Introduction to the focus issue on surface plasmon photonics

Surface plasmon photonics is a rapidly growing and evolving field on the cutting edge of optical science and engineering. The field is concerned with the interaction of light on metallic structures, from THz to UV wavelengths, and is driven by interests in fundamental questions and in applications such as biosensors for health care, light concentration for solar energy, devices for telecommunications, and near-field instrumentation for performing state-of-the-art research.

The 6th International Conference on Surface Plasmon Photonics (SPP6) was held recently in Ottawa, Canada from May 26 to 31, 2013 [1]. This independent series of biennial conferences is widely regarded as the premier series in the field, and the 6th edition maintained the tradition of excellence established at previous editions (SPP5 Busan, SPP4 Amsterdam, SPP3 Dijon, SPP2 Graz, SPP1 Granada, and the precursor to the series organized by T. W. Ebbesen in 2001). SPP6 brought together about 500 plasmonics experts from around the globe, both junior and senior, from academia and industry, to share their latest results and set the agenda for future developments in the field. About 520 abstracts were received from 34 countries across several science and engineering disciplines. The technical program of the conference was constructed from submissions on fundamental and applied topics. Papers reporting applications such as biosensors for health care, light concentration for solar energy, devices for telecommunications, and near-field instrumentation were presented, along with papers on fundamental aspects such as nonlocality, quantum effects, electron-plasmon interactions, loss compensation and plasmon lasing, and novel plasmonic materials. Papers were gathered into sessions on traditional topics such as waveguides, particles, nanoholes and metamaterials, and in sessions reflecting recent trends such as antennas, photovoltaics and graphene. The large number of high-quality abstracts received for SPP6, the representation of several science and engineering disciplines, the increasing international scope, and the emergence of new topics and trends, suggest that the field of plasmonics is very healthy indeed and that it continues to expand.

This Focus Issue collects several invited papers related to research presented at SPP6, and although the number of papers it comprises is small compared to the total number of papers presented at the conference, the issue is representative and provides a good snapshot of the field at this point in time. Plasmon-oriented topics covered in this Focus Issue include quantum effects, nonlocality, emission, nonlinearity, sensors, nano-particles, nano-antennas, waveguides, devices and ultrafast phenomena. We briefly describe in what follows the papers included in this Focus Issue [2–21].Quantum effects and nonlocality: The assumption that the electric displacement is simply proportional to the electric field at the same position (local description) is widely extended in the study of plasmonics, but recent work is demonstrating that it breaks down when small distances between metals or sharp metal elements are involved, where nonlocal effects become important. Nonlocality produces a smooth redistribution of the induced charges towards the bulk of the materials involved, rather than being confined to the interfaces, as predicted by local theories. This charge redistribution generally leads to weaker plasmon confinement and field enhancement, which can damage the prospects of plasmons for applications such as biosensing and short-distance waveguiding. At SPP6, Wiener et al. presented semi-analytical theory based on an elegant reformulation of the hydrodynamical model to describe the reduction of field enhancement at hourglass waveguides involving gap distances below 0.5 nm [2].

The drive for engineering ultimate field enhancement and confinement has recently entered a new realm as optical feed gaps are now routinely reaching sub-nanometer dimensions. In this interaction regime, the classical description of the plasmonic response based on Maxwell’s equations fails to capture the underlying physics. In their approach Teperik and associates used a full quantum mechanical approach to take into account electron tunneling to appropriately predict the optical response of two strongly coupled nanowires [3].

The regime of strong coupling is further discussed in the work of Rodriguez and Gómez Rivas [4]. Using energy-momentum spectroscopy, the authors investigate the coupling mechanisms between lattice resonances and organic excitons. They show that the lattice constant controls the properties of the hybridization and that the effective mass of the coupled system can be reduced. This is a prerequisite to achieve quantum condensation.

Raza et al. [5] have studied the blueshift of the surface plasmon resonance of Ag nanoparticles with decreasing particle diameter from 26 to 3.5 nm. A blueshift of 0.5 eV observed from EELS was in qualitative agreement with the nonlocal hydrodynamic model, for which a nonlocal Clausius-Mossotti factor was derived. The authors suggested the neglect of the intrinsic properties of silver as a possible mechanism for the observed quantitative deviation between theory and experiment.Emission and nonlinearities: By examining light emission from the junction of a STM in the presence of 20 nm topographical features in thin gold films, Divitt et al. [6] show that the variability in STM photoemission rates between a gold tip and a gold film under ambient conditions is due to the modification of localized gap plasmon modes. The electro-luminescence from gold clusters on the STM probe apex was typically negligible.

Light interaction with hole arrays, and in particular extraordinary optical transmission and resonant effects related to periodic arrangements of holes, have been a recurrent subject in the entire series of SPP conferences. At SPP6, van Exter et al. [7] presented evidence of lasing in the light emitted from metallic hole arrays surrounded by an optically pumped semiconductor. The emission took place at frequencies and directions of emission determined by the lattice resonances.

The enhancement of optical nonlinearities using plasmonic structures is of strong current interest. Khurgin and Sun [8] investigated the plasmonic enhancement of 3rd order nonlinear optical phenomena in metal-cladded dielectric waveguides, with the nonlinearity originating in the dielectric. They find that the effective nonlinear index is strongly enhanced relative to the bulk dielectric but that propagation losses limit the overall efficiency.Sensors: Because of their nature, surface plasmons are routinely used to probe and monitor minute changes in the refractive index of the bounding dielectric environment. With the quest to ever more sensitive platforms, Špačková and Homola numerically investigate the figure-of-merit of lattice resonances [9]. They found that while this parameter is comparable to that for surface sensing on non-ordered arrays of nanoparticles, lattice resonances outperform when the bulk surrounding refractive index changes.

The large field enhancement and strong focusing of light produced by localized plasmons allow using them as “lighthouses”, whereby any substance in the vicinity of the metallic particles sustaining plasmons are detected with a minimum of noise, as the surrounding environment is comparatively less exposed to the applied light. Using this principle, Punj et al. [10] have managed to detect micromolar concentrations in zeptomol volumes with improved sensitivity by combining it with fluorescence correlation analysis.Nano-particles and nano-antennas: Pors and Bozhevolnyi [11] investigate theoretically the absorption and scattering properties of an array composed of gap surface plasmon resonators. The authors introduced the concept of a birefringent metal surface to control the phase of polarized light and meta-scatterers to steer light beams in integrated nanophotonic systems.

Earl et al. [12] investigated tunable optical antenna arrays as resonating Ag nano-structures on films of VO₂. Tuning was achieved thermally by inducing a phase transition in VO₂, by heating the film above its critical temperature (68 °C). Tuning of resonant wavelengths by up to 110 nm was observed.

The field of plasmonics has witnessed an impressive, continued effort to devise simple analytical descriptions of the interaction of light with nanostructures. Following this tradition, a new method has been put forward by Bai et al. to analyze the scattering of light by resonant plasmonic antennas [13].

Active control over the plasmonic response of nanoantennas is also of paramount importance, in order to reconfigure the response of future adaptable nanoplasmonic devices. In particular, control through applied magnetic fields has been demonstrated by placing a magneto-optic structure close to the plasmonic structure. This results in so-called magneto-plasmonic activity, in which the plasmon inherits a susceptibility to external magnetic fields. In their paper, Armelles et al. have demonstrated an emulation of electromagnetically induced transparency using suitably designed magnetoplasmonic structures [14].Waveguides and devices: The ability of propagating plasmons to carry information encoded in their time-dependent intensity has stimulated a long search for suitable waveguide designs to realize plasmonic networks for on-chip optical communications. Inelastic plasmon attenuation is however a limiting factor, especially when the plasmons are tightly confined to laterally narrow waveguides. New designs are constantly challenging the tradeoff between the degree of confinement, which is beneficial for high integration density, and the propagation distance. Takahara and Miyata [15] propose and demonstrate conversion of short-range, tightly confined plasmons into long-range, loosely confined plasmons and vice-versa. This is an interesting solution this tradeoff, where each type of plasmon is used wherever it can perform better.

Weeber et al. [16] investigated the thermo-optic dynamics of polymer-loaded plasmonic waveguides driven photo-thermally on ns time scales. They demonstrated the thermally-modulated absorption of SPP modes, by thermally modulating the absorption of the metal (i.e., they exploit the temperature-dependent Ohmic loss of metals). Modulation depths of up to 50% were observed, with 2 and 800 ns characteristic times. Racetrack-shaped resonators were also investigated.

Babicheva et al. [17] propose ultra-compact plasmonic modulators using alternative plasmonic materials, such as transparent conducting oxides and titanium nitride, with a view towards eventual integration with silicon electronics. They exploit the carrier refraction effect in a conducting oxide, whereby modulating the carrier density therein modulates the loss of the SPPs. They predict extinction ratios of up to ~45 dB/μm.

Schmidt et al. [18] propose and demonstrate the use of a deformable mirror to increase the excitation efficiency to SPPs by a grating coupler fabricated on a conical ultra-sharp gold taper. The deformable mirror adaptively controls the wave front of the incident far field light. The shape of the mirror was optimized to maximize the intensity of the light scattered from the tip by nanofocused SPPs converging thereon. They find that a highly astigmatic beam profile maximizes the nanofocusing efficiency of their structure.Ultrafast phenomena: Lemke et al. [19] demonstrated a plasmonic autocorrelator for SPP pulse characterization based on a wedge-shaped structure which continuously increases the time delay (0 ~ 30 fs) between two interfering SPPs. The autocorrelation signal monitored by non-linear two-photon photoemission electron microscopy also provided the mapping of SPP field amplitudes in a direct manner.

Zerula and co-workers [20] introduce an engineered periodic surface to efficiently excite surface plasmons over the entire spectral content of a sub-20 fs laser pulse. A nearly dispersion-less mode is identified by energy-momentum spectroscopy. The ability to excite propagating broadband modes with no chirp is an important step for controlling the ultrafast dynamics of surface plasmons.

The visualization of plasmonic fields with high spatial and temporal resolution is a necessity to understand and design the next generation of plasmonic devices. Imaeda and Imura [21] introduce a near-field optical microscope to interrogate the lifetime of plasmon excitations. They show that interference dictates the spatial distribution and can thus be controlled by a phase-modulation of the ultrafast incident pulses.

In closing, we hope that researchers will enjoy reading this Focus Issue, and we extend to all an enthusiastic invitation to the 7th edition of the SPP conference series which will be held in Jerusalem May 31 to June 5, 2015.