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.     

The Tunability of Surface Plasmon Polaritons in Graphene Waveguide Structures

The tunability of propagation properties of surface plasmon polariton (SPP) modes in a waveguide formed by two parallel graphene layers separated by a dielectric layer is studied. For this purpose, the dispersion equation of the structure is numerically solved and the effects of applied bias voltage, the role of effective structural parameters, and electron–phonon scattering rate on the propagation of symmetric and antisymmetric SPP waves are investigated. The results of calculations show that considering the electron–phonon scattering rate as a function of Fermi energy and temperature leads to a considerable decrease in the propagation length of SPPs. As the main result of this work, tuning the propagation characteristics of SPPs is possible by varying any of the parameters such as applied voltage, thickness of insulating layer between two graphene layers and permittivities of dielectric layers, and finally the temperature. It is found that antisymmetric mode benefits from a larger propagation length in comparison with that of the symmetric mode.

     

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.     

Signal peptide peptidases: A family of intramembrane-cleaving proteases that cleave type 2 transmembrane proteins

Five genes encode the five human signal peptide peptidases (SPPs), which are intramembrane-cleaving aspartyl proteases (aspartyl I-CLiPs). SPPs have been conserved through evolution with family members found in higher eukaryotes, fungi, protozoa, arachea, and plants. SPPs are related to the presenilin family of aspartyl I-CLiPs but differ in several key aspects. Presenilins (PSENs) and SPPs both cleave the transmembrane region of membrane proteins; however, PSENs cleave type 1 membrane proteins whereas SPPs cleave type 2 membrane proteins. Though the overall homology between SPPs and PSENs is minimal, they are multipass membrane proteins that contain two conserved active site motifs YD and GxGD in adjacent membrane-spanning domains and a conserved PAL motif of unknown function near their COOH-termini. They differ in that the active site YD and GxGD containing transmembrane domains of SPPs are inverted relative to PSENs, thus, orienting the active site in a consistent topology relative to the substrate. At least two of the human SPPs (SPP and SPPL3) appear to function without additional cofactors, but PSENs function as a protease, called γ-secretase, only when complexed with Nicastrin, APH-1 and Pen-2. The biological roles of SPP are largely unknown, and only a few endogenous substrates for SPPs have been identified. Nevertheless there is emerging evidence that SPP family members are highly druggable and may regulate both essential physiologic and pathophysiologic processes. Further study of the SPP family is needed in order to understand their biological roles and their potential as therapeutic targets.     

Excitation and Optimization Modeling of Surface Plasmon Polaritons in a Concentric Circular Metallic Grating Film

Interaction behavior between surface plasmon polaritons (SPPs) and Hankel-distributed diffracted waves (DWs) on a silver concentric circular grating film is studied using a rigorous coupled-wave technique for circular structure. It is shown that the numerical technique reveals the excitation characteristics of SPPs in the circular metal grating as well as provides an accurate calculation of SPP intensities for further optimization designs. Results show that the SPPs can be excited by various DWs through the control of wavelength and angle of the incident light. The most efficient excitation of SPPs from this circular metal grating structure can be obtained from the +1^st-order DW under a normal incidence with wavelength close to the grating period, and the optimal thickness and duty cycle of the grating are found to be 370 and 0.5 nm, respectively. It is shown that the optimized intensity of SPPs excited from circular metal grating can be higher than that from strip metal grating by over one order of magnitude.     

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.     

Imaging and Characterizing Long-Range Surface Plasmon Polaritons Propagating in a Submillimeter Scale by Two-Color Two-Photon Photoelectron Emission Microscopy

Long-range surface plasmon polaritons (SPPs), which propagate along metal/dielectric interfaces to submillimeter distances in the range of near-infrared (NIR) excitation wavelength, were examined by two-color two-photon photoelectron emission microscopy (2P-PEEM). Interferences between incident NIR photons and SPPs excited by the NIR photons at surface defects were imaged by detecting photoelectrons emitted from a gold surface, assisted by simultaneously irradiated ultraviolet photons which are to overcome the workfunction of the surface. The wavelength of the interference beat depends sensitively on the NIR wavelength. By analyzing the interference beat, the dispersion curve as well as phase and group velocities of SPP’s were experimentally obtained. The results closely match the theoretical one based on the Drude free electron model, indicating that two-color 2P-PEEM is applicable not only to the visualization of NIR-excited SPPs but also to the quantitative analysis of its physical properties. This method will be widely used to observe SPPs for various artificial plasmonic devices.     

Directional Excitation of Surface Plasmon Polaritons by Circularly Polarized Vortex Beams

We investigate the excitation of surface plasmon polaritons (SPPs) using a metallic nanoaperture array illuminated by circularly polarized Laguerre-Gaussian (LG) vortex beams. The direction of SPP excitation is tunable by changing the circular polarization and topological charge of LG beams. The left- or right-handed circular polarization determines SPP propagation on either side of the nanoaperture array. Furthermore, varying the topological charge of LG beam will result in beam splitting of SPPs. We also utilize a composite nanoaperture array with different periods to achieve unidirectional excitation of SPPs. The study provides an interesting approach to control the excitation direction of SPPs and may find great applications in SPP generators and optical switches.

     

Optical Nanofocusing on Tapered Metallic Waveguides

Tapered metal wires show a remarkable ability to ‘squeeze’ the lateral extent of a propagating surface-plasmon-polariton mode as it travels toward the tip of the taper. The transformation can be continued well below the diffraction limit to terminate at a nanoscale apex where intense near-fields are created. We perform the first full numerical simulations to investigate and quantify this phenomenon. We find optimal angles for maximal tip-field enhancement on conical wires by considering absorption, scattering to radiation and reflection. The optimal parameters we obtain contradict the conditions for adiabatic tapering, thereby advocating the use of numerical simulations. Despite the influence of losses, nanofocusing is still highly efficient for a broad range of practical metals, visible wavelengths and taper geometries. Diverse nano-optic applications can benefit directly and significantly from the results.     

Efficient Unidirectional Launching of Surface Plasmons by Multi-Groove Structures

Efficiency is an important criterion in developing a practical surface-plasmon-polariton (SPP) unidirectional launcher. In this paper, we show that multi-groove structures can efficiently launch SPPs by numerically optimizing structural parameters and normal incident light. Experimentally, a high efficiency of 58.4 % is demonstrated in a six-groove structure with a lateral dimension of 3.9 μm. For a three-groove structure with even smaller lateral dimension of 1.35 μm, the efficiency presents a broadband response, which remains higher than 42 % from 720 to 860 nm. The proposed multi-groove structures with high SPP launching efficiency and small size exhibit potential in highly integrated plasmonic circuits.     

Multi-Directional Plasmonic Splitter and Polarization Analyzer Based on the Catenary Metasurface

Owing to the unique properties of strongly confined and enhanced electric fields, surface plasmon polaritons (SPPs) provide a new platform for the realization of ultracompact plasmonic circuits. However, there are challenges in coupling light into SPPs efficiently and subsequently routing SPPs. Here, we propose a multi-directional SPP splitter and polarization analyzer based on the catenary metasurface. Based on the abundant electromagnetic modes and geometric phase modulation principle of catenary structure, the device has realized high-efficiency beam splitting for four different polarization states (x-polarization, y-polarization, LCP, and RCP). The central wavelength of the device is 632 nm and the operation bandwidth can reach 70 nm (585–655 nm). Based on the phenomenon of SPP beam splitting, we present a prototype of a polarization analyzer, which can detect the polarization state of incident light by adding photodetector with light intensity logic threshold in four directions. Moreover, by combining this device with dynamic polarization modulation techniques, it is possible to be served as a router or switch in integrated photonic circuits.

     

A Surface Plasmon Microcavity Between the Toroidal and Flat Metallic Surfaces

We consider the formation of the surface plasmon polariton (SPP) mode in the structure with a metallic torus and a metallic flat surface separated by a dielectric medium. The energy of the wave field is mainly concentrated in the dielectric medium at the vicinity of the minimum thickness of the gap between the metallic surfaces. The dependence of the resonant frequency on parameters of the structure was determined. The strongly localized SPP mode in the transverse direction contributes to the increase in the Purcell factor that is crucial for enhancement of the spontaneous emission rate.     

Terahertz Surface Plasmon-Polariton Superfocusing in Coaxial Cone Semiconductor Structures

It is shown that in coaxial cone semiconductor structures, the strong localization of surface plasmon polariton (SPP) is possible in terahertz wave regions. During the propagation through this structure, the SPP acquires the following peculiarities: its wavelength essentially decreases and the diffraction processes do not hinder its localization in very small regions; the wave fields noticeably increase. The dissipative processes are also essential, and, having a noticeable role in terahertz regions, do not hinder the strong localization of the SPP. On the basis of such structure, it is possible to create a scanning THz microscope with the resolution equal to a few microns.     

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.     

Excitation of Multiple Surface Plasmon-Polaritons by a Metal Layer Inserted in an Equichiral Sculptured Thin Film

Excitation of multiple surface plasmon-polaritons (SPPs) by an equichiral sculptured thin film with a metal layer defect was studied theoretically in the Sarid configuration, using the transfer matrix method. Multiple SPP modes were distinguished from waveguide modes in optical absorption for p-polarized plane wave. The degree of localization of multiple SPP waves was investigated by calculation of the time-averaged Poynting vector. The results showed that the long-range and short-range SPP waves can simultaneously be excited at both interfaces of metal core in this proposed structure which may be used in a broad range of sensing applications.     

Crystal structure of Bacillus subtilis SPP1 phage gp23.1, a putative chaperone

SPP1 is a siphophage infecting the gram‐positive bacterium Bacillus subtilis. The SPP1 tail electron microscopy (EM) reconstruction revealed that it is mainly constituted by conserved structural proteins such as the major tail proteins (gp17.1), the tape measure protein (gp18), the Distal tail protein (Dit, gp19.1), and the Tail associated lysin (gp21). A group of five small genes (22–24.1) follows in the genome but it remains to be elucidated whether their protein products belong or not to the tail. Noteworthy, an unassigned EM density accounting for ～245 kDa is present at the distal end of the SPP1 tail‐tip. We report here the gp23.1 crystal structure at 1.6 Å resolution, a protein that lacks sequence identity to any known protein. We found that gp23.1 forms a hexamer both in the crystal lattice and in solution as revealed by light scattering measurements. The gp23.1 hexamer does not fit well in the unassigned SPP1 tail‐tip EM density and we hypothesize that this protein might act as a chaperone.     

The expression of native and oxidized LDL receptors in brain microvessels is specifically enhanced by astrocytes-derived soluble factor(s)

Based on the functional characterization of sucrose biosynthesis related protiens[SBP: sucrose-phosphate synthase (SPS), sucrose-phosphate phosphatase (SPP), and sucrose synthase (SuS)] in Anabaena sp. PCC7120 and sequence analysis, we have shown that SBP are restricted to cyanobacterium species and plants, and that they are multidomain proteins with modular architecture. Anabaena SPS, a minimal catalytic SPS unit, defines a glucosyltransferase domain present in all SPSs and SuSs. Similarly, Anabaena SPP defines a phosphohydrolase domain characteristic of all SPPs and some SPSs. Phylogenetic analysis points towards the evolution of modern cyanobacterial and plant SBP from a bidomainal common ancestral SPS-like gene.     

Tunable Multi-Port Surface Plasmon Polariton Excitation with Nanostructures

Surface plasmon polaritons (SPPs) have appealing features such as tighter spatial confinement and higher local field intensity. Manipulation of surface plasmon polaritons on metal/dielectric interface is an important aspect in the achievement of integrated plasmonic circuit beyond the diffraction limit. Here, we introduce a design of pin cushion structure and a holographic groove pattern structure for tunable multi-port SPPs excitation and focusing. Free space light is coupled into SPPs through momentum matching conditions. Both nanostructures are capable of tunably controlling of SPPs depending on the incident polarizations, while the holographic method provides more flexibility of wavelength-dependent excitations. Furthermore, a quantitative method is applied to calculate the efficiencies of excitation for both nanostructures under different conditions, including radially polarized incident beams. These results can work as a guidance and be helpful to further choice of the suitable design strategies for variable plasmonic applications such as beam splitter, on-chip spectroscopy, and plasmonic detectors.     

Nanofocusing of Surface Plasmon Polaritons on Metal-Coated Fiber Tip Under Internal Excitation of Radial Vector Beam

We theoretically present the nanofocusing of the metal-coated fiber tip under internal excitation of the radial vector beam within visible band based on the finite difference time domain (FDTD) analysis. The electric field intensity enhancement factor of the localized surface plasmons (LSP) mode at the tip apex is quantitatively shown in relation with incident wavelength, coating material, conical angle of tip, and coating film thickness/length. Specially, the evolution of fiber radial vector mode to surface mode with respect to the radius of metal-coated fiber tip is calculated under typical excitation wavelengths of 633 nm and 785 nm. Furthermore, the reason of the tip eliminating far-field background signal is explained, and the transverse electric field distributions of LSP mode and the tip-substrate coupling are also given at the optimal excitation wavelength. These calculation results will be a good reference for the fabrication of metal-coated fiber tips and for the experimental design of the tip-enhanced spectroscopy (TES) system.

     

Making noise: Emergent stochasticity in collective motion

Individual-based models of self-propelled particles (SPPs) are a popular and promising approach to explain features of the collective motion of animal aggregations. Many models that capture some features of group motion have been suggested but a common framework has yet to emerge. Key to all of these models is the inclusion of “noise” or stochastic errors in the individual behaviour of the SPPs. Here, we present a fully stochastic SPP model in one dimension that demonstrates a new way of introducing noise into SPP models whilst preserving emergent behaviours of previous models such as coherent groups and spontaneous direction switching. This purely individual-to-individual, local model is related to previous models in the literature and can easily be extended to higher dimensions. Its coarse-grained behaviour qualitatively reproduces recently reported locust movement data. We suggest that our approach offers an alternative to current reasoning about model construction and has the potential to offer mechanistic explanations for emergent properties of animal groups in nature.