Surface Plasmon Waveguide Research Articles

Terahertz Spoof Surface Plasmonic Logic Gates.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

Spoof surface plasmon analysis based on Marcatili’s method

Motivated by surface plasmon polariton waveguides in the optical regime, spoof surface plasmon (SSP) waveguides have received a lot of attention in terahertz and millimeter wave frequencies. Most research on these kinds of waveguides is numerical. However, some limited analytical work can be seen in the literature. In this paper, one type of SSP waveguide that is composed of a rectangular corrugation with finite lateral width on the ground is considered, and an analytical method, which is inspired by Marcatili’s method, is proposed in order to calculate the dispersion curve of the first mode. The results of this analytical method and a numerical commercial eigenmode solver are compared. The accuracy of this method by varying different geometrical parameters is shown. Finally, it is shown that this method works well in predicting the electromagnetic fields of the first mode.

Broadband bandpass filter with double‐layered spoof surface plasmon waveguide as main transmission line

In this article, double-layered spoof surface plasmon waveguide (DLSSPW) which consists of double-layered metal gratings is used as a main transmission line for realizing a broadband bandpass filter with a center frequency of 1.5GHz and a relative bandwidth of 80%. Four equally spaced short-circuited double-stripped lines are used as resonators and mounted on the main transmission line. Efficient conversion from the quasi-transverse electromagnetic (TEM) waves in the coplanar waveguide to spoof surface plasmon polaritons (SSPPs) in the DLSSPW is achieved by using gradient double-layered metal gratings and sector-structure impedance matching circuit. This filter has lower insertion loss than using single-layered spoof surface plasmon polaritons, as the DLSSPW has tighter field confinement and lower radiation loss. Moreover, due to the slow-wave effect of DLSSPW, the main-part length of the filter is shorter than microstrip-form counterpart by 30%. The measured S-parameters of the plasmonic bandpass filter agree well with the simulated ones, validating the good features.

Entanglement created in three-quantum dot system coupled to plasmonic waveguide

We investigate the entanglement properties of three-quantum dot (QD) system coupled to surface plasmonic waveguide by means of the concurrence with fully quantum-mechanical approach. High bipartite entanglement of three QDs could be achieved by adjusting the interparticle distance of the QDs, the detuning and the QD-waveguide coupling strength. We show that the bipartite entanglement of three-QD system is higher and its switching effect is better than the case of two-QD system. The discussed system with three QDs provides us rich way to realize the quantum entanglement used in the quantum information processing such as quantum computation and quantum communication.

Systematic Evolution of Resonant Coupling Behavior Between Surface Plasmon Polaritons and Multi-waveguide Modes in Metal-Dielectric Multi-layers

A multi-layer structure consisting of metal and dielectric layers which allows coupling between surface plasmon polaritons and waveguide modes is studied by calculating reflectivity curves of incident light. A wide waveguide structure is employed to ensure SPP resonance with different order waveguide modes when varying coupling layer thicknesses. Multi-hybrid plasmonic waveguide modes (HPWG) are formed in the coupled system. The weight fractions of metal SPP deduced from losses of HPWG modes are calculated for each hybridized mode, which can be considered as a parameter of evaluating coupling strength. Three different evolution stages for hybrid modes are clearly distinguished based on the variation of loss with spacer layer thickness. The interaction between SPP and waveguide experiences a series of change from weak Fano resonance, via strong coupling, finally to weak coupling process. In case of strong coupling regime, the contribution of metal SPP for low order hybrid modes starts to increase and then decreases with decreasing spacer layer thicknesses. Furthermore, difference of resonant behavior for hybrid modes in both weak coupling stages is analyzed in detail.

A broadband reconfigurable bandpass filter based on half-mode substrate integrated waveguide and spoof surface plasmon polarization structure

In this paper, a design method of reconfigurable broadband bandpass filter, with the advantage of adjustable bandwidth, is proposed by fabricating a hybrid sharing waveguide which is consist of half mode substrate integrated waveguide (HMSIW) and spoof surface plasmon polarization (SSPP). By adding a U-shaped resonant unit and a container structure to the bandpass filter, then through the movement of the slide on U-shaped groove and the alteration of the medium parameters in the container, the lower cut-off frequency of HMSIW and upper cut-off frequency of SSPP can be independently controlled by the fine tuning of the structures easily without changing the filter parameters. A prototype sample is fabricated to demonstrate and confirm the validity of the proposed method. The measured largest passband ranges from 11.5 to 14.5 GHz with the relative bandwidth of 23% which can be adjusted within a certain scope. The design of reconfigurable filter is realizing by the combination of HMSIW–SSPP structure and the concept of tunable filter. The proposed method can be used to develop different broadband bandpass filters.

Study of hybrid surface Plasmon modes in metallic circular waveguide filled with magnetized plasma

We demonstrated the theoretical analysis of the characteristics of a surface Plasmon of a metallic circular waveguide with the core region filled with magnetized plasma material. The dispersion relations are derived using Maxwell's equation and suitable boundary conditions. The cutoff frequency of the Surface Plasmon Polariton (SPP) wave at circular metal-magnetized plasma interface have been obtained computationally. The influence of the radius of cylinder, cyclotron frequency, and plasma frequency of the magnetized plasma on effective mode index, and the propagation length have been analyzed. We observed that the existence of cutoffs in different modes at the circular metal-magnetized plasma interface may facilitate the design of the mode-selective surface Plasmon waveguides.

Spatial Multi-Polarized Leaky-Wave Antenna Based on Spoof Surface Plasmon Polaritons

A leaky-wave antenna (LWA) is proposed on the basis of an ultrathin spoof surface plasmonic (SSP) waveguide, which can convert SSP waves into spatial multi-polarized radiation waves. The SSP waveguide is composed of a metallic strip line with periodic unit cells, and each unit consists of two orthogonally beveled slots. The feature of the single-conductor SSP waveguide determines that the leaky-wave radiation pattern is omnidirectional in the azimuth direction around LWA. Due to the design of orthogonally beveled slots, different polarized radiations can be achieved along the azimuth direction. Simulation results show that left-handed circularly polarized (LHCP), right-handed circularly polarized (RHCP), and linearly polarized (LP) radiations are achieved in different azimuth directions simultaneously, which are further validated by the experimental measurement. We also demonstrate a purely single polarized radiation by placing a metal reflector close to LWA, and the polarization is reconfigurable among LHCP, RHCP, and LP by mechanically rotating LWA.

Graphene Hybrid Surface Plasmon Waveguide with Low Loss Transmission

In this paper, a novel graphene hybrid surface plasmon waveguide structure is designed. Based on the finite element method, the mode characteristics, the quality factor, and the gain threshold of the waveguide structure are analyzed. The results show that the optical field constraint of the designed waveguide can reach a better level of deep sub-wavelength under the optimal parameters of 1550-nm working wavelength. The structure is applied to a laser, and the high quality factor, the low energy loss, the low threshold limit, and the ultra-small effective mode field area are obtained by adjusting waveguide design parameters. Compared with the common waveguide structure, this structure has stronger optical field limiting ability and microcavity binding ability. It provides theoretical and technical support for the development of new high-efficiency nano-laser devices and is expected to be applied to fields such as on-chip interconnects, photonic integrated circuits, optical storage, and optical signal processing.

Plasmonic and photonic enhancement of photovoltaic characteristics of indium-rich InGaN p-n junction solar cells.

In this paper, we demonstrate via Finite-difference time-domain (FDTD) simulations that the performance of indium-rich InxGa1-xN (x = 0.6) p-n junction thin-film solar cells is improved by incorporating an integrated structure of a 2-dimensional (2D) array of ITO nanodiscs on the top surface and a 2D array of Ag nanodiscs in the active layer above the Ag back reflector of the solar cell. The bottom Ag nanodiscs primarily enhance the absorption of longer wavelengths by coupling incident light into surface plasmon resonance (SPR) and waveguide modes. The top ITO nanodiscs enhance the middle wavelengths (400 nm to 800 nm) by coupling the incident light to photonic modes in the active layer. Thus, the integrated structure of nanodisc arrays leads to a very high absorption in the active region in broad spectral range (> 0.85 for wavelengths lying between 350 nm and 800 nm), significantly increasing the short circuit current density (Jsc) and power conversion efficiency (PCE) of the solar cell. In the proposed solar cells, the geometries of the silver and ITO nanodiscs were optimized to obtain the maximum possible values of the Jsc. The highest enhancements in Jsc and PCE of ∼25% and ∼26%, respectively, were obtained in a solar cell containing the integrated structure of ITO and Ag nanodisc arrays. Moreover, the performance of these cells was examined under oblique light incidence and it was observed that the solar cells containing the integrated structure of nanodisc arrays have a significantly larger value of Jsc when compared to the cells having no nanostructures or having only the top ITO nanodisc array or only the bottom Ag nanodisc array.

Improving the out-coupling efficiency of polymer light-emitting diodes with soft nanoimprinted random corrugated structures

A simple method is demonstrated in this study by applying random corrugated structures into the polymer emitting layer via soft nanoimprint technology to enhance the light out-coupling efficiency of polymer light-emitting diodes (PLEDs). The device with the random corrugated structures presents high efficiency and stable emission close to the Lambertian pattern. Compared with the flat device, about 52% efficiency enhancement is achieved in the PLEDs with the random corrugated structures, which can be understood by extraction of waveguide mode and surface plasmon polariton mode losses simultaneously. With these advantages, the simple random corrugated structures have the potential to stimulate the development of low-cost and large-area flexible PLEDs with high efficiency in the applications of solid-state light sources and full-color displays.

Ultrathin InP annular nanohole arrays for efficient light absorption solar cells

This Letter proposes ultrathin InP films consisting of annular nanohole arrays (ANAs) for highly efficient solar cells. By tailoring the inner and outer radii of ANAs properly and combining antireflection coating (ZnO) and back-reflector (Ag), the photocurrent generated in the InP based dielectric-semiconductor-metal (DSM) configuration can be increased dramatically by 124.6% with respect to the planar solar cell of equal geometric thickness (100 nm). The fact that the DSM-ANA structure is able to harvest broadband and wide-angle incident light can be attributed to the excitation of leaky waveguide modes, Bloch modes, and surface plasmon polariton modes. This work provides a promising and feasible way to design and fabricate efficient ultrathin InP photovoltaic and other optoelectronic devices.

Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides.

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm-1 at 300 K reducing to 14.5 cm-1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1-xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm-1 and 23.8 cm-1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings.

Electrically pumped dielectric loaded surface plasmon waveguides

Several schemes for the electrical pumping of dielectric loaded surface plasmon polariton waveguides are analysed. Two schemes that are designed to move the electrical contact away from the plasmon mode are compared to a Schottky contact scheme which has the electrical contact at the same metal/semiconductor interface which supports the surface plasmon mode. One scheme involves transporting carriers from lateral contacts via a thin semiconductor pathway. The other scheme involves burying the electrical contact inside the metal surface and having the surface plasmon mode skip over the break in the metal/semiconductor interface. The Schottky contact scheme is shown to have the best confinement of the surface plasmon mode on the gain medium. However, the other two alternative schemes can be engineered to require significantly less gain to overcome metal losses. Current densities required in the non Schottky contact schemes are found to be compatible with current semiconductor laser electrical contacts.

Controllable Filtering Structure Using Magnetic Coupling Between Spoof Plasmonic Waveguide and Solid Dielectric Resonators

The structure and electrical characteristics of a dielectric resonator (abbreviated as DR hereafter) loaded plasmonic waveguide with controllable bandwidth are investigated in the present research. By loading the high permittivity dielectric medium on a spoof surface plasmonic waveguide, the working bandwidth of the filtering structure can be regulated flexibly due to the resonant effect of the loaded cylindrical DR. The DR is home-made and possesses the permittivity of 25.66, tanδ of 5.3 × 10 -5 and stable temperature coefficient of -6.3 ppm/°C in microwave region. After the DR is loaded, the filtering structure can form the transmission zero on the edge of the pass band, regulating the bandwidth as well as increasing the out-of-band rejection. By controlling the height between the loaded DR and the substrate, or adjusting the inter-distance between the DRs, the relative bandwidth of the filtering structure can be effectively tailored.

Optical Properties of Bowtie-Type Nanoantennas Integrated Onto a Silicon Waveguide Platform

In this paper, we provide a detailed three-dimensional numerical analysis of the optical properties of common and modified bowtie aperture antennas integrated onto a silicon waveguide platform, to discuss the influence of geometrical parameters on the electric field enhancement factor and waveguide transmission when such antennas are excited by the evanescent field of the Si waveguide mode. We demonstrate that waveguide transmission is severely affected by the interference between Si waveguide modes and surface plasmon polariton modes excited in the antenna, while the antenna's field enhancement factor is mainly determined by the localized surface plasmon resonance occurring in its nano-gap. These mechanisms lead to a mismatch between the wavelength at which the antenna's field enhancement factor is maximized, and the wavelength at which transmission through the Si waveguide is minimized, suggesting that in some multi-mode cases, the optical properties of integrated nanoantennas determined through direct measurement of Si waveguide transmission may be misleading. Methods for improving the electric field enhancement (such that it has a bigger modulation depth) that have minimal impact on the resonant wavelength, and for improving the shape and location of the corresponding hot spot of the bowtie aperture antennas, are also discussed and analyzed. We believe that this analysis will be helpful in design of on-chip bowtie-type optical antennas for surface enhanced Raman spectroscopy, near-field optical microscopy, high sensitivity detection, and plasmonic optical tweezers.

A Band-Pass Filter Based on Half-Mode Substrate Integrated Waveguide and Spoof Surface Plasmon Polaritons

In this paper, a band-pass filter based on half-mode substrate integrated waveguide (HMSIW) and double-layer spoof surface plasmon polaritons (SSPPs) consisting of two corrugated metal strips is proposed, which can realize band-pass transmission by etching periodic grooves at the top and bottom metal layers of the HMSIW. Moreover, the influences of important parameters on the performance of the proposed band-pass filter are analyzed by parametric study. By changing the key parameters, the low and high cut-off frequency can be controlled independently. The corresponding equivalent circuit of the proposed band-pass filter is put forward to explain the physical mechanism. Compared with the previous structures, this structure features smaller size, wider bandwidth and lower loss. Simulated results show that the proposed band-pass filter achieves a bandwidth (for |S11| < −10 dB and |S21| > −0.8 dB) of about 69.77% (15.6–32.1 GHz). The measured results have good agreements with the simulated ones, which verify that the proposed band-pass filter has good performances and potential applications at the microwave frequencies.

Identification of waveguide mode and surface plasmon resonance mode using Fourier cross-correlation analysis.

The light reflected into the back focal plane of a microscope objective allows one to gather a great deal of information about the resonant modes excited on a sample. These dips represent modes excited on the sample, which are related to both the material properties and the structure. Automatic identification of these resonances is a vital stage in developing automated machine-learning techniques for high-throughput sample characterization. In previous work, identification of a single isolated mode was demonstrated; here we show how multiple modes can be separately identified using an automated centering procedure in a process we call radial thresholding. Once the center was determined, the radial thresholding process was modified and combined with interpolation to locate the precise modal positions. We show that this method is capable of resolving very closely spaced modes and is sensitive to nanometric changes in sample dimensions. The processing time for the method is sufficiently fast to ensure that it is suited for rapid sample identification.

Integrated multichannel Young’s interferometer sensor based on long-range surface plasmon waveguides

Two integrated Young's interferometer (YI) sensors based on long-range surface plasmon polariton (LRSPP) waveguides are presented. The first sensor is single-channel and based on a Y-junction splitter, and the other is multi-channel and based on a corporate feed structure. The multichannel YI enables simultaneous and independent phase-based monitoring of refractive index changes in multiple channels. The diverging output beams from the waveguides are overlapped in the far field to form interference patterns which are then post-processed using the fast Fourier transform (FFT) algorithm to extract phase values. The sensing capability of these YIs was demonstrated through sequential injection of solutions with increasing refractive index into the sensing channels. A detection limit of ∼ 1 × 10-6 RIU was obtained for both LRSPP based YIs, a significant improvement over measurements from similar structures using attenuation-based sensing.

Tunable Fano resonances and improved sensitivity in waveguide-coupled surface plasmon resonance sensors with a bimetallic layer

A multilayer structure consisting of Au–Ag bimetal and dielectric layers which allows coupling between surface plasmon polaritons and waveguide modes is studied by calculating reflectivity curves of incident light. The coupling results in asymmetric resonance modes, which can be attributed to Fano resonances (FR). The variation trend of FR is investigated in detail by fitting numerical results with an analytic model when changing both waveguide widths and silver thicknesses. It is demonstrated that changing the thickness ratio of Au–Ag bilayers can tune the coupling strength and the line shape of FR. We build the resonant mode diagram according to the different asymmetric line shape which can be depicted by the sign of the fitting Fano constant q. At a critical Ag thickness, the coupling gives rise to a q = 0 symmetrical resonance state (plasma induced transparency). Finally, the best optimized thickness combination (Ag/Au) in the structure is found when the refractive index of the sensing layer medium changes. The sensitivity of the optimized sensor is twice as high as that of the single Au film structure.