Plasmonic directional couplers using channel waveguides in random arrays of metal nanoparticles.

Plasmonic directional couplers based on channel waveguides embedded in random arrays of metal nanoparticles (NPs) operating in near-infrared are fabricated using electron-beam lithography and investigated experimentally characterizing their performance with leakage-radiation microscopy. The power exchange between coupled waveguides, its spatial period and efficiency, along with the overall power transmission, are determined in the wavelength range from 700 to 800 nm. We introduce a simple coupled-mode approach based on three coupled waveguides. The composite system considers a waveguide consisting of NP-filled stripe with characteristics distinctly different from those of the channel waveguides. Using this model, we describe the performance of investigated composite plasmonic configurations and obtain good qualitative agreement with experimental observations.

Partial loss compensation in dielectric-loaded plasmonic waveguides at near infra-red wavelengths.

We report on the fabrication and characterization of straight dielectric-loaded surface plasmon polaritons waveguides doped with lead-sulfide quantum dots as a near infra-red gain medium. A loss compensation of ~33% (an optical gain of ~143 cm⁻¹) was observed in the guided mode. The mode propagation, coupling efficiency and stimulated emission were characterized using leakage radiation microscopy. The guided mode signature was separated using spatial filters in the Fourier plane of the microscope for quantitative measurements of stimulated emission.

0.48Tb/s (12x40Gb/s) WDM transmission and high-quality thermo-optic switching in dielectric loaded plasmonics.

We demonstrate Wavelength Division Multiplexed (WDM)-enabled transmission of 480Gb/s aggregate data traffic (12x40Gb/s) as well as high-quality 1x2 thermo-optic tuning in Dielectric-Loaded Surface Plasmon Polariton Waveguides (DLSPPWs). The WDM transmission characteristics have been verified through BER measurements by exploiting the heterointegration of a 60 µm-long straight DLSPPW on a Silicon-on-Insulator waveguide platform, showing error-free performance for six out of the twelve channels. High-quality thermo-optic tuning has been achieved by utilizing Cycloaliphatic-Acrylate-Polymer as an efficient thermo-optic polymer loading employed in a dual-resonator DLSPPW switching structure, yielding a 9 nm wavelength shift and extinction ratio values higher than 10 dB at both output ports when heated to 90°C.

Surface-enhanced Raman spectroscopy: nonlocal limitations.

Giant field enhancement and field singularities are a natural consequence of the commonly employed local-response framework. We show that a more general nonlocal treatment of the plasmonic response leads to new and possibly fundamental limitations on field enhancement with important consequences for our understanding of surface-enhanced Raman spectroscopy (SERS). The intrinsic length scale of the electron gas serves to smear out assumed field singularities, leaving the SERS enhancement factor finite, even for geometries with infinitely sharp features. For silver nanogroove structures, mimicked by periodic arrays of half-cylinders (up to 120 nm in radius), we find no enhancement factors exceeding 10 orders of magnitude (10(10)).

Efficient unidirectional ridge excitation of surface plasmons.

Using leakage-radiation microscopy, we characterize the efficiency of unidirectional surface-plasmon excitation with periodic (800 nm) arrays of 130-nm-high and 330-nm-wide gold ridges on a thin gold film illuminated with a focused (5-microm-wide) laser beam. We demonstrate that, at the resonant wavelength of 816 nm, the excitation efficiency of > 0.4 can be obtained with >or= 5 ridges by adjusting the beam position. Conducting numerical simulations, we account for the experimental results and calculate the electric-field enhancement achieved near the gold surface.

Plasmon-polariton nano-strip resonators: from visible to infra-red.

Dispersion of the resonant properties exhibited by silver and gold nano-strips in a wide range of wavelengths is considered. The tunability and Q-factor of scattering resonances as well as the field enhancement achieved at strip terminations are analyzed in the wavelength range from visible to near infrared (400-1700 nm), confirming that the resonant behaviour is dominated by dispersion properties of short-range surface-plasmon polaritons (SR-SPPs) propagating along the strip. It is found that, while the Q-factor decreases for longer wavelengths due to the SR-SPP dispersion curve moving closer to the light line, the field enhancement depending also on the metal susceptibility magnitude remains largely unaffected. The results obtained are also used to estimate the phase change involved in the SR-SPP reflection by strip terminations.

Focusing and directing of surface plasmon polaritons by curved chains of nanoparticles.

Excitation, focusing and directing of surface plasmon polaritons (SPPs) with curved chains of nanoparticles located on a metal surface is investigated both experimentally and theoretically. We demonstrate that, by using a relatively narrow laser beam (at normal incidence) interacting only with a portion of a curved chain of nanoparticles, one can excite an SPP beam whose divergence and propagation direction are dictated by the incident light spot size and its position along the chain. It is also found that the SPP focusing regime is strongly influenced by the chain inter-particle distance. Extensive numerical simulations of the configuration investigated experimentally are carried out for a wide set of system parameters by making use of the Green's tensor formalism and dipole approximation. Comparison of numerical results with experimental data shows good agreement with respect to the observed features in SPP focusing and directing, providing the guidelines for a proper choice of the system parameters.

Efficient electro-optic modulation in low-loss graphene-plasmonic slot waveguides.

Surface plasmon polaritons enable light concentration within subwavelength regions, opening thereby new avenues for miniaturizing the device and strengthening light-matter interactions. Here we realize efficient electro-optic modulation in low-loss plasmonic waveguides with the aid of graphene, and the devices are fully integrated in the silicon-on-insulator platform. By advantageously exploiting low-loss plasmonic slot-waveguide modes, which weakly leak into a substrate while featuring strong fields within the two-layer-graphene covered slots in metals, we successfully achieve a tunability of 0.13 dB µm-1 for our fabricated graphene-plasmonic waveguide devices with extremely low insertion loss, which outperforms previously reported graphene-plasmonic devices. Our results highlight the potential of graphene plasmonic leaky-mode hybrid waveguides to realize active ultra-compact devices for optoelectronic applications.

Long-range surface plasmon polariton nanowire waveguides for device applications.

We report an experimental study of long-range surface plasmon polaritons propagating along metallic wires of sub-micrometer rectangular cross-sections (nanowires) embedded in a dielectric. At telecom wavelengths, optical signals are shown to propagate up to several millimeters along such nanowires. As the wires approach a square cross-section, the guided mode becomes more symmetric and can, for example, be tuned to match closely the mode of a standard single-mode optical fiber. Furthermore, symmetric nanowires are shown to guide both TM and TE polarizations. In order to illustrate the applicability of plasmonic nanowire waveguides to optical circuits, we demonstrate a compact variable optical attenuator consisting of a single nanowire that simultaneously carries light and electrical current.

Hybrid graphene plasmonic waveguide modulators.

The unique optical and electronic properties of graphene make possible the fabrication of novel optoelectronic devices. One of the most exciting graphene characteristics is the tunability by gating which allows one to realize active optical devices. While several types of graphene-based photonic modulators have already been demonstrated, the potential of combining the versatility of graphene with subwavelength field confinement of plasmonic waveguides remains largely unexplored. Here we report fabrication and study of hybrid graphene-plasmonic waveguide modulators. We consider several types of modulators and identify the most promising one for telecom applications. The modulator working at the telecom range is demonstrated, showing a modulation depth of >0.03 dB µm(-1) at low gating voltages for an active device area of just 10 µm(2), characteristics which are already comparable to those of silicon-based waveguide modulators while retaining the benefit of further device miniaturization. Our proof-of-concept results pave the way towards on-chip realization of efficient graphene-based active plasmonic waveguide devices for optical communications.

Gap and channeled plasmons in tapered grooves: a review.

Tapered metallic grooves have been shown to support plasmons - electromagnetically coupled oscillations of free electrons at metal-dielectric interfaces - across a variety of configurations and V-like profiles. Such plasmons may be divided into two categories: gap-surface plasmons (GSPs) that are confined laterally between the tapered groove sidewalls and propagate either along the groove axis or normal to the planar surface, and channeled plasmon polaritons (CPPs) that occupy the tapered groove profile and propagate exclusively along the groove axis. Both GSPs and CPPs exhibit an assortment of unique properties that are highly suited to a broad range of cutting-edge nanoplasmonic technologies, including ultracompact photonic circuits, quantum-optics components, enhanced lab-on-a-chip devices, efficient light-absorbing surfaces and advanced optical filters, while additionally affording a niche platform to explore the fundamental science of plasmon excitations and their interactions. In this Review, we provide a research status update of plasmons in tapered grooves, starting with a presentation of the theory and important features of GSPs and CPPs, and follow with an overview of the broad range of applications they enable or improve. We cover the techniques that can fabricate tapered groove structures, in particular highlighting wafer-scale production methods, and outline the various photon- and electron-based approaches that can be used to launch and study GSPs and CPPs. We conclude with a discussion of the challenges that remain for further developing plasmonic tapered-groove devices, and consider the future directions offered by this select yet potentially far-reaching topic area.

A generalized non-local optical response theory for plasmonic nanostructures.

Metallic nanostructures exhibit a multitude of optical resonances associated with localized surface plasmon excitations. Recent observations of plasmonic phenomena at the sub-nanometre to atomic scale have stimulated the development of various sophisticated theoretical approaches for their description. Here instead we present a comparatively simple semiclassical generalized non-local optical response theory that unifies quantum pressure convection effects and induced charge diffusion kinetics, with a concomitant complex-valued generalized non-local optical response parameter. Our theory explains surprisingly well both the frequency shifts and size-dependent damping in individual metallic nanoparticles as well as the observed broadening of the crossover regime from bonding-dipole plasmons to charge-transfer plasmons in metal nanoparticle dimers, thus unravelling a classical broadening mechanism that even dominates the widely anticipated short circuiting by quantum tunnelling. We anticipate that our theory can be successfully applied in plasmonics to a wide class of conducting media, including doped semiconductors and low-dimensional materials such as graphene.

Graphene-protected copper and silver plasmonics.

Plasmonics has established itself as a branch of physics which promises to revolutionize data processing, improve photovoltaics, and increase sensitivity of bio-detection. A widespread use of plasmonic devices is notably hindered by high losses and the absence of stable and inexpensive metal films suitable for plasmonic applications. To this end, there has been a continuous search for alternative plasmonic materials that are also compatible with complementary metal oxide semiconductor technology. Here we show that copper and silver protected by graphene are viable candidates. Copper films covered with one to a few graphene layers show excellent plasmonic characteristics. They can be used to fabricate plasmonic devices and survive for at least a year, even in wet and corroding conditions. As a proof of concept, we use the graphene-protected copper to demonstrate dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon resonances. Our results are likely to initiate wide use of graphene-protected plasmonics.

Near-field optical microscopy of localized excitations on rough surfaces: influence of a probe.

Starting from the general principles of near-field optical microscopy. I consider the influence of a probe when being used to image localized dipolar excitations and suggest a way of evaluating the perturbation thus introduced. Using the rigorous microscopic (electric) point-dipole description, I calculate the self-consistent field intensity at the site of a probe dipole scanning over resonantly interacting object dipoles and show that the intensity distribution deviates from that existing in the absence of a probe. I demonstrate that this difference increases with an increase in the polarizability of the probe dipole. resulting eventually in a completely different intensity distribution. The calculations also show that the perturbation of the intensity distribution due to the presence of a probe decreases with an increase in the probe-sample distance. In order to evaluate the degree of perturbation. I suggest comparing the images obtained at different probe-sample distances. Finally, I formulate a simple rule of thumb that allows one to roughly estimate the probe-sample coupling when imaging localized excitations.

Near-field mapping of surface polariton fields.

Using the general approach to image formation in collection near-field optical microscopy, I derive the symmetry relations for the amplitude coupling coefficients in the case of a weakly guiding single-mode fibre terminated with a probe tip possessing axial symmetry. It is shown that, for the symmetrical detection configuration, six elements of the coupling matrix can be expressed by using only three independent coupling coefficients. The obtained relations are further applied to describe near-field mapping of surface plasmon polariton (SPP) fields. I demonstrate that, for the symmetrical detection configuration, the near-field optical image reflects the intensity distribution of the SPP field components parallel to the surface plane, even though the strong perpendicular component is also being detected. This conclusion is supported with numerical simulations that elucidate the influence of symmetry of the fibre probe on the resulting near-field optical image. The near-field optical images simulated for scattering systems typical for SPP microoptics and localization are presented. It is found that the presence of asymmetry in the detection configuration increases the contribution of the perpendicular field component and results in the images approaching the corresponding SPP intensity distributions.

Near-field imaging of the interference pattern of counterpropagating evanescent waves.

It is generally accepted that measurement of the contrast of the intensity interference pattern formed by two counterpropagating evanescent waves can be used to characterize the resolving power of a collection near-field microscope. We argue that, if the light collected by a fiber probe propagates toward a detector in the form of guided fiber modes, then this perception is wrong. Using symmetry arguments and simple calculations, we show that, if an extended fiber probe is axially symmetric and oriented perpendicular to the propagation direction, then the measured contrast should always be equal to the contrast of the interference pattern.

Characterization of near-field optical probes.

Radiation and collection characteristics of four different near-field optical-fiber probes, namely, three uncoated probes and an aluminum-coated small-aperture probe, are investigated and compared. Their radiation properties are characterized by observation of light-induced topography changes in a photosensitive film illuminated with the probes, and it is confirmed that the radiated optical field is unambiguously confined only for the coated probe. Near-field optical imaging of a standing evanescent-wave pattern is used to compare the detection characteristics of the probes, and it is concluded that, for the imaging of optical-field intensity distributions containing predominantly evanescent-wave components, a sharp uncoated tip is the probe of choice. Complementary results obtained with optical phase-conjugation experiments with the uncoated probes are discussed in relation to the probe characterization.

Optical characterization of probes for photon scanning tunnelling microscopy.

The photon scanning tunnelling microscope is a well-established member of the family of scanning near-field optical microscopes used for optical imaging at the subwavelength scale. The quality of the probes, typically pointed uncoated optical fibres, used is however, difficult to evaluate in a direct manner and has most often been inferred from the apparent quality of recorded optical images. Complicated near-field optical imaging characteristics, together with the possibility of topographically induced artefacts, however, has increased demands for a more reliable probe characterization technique. Here we present experimental results obtained for optical characterization of two different probes by imaging of a well-specified near-field intensity distribution at various spatial frequencies. In particular, we observe that a sharply pointed dielectric probe can be highly suitable for imaging when using p-polarized light for the illumination. We conclude that the proposed scheme can be used directly for probe characterization and, subsequently, for determination of an optical transfer function. which would allow one to deduce from an experimentally obtained image of a weakly scattering sample the field distribution existing near the sample surface in the absence of the probe.

Coupling of surface-plasmon polaritons to directional far-field radiation by an individual surface protrusion.

An experimental study of surface-plasmon polariton scattering by an individual surface protrusion on a silver film has been performed. Both in- and out-of-plane scattering have been investigated by imaging with a photon-scanning tunneling microscope at various probe-to-sample distances. For imaging in close proximity we observed a bright spot with rims on the shadow side, but, for imaging at larger distances, the location of the bright spot shifts away from the direction of the incoming surface-plasmon polariton. We estimate a pronounced peak in the far-field scattering pattern at an angle of ~20 degrees from the sample plane, which is in good agreement with recent theoretical studies and indicates that the same protrusion could possibly be used for local launching of surface-plasmon polaritons in a reversed illumination configuration.

Experimental statistics of near-field intensity distributions at nanostructured surfaces.

Scanning near-field optical microscopy is a technique in which the resolution is primarily determined by the size of a probe and not by the wavelength of illumination as in classical (far-field) microscopy. However, the relationship between a sample and its near-field optical image is usually rather complex. Typical factors responsible, at least partially, for such a complexity are the conditions of illumination and detection, sample characteristics (e.g. roughness and dielectric constant) and optical properties of the probe. Theoretical and experimental works conducted to improve our understanding of the relation between the object and the image have been reported (Greffet & Carminati, 1997). Recently, with the help of a photon scanning tunnelling microscope we have carried out an extensive study of the resultant near-field intensity distributions due to the elastic (in the plane) scattering of surface plasmon polaritons (SPPs) at metal film surfaces. We have also directly observed (in similar experimental conditions) localized dipolar excitations in silver colloid fractals (Bozhevolnyi et al., 1998). In both cases, the studied phenomena are intimately related to the regime of multiple light scattering, in which the interference effects are rather complicated and therefore a proper interpretation of them was far from being trivial. Thus, even though a certain understanding of many features inherent to the subwavelength light interference phenomena was gained (Bozhevolnyi & Coello, 1998; Bozhevolnyi et al., 1998; Coello & Bozhevolnyi, 1999), it is clear from the outcome of the investigations that more systematic studies in this context are still needed. A different and more powerful approach may be a statistical study of the recorded near-field intensity distributions. In this work, we report what we believe to be the first results on experimental statistics of near-field optical images exhibiting localized optical excitations (related to the regime of multiple scattering of light). We investigated optical images obtained with SPPs excited at different light wavelengths and scattered at different film surfaces, and with different polarizations and wavelengths of light scattered by silver colloid fractal structures. We have found significant differences in statistics between near-field intensity distributions taken at rough and smooth metal film surfaces and fractal structures. Finally, our predictions seem to be in agreement with theoretical studies reported by other authors (Sanchez-Gil & Garcia-Ramos, 1998).