Surface Plasmon Enhancement of Optical Absorption in Thin-Film Silicon Solar Cells

Strong electromagnetic field enhancement that occurs under conditions of the surface plasmon excitation in metallic nanoparticles deposited on a semiconductor surface is a very efficient and promising tool for increasing the optical absorption within semiconductor solar cells and, hence, their photocurrent response. The enhancement of the optical absorption in thin-film silicon solar cells via the excitation of localized surface plasmons in spherical silver nanoparticles is investigated. Using the effective medium model, the effect of the nanoparticle size and the surface coverage on that enhancement is analyzed. The optimum configuration and the nanoparticle parameters leading to the maximum enhancement in the optical absorption and the photocurrent response in a single p-n junction silicon cell are obtained. The effect of coupling between the silicon layer and the surface plasmon fields on the efficiency of the above enhancement is quantified as well.     

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.     

On the Surface Plasmon Resonance Modes of Metal Nanoparticle Chains and Arrays

A method for estimating the surface plasmon resonance modes of metal nanoparticle chains and arrays within a multilayered medium is proposed. In this fully retarded point-dipole method, an inhomogeneous background is replaced with a homogeneous one, based on an effective refractive index approximation. The proposed method includes the effects of retardation, radiative damping, and dynamic depolarization due to the finite size of the nanoparticles. The use of diagonal terms of dyadic Green’s functions and different polarizability coefficients along the semi-axes of ellipsoidal nanoparticles provides a complete set of both longitudinal and transverse resonance modes. Numerical results are compared with experimental results found in the literature.     

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.

     

Position Dependent Plasmonic Interaction Between a Single Nanoparticle and a Nanohole Array

The interaction of surface plasmons supported on a nanohole array and a single nanoparticle affixed to an atomic force microscopy (AFM) probe was studied for optimizing gap mode enhancement of the plasmonic field. Scanning probe microscopy controlled the AFM probe position, and the location specific interaction of the single nanoparticle (SNP) probe-nanohole array surface plasmons, was measured by darkfield spectroscopy. Raster-scanned darkfield imaging of the surface plasmons on the nanohole array is demonstrated, as well as image formation from measuring the SNP interaction at various (X, Y) locations relative to the nanohole. Coupling of the nanoparticle to the nanohole array exhibited maximal coupling when the SNP resided within a nanohole, resulting in a maximum SPR wavelength shift of 17 nm and an increase in scatter intensity of 137×. This technique may be expanded to mapping nanostructure coupling across three dimensions to determine optimal coupling conditions for applications in biosensing and surface enhanced spectroscopy. This contribution presents the first empirical observations of scanning probe microscopy (SPM) controlled gap mode enhancement of more complex nanostructures, a method for positioning optimization prior to sensing applications and experimental evidence for optimal lateral SNP-nanohole array positioning.     

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.     

Scattering Efficiency and LSPR Tunability of Bimetallic Ag,Au, and Cu Nanoparticles

Scattering efficiencies of Ag–Cu, Ag–Au, and Au–Cu alloy nanoparticles are studied based on Mie theory for their possible applications in solar cells. The effect of size (radius), surrounding medium, and alloy composition on the scattering efficiency at the localized surface plasmon resonance (LSPR) wavelengths has been reported. In the alloy nanoparticles of Ag_1?x Cu _x , Au_1?x Cu _x and Ag_1?x Au _x ; the scattering efficiency gets red-shifted with increase in x. Moreover, the scattering efficiency enhancement can be tuned and controlled with both the alloy composition and the surrounding medium refractive index. A linear relationship which is in good agreement to the experimental observations between the scattering efficiency and metal composition in the alloys are found. The effect of nanoparticle size and LSPR wavelength (scattering peak position) on the full width half maxima and scattering efficiency has also been studied. Comparison of Au–Ag, Au–Cu, and Ag–Cu alloy nanoparticles with 50-nm radii shows the optical response of Ag–Cu alloy nanoparticle with wide bandwidth in the visible region of the electromagnetic spectrum making them suitable for plasmonic solar cells. Further, the comparison of Ag–Cu alloy and core@shell nanoparticles of similar size and surrounding medium shows that Cu@Ag nanoparticle exhibits high scattering efficiency with nearly the same bandwidth.     

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.     

Multiple Surface Plasmon Resonances in Compound Structure with Metallic Nanoparticle and Nanohole Arrays

The optical properties of a compound structure with metallic nanoparticle and nanohole arrays are numerically investigated by the means of finite-difference time domain method. We report on the observation of multi-valleys in the reflection spectra due to the excitation of surface plasmon (SP) resonant modes of the compound structure. Simulation results show that multiple SP resonances consist of surface plasmon polaritons on the gold film, localized surface plasmons on the nanoparticles, and coupling mode between them. These findings are important for applications utilizing multiple surface plasmon resonances.     

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.     

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.     

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

     

Non-local Quantum Plasmon Resonance in Ultra-small Silver Nanoparticles

Understanding the mechanisms of light–matter interactions in ultra-small plasmonic nanoparticles (USNP) represents a major challenge because of the importance of size dependence and quantum effects. The plasmon resonance in such small metallic nanoparticles (< 5 nm) exhibits substantial deviation from classical theory predictions, with evident frequency shifts to a higher energy. This is due to the quantum nature of the free charge carriers and the dynamic response of metallic nanoparticle to the self-consistent electromagnetic fields. Such phenomena have so far been poorly understood in experiments while classical theory has mostly focused on nanostructures and sidestepped the size dependence. Here we report a quantum mechanical model of the metal permittivity to describe the USNP behaviour and experimental evidence. The proposed non-local quantum model of the permittivity for the propagation of plasmon waves in quantum-confined silver nanoparticles has no size limitations in the UNSP range.

     

Fluorescence Resonance Energy Transfer Near Thin Films on Surfaces

A theory is developed for resonant energy transfer between donor and acceptor molecules outside of a solid coated with a thin film. The energy transfer rate is expressed in terms of a second-rank tensor, allowing one to consider all possible orientations of the transition dipole moments of the molecules. The theory of images is employed to construct expressions valid in the near-field approximation. This theory is extended to the full electrodynamic theory valid over all distances. Connections are made between the expressions for the image charges and the Fresnel coefficients from optics. It is found that the energy transfer rates are strongly influenced by surface resonances, including the interfacial surface plasmons and the two-dimensional plasmon of a metallic film. The possibility of the film supporting Fabry–Perot resonator modes is discussed.     

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.

     

Surface Plasmon Resonant THz Wave Transmission on Carbon Nanotube Film

The properties of the terahertz resonant surface plasmons wave on the carbon nanotube film and dielectric interface have been investigated. As a first step towards engineering terahertz SPPs-like surface modes, we present a computer experiment to demonstrate that the carbon nanotube film surface can also be employed to concentrate and guide the terahertz SPPs wave. The carbon nanotube film is modeled in an experimentally realizable geometry. It is shown that a unique electromagnetic surface mode in terahertz region can be supported along the carbon nanotube film/dielectric interface when the free-space broadband terahertz pulse is incident on the carbon nanotube film with subwavelength gratings. Comparing with noble metals, plasmonic nano-structure materials based on carbon nanotube film offer a potentially more versatile approach to engineering tightly confined surface modes in the THz regime.     

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.     

Study of Circular Dichroism Modes Through Decomposition of Planar Nanostructures

We extend the plasmon hybridization method from a single nanoparticle to a complex planar nanostructure, decomposing the complex nanostructure into fundamental nanoparticle building blocks. Using gammadion nanostructure as an example, we validated the theory by comparing the field profile in the gammadion’s arms under the influence of an incident circularly polarized wave. This allows us to address the origin of the plasmonics modes in the circular dichroism (CD) spectrum. The use of this hybridization method provides a simple and intuitive explanation on how conductive and inductive coupling may result from complex planar nanostructures, allowing us to study its optical properties. Using our approach, top down hybridization studies can be applied to other complex planar structures to gain further insight on the origin of the CD modes and enhance ultrasensitive sensing of chiral micro and macro molecules.     

Rapid Catch and Signal (RCS) Technology Platform: Multiplexed Three-Color, 30 s Microwave-Accelerated Metal-Enhanced Fluorescence DNA Assays

We present an innovative “Rapid Catch and Signal” (DNA-RCS) technology for the simultaneous highly selective detection of multiple specific DNA sequences in solution. The DNA-RCS technology combines advantages of microwave-accelerated DNA hybridization (Rapid Catch) with the metal-enhanced fluorescence (MEF) technology (Signal), to enable specific DNA’s to be detected at high sensitivity within seconds. Fluorescent DNA labels, which play the role of molecular sensor probes, show a strong response upon DNA hybridization, due to fluorophore coupling with nanoparticle plasmons at a short (10–30 nm) distance from the surface. We have also shown that the fluorophore sensor probes demonstrate high photostability due to close proximity to a SiF surface, which significantly increases the total stability and reliability of the assay. Applications of the DNA-RCS technology in the life sciences, its advantages and benefits as compared to other DNA detection schemes, such as PCR, are subsequently discussed.