields to detect smallest traces of substances in the gap using spectroscopy and surface-enhance Raman scattering.

This is an open access article published under a Creative Commons Attribution (CC-BY) License.-- et al., Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids., This work was supported by the UK EPSRC grants EP/G060649/1, EP/L027151/1, Defence Science and Technology Laboratory (DSTL), and ERC grant 320503 LINASS. C.T. and J.A. acknowledge financial support from Project FIS2013-41184-P from MINECO, ETORTEK 2014-15 of the Basque Department of Industry and IT756-13 from the Basque consolidated groups.

This work is licensed under a Creative Commons Attribution 4.0 International License.-- et al., Nanomaterials find increasing application in communications, renewable energies, electronics and sensing. Because of its unsurpassed speed and highly tuneable interaction with matter, using light to guide the self-assembly of nanomaterials can open up novel technological frontiers. However, large-scale light-induced assembly remains challenging. Here we demonstrate an efficient route to nano-assembly through plasmon-induced laser threading of gold nanoparticle strings, producing conducting threads 12±2nm wide. This precision is achieved because the nanoparticles are first chemically assembled into chains with rigidly controlled separations of 0.9nm primed for re-sculpting. Laser-induced threading occurs on a large scale in water, tracked via a new optical resonance in the near-infrared corresponding to a hybrid chain/rod-like charge transfer plasmon. The nano-thread width depends on the chain mode resonances, the nanoparticle size, the chain length and the peak laser power, enabling nanometre-scale tuning of the optical and conducting properties of such nanomaterials., We acknowledge financial support from EPSRC grants EP/G060649/1, EP/K028510/1 and EP/L027151/1, ERC grants LINASS 320503 and ASPiRe 240629, and project FIS2010-19609-C02-01 from the Spanish Ministry of Science and Innovation. J.S.B. acknowledges the School of Physical Science, University of Cambridge, for the funding of the transmission electron microscope. S.K. acknowledges funding from the Biochemical Society (Krebs Memorial Scholarship) and the Cambridge Commonwealth Trust.

We present an extended version of the layer-multiple-scattering method, which is ideally suited for the study of photonic crystals of different kinds of particles, encompassing homogeneous and multicoated chiral and nonchiral spheres, gyrotropic spheres, as well as homogeneous nonspherical particles. The efficiency of the method is demonstrated on specific examples of planar magnetoplasmonic nanostructures that lack, simultaneously, time-reversal and space-inversion symmetries. Nonreciprocal transport of light at the (001) surface of a semi-infinite face centered cubic (fcc) crystal of plasma nanospheres under the action of an external, in-plane, static magnetic field and of surface plasmon polaritons at the surface of a plasmonic material coated with an overlayer of magnetized garnet nanospheres is demonstrated in the Voigt geometry. © 2014 Elsevier Ltd.

We report on the calculation of the fundamental plasmon waveguide modes in linear periodic chains of finite silver nanorods, aligned perpendicular to the chain. The results of rigorous full-electrodynamic calculations by the layer-multiple-scattering method are discussed in conjunction with the results of the widely used coupled-dipole model and a critical evaluation of the latter is provided. More specifically, it is shown that both diameter and height of the nanorods must be much smaller than the interparticle distance; otherwise, for relatively long nanorods close to each other, the coupled-dipole model can fail completely to predict the waveguide dispersion diagram. Moreover, the model systematically underestimates the effect of dissipative losses and cannot describe the effect of a supporting substrate, which is always present in realistic cases and induces considerable changes in the waveguide dispersion diagram. © 2012 Optical Society of America.

We report on the occurrence and properties of photonic surface states in fcc crystals of metallic nanoshells, by means of full-electrodynamic calculations using the layer-multiple-scattering method, properly extended. Detailed dispersion diagrams of the surface states associated with the (001) and (111) surfaces are calculated for such semi-infinite crystals and corresponding finite slabs, and convergence by increasing the slab thickness is discussed. It is shown that these states can be tuned over a broad frequency range by varying the shell thickness and can be characterized, along high-symmetry directions, according to their symmetry. Absorption in the metallic material limits the propagation length which can, however, be as long as several tens of lattice constants for low-loss metals and relatively broad bands. © 2011 American Physical Society.

By applying a homogenization method based on systematic full-electrodynamic complex-band-structure calculations, we deduce the effective permittivity tensor of a uniaxial photonic crystal consisting of consecutive hexagonal arrays of aligned metallic nanorods of finite length. The form of the obtained permittivity tensor over a relatively broad low-frequency region, where homogenization is applicable, suggests the occurrence of unconventional refractive behavior, namely, negative refraction and self-collimation. This behavior is corroborated by straightforward calculation of the relevant group velocities in the actual photonic crystal. Moreover, it is shown that, in the frequency region where negative refraction occurs, a finite slab of the crystal possesses eigenmodes that form flat bands outside the light cone, as many as the number of its constituent layers. These eigenmodes allow for transfer of the evanescent components of an incident wave field to the other side of the slab, thus enabling subwavelength imaging. © 2010 Optical Society of America.

We report on the effective optical response of single- and multilayer periodic structures of metallodielectric nanosandwiches on the basis of rigorous, full-electrodynamic calculations by the extended layer-multiple- scattering method. It is shown that the complex photonic band structure and the reflection coefficient of the infinite and semi-infinite crystal, respectively, provide reliable bulk effective parameters, which can be used as a reference in order to resolve ambiguities and problems in the determination of these parameters for finite slabs by the S-matrix retrieval procedure. Our results show that the structures under consideration exhibit strong artificial optical magnetism and thin films consisting of a few layers already behave like the bulk metamaterial. © 2010 Elsevier B.V.

We apply the layer-multiple-scattering method to study the optical properties of different plasmonic architectures; namely two- and three-dimensional periodic arrays of metallic nanocylinders and of metallodielectric nanosandwiches. These structures exhibit various types of collective plasmonic resonances, tunable over a broad spectral range from infrared to visible frequencies, which cause large enhancement of the local field and give rise to interesting phenomena that we discuss and provide a consistent interpretation of the underlying physics. We analyze extinction spectra of finite slabs of the structures under consideration and explain the different spectral features. In relation to optical metamaterials, we deduce effective electromagnetic parameters by the S-matrix retrieval procedure for single- and multi-layer slabs of periodic arrays of metallodielectric nanosandwiches and propose a method to resolve ambiguities in the determination of the effective refractive index, which become prominent for thick slabs, based on the complex band structure of the corresponding infinite crystal. © 2010 Elsevier B.V. All rights reserved.

We report on the effective optical response of a uniaxial crystal of metal-dielectric-metal nanosandwiches, which exhibits artificial optical magnetism, through full-electrodynamic simulations by the extended layer-multiple-scattering method. Using a recently developed all-angle homogenization procedure, which is based on rigorous results of complex-band-structure and reflection-coefficient calculations, we deduce local effective permittivity and permeability tensors, appropriate for this crystal. We show that the effective-medium description breaks down as we approach the region of the magnetic resonance. In a frequency region close to the resonance the retrieved effective parameters, though doubtful, indicate that the crystal under consideration may exhibit negative refraction. This behaviour is demonstrated by rigorous calculation of the isofrequency surfaces of the actual crystal and determination of the relevant group velocities.

We propose a method for calculating average local effective permittivity and permeability tensors for anisotropic photonic crystals through least-squares fits of sets of data points, obtained by rigorous, systematic complex-band-structure, and reflection calculations for all propagation directions, to appropriate analytic expressions. The proposed methodology is applied on a specific example of a tetragonal structure of metallic nanoshells, which is a uniaxial photonic crystal of resonant units. Our results demonstrate the efficiency of the method at low and moderate frequencies and, at the same time, reveal the inability to define local effective constitutive parameters in regions of resonance gaps. © 2010 The American Physical Society.

We present a thorough theoretical study of the optical properties of periodic structures built of silver and silica nanodisks in a sandwich-like configuration, by means of full electrodynamic calculations using the extended layer-multiple-scattering method. The strong coupling of the metallic nanoparticles and the resulting plasmon hybridization lead to collective electric and magnetic resonant modes, which can be tuned by changing the structural parameters, such as nanoparticle size and lattice constant. We analyze the response of single-and multi-layer architectures of ordered arrays of such nanosandwiches on a dielectric substrate to externally incident light and evaluate the corresponding effective permittivity and permeability functions. Our results reveal the existence of optical magnetism, with a strong negative effective permeability over a tunable spectral range at near-infrared and visible frequencies. We introduce the complex photonic band structure as a tool in the study of three-dimensional metamaterials and establish additional criteria for the validity of their effective-medium description. Our work demonstrates the efficiency of the recently developed extended layer-multiple-scattering method in the study of metamaterials of composite metal-dielectric particles of arbitrary shape.

After a brief description of the multiple-scattering method for photonic crystals, we present some results obtained by this method, relating to various types of plasmonic nanostructures in one, two and three dimensions: cavity plasmon waveguides, systems of metallic particles, and arrays of metallic shells. We analyse the optical response of these structures and emphasise some interesting aspects of the underlying physics. copyright © 2009 Inderscience Enterprises Ltd.

We report a thorough theoretical study of the optical response of two- and three-dimensional periodic assemblies of metallic nanorods by means of full-electrodynamic calculations using the extended layer-multiple-scattering method. We show that these systems support various types of resonant- and bound-collective plasmon modes, which are tunable over a broad spectral range, and provide a consistent interpretation of the underlying physics. In particular, we reveal the existence of slab plasmon modes with zero group velocity, which can cause evanescent-wave enhancement and enable subwavelength imaging. We discuss extinction spectra of single-layer and multilayer slabs of nanorods in conjunction with relevant complex band-structure diagrams and present a rigorous analysis of the results using group theory. Moreover, we explain some peculiar spectral features which are due to the existence of surface resonances. These can modify the optical response of the system in a controllable manner by using a supporting substrate. © 2009 The American Physical Society.

Collective plasmonic modes in two-and three-dimensional periodic assemblies of metallic nanoshells are studied by means of full electrodynamic calculations using the layer-multiple-scattering method. We consider structures made of a single type of nanoshell as well as binary heterostructures made of two different types of nanoshells. The complex photonic band structure of such three-dimensional photonic crystals is analyzed in conjunction with relevant transmission diagrams of corresponding finite slabs and the physical origin of the different optical modes is elucidated. Moreover, we discuss associated absorption spectra and provide a consistent interpretation of the underlying physics. In the case of the binary systems, the plasmonic modes of the two building components coexist, leading to a rich structure of resonances over an extended frequency range and to broadband absorption.

Periodic nanostructures for plasmonic engineering, comprising one or two types of silica core - metallic shell spherical particles, are studied by means of full electrodynamic calculations using the layer-multiple-scattering method. The complex photonic band structure of such three-dimensional crystals is analyzed in conjunction with relevant transmission spectra of corresponding finite slabs and the physical origin of the different optical modes is elucidated, providing a consistent interpretation of the underlying physics. In the case of binary structures, collective plasmonic modes originating from the two building components coexist, leading to broadband absorption and a rich structure of resonances and hybridization gaps over an extended frequency range.

Increasing Yb3+ absorption efficiency is currently desired in a number of applications including bioimaging, photovoltaics, near infrared driven photocatalysis or ultra-short pulsed solid-state lasers. In this work, silver nanoparticles, which are connected forming disordered networks, have been self-assembled on Yb3+ doped RbTiOPO4 crystals to produce a remarkable enhancement of Yb3+ absorption, and hence in the photoluminescence of this ion. The results are interpreted taking into account the near-field response of the plasmonic networks, which display strong amplification of the electric field at the maximum of Yb3+ excitation at around 900 nm, together with the anisotropic character of the Yb3+ transitions in RbTiOPO4. We show that in the near field regime, the scattering of the plasmonic networks produces additional polarization field components to those of the incident field, which allows access to the largest transition dipolar moment of Yb3+ ions in RbTiOPO4. As a result, a much more efficient route for Yb3+ excitation takes place at the immediacy of the plasmonic networks. This work provides fundamental insights for improving the optical properties of rare earth ions by the suitable design of metallic nanoparticle arrangements, and constitutes a promising step towards the development of new multifunctional solid-state lasers.