Highlights from Faraday Discussion 178: Nanoplasmonics, London, UK, February 2015

In February 2015 Burlington House hosted Faraday Discussion 178, gathering over 130 scientists from all over the world to highlight the most recent advances in the exciting field of nanoplasmonics but also to define the most challenging problems for the future. Discussions between the attendees started straight after registration, mostly about the cutting-edge research in plasmonics but also about the unusual format of the conference, where research papers written by the speakers were distributed to all participants before the meeting, and most of the meeting was devoted to discussing the papers.

Taking advantage of new nanofabrication techniques and subwavelength optical characterisation, nanoplasmonics has become a wide area of research. To catch most of what is going on in the field at the moment, this Faraday Discussion was divided into five major themes. The first one, about new developments of plasmonic materials, included new approaches for the design and fabrication of tailored systems capable of achieving strong field confinement and enhancement as well as numerical modelling techniques used to predict their plasmonic properties. The second main topic of the meeting covered surface plasmon enhanced spectroscopy and its applications such as surface enhanced Raman scattering, surface enhanced infrared absorption, surface plasmon enhanced fluorescence and the enhanced nonlinear optical properties of plasmonic nanostructures. Cutting edge developments in plasmonics dealing with the quantum nature of plasmonic excitations and their interaction with quantum objects, such as quantum dots or molecules were then tackled before finishing with two sessions dedicated to different applications in nanophotonics as well as the sensing, imaging and chemistry applications of plasmonics, leading to discussions about label-free biosensing techniques, optical trapping using plasmonic forces or high-resolution cell imaging, for example. The diversity in the domains covered by plasmonics along with all the applications emerging from the research conducted over recent years shows the necessity of such meetings in order to stay aware of the possibilities opened up by such an active field of research.

After the official opening and welcome by Prof. Anatoly Zayats and Prof. Stefan Maier, an introductory lecture (DOI: 10.1039/C5FD90020D) on nanoplasmonics was given by Prof. Mark Brongersma. He started his presentation with a brief historical development of the field and emphasized the fact that Michael Faraday's work marked the birth of research in plasmonics in the 1850s. Taking into account the major progress achieved by the researcher community up to now, Mark Brongersma highlighted five key strengths of nanoplasmonics, which also constitute the biggest challenges at the moment and can be described as follows: light confinement and manipulation below the diffraction limit, the high tunability of the geometrical parameters of the plasmonic nanostructures and thus of their electromagnetic properties, the design flexibility to achieve a broad range of optical functions, efficient light-to-heat conversion and the multi-functionality of the structures. He concluded his talk by noting that with a wealth of over 150 years of experience, researchers are currently bringing nanoplasmonics towards a new range of applications, linking this exciting field with other areas of science and technology.

The first session of this Faraday Discussion was mostly dedicated to the research on new plasmonic materials. Material limitations form one of the main challenges in the development of new devices. Work in the past few decades has mostly been based on the use of noble metals such as gold or silver which exhibit among the lowest, but still non-negligible, losses. In his paper, Vladimir Shalaev (DOI: 10.1039/C4FD00208C)(Purdue University) investigated a new class of materials: transition metal nitrides such as titanium nitride (TiN) or zirconium nitride (ZrN). These new materials exhibit plasmonic properties very similar to those of gold but have the advantages of being refractory materials with high temperature durability and mechanical hardness and also being CMOS compatible and chemically stable. An extensive comparison between gold and refractory transition metal nitrides shows that both type of materials have similar properties in the visible and near-infrared regimes, with the latter having losses coming close to those of gold. Using heat assisted magnetic recording for data storage as an example of a high temperature plasmonic application, the authors showed that TiN and gold can perform similarly in terms of plasmonic properties but TiN provides required advantages due to its refractory characteristics and thus could be the key to the development of compact and durable plasmonic devices.

The next paper of this session on new plasmonic materials was presented by Jacob B. Khurgin (DOI: 10.1039/C4FD00193A) (Johns Hopkins University) in which he investigated the ultimate limit of field confinement by surface plasmon polaritons. One of the challenges in the development of nanoplasmonics and its applications relies on the possibility to concentrate the optical field into sub-wavelength dimensions. Unfortunately, plasmonic materials always suffer losses which can have different origins such as scattering, absorption or surface imperfections. In the attempt to find a novel plasmonic material with lower losses in the optical range, the main question remains: what would be the maximum attainable degree of field concentration? In this work, the authors considered the relative importance of the different loss mechanisms in the light confinement challenge and showed that the Landau damping remains the most important source of limitations in plasmonic systems.

The research into new plasmonic materials has recently led scientists to the study of graphene which has shown its potential for various electro-optics applications such as fast light modulation. Even with a strong rise in experimental efforts from the community, graphene plasmons have so far been observed only at mid-infrared and lower frequencies. The second paper of this session, from the group of F. Garcia de Abajo (DOI: 10.1039/C4FD00216D) (ICFO), was dedicated to the analytical study of other potentially interesting atomically thin materials. They particularly explored one-atom-thick noble-metal layers such as disks or ribbons, allowing the electrical modulation of light at visible and near-infrared frequencies as well as the strong coupling with optical quantum emitters.

Continuing the research conducted on graphene and the phenomena arising from the Dirac cone dispersion of electrons, interest has been shown in finding optical systems with equivalent behaviour. Several systems are known to support Dirac cone dispersions such as coupled waveguides, photonic crystals or metamaterials. However, these structures exhibit spatial variations in two or three dimensions which make their practical use restricted. With the aim of overcoming these limitations, Evgenii Narimanov (Purdue University) demonstrated that photonic hypercrystals can support Dirac cone dispersion (DOI: 10.1039/C4FD00207E). These planar optical composites are hyperbolic metamaterials, with a periodic spatial variation of the dielectric permittivity on a subwavelength scale, and thus combine the properties of hyperbolic metamaterials and photonic crystals. Their high geometrical tunability allows the fine control of their electromagnetic properties and their accessible fabrication requirements give the opportunity to optically study the physical concepts behind Dirac cone dispersion such as Klein's tunneling or Schrödinger’s “zitterbewegung” (Fig. 1).

Fig. 1 The dispersion diagram of a photonic hypercrystal. The hypercrystal unit cell is formed by 250 nm of an In_0.53Ga_0.47As:Al_0.48In_0.52As semiconductor hyperbolic metamaterial with a 5 μm plasma wavelength, followed by 250 nm of an Al_0.48In_0.52As dielectric layer (DOI: 10.1039/C4FD00207E).

Viktor A. Podolskiy (DOI: 10.1039/C4FD00186A) (University of Massachusetts Lowell) then presented analytical and computational studies of light emission in nonlocal plasmonic metamaterials formed by arrays of aligned nanowires. In this work, they compare the results of the enhancement of the decay rate for a point dipole emitting inside the metamaterial using the local and nonlocal effective medium theories (EMTs). The results show a disagreement between the predictions of the two theories which the authors explain by the presence of an additional plasmonic mode missed in the local EMT. This non-locality strongly influences the light emission in the material leading to an increase in the photonic density of states in the elliptic regime. In contrast with previous studies predicting the maximum enhancement of the Purcell effect in the hyperbolic regime, these results suggest a significant enhancement of the decay rate where the metamaterial exhibits an elliptic response.

The fine tuning of the electromagnetic properties of materials has recently been at the heart of the research in nanoplasmonics in order to engineer the interactions between light and matter. In the paper of Nader Engheta (University of Pennsylvania) (DOI: 10.1039/C4FD00205A), materials with relative permittivities and permeabilities exhibiting near-zero values, called epsilon-and-mu-near-zero (EMNZ) media, are considered. He theoretically and computationally investigated the electromagnetic behaviour of the end-points of bound structures filled with an hypothetical EMNZ medium before moving on to examples of effective media. Peculiar properties are shown in such EMNZ media: a decoupling between electric and magnetic fields while they vary sinusoidally with time and a uniformity of the phase across the material open up the possibilities for coherent radiation.

The end of the first day of discussions was dedicated to lightning presentations and a poster session where 60 students presented their most recent research in the field of nanoplasmonics, including theoretical as well as experimental studies. The posters stayed throughout the whole conference, which delivered great opportunities for the students to discuss their work with each other and senior researchers with all kinds of scientific backgrounds.

Session 2, chaired by Thomas Ebbesen, on surface plasmon enhanced spectroscopy focused on the very limit of nm-scale plasmonics. The behaviour of light in gaps in the single and sub-nm range and its uses in spectroscopy were discussed. Javier Aizpurua (DIPC) started the session by presenting theoretical work on the different mechanisms that have an influence on the optical response of a gap between two plasmonic structures and their relative importance for different gap sizes (DOI: 10.1039/C4FD00196F). A classical model, the Quantum Corrected Model (QCM), was used to model the electron tunneling across the gap by introducing an effective conductive medium in it. The significant computational advantage of this over full quantum calculations allowed the authors to work with more realistic plasmonic structures and also nonlocal effects were added to the QCM. Three different regions of inter-particle distance were identified with these models. For gap sizes larger than 2–5 nm, Maxwell's equations with local material properties reliably describe the optical response of plasmonic dimers. For smaller distances, nonlocal effects need to be introduced to account for the quantitative changes, while below ≈0.35 nm the QCM was needed to model the radical qualitative changes created by quantum tunneling. Jeremy Baumberg (Cambridge University) continued the session by introducing a new optical method, normalising plasmon resonance (NPR) spectroscopy, which uses the intense plasmonic hot spots created between a gold surface and gold nanoparticles to measure both the properties of the material within and the size of the gap (DOI: 10.1039/C4FD00195H). These structures typically contain two plasmonic resonance modes: the transverse one with charge oscillations parallel to the gold surface, making it largely insensitive to the gap properties, and the coupled mode, with the optical mode largely confined within the gap. Normalizing the gap-sensitive latter mode by the former mode allowed the authors to measure spacer thicknesses below 1 nm with a 0.1 nm accuracy and to distinguish between different materials using the refractive index, with graphene as a highly useful example (Fig. 2).

Fig. 2 Depiction of a gold nanoparticle on a gold surface with a 0.34 nm thick graphene spacer. The locations of the transverse mode (green) and the coupled mode (red) are shown schematically (DOI: 10.1039/C4FD00195H).

The last talk of the first part of the session, presented by Sang-Hyung Oh (University of Minnesota), was on the extraordinary optical transmission of terahertz waves through arrays of nanogaps and the use of the created field enhancement for the sensing of films of single-walled carbon nanotubes (DOI: 10.1039/C4FD00233D). By using a novel atomic layer lithography technique they managed to fabricate wafer-scale closely packed and well-defined periodic arrays of nanogap loops made of Al₂O₃ with gap widths of only 2 nm and separations going down to 5 μm. These periodic loops can sustain terahertz waves along their contours, creating extraordinary optical transmission through the gaps and broadband field enhancement in them. With aerial coverage ratios of 0.1%, amplitude transmissions of 50% are reached. A 10 nm thick layer of carbon nanotubes on top of the structure was used to show an improved extinction of 43% compared to 2% for the same layer on a glass substrate, convincingly showing the possibilities for THz biological and chemical sensing.

Masayuki Futamata (Saitama University) started the next part of the session with the presentation of their work on the importance of gap modes in surface enhanced Raman scattering (DOI: 10.1039/C4FD00188E). Three different geometries were investigated: gap modes between silver nanospheres by flocculation-SERS, direct excitation of gaps between gold nanoparticles and a wide variety of metal substrates, and excitation of the gaps in an ATR geometry through propagating surface plasmons. In-depth experimental results were compared with FDTD computational results so as to establish gap mode SERS as an analytical tool. It was shown that substrates with large damping factors or even dielectric materials with sufficiently large optical constants provide efficient Raman scattering. Because of this, investigating absorbed states on smooth surfaces of, for example, Au or catalytic reactions on transition metals like Pt and Fe is possible. Gap modes excited in an ATR set-up provide even an additional enhancement due to the coupling of the propagating surface plasmons with the gap modes.

The possibilities of SERS and tip enhanced Raman scattering (TERS) were further elucidated in the work by Wang et al. Zachary Schultz (University of Notre Dame) presented their results on the use of plasmonics for TERS detection and imaging (DOI: 10.1039/C4FD00190G). SERS spectra from purified molecules were used to image proteins on surfaces and in cell membranes, serving as a reference for the spectra obtained from TERS, where the AFM tip delivers nanometer resolution in combination with high sensitivity due to the gap plasmon modes between the tip and the gold nanoparticles, allowing the exploration of complex and chemically heterogeneous systems. It was shown that with TERS the location of a biomolecule in a matrix can be detected and the significant influence of the binding amino acids in the signal when using protein mutants can be measured.

Riccardo Sapienza (Kings College London) introduced their results on the influence of both global long-range interactions and local near-field interactions on the light emission of fluorescent probes placed in percolating plasmonic networks (DOI: 10.1039/C4FD00187G). Self-assembled percolating networks were made by thermal evaporation of gold on a dielectric surface, where at the percolation threshold (the moment large connected components appear) a continuous conducting path of gold is formed. Confocal microscopy was used for the fluorescent dynamics measurements of dye-doped beads coupled to these self-assembled films and these were compared to the dynamics coming from coupling the beads to a plasmonic network of gold rectangular nano-antennas made by Electron Beam Lithography (EBL). The latter provides the advantage of having more controllable parameters (Fig. 3). No strong evidence for the influence of long-range interactions linked to the degree of percolation were found, indicating that the local density of states was the dominant effect on the fluorescence of the dipoles. This conclusion mainly holds for the smartly chosen EBL manufactured networks, where fewer uncontrollable parameters influence the results.

Fig. 3 SEM image of self assembled (a) and EBL manufactured (b) plasmonic networks (DOI: 10.1039/C4FD00187G).

Under the supervision of Professor Nader Engheta, session 3 on quantum plasmonics, gain and spasers started with a presentation by Thomas Ebbesen (Strasbourg University) on the dynamics and spectroscopy of several molecular materials ultra-strongly coupled to an optical cavity (DOI: 10.1039/C4FD00197D). Three different kinds of molecules, TDBC in J-aggregated form, BDAB and fluorescein, were placed in high concentration in the strongly confined fields of metallic micro cavities, where the ultra-strong coupling with the optical mode not only creates polaritonic states, but also perturbs the other states of the system, including the ground state. Absorption and emission spectra of the coupled systems were compared with those of the bare molecules, with a special focus on the transition probabilities to the lower polaritonic state. The lifetime of this state was also probed. Several questions were raised, and very surprising was the lack of emission or transient absorption from the lower polaritonic branch in the presence of a strong static absorption peak to this branch. After a very constructive discussion, it seems that much progress still has to be made in the topic of strong light–matter interactions in molecular materials, both experimentally and theoretically.

The subject changed from many to single emitter interactions with a paper by Sergey Bozhevolnyi (University of Southern Denmark) on the relaxation dynamics of a quantum emitter resonantly coupled to a coherent state of localized surface plasmons (DOI: 10.1039/C4FD00165F). The situation of a three-level quantum dipole emitter excited by a external laser and weakly coupled to a dipolar Localized Surface Plasmon (LSP) described by a coherent state was solved analytically, which is possible when the decay is considerably faster than in free space while still considerably slower than the LSP decay. It was claimed that this weak coupling regime is the result of a coherent interaction between the quantum dipole emitter and the coherent state of the resonant LSP, resulting in relaxation dynamics that also exhibit strong coupling features. Several interesting outcomes were obtained, for example a deviation of the expected exponential decay was shown, including a radiation emission that reaches a maximum with a considerable time delay.

Prof. Ortwin Hess (Imperial College London) ended this session with another jump in subject to stopped-light lasing in a nanoplasmonic structure (DOI: 10.1039/C4FD00181H). The Maxwell–Bloch Langevin approach was used to study dispersionless stopped-light in metal–dielectric waveguides and the spatio-temporal dynamics of both stopped-light lasing and stopped-light surface plasmon polariton lasing. In these situations, not a cavity, but a closed-loop energy vortex creates the lasing mode by dynamically phase-locking the continuum of k-modes around the stopped light points. Interesting dynamics develop, with stable lasing for structures with cores less than 200 nm, but more complex spatio-temporal oscillations are seen in wider structures. Thanks to the extremely strong light localization and fast dynamics that come with the plasmonic structures, the idea of stopped-light nanoplasmonic lasers could further open the way to ultrafast nanolasing, ultra-thin lasing structures and cavity-free quantum-electrodynamics.

Ortwin Hess as chair opened session four on applications in nanophotonics. The rather broad title promised a diverse 90 minutes, with talks on the electrical control of Faraday rotation, second harmonic generation in a metamaterial and to start with, a theoretical discussion presented by David Zueco (Universidad de Zaragoza) on nonlinear quantum optics in the strong and ultra strong light–matter coupling in a waveguide-QED setting (DOI: 10.1039/C4FD00206G). In these regimes, where nonlinear effects are expected at the level of a few photons, it is crucial to describe both matter and light quantum mechanically. Numerical solutions of the quantum evolution for a wide range of different ratios of the number of photons to qubits and different coupling strengths were obtained with the matrix product states technique. The transition from linear to non-linear scattering was quantified and it was shown that while transmitted currents already give linear behaviour when the amount of qubits is close to the amount of photons, two photons interacting with 20 qubits can still give rise to photon–photon interactions. Furthermore, new phenomena were shown in the ultra-strong coupling regime, where the coupling strength enters the energy scales of the qubits and the rotating wave approximation breaks down. In the current quest of experimentally realizing stronger and stronger coupling and creating nonlinear interactions at lower photon levels, theoretical results such as these are indispensable for further understanding and progress.

Alessandro Belardini (Universita di Roma La Sapienza) showed the results from second harmonic generation measurements on a self-assembled metasurface (DOI: 10.1039/C4FD00200H). By evaporating gold onto a silicon substrate positioned at an angle, self-ordered arrays of gold standing nanowires were grown. Thanks to this bottom-up fabrication approach, these samples have a strong advantage in fabrication time and cost. The symmetry breaking coming from the surface geometry was investigated by measuring the power and polarization of light at 400 nm coming from the surface when irradiated with elliptically polarized pulsed light at 800 nm. This well established technique to study the optical chirality of materials allowed the authors to quantify the chirality of the surface. Considering the importance of chiral molecules in the life sciences, boosting the chirality using patterned surfaces like this metasurface has a wide range of applications.

Another jump in field was made in the final paper of the day by Michael Flatte (University of Iowa), in which the results of a theoretical development of the Faraday rotation of light by magnetic nanoparticles at a liquid–liquid interface were discussed (DOI: 10.1039/C4FD00210E). An externally applied electrical potential can switch yttrium iron garnet nanospheres between the interface of two immiscible electrolyte solutions (ITIES), like an aqueous and organic oil salt solution interface, and the bulk of the aqueous phase. When at the interface, the closely packed particles create an enhanced Faraday rotation due to the interaction between them, creating the possibility of electrically modulating the rotation. It was shown that for the right choice of applied voltage and particle size, this kind of mechanism could effectively be used to switch an optical cavity from fully transparent to fully reflective with a change in voltage as low as 0.5 V with small cell sizes, a feature highly useful for devices that require the modulation of light such as optical isolators and circulators (Fig. 4).

Fig. 4 Positive and negative voltages applied over the electrolytic solutions move them to the bulk of the aqueous phase or to the interface between the two liquids, respectively (DOI: 10.1039/C4FD00210E).

The final session of the meeting, on Wednesday morning, was chaired by Jeremy Baumberg (University of Cambridge) and centered on new applications of plasmonics. Strong field enhancements in the areas of plasmonic nanoparticles and surfaces are exploited to develop new techniques in various domains such as sensing, imaging and chemistry.

In his paper, David Richards (DOI: 10.1039/C4FD00198B) (King's College London) outlined how the changes in the fluorescence lifetime of molecules above a nanostructured plasmonic substrate can be used as a “nanoruler” allowing the determination of molecular location inside a biological cell up to 100 nm above the substrate surface with an axial position sensitivity reaching 6 nm. Compared to a similar study performed on flat metallic films, where the reduction of the lifetime with respect to the separation of the film is due to the enhancement of the non-radiative decay rate, radiative decay processes dominate in this case, leading to a fluorescence signal with high intensity even close to the substrate. This approach has been employed to map the topography of a cell membrane and has demonstrated its potential in the investigation of receptor-mediated endocytosis.

To follow on from the use of plasmonics in biology related applications, Yu Chen (DOI: 10.1039/C4FD00199K) (Strathclyde University) took advantage of the strong two-photon luminescence of gold nanoparticles to study the surface plasmon enhanced energy transfer between gold nanorods (GNRs) and fluorophores. Using a Fluorescence Lifetime Imaging Microscopy (FLIM) based method, the authors studied the internalization of GNRs in HeLa cells using an early endosome marker coupled with GFP. The energy transfer between the GNRs and GFP, reducing the fluorescence lifetime of GFP and modifying the lifetime distribution, reflects the involvement of endocytosis in the GNR uptake. The authors also investigated GNR based energy transfer for ribonucleic acid sensing with a novel nanoprobe based on oligonucleotide functionalized GNRs for cancer diagnosis and prognosis (Fig. 5).

Fig. 5 FLIM images of Rab5a-GFP treated HeLa cells incubated with CTAB capped gold nanorods for (a) 30 minutes, (b) 45 minutes, and (c) 1 hour; (d) lifetime distribution of GFP in overlapping areas.

The last speaker before a short break was Mikhail Noginov (DOI: 10.1039/C4FD00184B) (Norfolk State University) who also addressed the topic of Förster resonance energy transfer by studying the lifetime changes of donor–acceptor pairs in the vicinity of metallic structures. Any photonic media with homogeneous or inhomogeneous dielectric permittivities, such as metamaterials, exhibit a local density of states different from that in a vacuum. Therefore, these metamaterials are known to allow the control of spontaneous emission rates. In this study, the authors research the effect of the modification of the dielectric environment on the energy transfer between donor and acceptor molecules. The main result shows that the presence of metallic surfaces and hyperbolic metamaterials, allowing an enhancement of the spontaneous emission rate of molecules placed in their vicinity, leads to the inhibition of the Förster energy transfer rate of the donor–acceptor pairs. Recently, the investigation of the influence of the local density of states on the Förster energy transfer has led to several controversial results and remains an open topic of discussion.

Not only has it been receiving a lot of attention in biology related domains, but the use of plasmonic nanostructures has shown a definite potential in solar energy conversion applications. Known for its ability to improve photovoltaic light trapping thanks to strong localized surface plasmon resonances, nanoplasmonics has been mainly exploited to improve the efficiencies of conventional semiconductor-based systems. Here Martin Moskovits (DOI: 10.1039/C4FD00185K) (University of California) discussed the possibility of using the hot electrons and holes generated as a result of surface plasmon decay in plasmonic nanostructures as a radically different approach in photovoltaic devices. The current designs show low efficiencies but manifest several interesting characteristics such as a high tunability over the entire solar spectrum as well as an improved longevity, thanks to the use of a liquid junction reducing the dielectric breakdown in the oxide layers involved. The presented device consisted of a 2D array of gold nanoparticles combined with a thin electrolytic liquid junction with redox active molecules, resulting in photocurrent densities in excess of 40 μA cm⁻². The presentation was then followed by the intervention of Prineha Narang (Caltech) who was invited to make some comments related to computational work on the analysis of plasmon-driven hot carrier generation. She discussed the implications of multi-plasmon decays on the design of hot carrier devices and plasmon-driven chemistry applications.

The final talk of the session was delivered by Olivier Martin (DOI: 10.1039/C4FD00224E) (EPFL) on optical forces in nanoplasmonic systems and how they can be used as the driving mechanism in the control of the manufacturing steps in a nanofactory. Plasmonic trapping, enabled by the strong gradient produced in the near-field of plasmonic structures, has recently shown its potential in the trapping and manipulation of extremely small objects. In this paper, the authors studied the optical forces produced by such systems and reviewed the numerical techniques used to study them. In particular, they describe a method based on the combination of the surface integral equation and Maxwell's stress tensor and apply it to the study of realistic optical antennas and heptamer nanostructures. The detailed knowledge of the optical forces in the nanostructures led to the futuristic vision of a nanofactory, where plasmonic trapping, surface chemistry and even electrical functions could be used to modify an original structure and produce a more advanced system. Fransisco Rodríguez Fortuño (King's College London) was also invited to present some comments related to the possibility of achieving a lateral force acting on a centrosymmetric particle near a surface. This force, perpendicular to the illumination direction and parallel to the surface, changes direction according to the polarization of the incident light. After an interesting discussion, this fifth and last session ended with a more general discussion about the topics raised at the meeting and the development of various techniques generally used in plasmonic studies such as transmission electron microscopy, electron energy loss spectroscopy or cathodoluminescence.

Niek Van Hulst (ICFO) presented the concluding remarks (DOI: 10.1039/C5FD90021B). We were reminded of the remarkable journey that research in electromagnetism has made since Faraday's time. Even though Faraday himself was already strongly interested in the interaction of light with metal particles, particularly gold, and the results in plasmonics show that Maxwell's equations don't break down at the nm scale and in the petaherz range, new techniques and ideas have allowed the field to evolve in many different directions. New materials, increasingly controllable sub-nm features and ultrafast spectroscopy all contribute to the new research and ultimately the new exciting applications that will hopefully one day make large contributions in nanoprobing, nanosensing, nanoruling and nanoimaging.

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