Plasmonics in Biology and Plasmon-Controlled Fluorescence

Fluorescence technology is fully entrenched in all aspects of biological research. To a significant extent, future advances in biology and medicine depend on the advances in the capabilities of fluorescence measurements. As examples, the sensitivity of many clinical assays is limited by sample autofluorescence, single-molecule detection is limited by the brightness and photostability of the fluorophores, and the spatial resolution of cellular imaging is limited to about one-half of the wavelength of the incident light. We believe a combination of fluorescence, plasmonics, and nanofabrication can fundamentally change and increase the capabilities of fluorescence technology. Surface plasmons are collective oscillations of free electrons in metallic surfaces and particles. Surface plasmons, without fluorescence, are already in use to a limited extent in biological research. These applications include the use of surface plasmon resonance to measure bioaffinity reactions and the use of metal colloids as light-scattering probes. However, the uses of surface plasmons in biology are not limited to their optical absorption or extinction. We now know that fluorophores in the excited state can create plasmons that radiate into the far field and that fluorophores in the ground state can interact with and be excited by surface plasmons. These reciprocal interactions suggest that the novel optical absorption and scattering properties of metallic nanostructures can be used to control the decay rates, location, and direction of fluorophore emission. We refer to these phenomena as plasmon-controlled fluorescence (PCF). We predict that PCF will result in a new generation of probes and devices. These likely possibilities include ultrabright single-particle probes that do not photobleach, probes for selective multiphoton excitation with decreased light intensities, and distance measurements in biomolecular assemblies in the range from 10 to 200 nm. Additionally, PCF is likely to allow design of structures that enhance emission at specific wavelengths and the creation of new devices that control and transport the energy from excited fluorophores in the form of plasmons, and then convert the plasmons back to light. Finally, it appears possible that the use of PCF will allow construction of wide-field optical microscopy with subwavelength spatial resolution down to 25 nm.     

Curved Gratings as Plasmonic Lenses for Linearly Polarised Light

The ability of curved gratings as sectors of concentric circular gratings to couple linearly polarised light into focused surface plasmons is investigated by theory, simulation, and experiment. The experimental and simulation results show that increasing the sector angle of the curved gratings decreases the width of the lateral distribution of surface plasmons resulting in focusing of surface plasmons, which is analogous to the behaviour of classical optical lenses. We also show that two faced curved gratings, with their groove radius mismatched by half of the plasmon wavelength (asymmetric configuration), can couple linearly polarised light into a single focal spot of concentrated surface plasmons with smaller depth of focus and higher intensity in comparison to single curved gratings. The major advantage of these structures is the coupling of linearly polarised light into focused surface plasmons with access to, and control of, the plasmon focal spot, which facilitate their potential applications in sensing, detection, and nonlinear plasmonics.     

Splitting of Plasmon Frequency in Spherical Metal Nanoparticles in Anisotropic Medium

The problem of plasmon resonance in spherical metal nanoparticles incorporated into an anisotropic dielectric medium is studied theoretically. Analytic solution is obtained in long-wavelength approximation for the case of weak uniaxial anisotropy. It is shown that medium anisotropy causes shifts of plasmon frequency from its position in an isotropic medium, which are different for plasmons with dipole momenta parallel and perpendicular to the axis of the medium. For the parallel and perpendicular orientations, which are determined by polarization of incident light, plasmon shifts differ by a factor of 4/3. Analytic expressions for field enhancement in the vicinity of a metal nanoparticle is obtained and analyzed for the cases of noble metals. The perspectives of experimental check of the obtained results and possible applications of plasmon anisotropy in plasmonics and sensorics are discussed.     

Efficient Extraction of Fluorescence Emission Utilizing Multiple Surface Plasmon Modes from Gold Wire Gratings

We demonstrate directional enhanced fluorescence emission from fluorophores located above gold wire gratings. In contrast to previous studies on corrugated films, efficient coupling was recorded for multiple plasmon modes associated with both the active and substrate side of the wires. This difference is likely due to the subtle differences in how light interacts with corrugated films versus metal films with periodic subwavelength slots. For corrugated films, coupling between modes on opposite sides of the grating are out of phase, and therefore plasmon modes on the opposite side of the grating are only weakly excited. For wire gratings, transmission and reflection features have been modeled well with a dynamical diffraction model that includes surface plasmons, which allows for efficient coupling to surface plasmon modes on both sides of the grating. We also compared the two mechanisms for fluorescent enhancement, namely the intense electromagnetic field associated with surface plasmons and excited fluorophores radiating via surface plasmon modes. We found the latter mechanism clearly dominant.     

Plasmon analysis of Triticum (wheat) and Aegilops. 2. Characterization and classification of 47 plasmons based on their effects on common wheat phenotype

This article comprises our final remarks on the phenotypic effects of alien plasmons on common wheat. Twenty-one vegetative, reproductive, and seed characters of 551 alloplasmic lines of 12 common wheat genotypes with 46 alloplasmons, and as the control, their euplasmic lines were investigated. Effects of genotype, plasmon, and their interaction had high statistical significance for all the characters investigated, whereas phenotypic variations attributable to the alien plasmons were relatively small. Individual plasmon types are characterized by their primary effects on 21 characters. Genotype x plasmon effects on two representative characters, heading date and plant height, are described in detail. Cluster and principal component analyses of the phenotypic effects of the 47 plasmons yielded 22 groups. The relationships between these phenotype-based groups and those defined by molecular differences in organellar genomes were examined. A significant correlation was found with some explainable discrepancies. For efficient plasmon identification, use of six of the present 12 genotypes is proposed. The key for plasmon classification is provided. Our findings indicate that alien plasmons may be of limited value in future wheat breeding, but that the plasmon diversity that exists in Triticum and Aegilops species is of great significance for understanding the evolution of these genera.     

Scattering-Suppressed Plasmonic Bends and Adapters with Gradient Refractive Index Medium

We propose the designs of plasmonic bends and adapters with low scattering loss in visible region theoretically. Tens of nanometers thick gradient refractive index medium is deposited on the metallic surface, which can confine and release the surface plasmon polaritons (SPPs). When SPPs can be strongly confined the metallic surface and propagate along the corners of the plasmonics devices, the scattering loss can be dramatically suppressed. Full wave simulations based on a finite element method have been performed to validate our proposal. Compared with the same class of design, our method can be achieved only with isotropic materials.     

Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission

Metallic particles and surfaces display diverse and complex optical properties. Examples include the intense colors of noble metal colloids, surface plasmon resonance absorption by thin metal films, and quenching of excited fluorophores near the metal surfaces. Recently, the interactions of fluorophores with metallic particles and surfaces (metals) have been used to obtain increased fluorescence intensities, to develop assays based on fluorescence quenching by gold colloids, and to obtain directional radiation from fluorophores near thin metal films. For metal-enhanced fluorescence it is difficult to predict whether a particular metal structure, such as a colloid, fractal, or continuous surface, will quench or enhance fluorescence. In the present report we suggest how the effects of metals on fluorescence can be explained using a simple concept, based on radiating plasmons (RPs). The underlying physics may be complex but the concept is simple to understand. According to the RP model, the emission or quenching of a fluorophore near the metal can be predicted from the optical properties of the metal structures as calculated from electrodynamics, Mie theory, and/or Maxwell's equations. For example, according to Mie theory and the size and shape of the particle, the extinction of metal colloids can be due to either absorption or scattering. Incident energy is dissipated by absorption. Far-field radiation is created by scattering. Based on our model small colloids are expected to quench fluorescence because absorption is dominant over scattering. Larger colloids are expected to enhance fluorescence because the scattering component is dominant over absorption. The ability of a metal's surface to absorb or reflect light is due to wavenumber matching requirements at the metal-sample interface. Wavenumber matching considerations can also be used to predict whether fluorophores at a given distance from a continuous planar surface will be emitted or quenched. These considerations suggest that the so called "lossy surface waves" which quench fluorescence are due to induced electron oscillations which cannot radiate to the far-field because wavevector matching is not possible. We suggest that the energy from the fluorophores thought to be lost by lossy surface waves can be recovered as emission by adjustment of the sample to allow wavevector matching. The RP model provides a rational approach for designing fluorophore-metal configurations with the desired emissive properties and a basis for nanophotonic fluorophore technology.     

Plasmon Modes Management

A new plasmonic structure based on bimetallic layer is proposed. We analyze the structure and show that bimetallic film plays a crucial role in the management of surface plasmons. The roll of the buffer is discussed, as well. Up to three surface plasmons can be excited simultaneously in the structure. Two of plasmons can be used for two-plasmon spectroscopy. The third plasmon can be used for controlling the temperature of the structure.     

Experimental Study on Localized Surface Plasmon Mode Hybridization in the Near and Mid Infrared

We investigate plasmon excitations within a regular grating of double-layered gold/insulator nanoparticles in the infrared and visible spectral region. Provided a flat gold film as substrate, strong coupling between the localized surface plasmon modes and their image-like excitations in the metal is observed. The interaction results in a strong red shift of the plasmon mode as well as the splitting of the modes into levels of different angular momenta, often referred to as plasmon hybridization. The diameters of the nanoparticles are designed in a way that the splitting of the resonances occurs in the spectral region between 0.1 and 1 eV, thus being accessible using an infrared microscope. Moreover, we investigated the infrared absorption signal of gratings that contain two differently sized nanoparticles. The interaction between two autonomous localized surface plasmon excitations is investigated by analyzing their crossing behavior. In contrast to the interaction between localized surface plasmons and propagating plasmon excitations which results in pronounced anticrossing, the presented structures show no interaction between two autonomous localized surface plasmons. Finally, plasmon excitations of the nanostructured surfaces in the visible spectral region are demonstrated through photographs acquired at three different illumination angles. The change in color of the gratings demonstrates the complex interaction between propagating and localized surface plasmon modes.     

Single-Particle Spectroscopy of Supported Silver Clusters on Silicon: Substrate Effects on Localized Surface Plasmons

The presence of a surrounding medium strongly affects the spectral properties of localized surface plasmons at metallic nanoparticles. Vice versa, plasmonic resonances have large impact on the electric polarization in a surrounding or supporting material. For applications, e.g., in light-converting devices, the coupling of localized surface plasmons with polarizations in semiconducting substrates is of particular importance. Using photoemission electron microscopy with tunable laser excitation, we perform single-particle spectroscopy of silver nanoclusters directly grown on Si(100). Two distinct localized surface plasmon modes are observed as resonances in the two-photon photoemission signals from individual silver clusters. The strengths of these resonances strongly depend on the polarization of the exciting electric field, which allows us to assign them to plasmon modes with polarizations parallel and perpendicular, respectively, to the supporting silicon substrate. Our mode assignment is supported by simulations which provide insight into the mutual interaction of charge oscillations at the particle surface with electric polarizations at the silver/silicon interface.

     

Bragg-Scattered Surface Plasmon Microscopy: Theoretical Study

We present a new approach to surface plasmon microscopy with high refractive index sensitivity and spatial resolution that is not limited by the propagation length of surface plasmons. It is based on a nanostructured metallic sensor surface supporting Bragg-scattered surface plasmons. We show that these non-propagating surface plasmon modes are excellently suited for spatially resolved observations of refractive index variations on the sensor surface owing to their highly confined field profile perpendicular to as well as parallel to the metal interface. The presented theoretical study reveals that this approach enables reaching similar refractive index sensitivity as regular surface plasmon resonance (SPR) microscopy and offers the advantage of improved spatial resolution when observing dielectric features with lateral size <10???m for the wavelength around 800?nm and gold as the SPR-active metal. This paper demonstrates the potential of Bragg-scattered surface plasmon microscopy for high-throughput SPR biosensing with high-density microarrays.     

Structure Metallic Surface for Terahertz Plasmonics

Plasmonics is the field of study of the interaction between incident light and electrons in metals. It is used widely for developing nanophotonic devices. The structured metallic surface such as metamaterials can be used to produce spoof surface plasmons at any frequencies with the dimensions of unit cell less than the incident wavelength. Terahertz plasmonics is attracted to the field of research since it is used for sensing biological components even in a weak environment. The issue with planar metamaterials is a lower quality factor value. Several methods have been adopted for obtaining high Q-value in metamaterials. Among them, Fano- and Toroidal-based metamaterials offer high Q-factor and string localized field enhancement. This article discusses the importance and developments in the field of high-Q terahertz metamaterial for plasmonics applications. The nonlinear responses of terahertz metamaterial under high-intense THz pulses are also discussed.

     

Low Energy Intraband Plasmons and Electron Energy Loss Spectra of Single and Multilayered Graphene

We present a theory for the calculation of the low energy intraband plasmon frequencies and the electron energy loss (EEL) spectra of single layer and multilayer graphene sheets. Our calculation shows that the number of plasmons that can be excited is equal to the number of graphene layers in the sample. One of these is the dominant in-phase plasmon having a square root dependence on the wave number at low wave vectors, whereas the others are out-of-phase plasmons having near linear dependences on the wave number. The EEL spectra of a single layer graphene shows a single peak at the plasmon frequency, which has been observed experimentally. The EEL spectra of all multilayer graphenes have two peaks, one corresponding to the dominant in-phase plasmon and the other due to the out of phase plasmons. We predict that careful measurement of the EEL of multilayer graphene will show both peaks due to the low energy intraband plasmons.

     

Tunnel Light Through a Continuous Optically Thick Metal Film Utilizing Higher Order Magnetic Plasmon Resonance

In this letter, we investigate the extraordinary optical transmission behavior of a flat continuous metal film sandwiched by magnetic plasmonic structures. A new mechanism by utilizing higher order magnetic plasmon resonance is proposed to enhance the transmission. Numerical simulation results show that 80 % electromagnetic energy can be transmitted through the middle 50-nm-thick continuous gold film in near-infrared regime. The excitation of the second-order magnetic plasmons and the propagating surface plasmons, as well as the interaction between them accounts for such a high transmission. The interaction of magnetic plasmons and surface plasmons leads to new hybrid modes, and the coupled oscillator model is introduced to analyze this hybridization. This work extends the application range of higher order magnetic plasmons and may have potential in transparent electrode and electromagnetic energy transfer applications.     

Impact of Nonuniform Electron Density on Plasmonic Properties of Metal Nanoparticles

The interfacial nonuniformity of the electron density that occurs in metals as a result of atomic imperfections can strongly affect the plasmonic properties of metallic nanostructures. Under certain conditions, it induces the bulk plasmon resonance in the transition area and can significantly change scattering and absorption of light by metallic nanostructures in a broad frequency range. This effect is numerically demonstrated for radially nonuniform spherical silver nanoparticles and analytically investigated with respect to the resonant coupling with the dipolar surface plasmons of the metal core.     

Dipole Decay Rates Engineering via Silver Nanocones

A full control of the interaction between confined plasmons and point sources of radiation is a central issue in molecular plasmonics. In this paper, a theoretical contribution towards a physical understanding on the localized surface plasmons excited into metallic nanocones by a point dipole is given. A numerical approach based on the discrete dipole approximation is applied to determine the modifications of the dipole decay rates for varying geometrical parameters of the dipole-metal nanoparticle system. Results declare the centrality of the cone aperture to control the plasmon resonances and to handle the effects it induces on the lifetime of a point emitter. A full spectral tuning of the resonances in the decay rates can be achieved by operating on a unique spatial degree of freedom: by tailoring the aperture alone, total decay rates 10⁵ times higher than the free-space value can be obtained at short distances from the metal in a large region of the spectral range. Quite unexpectedly, size dependence of the antenna is found to have a marginal role if only a lifetime manipulation is desired. It becomes, instead, a crucial aspect of the problem when large quantum yields are required. Results presented in this work shed light on spontaneous emission modification due to interaction with plasmonic nanocones of different shapes and are relevant for a number of applications in the fields of nanoplasmonics and fluorescence microscopy.     

Fluorescence Enhancement Caused by Plasmonics Coupling Between Silver Nano-Cubes and Silver Film

In this article, we experimentally investigated the plasmonics interaction in the system composed of Ag nano-cubes on Ag film with controlled distance. The distance is controlled by Rhodamine B (RhB)-doped polymethylmethacrylate (PMMA) film as the spacer, whose fluorescence intensity was then enhanced by the plasmonics interaction. Experimental results show that the fluorescence enhancement is sensitive to the thickness of the spacer. The largest enhancement factor obtained is 521 with the RhB-doped PMMA film of 10 nm thickness. For comparison, we also presented the fluorescence enhancement caused by only the localized surface plasmons from Ag nano-cubes on glass substrate coated with RhB-doped PMMA film, which gives out lower enhancement factors at the same thick spacer. Our experimental results are consistent with previous theoretical investigation and shows promising applications in fluorescence based bio-sensing or bio-imaging.     

Giant Infrared Sensitivity of Surface Plasmon Resonance-Based Refractive Index Sensor

Surface plasmons (SPs), the coherent charge density oscillations of the electrons bound to the metal-dielectric interface, are dominating the research field of optics. One of the ubiquitous applications of SPs is in sensing. In the present work, we have theoretically studied a couple of surface plasmon resonance (SPR)-based fiber-coupled ultra-sensitive refractive index sensors working in the infrared (IR) region. Either of the copper (Cu) and aluminum (Al) is used as surface plasmon exciting layers in these sensing probes. On the top of the metal layer, field-enhancing graphene and silicon layers are considered. The probes are characterized in terms of sensitivity and detection accuracy (DA). The sensitivities of Cu- and Al-based optimized probes are obtained respectively to be 23.50 and 24 μm/refractive index unit (RIU). To ensure the probes’ compatibility with bio-samples, an extra bio-recognition layer of graphene has been considered over the silicon layer which resulted into the respective sensitivities of 20 and 19.50 μm/RIU for Cu- and Al-based probes with appreciably good DAs.     

Sensitive label-free biosensors by using gap plasmons in gold nanoslits

The detection sensitivities of gap plasmons in gold nanoslit arrays are studied and compared with surface plasmons on outside surface. The nanoslit arrays were fabricated in a 130 nm-thick gold film with various slit widths. For transverse-magnetic (TM) incident wave, the 600 nm-period nanoslit array shows two distinguishable transmission peaks corresponding to the resonances of gap plasmons and surface plasmons, respectively. The surface sensitivities for both modes were compared by coating thin SiO(2) film and different biomolecules on the nanoslit arrays. Our experimental results verify gap plasmons are more sensitive than conventional surface plasmons. Its detection sensitivity increases with the decrease of slit width. The gap plasmon is one order of magnitude sensitive than the surface plasmon for slit widths smaller than 30 nm. We attribute this high sensitivity to the large overlap between biomolecules and nanometer-sized gap plasmons.