Long-Range Surface Plasmon Supported by Asymmetric Bimetallic Structure

With this study, we prove that an asymmetric bimetallic structure can support long-range surface plasmon (LRSP) and propose a procedure for its optimization. By applying different criteria we prove that the plasmon which is supported by the structure is indeed LRSP. Unlike all known asymmetric structures supporting LRSP, our structure provides prism excitation and can be used as a biochip for biosensing. Moreover, we show that the structure supports a plasmon with the same propagation constant as LRSP and which is excited at the interface of metal and buffer. This plasmon can be used as a reference channel providing information for the temperature of the structure.     

Enhanced SPR Sensitivity with Nano-Micro-Ribbon Grating—an Exhaustive Simulation Mapping

In this study, we theoretically investigate the sensing potential of 2D nano- and micro-ribbon grating structuration on the surface of Kretschmann-based surface plasmon resonance (SPR) biosensors when they are employed for detection of biomolecular binding events. Numerical simulations were carried out by employing a model based on the hybridization of two classical methods, the Fourier modal method and the finite element method. Our calculations confirm the importance of light manipulation by means of structuration of the plasmonic thin film surfaces on the nano- and micro-scales. Not only does it highlight the geometric parameters that allow the sensitivity enhancement compared with the response of the conventional SPR biosensor based on a flat surface but also describes the transition from the regime where the propagating surface plasmon mode dominates to the regime where the localized surface plasmon mode dominates. An exhaustive mapping of the biosensing potential of the 2D nano- and micro-structured biosensors surface is presented, varying the structural parameters related to the ribbon grating dimensions, i.e., the widths and thicknesses. The nano- and micro-structuration also leads to the creation of regions on biosensor chips that are characterized by strongly enhanced electromagnetic (EM) fields. New opportunities for further improving the sensitivity are offered if localization of biomolecules can be carried out in these regions of high EM fields. The continuum of nano- and micro-ribbon structured biosensors described in this study should prove a valuable tool for developing sensitive and reliable 2D-structured plasmonic biosensors.     

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.     

Gain-Assisted Propagation and Angular Response of Surface Plasmon Resonance in Nonlinear Kretschmann Configuration

The present study investigates the optical characteristics and angular response of gain-assisted surface plasmon resonance incorporating a nonlinear Kretschmann configuration. Nonlinear susceptibility of two-level atoms is used to describe the gain and nonlinear characteristics of amplifying medium. The structure is investigated and compared in both linear and nonlinear regimes. Our theory presents surface plasmon polaritons (SPPs) amplification accounting for saturation of linear gain by nonlinear losses. Reflectivity curve of the Kretschamnn configuration, attenuation constant, propagation length, and magnetic field intensity of plasmonic wave at the interface provide direct proofs of saturated gain induced by nonlinear absorption loss. Linear analysis predicts enhanced total reflection (ETR) in the presence of gain while in nonlinear regime, no ETR phenomenon occurs due to nonlinear loss absorption. Our analysis verifies that in the presence of nonlinear absorption loss, a saturation tendency of propagation length and plasmonic intensity is inevitable which is coincident with the practical observations.     

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.     

Plasmonic Planar Lens Based on Slanted Nanoslit Array

The novel plasmonic lenses based on slanted nanoslits have been proposed theoretically. The slanted nanoslits with different slant angles can provide unequal propagation distances for the surface plasmon polaritons excited by incident light. The phase retardation for wavefront shaping can be obtained to realize constructive interference on a preset single spot. We can actively modulate the position of the optical focus by adjusting the slits slant angles properly. The simulation results of the finite element method are used to verify our proposals.     

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.     

Plasmonic Variable Optical Attenuator Based on Liquid-Crystal Tunable Stripe Waveguides

A long-range surface plasmon polariton variable optical attenuator based on available nematic liquid crystals and polymers is proposed and theoretically investigated. It is demonstrated that the electro-optic control of the nematic molecular orientation is capable of tuning the level of index asymmetry of an Au stripe waveguide and the key properties of the fundamental long-range plasmonic mode, such as modal profile and propagation losses. By proper structural design and material selection, plasmonic in-line intensity modulators are designed, which exhibit very low power consumption, extinction ratios in excess of 30 dB, and insertion losses as low as 1 dB for a device length in the millimeter range. Such active plasmonic elements are envisaged to be used in interchip photonics bus interconnects.     

Design of Full-Adder and Full-Subtractor Using Metal-Insulator-Metal Plasmonic Waveguides

The metal-insulator-metal (MIM) waveguides are considered best among all plasmonic waveguides for propagation of optical signal to deep sub-wavelength scale. In this paper, MIM plasmonic waveguides based Mach-Zehnder interferometer (MZI) is developed. It possesses nonlinear Kerr material in one of its linear arm for controlling of optical signal with light intensity. Self phase modulation (SPM) and cross phase modulation (XPM) processes inside nonlinear MZI are used to design novel and compact full-adder and full-subtractor. Analysis and verification of proposed devices are carried out using FDTD and MATLAB simulations.     

Efficient Surface Plasmon Polariton Excitation with the Beam Generated by Micro Triangular Prism

The optical beam generated by a micro triangular prism is presented to excite surface plasmon polaritons (SPPs) on a single silver nano slit. The electromagnetic fields generated by the micro triangular prism and the excited surface plasmon polaritons are simulated with finite-difference time-domain method. Compared with directly normal incident beam, the efficiency of SPPs’ excitation with the beam generated by the micro triangular prism is highly improved.     

Plasmonic Lens Based on Rectangular Holes

A compact plasmonic lens is proposed in this paper. This plasmonic lens consists of rectangular holes etched on the silver film and arranged on one straight line and possesses the characteristics of short focus length, ultrathin thickness, and strong focus ability. The theoretical design for the plasmonic lens abides by the constructive interference theorem, and the surface plasmon polaritons excited by the holes with linearly polarized light illumination focuses effectively. The plasmonic lenses with single and double focus spots are provided, and the simulation experiment gives the powerful verification. The distinct structure feature and the excellent focusing characteristic of this plasmonic lens are benefit for its applications in optical integration.     

Optical Bifacial Transmission by Asymmetric Charge-Oscillation-Induced Light Transmission Through a Plasmonic Structure

From first-principles computation, we reveal that optical bifacial transmission can be induced within an asymmetric metallic subwavelength structure. This phenomenon can be explained by a concrete picture in which the intensity of the driving forces for surface plasmon or charge wave is asymmetric for the two incident directions. Two distinguished different numerical methods, finite difference time domain (FDTD), and rigorous coupled wave analysis (RCWA) are utilized to verify that optical bifacial transmission can exist for linear plasmonic metamaterial. Previous results are also reviewed to confirm the physical meaning of optical bifacial transmission for a planar linear metamaterial. The incident light can provide direct driving forces for surface plasmon in one direction. While in the opposite direction, forces provided by the light diffraction are quite feeble. With the asymmetric driving forces, the excitation, propagation, and light-charge conversion of surface plasmon give the rise of bifacial charge-oscillation-induced transmission. In periodic a structure, the excitation of surface plasmon polariton can lead to the spoof vanish of such phenomenon. The transmissions for two incident directions get the same in macroscopic while the bifacial still exists in microscale.     

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.     

Plasmon Modes Management

A new plasmonic structure based on bimetallic layer is proposed. We analyze the structure and show that bimetallic film plays a crucial role in the management of surface plasmons. The roll of the buffer is discussed, as well. Up to three surface plasmons can be excited simultaneously in the structure. Two of plasmons can be used for two-plasmon spectroscopy. The third plasmon can be used for controlling the temperature of the structure.     

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.     

Beam Manipulating via an Array of Nanoslits Modified by Perpendicular Cuts and Bumps

We report the observation of focusing and deflection phenomena by employing a novel technique to perform phase front profile design in nanoslit-based planar plasmonic lenses and beam deflectors. Introducing perpendicular cuts and bumps to the perforated nanoslits on a thin metallic film is utilized to change the effective depth of the nanoslits which provide the possibility of manipulating the phase front profile based on the propagation property of the surface plasmon polaritons in the metal–insulator–metal waveguides. Using the dispersive finite-difference time-domain numerical method, simulations are conducted to explore the beam focusing and deflection phenomena, and the performance parameters of the lens and beam deflector include the focal length, full-width half-maximum, depth of focus, the efficiency of focusing, and the deflection angle. The whole structure is formed on a planar thin film which is convenient for miniaturization and high density integration besides that it can be fabricated by well-known techniques such as focused ion beam milling.     

Higher Order Tunable Fano Resonances in Multilayer Nanocones

We present a computational study of the plasmonic response of a gold–silica–gold multilayered nanostructure based on truncated nanocones. Symmetry breaking is introduced by rotating the nanostructure and by offsetting the layers. Nanocones with coaxial multilayers show dipole–dipole Fano resonances with resonance frequencies depending on the polarization of the incident light, which can be changed by rotating the nanostructure. By breaking the axial symmetry, plasmonic modes of distinct angular momenta are strongly mixed, which provide a set of unique and higher order tunable Fano resonances. The plasmonic response of the multilayered nanocones is compared to that of multishell nanostructures with the same volume and the former are discovered to render visible high-order dark modes and to provide sharp tunable Fano resonances. In particular, higher order tunable Fano resonances arising in non-coaxial multilayer nanocones can vary the plasmon lines at various spectral regions simultaneously, which makes these nanostructures greatly suitable for plasmon line shaping both in the extinction and near field spectra.     

Effects of Dielectric Anisotropy on Surface Plasmon Polaritons in Three-Layer Plasmonic Nanostructures

The effects of highly anisotropic dielectric on surface plasmon polaritons (SPPs) are investigated in several three-layer plasmonic nanostructures. Dispersion relations of SPPs in anisotropic-dielectric-metal (ADM), dielectric-anisotropic-metal (DAM), and metal-anisotropic-metal (MAM) structures are analytically derived. The numerical results in the visible indicate that, in ADM, the propagation length of a conductor-gap-dielectric mode is changed from 5.9 to 91 μm and its cutoff thickness from 83 to 7 nm with varying the optical axis, while in DAM, the influences of anisotropic dielectric are reversed on propagation length and cutoff thickness. In MAM, by tuning the optical axis, the light confinement of symmetry SPPs mode varies about 10 %. Further numerical calculations show that the above results induced by the anisotropy of dielectric can be extended to the telecommunication frequency. The improved mode properties may be used in plasmonic-based nanodevices and tunable single surface plasmon sources.     

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.     

Highly Sensitive Plasmonic Temperature Sensor Based on Photonic Crystal Surface Plasmon Waveguide

We propose a highly sensitive temperature sensor based on photonic crystal surface plasmon waveguides comprising different plasmonic active metals such as gold, silver, and aluminum, utilizing surface plasmon resonance phenomenon. We found that the resonance wavelength can be easily and substantially tuned over a broad spectral range by changing the temperature and also by judiciously choosing the different plasmonic metals. Employing coupled mode theory, we found that the proposed sensor can be used in harsh environment with sensitivity as high as ～70 pm/K around telecommunication window.