Surface Plasmon Polariton Enhancement in Silver Nanowire–Nanoantenna Structure

The surface plasmon polariton (SPP) coupling and enhancement in silver nanowire–nanoantenna structure is proposed and simulated by using finite difference time domain method. The results demonstrate that three-arm antenna can effectively enhance the coupling efficiency at the incident end and the SPP field intensity at the emission end. The enhancement factor, which is defined as the ratio of the SPP field intensity at the emission end with and without the three-arm antenna, for the various antenna arm lengths and incident wavelengths under different incident angles are calculated. The suggested structure can be served as an enhanced plasmonic waveguide for the nanophotonic and plasmonic circuits in the future.     

Surface Plasmon Polariton Waveguide by Bottom and Top of Graphene

Semiconductor surface plasmon polariton (SPP) waveguide has unique optical properties and compatibility with existing integrated circuit manufacturing technology; thus, SPP devices of semiconductor materials have wide application potential. In this study, a new integrated graphene SPP waveguide is designed using the bottom and top roles of graphene. Moreover, a T waveguide structure is designed by InGaAs of semiconductor gain, with rectangular GaAs material on both sides. The structure adopts light to stimulate the SPP, where its local area is enhanced by the interaction between two interface layers and a semiconductor gain and where its frequency can be adjusted by the thickness of the graphene. Characteristic analysis reveals the coupling between the T semiconductor gain and the SPP mode. The propagation distance of the waveguide can reach 75 cm, the effective mode field is approximately 0.0951λ ², the minimum of gain threshold is approximately 2992.7 cm^?1, and the quality factor (FOM) can reach 180. The waveguide structure which provides stronger localization can be compatible with several optical and electronic nanoscale components. That means, it can provide light for surface plasmon circuit and also can provide a great development in the low-threshold nanolaser.     

Three-Dimensional Manipulations of Surface Plasmon Polariton Wave Propagation

The plasmonic integrated circuit, a potential application of surface plasmon polaritons (SPPs), can manipulate an SPP wave propagating on a metal surface in a way similar to electronic circuits. Here, we propose the concept of three-dimensional (3D) SPP wave manipulation: control of an SPP wave propagating in both the horizontal direction and the vertical direction. A hole set in the film can guide an SPP wave in the vertical direction. In the horizontal direction, two holographic groove patterns are used to focus an incident SPP wave on one surface of the film to the hole and control the divergent SPP waves transmitted from the hole on the other metal surface, respectively. The holographic groove patterns are designed via the methodology of surface electromagnetic wave holography. 3D finite-difference time-domain method simulations show a good performance of the 3D manipulation via these designed holographic groove patterns.

     

Coupled Plasmonic Cavities on Moire Surfaces

Surface plasmon polariton (SPP) waveguides formed by coupled plasmonic cavities on metallic Moire surfaces have been investigated both experimentally and numerically. The Moire surface, fabricated by interference lithography, contains periodic arrays of one-dimensional cavities. The coupling strength between the cavities has been controlled by changing the periodicities of the Moire surface. The ability to control the coupling strength allows us to tune the dispersion and the group velocity of the plasmonic coupled cavity mode. Reflection measurements and numerical simulation of the array of SPP cavities have shown a coupled resonator type plasmonic waveguide band formation within the band gap. Coupling coefficients of cavities and group velocities of SPPs are calculated for a range of cavity sizes from weakly coupled regime to strongly coupled regime.     

Hybrid Plasmonic Waveguide Based on Tapered Dielectric Nanoribbon: Excitation and Focusing

The nanofocusing of light source was proposed and simulated using the dielectric-loaded surface plasmon polariton (SPP) model with various laterally tapered planar dielectric architectures on the top surface of the metal. By using finite-difference time-domain method, enhancement factor for the local electric field under distinctive incident polarization was analyzed with different taper apexes under various incident wavelengths and incident angles of the excitation laser. The SPP dispersion and the effect of dissipation on adiabatic nanofocusing of SPP in a sharp taper structure were used to predict the optimal taper angles of the structure and to explain the phenomena of SPP wave slowing down as it propagating toward the taper end. This SPP nanofocusing process was also experimentally realized by illuminating the structure of a tapered CdS nanoribbon deposited on the Ag surface. As the emission of the focused SPP at the taper end, the proposed plasmonic structure can be severed as a light nanosource emitter in the future optical integrated circuits.     

Optimization of Optoelectronic Plasmonic Structures

We discuss the interplay between surface plasmon polaritons (SPPs) and localized shape resonances (LSRs) in a plasmonic structure working as a photo-coupler for a GaAs quantum well photodetector. For a targeted electronic inter-subband transition inside the quantum well, maximum photon absorption is found by compromising two effects: the mode overlapping with incident light and the lifetime of the resonant photons. Under the optimal conditions, the LSR mediates the coupling between the incident light and plasmonic structure while the SPP provides long-lived resonance which is limited ultimately by metal loss. The present work provides insight to the design of plasmonic photo-couplers in semiconductor optoelectronic applications.     

Unidirectional Surface Plasmon Polariton Excitation on Single Slit with Oblique Backside Illumination

We have theoretically investigated the unidirectional surface plasmon polariton (SPP) excitation on single slits with oblique backside illumination. An aperture diffraction method is devised, from which the conditions of slit width and beam illumination angle for the unidirectional SPP excitation are formulated analytically. The derived unidirectional conditions are validated with vectorial electromagnetic simulation using the rigorous coupled wave analysis.     

The Periodically Graded Metal–Insulator–Metal Gap Structure for Plasmonic Waveguides

In this paper, we explore the potential of the plasmonic metal–insulator–metal (MIM) periodically graded structure. Based on the coupled modes approach, an analytical model has been observed for the surface plasmon polariton (SPP) propagation. The band modes of SPP can be also supported by the MIM structure and we have analyzed the strong dependence of band width on structure parameters. The obtained analytical expressions allow one to easily choose the structure parameters for the desired band width.     

Ultrahigh Contrast One-Way Optical Transmission Through a Subwavelength Slit

We computationally demonstrate one-way optical transmission characteristics of a subwavelength slit. We comparatively study the effect in single layer and double layer metallic corrugations. We also investigate the effect of a dielectric spacer layer between double corrugations to control the volumetric coupling of plasmon and optical modes. We computationally show unidirectional transmission behavior with an ultrahigh contrast ratio of 53.4 dB at λ?=?1.56 μm. Volumetric coupling efficiency through the nanoslit strongly depends on the efficient excitation of both the surface plasmon resonance and metal–insulator–metal waveguide modes. We show that the behavior is tunable in a wide spectral range.     

Plasmonic Band Gap: Role of the Slit Width in 1D Metallic Grating on Higher Refractive Index Substrate

This paper reports the excitation of surface plasmon polaritons (SPPs) and associated plasmonic band gap (PBG) while using TM plane wave interacting with 1D metallic grating on higher refractive index GaP substrate. A simple method is introduced to estimate the PBG which is crucial for many plasmonic devices. The PBG is estimated by measuring the transmission spectra obtained through the plasmonic grating structures when slit width is varied while periodicity and the thickness of the gold (Au) film remained fixed. The PBG is observed for the grating devices whose slit width is less than one third of the periodicity which is caused by the presence of a higher plasmonic mode. The PBG is absent for the grating device whose slit width is slightly less than half and greater than one third of the periodicity. Such grating devices support only a fundamental plasmonic mode because the profile/shape of the slit in the grating device is more like a sinusoidal nature. Furthermore, such grating offers intermediate scattering to the incident light and the SPP as well which in turn couple more incident energy to the SPPs. Far-field modelling results also support the results obtained through experiment.

     

Photocurrent Enhancement in Si-Ge Photodetectors by Utilizing Surface Plasmons

A circular slit-groove surface plasmon polaritons (SPPs) launcher surrounding a photodetector is employed theoretically to enhance the photocurrent of atypical Si-Ge photodetectors. The slit and grooves are designed such that the SPPs are focused at the center of the absorption layer of the photodetector to result in additional electric current. Fabry–Perot resonance condition accurately calculates the period of the groove, slit-groove distance, photodetector radius, and slit-photodetector distance. The manipulation leads to constructive interference between the incident light impinging from the top and the SPPs propagating toward the photodetector. Simulation result shows that photocurrent increases by approximately 13-fold when the SPPs are introduced.     

Nanoscale Surface Plasmon All-Optical Diode Based on Plasmonic Slot Waveguides

A nanoscale surface plasmon all-optical diode is proposed based on a plasmonic slot waveguide having an asymmetric plasmonic grating in the center. The asymmetric configuration of the plasmonic grating and the unique dispersion relations of the plasmonic slot waveguide ensure the nonreciprocal transmission properties. High transmittance contrast ratio of 1,150 is achieved theoretically. The performance of the surface plasmon all-optical diode does not have any high power requirement. This may open a new way for the study of integrated photonic devices based on surface plasmons.     

Analysis of the Scattering of Surface Plasmon Polariton Waves from a Perfectly Conducting Circular Cylinder

Surface plasmon polariton (SPP) waves are the most extensively studied waves among various types of surface waves because they are easy to excite and have been used in many optical applications particularly for plasmonic communication, sensing, and harvesting solar energy. In all these applications, especially on-chip plasmonic communication, scattering can be an important issue to deal with. Therefore, this paper aimed to theoretically inspect the scattering pattern of SPP waves from a perfect electric conductor (PEC) cylindrical scatterer. The cylindrical wave approach is used to solve the scattering problem by a cylindrical object placed below a metallic layer. The scattering is investigated thoroughly by changing the size of the scatterer and its distance from the interface along which the SPP wave is excited. As the size of the scatterer increases, the scattering increases significantly. On the other hand, when the distance of the scatterer from the interface is increased, the scattered field becomes small. Two-dimensional field maps are produced for the incident angle at which SPP is excited. Numerical results are also presented for other incident angles to make a comparison. Furthermore, the forward and backward far-fields are significantly enhanced if the SPP wave is scattered in comparison with when the SPP wave is not present.

     

Effective Coupling of Incident Light Through an Air Region into an S-Shape Plasmonic Ag Nanowire Waveguide with Relatively Long Propagation Length

Coupling of incident light through an air region into an S-shape silver (Ag) plasmonic nanowire waveguide (SSAPNW) is a highly difficult challenge of light guiding on the surface of metal nanowire. In this paper, we numerically analyze the coupling effect of an SSAPNW which is covered by a dielectric medium using a finite element method. The coupling effect can be modulated by adjusting the Ag nanowire diameter and the covering dielectric medium width and wavelength of incident light, and the propagation length of surface plasmon (SP) coupling can be maximized. Simulation results reveal that the field confinement can be significantly improved and the majority of the electric field can be carried on the surface of a bending Ag nanowire. The effect of electric field transport along an SSAPNW due to SP coupling and Fabry-Perot resonance is investigated for different dimensions and lengths. Accordingly, long propagation lengths of about 41.5 μm for 10?×?SSAPNW at an incident wavelength of 810 nm and longer propagation length can be achieved if more sections of an SSAPNW are used. Simulation results offer an efficient method for optimizing SP coupling into bending metal nanowire waveguides and promote the realization of highly integrated plasmonic devices.     

Polarization Properties of Selectively Gold-filled Suspended Core Microstructured Optical Fibers

We study the polarization properties of suspended core microstructured optical fibers (SC-MOFs) with hexagonal lattice structure and high air-filling fraction having a single gold-filled hole along the horizontal axis. The interaction between the core-guided light and metal leads to surface plasmon resonance (SPR) at particular frequencies where the phase-matching condition is satisfied. We observe from the modal analysis that MOFs with high air-filling fraction offer the possibility of coupling of the fundamental mode with the first-order surface plasmon polariton (SPP) mode. With the increase in the suspension factor (SF), the fundamental mode couples with higher order SPP modes and the coupling strength also enhances. It also leads to an increase in modal birefringence. Reduction in beat length by an order of magnitude compared to the reported values is being reported for the first time to our knowledge. We have achieved the lowest beat length of 0.0105 mm at 1 μm wavelength for the structure having d/Λ = 0.85 and SF = 1.65. The results show that such plasmonic SC-MOFs may perform as efficient in-fiber polarizers and polarization filters.

     

Surface Plasmon Resonance Intensification by Cavity-Ring Plasma Structure in the Giga-Hertz Regime

In this paper, we propose a novel sub-wavelength plasma structure that can effectively enhance surface plasmon resonance (SPR) to achieve a significant local field. On the basis of a plasma ring structure, we add a slit and two thin plasma layers, working as a metal-insulator-metal (MIM) waveguide at a specific incident wave frequency and generate the Fabry-Perot resonance (FPR). The structure thus couples the incident wave energy to the vicinity of the slit and intensifies the SPR inside the plasma ring. In addition, we also find the coupling and competing between SPR and FPR. For the coupling mode, the average field enhancement in the ring is up to a factor of 9.7. Moreover, the optimized thickness of the plasma layer is much thinner than the skin depth of the plasma to ensure the incident wave easily entering the MIM waveguide. We further calculate the dispersion relationship of surface plasmon polaritons in the waveguide cavity. The simulation results and theoretical dispersion function are in good agreements.

     

A Three-Dimensional Plasmonic Nanostructure with Extraordinary Optical Transmission

We report a 3D plasmonic nanostructure having an extraordinary optical transmission due to localized surface plasmon (LSP) coupling between nanoholes and nanodisks. The nanostructure contains a free-standing gold nanohole array (NHA) film above a cavity and an array of nanodisks at the bottom of the cavity that is aligned with the NHA. For the device, the LSP-mediated resonance position was dependent on the hole and nanodisk diameter as well as the separation distance. Also, the effect of LSP coupling between each hole and corresponding nanodisk became negligible for cavities deeper than 200 nm as observed as a disappearance of the LSP resonance. The greatest LSP resonance transmission and the highest electric field intensity were observed for the structure with the shallowest cavity. In addition, the structure had high surface plasmon resonance sensitivity and may have potential for surface-enhanced Raman spectroscopy and optical trapping applications.     

Tailoring Plasmon Lifetime in Suspended Nanoantenna Arrays for High-Performance Plasmon Sensing

We demonstrate significantly longer plasmon lifetime and stronger electric field enhancement by lifting the nanoantenna arrays above the substrate by dielectric nanopillars. The role of the pillar is to offer a more homogeneous dielectric background allowing stronger diffraction coupling among plasmonic nanoantennas leading to a Fanolike asymmetric lineshape. It is found that the electric fields around the nanoantennas can be greatly enhanced when the Fanolike resonance is excited, and a 4.2 times enhancement is achieved compared with the pure resonance in individual nanoantennas. Furthermore, only a collective surface mode with its electric fields of the same direction as the induced electric moment in the nanoantennas could mediate the excitation of such a Fanolike resonance. More importantly, the sensitivity and the figure of merit (FOM) of this plasmonic structure can reach as high as 900 nm/RIU and 53, respectively. Our study offers a new, simple, and efficient way to design the plasmonic systems with desired electric field enhancement and spectral lineshape for different applications.     

Efficient and Directional Excitation of Surface Plasmon Polaritons by Oblique Incidence on Metallic Ridges

For many years, the search for efficient surface plasmon polariton (SPP) excitation mechanisms has been a recurring matter in the development of compact plasmonic devices. In this work, we excited SPPs illuminating a subwavelength metallic ridge with a focused spot to characterize the coupling efficiency by varying the incidence angle of the excitation beam from ??50 to 50°. The intensity distribution of the excited SPPs was measured using leakage radiation microscopy to determine the relative coupling efficiency in the wavelength interval from 740 to 840 nm. We modeled the excitation efficiency as a function of the incidence angle using a simple analytical diffraction model. Two ridges of different width (200 and 500 nm) were used to compare results and validate the model. The experimental results show a higher coupling efficiency at oblique incidence, where the coupling was enhanced by factors of 2× for the 500-nm-wide ridge, and 3× for the 200-nm-wide ridge, as well as unidirectional SPP excitation. The experimental results are in good agreement with the proposed model.     

Complicated Wavefront Shaping of Surface Plasmon Polaritons on Metal Surface by Holographic Groove Patterns

Surface plasmon polaritons (SPPs) manipulation on metal surfaces is important for constructing ultracompact integrated micro/nano-optical devices and systems. We employ the methodology of surface electromagnetic wave holography (SWH) to design holographic groove patterns for controlling SPPs with complicated wavefronts traveling on metal surface. SPPs are scattered by these deli groove patterns and interfere with each other to form desired SPP wavefronts. Several devices are demonstrated to control the intensities and phases of SPPs, such as focusing a plane SPP or diverging SPPs to two points with different phases, and focusing SPPs with complicated beam profile to a point. The finite-difference time-domain simulations show that in all cases, the predesignated functionalities are fully achieved by the designed plasmonic holographic structures. The results strongly support the power of SWH for shaping the complicated wavefront of in-plane transporting SPPs.