Subradiant,Superradiant Plasmon Modes and Fano Resonance in a Multilayer Nanocylinder Array Standing on a Thin Metal Film

The optical properties of a novel nanostructure consisting of a hexagonal array of aligned vertically three-layered metal-dielectric-metal nanodisks on a silver film are theoretically studied through the finite-difference time-domain method. The novel nanostructure exhibits three obvious optical transmission bands due to the excitation of subradiant plasmon modes, superradiant plasmon modes, and Fano resonances. Surface plasmon polaritons of the underlying Ag film also play a significant role on these three optical transmission bands via coupling with localized surface plasmons of nanodisk pairs. Moreover, the nanostructure also exhibits a good tunability of optical response by modifying the sizes of cylinders, the thickness of underlying metal film, and the dielectric constant of middle layer. These results demonstrate the nanostructure with great advantages in optical sensors and filters.

     

Resonant Aluminum Nanodisk Array for Enhanced Tunable Broadband Light Trapping in Ultrathin Bulk Heterojunction Organic Photovoltaic Devices

A cost-effective approach to enhancing broadband light trapping in ultrathin bulk heterojunction organic photovoltaic (OPV) devices is proposed. This is achieved by simply inserting an array of Al nanodisks at the interface of the ITO anode and the organic active layer; forming circular plasmonic nanopatch cavities (between the nanodisks and the Al cathode) that sandwich the active layer. Through interactions between the surface plasmon polaritons localized at the nanodisk and the cathode, a tunable broadband resonance peak spanning 450?C700?nm in the scattering cross-section spectrum is formed, thereby enhancing the electromagnetic field in the active layer. Compared to an OPV device with a 60-nm-thick PCPDTBT/PC₆₀BM layer, our numerical simulations reveal that integrated absorption enhancements of up to 40?% can be achieved in an equivalent device integrated with an array of nanodisks with a diameter of 100?nm and a periodicity of 250?nm. From the analysis of the structure?Cperformance relationships, implications for the design of these nanopatch cavities for light harvesting in ultrathin OPV devices are discussed.     

Suppressed Optical Transmission Through an Array of Ultrathin Interlayer Metal Slits

In contrast to the enhanced peak transmission in a subwavelength metal hole array structure (Ebbesen et al., Nature 391:667–669, 1998), here we theoretically investigate the spectral transmission through an array of identical metal slits with ultrathin interlayers and surprisingly find the depressed optical transmission for both infinite and finite array case. Notably, in the latter system, the narrowband dip transmission is evidently produced with the accompaniment of selective field enhancement and phase jumping across the structure. Analyses suggest that this phenomenon is intrinsically related to the penetrant coupling of intracavity surface plasmon polaritons together with the slit termination effect.     

Extraordinary Transmission of Three-Dimensional Crescent-like Holes Arrays

We developed a method to fabricate a periodic array of three-dimensional crescent-like holes (3DCLH) via an inverted hemispherical colloidal lithography. It is found that there exists an extraordinary optical transmission in this non-planar perforated periodic array of 3DCLH when the electric field of the incident light is perpendicular to the cross-line of the crescent-like hole. This extraordinary optical peak is insensitive with the incident angles and sensitive with the angle between the electric field of the incident light to the cross-line of the 3DCLH. Numerical simulation based on finite-difference time-domain method reveals that this peak is caused by an asymmetric localized surface plasmon resonance. This structure might be useful for the optical sensing and optical-integrated circuits.     

Multiple Surface Plasmon Resonances in a Metal–Insulator–Metal Structure Assembled Via Silver–Copper Nanoparticle Arrays

The Ag–Cu nanoparticle arrays, prepared using the electrochemical deposition method, were assembled into the metal–insulator–metal (MIM) structure with polyvinyl alcohol acting as insulating layer, the transmission spectrum of the MIM structure was observed to support the multiple surface plasmon resonances in the wavelength range 1,000 to 2,600 nm. The multiple peaks were formed due to the superposition and coupling of the surface plasmon resonance of nanoparticles with various sizes in the metal layers. The newly found MIM structure in which multiple resonances exist has a potential application in multiband-pass filters and optical magnetic metamaterials at the resonance wavelength.     

Multifactor-Controlled Non-Monotonic Plasmon Shift of Ordered Gold Nanodisk Arrays: Shape-Dependent Interparticle Coupling

Surface plasmon resonance (SPR) absorption spectra of gold nanodisks hexagonally arranged in planar arrays have been studied by using coupled dipole method and quasi-static approximation. The calculation results reveal that the increasing aspect ratio (AR) of gold disks in the close-packed nanoarray leads to SPR blue shift firstly and then red shift. The critical AR corresponding to the maximum blue shift can be controlled by tuning the interparticle distance and particle size. The physical mechanism of this non-monotonic SPR shift is investigated based on the competition between the influences from shape factor and arranging structure of the array. Although increasing the semi-minor axis of gold disk reduces the AR and leads to a blue shift of SPR, this increasing semi-minor axis also reduces the average gap between two neighboring disks and enhances their coupling. Furthermore, the coulombic attraction between two neighboring disks introduces an additional plasmon damping and results in a red shift of SPR. This competition between AR and interparticle coupling improves the tuning ability of SPR in anisotropic metallic nanoparticle arrays and presents a potential for design and fabrication of optical biochip based on SPR.     

Design of a Slit-Groove Coupler for Unidirectional Excitation of the Guided Surface Plasmon Polaritons Through a Plasmonic Slot Waveguide

In this paper, a plasmonic-photonic nanostructure has been introduced for efficient unidirectional coupling of free-space radiation to surface plasmon polariton (SPP) waves under normal illumination on a subwavelength slit. The structure consists of a conventional metallic slit-groove nanostructure integrated with a plasmonic waveguide to support SPP waves along the desired direction with a remarkable lateral confinement. The unidirectional coupling is achieved by using an integrated plasmonic distributed reflector designed under Bragg condition. This reflector basically distributes part of the light coupled through the slit into the SPP modes of the waveguide. Numerical simulations show that up to 26 % of the normally incident light couples to the transversely localized field of the surface plasmon. In addition, the ratio of mode current density of the surface plasmon, launched in the desired direction, to that in the opposite direction can reach about 23 times. This structure shows a 2.5-fold improvement in coupling efficiency relative to a standard slit-groove structure. Also, the transmission distance for the new nanostructure is shown to be more than 8 times greater than that of the standard nanostructure.     

Enhancement of Emission Efficiency of Deep-Ultraviolet AlGaN Quantum Wells Through Surface Plasmon Coupling with an Al Nanograting Structure

The enhancement of the internal quantum efficiency (IQE) of deep-ultraviolet Al _x Ga_1-x N/Al _y Ga_1-y N (x < y) quantum wells (QWs) by fabricating one-dimensional Al nanogratings on a QW structure for inducing surface plasmon (SP) coupling is demonstrated. Through temperature-dependent photoluminescence (PL) measurement, the enhancements of IQE in different emission polarizations are illustrated. Due to the small difference in energy band level between the heavy/light hole and split-off valence bands, the IQEs of the transverse electric- (TE-) and transverse magnetic- (TM-) polarized emissions are about the same. When emission polarization is perpendicular to Al-grating ridges, the SP resonance mode for coupling with the QWs is dominated by localized surface plasmon (LSP). When emission polarization is parallel with Al-grating ridges, the coupled SP resonance mode may mix LSP and SP polariton. In this polarization, LSP can be excited because of the width fluctuation of a grating ridge. When the excitation laser polarization is perpendicular to Al-grating ridges, the strong LSP resonance at the excitation laser wavelength leads to stronger excitation and hence higher IQE levels.     

Engineering the Optical Response of the Novel Plasmonic Binary Nanohole Array

The phenomenon of extraordinary optical transmission (EOT) due to its advantages has been considered by researchers in various applications, and in recent years, many efforts have been made to engineer these structures to get the best possible response for desired applications. In this work, the optical properties of novel binary gold nanohole arrays are investigated theoretically. We engineered the optical response of the system by adjusting the ratio of contribution of surface plasmon polariton (SPP) to localized surface plasmon resonance (LSPR) through the manipulation of the geometrical properties. The changes in the topology of this nanohole array affected the intensity and the wavelength of transmission peaks. The sensitivity of the optical response to the refractive index was also investigated. The designed structure is a good candidate for use as a polarization-independent optical label-free sensor.

     

Plasmon Hybridization in Silver Nanoislands as Semishells Arrays Coupled to a Thin Metallic Film

We obtained experimentally strong plasmon interactions between localized surface plasmon with delocalized surface plasmon polaritons in a new nanosystem of silver semishells island film arrays arranged as a closed-packing structure coupled to an adjacent thin silver film. We show that plasmon interactions for such a nanosystem exhibits two pronounced resonances and interpret the coupling in terms of Fano resonances. The higher energy resonance is identified as a symmetric hybridization mode between localized plasmon resonances in the island semishell array and surface plasmon polaritons in the metal film and while the lower energy resonance is identified as a corresponding anti-symmetric hybridization mode. Increasing the size of the particle arrays enhances and red shifts the resonances. We show that adding a dielectric spacer between the semishell island array and the metal film results in a red shifting of the resonances and introduce an additional high energy spectral peak. The effect of the spacer layer is interpreted as a reduced hybridization and the generation of additional localized surface plasmon resonances.     

Spectrally Tunable Optical Transmission of Titanium Nitride Split Ring Resonators

In this paper, the optical properties of titanium nitride split ring resonators as an intermetallic metamaterial nanostructure were studied. Our simulation shows the presence of plasmon and LC resonances in the transmission spectrum of a cell consists of four u-shape split ring resonators. The effect of different parameters of resonator such as size, periodic constant, and the material between arms in addition to the polarization of incident beam was examined on the resonance behavior of the system. Also, the optical properties of a cell consist of four complementary split ring resonators within titanium nitride thin film were investigated. An excited mode was observed at λ = 840 nm that was attributed to the plasmon resonance. Changing the arrangement and configuration of the system from C _1v to C _2v symmetry led to the presence of the LC mode beside the plasmon mode in the transmission spectrum. Also, we explored a connection between the complementary split ring resonators and orderly perforated surface plasmon systems. It was determined that a transition occurred from resonator-type to surface plasmon behavior by increasing the size of resonator above 170 nm.     

InGaN/GaN MQW Photoluminescence Enhancement by Localized Surface Plasmon Resonance on Isolated Ag Nanoparticles

Spectroscopic study of photoluminescence (PL) enhancement due to the coupling of the light emitters in InGaN/GaN multiple quantum wells (MQWs) with the localized surface plasmon (LSP) resonance on silver (Ag) nanoparticles (NPs) is performed using the confocal microscopy and scanning near-field optical microscopy (SNOM) techniques. The paper is focused on revealing the emission enhancement due to coupling with a single metal nanoparticle. The enhancement is confirmed by time-resolved study of differential transmission (DT). The enhancement suppression caused by potential fluctuations due to the variations of indium content and quantum well (QW) width is also studied. A strong photoexcitation intensity dependence of the emission enhancement due to spectral runaway of the MQW emission from the resonance as carrier density increases is observed both in spatially integrated spectra and in the vicinity of a single nanoparticle.     

The Effect of Negative Curvature on the Plasmonic Coupling of Concentric Core–Shell Metallic Nanostructures

Negative curvature-dependent localized surface plasmon resonance (LSPR) properties of concentric core–shell metallic nanostructure have been studied using quasistatic approach and plasmon hybridization theory. Whether in single-layered gold nanoshell or double gold nanoshells, the oscillating surface charges always concentrate close to the poles of the metal surface with negative curvature, which results in the anisotropic local electric field distribution and affects both the inter-surface plasmonic coupling and inter-shell plasmonic coupling. Therefore, the change of the radius of the gold surface with negative curvature could modulate the plasmon hybridization and lead to the LSPR shifting. The physical mechanism of the negative curvature-dependent LSPR presents a potential for design and fabrication of nanoscale optical device based on core–shell type metallic nanostructures.     

Tuning the Fano Resonance Between Localized and Propagating Surface Plasmon Resonances for Refractive Index Sensing Applications

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other’s vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.     

Effect of Boundary Condition and Periodical Extension on Transmission Characteristics of Terahertz Filters with Periodical Hole Array Structure Fabricated on Aluminum Slab

Terahertz (THz) filters based on extraordinary optical transmission from periodical hole array structures fabricated on aluminum slab have been experimentally investigated by using THz time-domain spectroscopy. The incident THz pulses with frequency from 0.1 to 2.7 THz could be partly filtered, and the central peak was at ～0.26. The high frequency signal could be observed to decrease, especially for the frequency above ～1 THz. Moreover, the transmission peak from small-size sample with less hole arrays shifts to high frequency at ～0.53 THz due to both the effects of boundary condition and insufficient periodical extension. Furthermore, finite element method with surface plasmon polariton theory is employed to analyze this extraordinary optical transmission and filter phenomena.     

Spoof Localized Surface Plasmons Excited by Plasmonic Waveguide Chip with Corrugated Disk Resonator

We designed and fabricated a millimeter plasmonic chip consisted of coplanar waveguide (CPW) and plasmonic waveguide with one corrugated disk resonator (CDR). The spoof localized surface plasmon (LSP) resonance modes can be excited by the interaction between plasmonic waveguide and CDR. Fundamental and higher order sharp spoof LSP resonances (from dipole to dodecapole) were observed in the transmission coefficient spectrum. The Q-value as high as 268.3 (octupole) was experimentally obtained. Experimental results show good agreement with theoretical and simulated ones. All the results may have potential applications in microchip based sensing and filtering.     

Resonance of Different Waveguide Structures with Various Vertical Indirect Coupled Cavities

In the paper, resonances of different waveguide structures with various vertical indirect coupled cavities were investigated by FDTD (finite difference-time domain). In the silicon cavity, Fano resonance could be observed at about 1430 nm. The coupling distance for the gold cavity/air cavity had less effect on the transmittance of the main waveguide but had a great influence on the transmission for water cavity in the visible region, which showed that water cavity could adjust resonance of waveguide structures. In addition, with the increment of refractive index n, the resonance peak at about 850 nm moved to the long wavelength (redshift). Dispersion rate about 2 × 10^–3/nm indicated that the transparent dielectric selectively absorbed the surface plasmon polariton wave and the sensitivity of the waveguide structure designed in this paper has high stability for the refractive index of the main waveguide cavity. Obvious Fano resonance could be observed with the increase of refractive index for silicon cavity. Among the four dielectrics, silicon and water are suitable for studying Fano resonance and filter dielectrics.

     

Scaffold-Based Multi-nanoparticle Clusters as Tunable LSPR Substrates for SERS Applications

Tunable local surface plasmon resonance (LSPR) enhancement properties of scaffold-based multi-nanoparitcle clusters were investigated using finite-difference time-domain (FDTD) method with calculated optical spectra, near-field distribution, and average enhancement of hybrid nanostructures as slab/nanoparticls, cylinder/nanoparticles, and sphere/nanoparticles. Focusing on influence factors including surface curvature, coupling effect, and decorated particle number, several models were built for further understanding on the dominate contribution in complicate multi-particle nanostructure and to explore their potential for plasmonic enhancement applications such as surface-enhanced Raman spectroscopy (SERS), solar cells material, LSPR sensor, and nanoantenna.     

Interaction Between Two Perpendicular Fabry–Perot-Like Resonances of the Antenna–Dielectric–Slit Structure and Their Influences on the Transmission Enhancement

The interaction between the two perpendicular Fabry–Perot-like resonances of the antenna–dielectric–slit structure and their influences on the transmission enhancement are investigated with a finite-difference time-domain method. The transmission enhancement is found with the antenna width corresponding to a Fabry–Perot-like resonance condition in the antenna–dielectric–slit structure; otherwise, there is no such an enhancement even when the slit is positioned under the magnetic field maximum. On the other hand, the resonance characteristics of the vertical slit can also modify the field distribution in the horizontal cavity by changing the phase difference at the two antenna ends. It is shown that the enhanced transmission can be realized in a wide range of incident wavelengths from the visible to near-infrared regime for different slit geometries. The physical mechanism of extraordinary optical transmission is discussed with a theoretical dispersion relationship of surface plasmon polaritons based on a metal–insulator–metal cavity model.     

High Optical Transmission in a Hybrid Plasmonic-Optical Structure with a Continuous Metal Film

Metals are naturally opaque for electromagnetic (EM) waves below violet frequency due to the Coulomb screening effect. In this letter, we demonstrate high optical transparency of a seamless continuous metal film by sandwiching it in a hybrid plasmonic-optical structure. The proposed structure consists of a plasmonic array and an optical cavity, which exhibits magnetic plasmon (MP) resonance and optical Fabry-Perot (FP) resonance, respectively. An optical transparency of 84% in the near-IR regime is achieved making use of interaction between the plasmonic and optical modes. Furthermore, spectral tunability of the high transparency is demonstrated and robustness under oblique incidence is examined. This work may give insights into plasmonic-optical interactions and may be a potential candidate for transparent electrodes.