Features of the Secondary Emission Enhancement Near Plasmonic Gold Film

Plasmonic gold films (PGF) prepared by vacuum deposition of gold onto quartz slides possess unique property to enhance electromagnetic signal in the near field. Spectral tuning of PGF’s plasmon band to resonance with the electronic spectra of adsorbed molecules provides selective enhancement of fluorescence or surface-enhanced Raman scattering in the far field. Plasmon-enhanced fluorescence (PEF) of mitoxantrone (mitox) as a function of the distance between gold surface and adsorbed molecules for different polarization and incidence angle of exciting light is analyzed in this work. Spectrophotometric data reveal that probability of localized plasmon excitation in gold grains increases with growth of incidence angle for s-polarized and decrease for p-polarized excitation. This fact correlates well with oblate shape of gold particles detected by Atomic force microscope. However, the fluorescence intensity of dyes deposited at fixed distance from gold surface increase with angle of incidence of p-polarized light more noticeably than for s-polarized one. Nevertheless, the behavior of mitox PEF signal upon p-polarized laser excitation and different angle of incidence are similar in appearance to such phenomenon as selective photoelectric effect. According to this observation, the near-field interactions between plasmons and molecule as possible mechanism of PEF is discussed.     

Ring Gap Resonance Modes on Disk/Film Coupling System Caused by Strong Plasmon Interaction

Ring modes with large wave vectors cannot be easily excited on a single disk by the plane wave illumination with the polarization parallel to the disk interface. In this work, we show that special antisymmetric ring gap modes on the surface of the disk in close proximity to the metallic thin film can be excited in the visible light region of the electromagnetic spectrum. In the presence of the film, the strong plasmon interaction between disk and film causes ring gap modes to have lower energies and be more easily excited. We apply the plasmon hybridization method to illustrate the ring gap modes arising from the interaction between the localized disk plasmons and the continuum surface plasmons. The calculated hybridization data show good agreement with the results of finite element simulations. The excitation of ring gap modes provides further insight into the strong coupling of plasmons and the design of novel nanostructures.

     

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.

     

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.

     

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.     

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.     

Controllable Coupling of Localized and Propagating Surface Plasmons to Tamm Plasmons

We theoretically investigate the coupling between Tamm plasmons and localized surface plasmons (LSPs) as well as propagating surface plasmons (PSPs) in a multilayer structure consisting of a metallic nanowire array and a spatially separated metal–dielectric Bragg reflector (DBR). A clear anticrossing behavior of the resonances is observed in the dispersion diagram resulting from the coupling, which is well explained by the coupled oscillator model. The coupling also creates new hybrid LSP or PSP modes with narrow bandwidths and unique spectral features. Upon the excitation of these hybrid modes, the local fields underneath the nanowires for the hybrid LSPs or near the lower metal layer surface for the hybrid PSPs are both enhanced greatly as compared with those achieved in the structure without DBR, which has potential applications in nonlinear optics and surface-enhanced spectroscopies.     

Plasmon-Matter Interactions in Optoelectronic Metamaterials with Negative Refractive Index

Optoelectronic metamaterials composed of nanoscale metallic structures and semiconductor quantum structures constitute a powerful platform to explore light-matter interaction and new devices. In this work, we numerically study an optoelectronically coupled metamaterial consisting of metallic double fishnet (DF) layers and semiconductor quantum well (QW) spacing layer. When the electronic intersubband transition (ISBT) in the QW coincides with the plasmonic resonances of the DF structure, the plasmon-matter interaction (PMI) can modify the optical properties considerably. In case of the ISBT-matching localized surface plasmons (LSP), i.e., f _QW?=?f _LSP, the polarization-selection-rule forbidden ISBT absorption can be enabled due to the nonnegligible E _z ?field distributions while the retrieved optical constants remain almost unchanged. However, when the gap surface plasmons (GSP) are matched, i.e., f _QW?=?f _GSP, PMI exhibits a clear anti-crossing behavior implying strong coupling effects between ISBT and GSP resonance and formation of intersubband polaritons. The effective optical constants are therefore modulated appreciably. The large difference between GSP and LSP can be attributed to their distinctive resonance qualities (Q-factors) and polarization conversion ratios (99.28 % for GSP and 1.54 % for LSP) from the transverse electric (TE) to transverse magnetic (TM) mode. Our results provide insight into the physical mechanism of PMI in nanoscale semiconductor-plasmon hybrid systems and suggest an alternative means in tunable negative refractive index (NRI) applications.     

Collection mode surface plasmon fibre sensors: a new biosensing platform

Sensors based on surface plasmon resonance (SPR) allow rapid, label-free, highly sensitive detection, and indeed this phenomenon underpins the only label-free optical biosensing technology that is available commercially. In these sensors, the existence of surface plasmons is inferred indirectly from absorption features that correspond to the coupling of light into a thin metallic film. Although SPR is not intrinsically a radiative process, when the metallic coating which support the plasmonic wave exhibits a significant surface roughness, the surface plasmon can itself couple to the local photon states, and emit light. Here we show that using silver coated optical fibres, this novel SPR transducing mechanism offers significant advantages compare to traditional reflectance based measurements such as lower dependency on the metallic thickness and higher signal to noise ratio. Furthermore, we show that more complex sensor architectures with multiple sensing regions scattered along a single optical fibre enable multiplexed detection and dynamic self referencing of the sensing signal. Moreover, this alternative approach allows to combine two different sensing technologies, SPR and fluorescence sensing within the same device, which has never been demonstrated previously. As a preliminary proof of concept of potential application, this approach has been used to demonstrate the detection of the seasonal influenza A virus.     

Amplifying the Interaction Between Two Identical Metallic Nanoparticles with a Large Interface Distance Based on the Strong Coupling-Like Phenomenon Involving Molecular J-aggregates

We introduce a new way to amplify the interaction between two identical metallic nanoparticles with a large interface distance (≥the radius of each nanoparticle). The proposed structure consists of two identical metallic nanoparticles embedded in molecular J-aggregates and the strong coupling-like phenomenon is described by the scattering spectra. Finite difference time domain (FDTD) method is employed to simulate this structure and the simulation results match the experiment well (Eizner et al., Nano Lett 15:6215–6221 2015; Lin et al., Nano Lett 15:4699–4703 2015; Zengin et al., Phys Rev Lett 114:157401 2015). Molecular J-aggregates take important roles in the strong coupling-like phenomenon and can be used to amplify the interaction between the particles. The scattering spectra of this proposed structure have two separated peaks, whose shifts are larger than those in the air with the interface distance decreasing. The coupling strength between the nanoparticles and the amplification of the interaction can be tuned by the incident polarization. This structure has potential applications in the field of quantum communications such as the quantum network, the quantum key distributions, and so on.

     

Photoluminescence Quenching in Quantum Emitter,Metallic Nanoparticle,and Graphene Hybrids

We have developed a theory for photoluminescence quenching and plasmonic properties in hybrid nanosystems made from three nanosystems such as a quantum emitters, metallic nanoparticles, and graphene. The metallic nanoparticles and graphene have surface plasmons which couple with probe photons and create surface plasmon polaritons. Therefore, the excitons in the quantum emitters interact with surface plasmon polaritons via the dipole-dipole interaction. Due to this interaction, energy is exchanged between the nanosystems. The second quantized formulation and the quantum density matrix method have been used to calculate photoluminescence and the radiative and non-radiative decay processes in the presence of dipole-dipole interaction. We have compared our theory with experiments of two and three nanosystems, and a good agreement between theory and experiments is achieved. It has been found that the photoluminescence quenching in hybrid systems not only occurs through the direct non-radiative energy transfer from the quantum emitter to the metal nanoparticle and to graphene but also occurs through the indirect non-radiative energy transfer from quantum emitter to the metal nanoparticle via graphene and from the quantum emitter to graphene via metal nanoparticle. These are interesting findings and they can be used to fabricate nanoswitches and nanosensors for medical applications.     

Ultrafast Plasmonic Electron Emission from Ag Nanolayers with Different Roughness

We demonstrate ultrafast plasmonic electron emission and acceleration from Ag nanolayers having different roughness. We obtained the spectrum of the electrons and found that the surface roughness deeply influences the properties of the electron spectra. Numerical simulations on propagating surface plasmons coupled to localized plasmons on surface grains support the observations. Applications related to ultrafast electron sources and ultrafast photocathodes are envisaged.     

Radiative decay engineering 3. Surface plasmon-coupled directional emission

A new method of fluorescence detection that promises to increase sensitivity by 20- to 1000-fold is described. This method will also decrease the contribution of sample autofluorescence to the detected signal. The method depends on the coupling of excited fluorophores with the surface plasmon resonance present in thin metal films, typically silver and gold. The phenomenon of surface plasmon-coupled emission (SPCE) occurs for fluorophores 20-250 nm from the metal surface, allowing detection of fluorophores over substantial distances beyond the metal-sample interface. SPCE depends on interactions of the excited fluorophore with the metal surface. This interaction is independent of the mode of excitation; that is, it does not require evanescent wave or surface-plasmon excitation. In a sense, SPCE is the inverse process of the surface plasmon resonance absorption of thin metal films. Importantly, SPCE occurs over a narrow angular distribution, converting normally isotropic emission into easily collected directional emission. Up to 50% of the emission from unoriented samples can be collected, much larger than typical fluorescence collection efficiencies near 1% or less. SPCE is due only to fluorophores near the metal surface and may be regarded as emission from the induced surface plasmons. Autofluorescence from more distal parts of the sample is decreased due to decreased coupling. SPCE is highly polarized and autofluorescence can be further decreased by collecting only the polarized component or only the light propagating with the appropriate angle. Examples showing how simple optical configurations can be used in diagnostics, sensing, or biotechnology applications are presented. Surface plasmon-coupled emission is likely to find widespread applications throughout the biosciences.     

Plasmonic Properties of Gold Nanostructures on Gold Film

This paper reports on a systematic study of the plasmonic properties of periodic arrays of gold cylindrical nanoparticles in contact with a gold thin film. Depending on the gold film thickness, it observes several plasmon bands. Using a simple analytical model, it is able to assign all these modes and determine that they are due to the coupling of the grating diffraction orders with the propagating surface plasmons travelling along the film. With finite difference time domain (FDTD) simulations, it demonstrates that large field enhancement occurs at the surface of the nanocylinders due to the resonant excitation of these modes. By tilting the sample, it also observes the evolution of the spectral position of these modes and their tuning through nearly the whole visible range is possible. Such plasmonic substrates combining both advantages of the propagative and localised surface plasmons could have large applications in enhanced spectroscopies.

     

Theoretical Investigation of Plasmonic Properties of Quantum-Sized Silver Nanoparticles

Plasmonic nanoparticles (NPs) like silver (Ag) strongly absorb the incident light and produce enhanced localized electric field at the localized surface plasmon resonance (LSPR) frequency. Enormous theoretical and experimental research has focused on the plasmonic properties of the metallic nanoparticles with sizes greater than 10 nm. However, such studies on smaller sized NPs in the size range of 3 to 10 nm (quantum-sized regime) are sparse. In this size regime, the conduction band of the metal particles discretizes, thus altering plasmon properties of the NPs from classical to the quantum regime. In this study, plasmonic properties of the spherical Ag NPs in size range of 3 to 20 nm were investigated using both quantum and classical modeling to understand the importance of invoking quantum regime to accurately describing their properties in this size regime. Theoretical calculations using standard Mie theory were carried out to monitor the LSPR peak shift and electric field enhancement as a function of the size of the bare plasmonic nanoparticle and the refractive index (RI) of the surrounding medium. Comparisons were made with and without invoking quantum regime. Also, the optical properties of metallic NPs conjugated with a chemical ligand using multi-layered Mie theory were studied, and interesting trends were observed.

     

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.     

Quantum Theory of Surface Plasmon Polaritons: Planar and Spherical Geometries

A quantum theory of retarded surface plasmons on a metal–vacuum interface is formulated, by analogy with the well-known and widely exploited theory of exciton-polaritons. The Hamiltonian for mutually interacting instantaneous surface plasmons and transverse electromagnetic modes is diagonalized with recourse to a Hopfield–Bogoljubov transformation, in order to obtain a new family of modes, to be identified with retarded plasmons. The interaction with nearby dipolar emitters is treated with a full quantum formalism based on a general definition of modal effective volumes. The illustrative cases of a planar surface and of a spherical nanoparticle are considered in detail. In the ideal situation of absence of dissipation, as an effect of the conservation of in-plane wavevector, retarded plasmons on a planar surface represent true stationary states (which are usually called surface plasmon polaritons), whereas retarded plasmons in a spherical nanoparticle, characterized by frequencies that overlap with the transverse electromagnetic continuum, become resonances with a finite radiative broadening. The theory presented constitutes a suitable full quantum framework for the study of nonperturbative and nonlinear effects in plasmonic nanosystems.     

Plasmon light scattering in biology and medicine: new sensing approaches, visions and perspectives

Surface plasmons are collective oscillations of free electrons at metallic surfaces. These oscillations can give rise to the intense colors of solutions of plasmon resonance nanoparticles and/or very intense scattering. While the use of plasmonic particle absorption based bioaffinity sensing is now widespread throughout biological research, the use of their scattering properties is relatively ill explored. We refer to the use, utility and control of surface plasmons as plasmonics. In this review and forward-looking article, we discuss the current opinions and uses of plasmonics, as well as speculate on areas of future research. These include the use of plasmon scatter for long-range immunosensing and macromolecular conformation studies, as well as the ability to Stokes shift plasmon scatter, a plasmonics phenomenon recently referred to as metal-enhanced fluorescence.