Plasmon Waves Research Articles

Graphene based tunable bandpass filter for terahertz spectroscopy of polymers

In this paper, a tunable dual-band bandpass filter using graphene is designed and simulated at resonant frequencies 1 THz and 1.5 THz. The proposed design consists of a series of TEM-mode transmission-line resonators which are open and short circuited in alternate orientation. The bandpass filter is designed from the lowpass Chebyshev prototype. A graphene layer is placed in between the dielectric layer and conductor layer in order to facilitate the plasmonic wave propagation. The simulation results suggest that the proposed graphene-based bandpass filter can achieve the specified frequency response. By altering the chemical potential of the graphene a shift of about 0.14 THz is observed in the transmission response of the filter. The proposed filter has the potential to be used as ultra-compact dual-band BPF in non-destructive testing in polymer processing industries.

Spin–Orbit-Engineered Selective Transport of Photons in Plasmonic Nanocircuits with Panda-Patterned Transporters

The spin–orbit coupling of light, which can be utilized to manipulate the polarization and spatial degrees of freedom of photons, has shown unprecedented potential in realizing on-chip integrated nanocircuits and quantum information processing. Particularly, this chiral coupling mechanism can result in unidirectional transport of spin-dependent light at a subwavelength scale by designing specific nanostructures. Here we propose and demonstrate a dielectric-loaded plasmonic nanocircuit with a panda-patterned transporter to directionally couple and guide plasmonic waves through two-branched waveguides. Both simulation and experimental results reveal that the waveguide modes excited by circularly polarized light with different spin states can be selectively coupled to one of two branching waveguides, with a measured unidirectionality efficiency up to 0.95. The proposed configuration may open up more possibilities in developing multifunctional nanocircuits that could be utilized for exploiting chiral manipulation with flexible degrees of freedom.

Resonant Excitation Analysis of the Absorption Enhancement of L-Shaped Metasurfaces in Midinfrared Band

Surface plasmon waveguides have attracted a lot of attention in recent years due to their ability to conduct light in the subwavelength scale. A large number of metallic structures have been used as waveguides and applied in integrated photonic circuits. Among these structures, the metal-insulator-metal surface plasmon waveguide is considered to have unique advantages. Compared to other waveguide forms, it is more compact and thus easier to integrate into optical circuits. For these reasons, we investigated the transmission properties of surface plasmon waves in metal-insulator-metal waveguides and built a MIM absorber consisting of gold and dielectric. We developed an interference model for the MIM absorber and concluded that the interactions between the metal patch and the substrate are not negligible in the near field. In addition, magnetic resonance plays an equally important role in approaching uniform absorption. Absorption spectra with different structural parameters, incidence angles, and polarizations were investigated. A sharp absorption peak was also found to be caused by the Rayleigh anomaly. The study reported in this paper contributes to further understanding of the physical properties of the MIM absorber, and we expect to refine the theoretical model in the future to eliminate the bias caused by near-field coupling.

Coupled Plasmon Wave Dynamics beyond Anomalous Reflection: A Phase Gradient Copper Metasurface for the Visible to Near-Infrared Spectrum

In nanoscale photonic devices, the demand for multifunctionality from 2D metasurface optics has increased rapidly. To explore the required fine-tuning in the design metrics, we reinvestigated the trapezoid-shape copper metasurface using finite-difference time-domain simulation to efficiently utilize linearly polarized light for two different functionalities. From the plasmonic band structure, we could see how the degree of asymmetry in the geometry affected the efficient resonance coupling of the traveling plasmonic modes, along with the different types of mode hybridization profiles that were related to the nanoantenna’s geometric shape. By tuning the nanoantenna’s length, we could excite the effective plasmon mode that was supported by this configuration and guide surface waves unidirectionally from the normal incidence free-space light within the visible to infrared range. The directed surface plasmon polaritons had both antisymmetric and symmetric modes that oscillated between the top and bottom surfaces of the continuous metal layer, depending on the nanoantenna’s length and wavelength. This proposed copper metasurface was optimized for a far-field application of broadband (600–900 nm) anomalous beam steering for an average of 60% efficiency with a maximum angle of 64°. This work offers more understanding of a metasurface being implemented in small plasmonic devices, waveguide mode controlling and beam steering with wavelength-dependent functionalities.

Design of a plasmonic refractive index sensor based on the amplifier system with two plasmonic waveguides and four cavities with different dimensions and coordinates

In this research, we design a plasmonic refractive index sensor based on metal insulated metal waveguide (MIM) and examine it numerically. To design the structure of this sensor, we use five cavities with different dimensions and coordinates and two plasmonic waveguides. The resonant wavelengths and refractive index of the resonators are investigated and simulated by the finite difference time domain (FDTD) method. Due to the fact that the sensors and conduction characteristics of plasmonic waves are affected by the structure parameters, by changing the refractive index and changing the dimensions of the cavities, we can weaken or strengthen the passage coefficient in the resonant modes. As a result, we obtain the sensitivity coefficient, the competency digit (FOM) and the sensor quality coefficient. These plasmonic sensors with a simple frame and high optical resolution can be used to measure the refractive index in the medical, chemical and food industries, and due to the special correct distribution of resonances, they are highly sensitive to changes in the refractive index of the environment.

Numerical study of vegetable oil as dielectric in the generation of surface plasmon polaritons in metal: The case of double interfaces

Modified electromagnetic waves which is resulted from coupling of surface plasmon and initial electromagnetic waves are called surface plasmon polaritons (SPP). These type of polaritons are generated at the interface between metal and dielectric. Many studies are performed since SPP have potential application in many fields. The process of generating SPP was usually using dielectrics in the form of solid. However, the usage of liquid dielectric in generating SPP is very rare. In this study, we predict numerically the usage of liquid dielectrics by solving the dispersion relation of the SPP. The dispersion relation was derived using Maxwell equations and the continuity of the fields at the interfaces. The metal was immersed in the liquid dielectrics. We used parameters of castor oil as liquid dielectric in the numerical calculation. We found that the dispersion relation had two branches. One branch represented in phase condition while the other branch illustrated out of phase condition. This result agree with the previous research using solid dielectric.

Microstructured Optical Fiber Based on Surface Plasmon Resonance for Dual-Optofluidic-Channel Sensing

A surface plasmon resonance (SPR)-based microstructured optical fiber (MOF) is designed and theoretically investigated for dual-channel optofluidic sensing. The design of the MOF consists of a solid silica core surrounded by double air holes, and the hollow section is coated with thin Ag film to excite the surface plasmon waves on the dielectric interface. The optofluidic sensing characteristics of the individual channel have been analyzed based on the mode-coupling theory between core and plasma modes. By filling the dual channel with different liquid analytes, the resonance wavelengths are adjusted away from each other to achieve dual-optofluidic-channel sensing. When the refractive index (RI) of liquid samples varies from 1.33 to 1.39 RIU, the optofluidic sensing sensitivities of channel 1 and channel 2 in MOF are 1439.29 nm/RIU and 4003.57 nm/RIU, respectively. The proposed SPR-based MOF is appropriate for multi-channel biological and chemical sensing fields.

Calculation of spectra and plasmon wave distribution in one-dimensional periodic structure

Surface Plasmon Resonance (SPR) is a resonance phenomenon between electromagnetic waves and electrons, which is formed on the interface between metal and dielectric surface. SPR waves can be generated by using a coupler element, such as prisms and gratings. In this study, we analyzed the effect of groove filling in nanograting structures using computation work based on Rigorous Coupled Wave Analysis (RCWA) method. The structure periodicity is 700 nm with a groove depth of around 350 nm. The reflectance spectra calculation by the RCWA method produces the appearance of SPR dip for an empty groove (with air inside the groove) at around 550 nm - 650 nm. The dip wavelength varies on the incident angles from 30 up to 60 degrees. In addition, the shifting of those dips is also predicted from the calculation results when the groove is filled by a medium with a refractive index of 1.5, which can be the basis of its utilization as optical sensing elements.

The transverse magnetic electromagnetic bound mode in a black phosphorene inserted multilayer dielectric structure

The electromagnetic wave of the bound mode along the black phosphorene (BP) interface and the field distribution in a BP inserted planar multilayer dielectric structure have been investigated. The dispersion results indicate that the transverse magnetic surface plasmon wave is a bound mode and anisotropic with different electric field polarization directions. The effective refractive index increases as a function of the incident optical energy. It can also be tuned by the chemical potential and the thickness of the middle dielectric layer. The variable coupling strength between the light and matter based on the BP intraband optical conductivity makes the influence of the thickness of the middle dielectric layer significant or negligible in the lower and higher frequency regimes, respectively. The presented results can be applied to design BP-based promising optoelectronic devices.

Achieving high aspect ratio in plasmonic lithography for practical applications with sub-20 nm half pitch

Plasmonic lithography, which exploits a bowtie nanoaperture (BNA) for the purpose of subwavelength near-field focusing, has the capability of high-resolution patterning. However, the ultra-small feature size is achieved at the price of sharply decay of the surface plasmon waves (SPWs) in the photoresist (PR) layer, which directly leads to some unfavorable patterning issues, such as non-uniformity and shallow pattern depth even over small exposure areas. In this work, a special hybrid plasmonic waveguide (HPW) patterning system, which is composed of the plasmonic BNA-PR layer-silver reflector, is designed to facilitate high spatial frequency selection and amplify the evanescent field in the PR layer. Theoretical calculations indicate that the antisymmetric coupled SPWs and plasmonic waveguide modes excited by the HPW structure can remove the exponential decay and ensure uniform exposure over the entire depth of the PR layer. Importantly, the hyperbolic decaying characteristic of the SPWs in the PR layer plays a noticeable role in the improvement of achievable resolution, depth-of-field, and line array pattern profile. It is worth to note that the uniform periodic patterns in sub-20 nm feature can be achieved with high aspect ratio. Additionally, further numerical simulation results are presented to demonstrate the achievement of spatial frequency selection of high-k mode in HPW structure by controlling the PR thickness and gap size. Our findings may provide a new perspective on the manufacture of surface nanostructures and broaden the potential promising applications of plasmonic lithography in nanoscale patterning.

Microwave Spoof Surface Plasmon Sensor for Dielectric Material Characterization

A microwave planar resonator sensor design supporting spoof surface plasmon waves is used for wafer dielectric characterization. The proposed circuit guides the microwave as a surface wave across the metal signal line, from port to port of the device. The surface wave effectively interacts with the sample being measured, placed on top of the planar sensor. The proposed planar sensor is fabricated on an RF-35 substrate, resulting in a low-cost implementation. The sensor is used to discriminate between wafers with different dielectric permittivity and thickness. Quartz, borosilicate, BK7 glass, and Silicon wafers have been characterized using the proposed sensor. The sensor resonant frequency and losses change according to the permittivity and the thickness of the samples, with an achieved sensor sensitivity of 60.22 MHz/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ϵ_r</i> . Simulation and measurements demonstrate that the proposed spoof surface plasmon resonator sensor can be used to create a high sensitivity sensor for dielectric material characterization. The dielectric constant and loss tangent of the samples are inferred from a polynomial fit obtained from sample simulations and confirmed through measurements.

Tunable plasmonic band stop filter based in graphene nanoribbon

In this work we suggest and analyze a simple and compact structure that can work as a graphene-based plasmonic band stop filter for the THz region. The device is formed by graphene nanoribbons, where the propagation and blocking of surface polariton plasmon waves occurs, which fully return to the device’s input port. The graphene nanoribbons are deposited on a two-layer dielectric substrate formed by SiO2 (silica) and Si (silicon). The band stop filter can be dynamically controlled by applying an electrostatic field that changes the chemical potential of graphene, changing the filter’s operating range. Numerical simulations show that the insertion losses reach levels around -55 dB with a bandwidth of 7.3 THz, showing that the device has good performance for the frequency range used in our work and can be used for future applications.

Microchannel-Embedded D-Shaped Photonic Crystal Fiber-Based Highly Sensitive Plasmonic Biosensor

An improved design of a D-shaped photonic crystal fiber (PCF)-based refractive index (RI) sensor is proposed that uses the surface plasmon resonance phenomenon. The sensor consists of a large semicircular open channel that is inserted at the upside of the D-shaped PCF. A thin plasmonic sensing layer is deposited on the interior surface of the channel to excite the surface plasmon wave that eliminates the requirement of additional effort to fabricate a well-polished sensing layer of the D-shaped sensor. The sensor’s optical properties are numerically explored by the finite element method. The sensor is optimized to detect the RI of biological and biochemical analytes in the range of 1.33 to 1.44, shows spectral sensitivity as high as 63,000 nm/RIU with a spectral resolution of 1.59 × 10−06 RIU, and maximum amplitude sensitivity of 1439 RIU−1 with a resolution of 6.94 × 10−06 RIU. It is also found that the sensor’s linearity parameter is very high with a large figure of merit of about 839. Additionally, the sensor’s fabrication tolerance is studied by varying its structural parameters. Therefore, high sensing parameters with a wide detection range make this microchannel-based D-shaped PCF sensor an appropriate device for the application of biological and biochemical analyte detection.

Design of hybrid Au grating/TiO2 structure for NIR enhanced photo-electrochemical water splitting

Titanium oxide is commonly considered as an effective catalyst for photo-electrochemical water splitting due to its low cost, high activity, and perfect stability. However, a wide bandgap of TiO2 prevents the utilization of visible and near-infrared photons in the photo-electrochemical process. Photosensitization of TiO2 with plasmon active nanostructures was proposed as a way to solve this problem but the optimal design of coupled TiO2-plasmonic nanostructures, as well as the exact mechanism of plasmon triggering, are still under debate. In this work, we propose a plasmon-based TiO2 photosensitization, using the gold grating, covered by homogenous TiO2 layer. The gold grating supports the excitation and propagation of surface plasmon polariton wave (SPP), which is responsible for TiO2 triggering (unlike common localized plasmon resonance commonly used for TiO2 activation). For the optimization of the present structure design, we studied the impact of TiO2 layer thickness, illumination regime, and spatial distribution of plasmonic evanescent wave energy with regards to the mechanism of TiO2 activation. Obtained results proved that the proposed structure can be used for efficient photo-electrochemical water splitting and hydrogen production under irradiation with NIR photons of 700–1000 nm wavelengths. It was also verified that for a certain TiO2 layer thickness the activation of catalytic activity is mostly due to the plasmon electric field, which is concentrated at the electrolyte/catalyst surface.

Airy-Talbot plasmon: an accelerating self-imaging surface wave.

In this Letter, a new class of an accelerating self-imaging surface plasmonic wave, the Airy-Talbot plasmon, is introduced for the first time, to the best of our knowledge. Our research shows that such a surface wave propagates at the interface between metal (silver) and a dielectric material (air) and causes a strong interference along curved trajectories, which generates the Talbot effect in the surface. The propagation properties have potential value in nanoscale plasmonic devices. A scheme for generating this novel plasmon theoretically is proposed, and we prove it by finite difference time-domain numerical simulations.

Plasmon Wave Propagation Property of Metal Wedge Plasmonic Waveguides Covered by a Protective Oxide Layer

Guiding plasmon waves is based on dielectric/metal interfaces. The wedge-shaped interface shows an excellent capacity in the tight lightwave confinement at deep-subwavelength propagation mode size. Several types of metals have also been investigated for guiding plasmon waves. Among them, the Ag metal shows a plasmon wave guiding ability superior to other metals, however, it is sensitive to the operating medium and is easily oxidized. To overcome these drawbacks, the Ag wedge covered by a protective thin oxide layer is proposed. Numerically investigated results show that the propagation length of the Ag wedge covered by a protective thin silicon dioxide layer can be enhanced by a factor of 7.5 while its figure of merit is at least 1.7 times larger than that of the Au wedge waveguide. The advantage of the proposed interface is potential for developing plasmonic waveguide components.

High-Efficiency Plasmonic Lens Based on Archimedes-Spiral with Cross Section of an Asymmetric Slot

A high-efficiency plasmonic lens composed of a single Archimedes-spiral slot with a cross section of an asymmetric slot is proposed. By adding an auxiliary nanocavity under the primary spiral slot, unidirectional plasmonic waves can be efficiently transmitted in the inward direction and focused on a hot spot in the center. Due to the asymmetric slot, the finite-difference time-domain (FDTD) method is used to numerically optimize the geometric parameters of the single spiral slot, which can achieve high-intensity unidirectional inward focusing. The proposed structure can decrease background noises and prevent cross-talk of nearby components in optical networks, which significantly improves the integration level of nanophotonic circuits and devices.

High-resolution surface plasmon resonance holographic microscopy based on symmetrical excitation

Surface plasmon resonance holographic microscopy (SPRHM) is able to simultaneously obtain the amplitude- and phase-contrast surface plasmon resonance (SPR) images, showing great potentials in imaging near-field targets with high sensitivity. However, suffered by the decaying length of surface plasmon wave which can be as long as tens of microns, the spatial resolution of SPRHM is lower than that of traditional holographic microscopy. In this work, we propose to enhance the spatial resolution in SPRHM by exciting surface plasmon resonance in two symmetrical directions and detecting the complex amplitudes of the reflected light symmetrically. Through the Fourier analysis of the recorded composite hologram, the reconstruction schemes for high-resolution amplitude- and phase-contrast SPR images are established, respectively. The feasibility and advantages of the proposed method is verified by numerical simulations and experimental demonstrations of small-size particles and micro-structures.

Distributed MEMS Sensors Using Plasmonic Antenna Array Embedded Sagnac Interferometer

A micro Sagnac interferometer is proposed for electron cloud distributed sensors formed by an integrated (micro-electro-mechanical systems) MEMS resonator structure. The Sagnac interferometer consists of four microring probes integrated into a Sagnac loop. Each of the microring probes is embedded with the silver bars to form the plasmonic wave oscillation. The polarized light of 1.50 µm wavelength is input into the interferometer, which is polarized randomly into upstream and downstream directions. The polarization outputs can be controlled by the space–time input at the Sagnac port. Electrons are trapped and oscillated by the whispering gallery modes (WGMs), where the plasmonic antennas are established and applied for wireless fidelity (WiFi) and light fidelity (LiFi) sensing probes, respectively. Four antenna gains are 2.59 dB, 0.93 dB, 1.75 dB, and 1.16 dB, respectively. In manipulation, the sensing probe electron densities are changed by input source power variation. When the electron cloud is excited by the microscopic medium, the change in electron density is obtained and reflected to the required parameters. Such a system is a novel device that can be applied for brain-device interfering with the dual-mode sensing probes. The obtained WGM sensors are 1.35 µm−2, 0.90 µm−2, 0.97 µm−2, and 0.81 µm−2, respectively. The WGMs behave as a four-point probe for the electron cloud distributed sensors, where the electron cloud sensitivities of 2.31 prads−1mm3 (electrons)−1, 2.27 prads−1mm3 (electrons)−1, 2.22 prads−1mm3(electrons)−1, and 2.38 prads−1mm3(electrons)−1 are obtained, respectively.

Operando monitoring of ion activities in aqueous batteries with plasmonic fiber-optic sensors

Understanding ion transport kinetics and electrolyte-electrode interactions at electrode surfaces of batteries in operation is essential to determine their performance and state of health. However, it remains a challenging task to capture in real time the details of surface-localized and rapid ion transport at the microscale. To address this, a promising approach based on an optical fiber plasmonic sensor capable of being inserted near the electrode surface of a working battery to monitor its electrochemical kinetics without disturbing its operation is demonstrated using aqueous Zn-ion batteries as an example. The miniature and chemically inert sensor detects perturbations of surface plasmon waves propagating on its surface to rapidly screen localized electrochemical events on a sub-μm-scale thickness adjacent to the electrode interface. A stable and reproducible correlation between the real-time ion insertions over charge-discharge cycles and the optical plasmon response has been observed and quantified. This new operando measurement tool will provide crucial additional capabilities to battery monitoring methods and help guide the design of better batteries with improved electro-chemistries.