Long-range Surface Plasmon Polariton Research Articles

Double-layer graphene-based wedge and groove SPP waveguides with ultra-long propagation length

This study investigates long-range surface plasmon polariton (LRSPP) waveguides based on graphene in wedge and groove configurations, comprised of two graphene layers embedded in silica wedges and grooves, respectively. The finite-difference time-domain method is employed for numerical simulations. By combining coupled-mode perturbation theory with numerical results, we demonstrate the excitation of both LRSPP modes and short-range SPP modes when two wedge-shaped or two groove-shaped graphene layers are coupled. Our findings emphasize the significant advantage of the LRSPP fundamental mode, simultaneously achieving an ultra-long propagation length (∼10 μm) and an ultra-deep subwavelength confinement (∼10−7–10−6 times the diffraction-limited mode area). This novel waveguide emerges as a promising candidate for guiding mid-infrared electromagnetic waves in future photonic integrated circuits.

Amplification and excitation of surface plasmon polaritons via four-wave mixing process

We suggest a scheme for the excitation and amplification of surface plasmon polaritons (SPPs) along the interface between metal and semiconductor quantum well (SQW), employing a four-wave mixing (FWM) process. The SQW consists of four-level asymmetric double quantum wells that exhibit quantum interference effects, which leads to the coupler-free excitation of SPPs. In our proposed system, the inherent losses of SPPs are compensated by introducing gain through the FWM process, resulting in a significant enhancement in the propagation length and penetration depth of SPPs. We further analyze the effect of gain on the long-range and short-range SPPs and observe that the propagation distance and lifetime of both types of SPPs are increased.

Reflection and scattering of long-range surface plasmon-polaritons on boundary of free half-space

We study the long-range surface plasmon-polariton (LRSPP) propagating along a thin metal film embedded in a homogeneous semi-infinite medium. The reflection and scattering of the LRSPP by the end face of the film at the boundary of free half-space are discussed. We apply the self-consistent electromagnetic approach using the expansion of the plasmon and scattered fields by the eigenfunctions of the structure. The Fredholm integral equation of the first kind appearing in the model has been solved with using the variational Ritz-Galerkin procedure. It is shown that the LRSPP reflection coefficient can be approximated by the Fresnel formula of reflection for normal incidence of a plane TEM wave. The power of the back-scattered radiation is predicted to be a considerable part (tens of percents) of the reflected LRSPP power. We discuss the far-field patterns of both the back- and forward-scattered radiation. It is shown that the back-scattered radiation form two narrow lobes directed symmetrically with regard to the metal film plane. The radiation scattered in the forward direction is presented by the one-lobe far-field pattern; decreasing the metal layer thickness results in the narrower lobe of the forward-scattered radiation.

Excitation and modulation of long-range surface plasmons based on InSb multilayer structure

We proposed an Ag-based multilayer InSb and SiO2 system in the terahertz region. Based on the surface plasmon polariton (SPP) interaction among each interface, the spontaneous emission of a dipole in multilayer systems supporting SPP and waveguide modes is theoretically studied. It found that the coupling efficiency of long-range SPP (LRSPP) is about 78% with a propagation length of up to 28.25 mm. The thermally tunable characteristics of InSb enable LRSPP devices to be realized with adjustable coupling efficiency. Our work could be applied as a future guide for integrated plasmonic device design.

Black phosphorus mediated long-range SPR imaging sensor with ultrahigh imaging sensitivity, penetration depth and figure of merit

In this paper, we proposed a black phosphorus (BP) mediated long-range surface plasmon resonance (LRSPR) imaging sensor (Structure: 2S2G prism/Teflon/Ag/BP/Sensing medium) for refractive index (RI) sensing. MATLAB and COMSOL simulation software is used for analytical analysis of the proposed design at 633 nm characteristic wavelength. Use of Teflon dielectric buffer layer (DBL) and thin Ag film generates long-range surface plasmon polaritons (LRSPPs) at both interface of Ag film. BP layer is used as biorecognition element (BRE) layer for attachment of analytes due to its unique electrical, optical and electronic properties useful for sensing applications. The proposed sensor demonstrates maximum imaging sensitivity (29,971 RIU−1), detection accuracy (111 Deg.−1), angular figure of merit, FoMang. (2568 RIU−1), imaging figure of merit, FoMimg (810,027 RIU−1) and penetration depth, PD (1957 nm). Ultrahigh imaging sensitivity, FoMimg and micron size PD of the proposed sensor can be efficiently utilized for the deep analytes sensing even in smaller concentrations. Thus, BP mediated proposed RI sensor could provide potential application for micron size biomolecule sensing.

The Design of a Glycerol Concentration Sensor Based on an LRSPP Hybrid Photonic Biosensor.

A refractive index sensor based on an on-chip silicon nitride (Si3N4) ridge waveguide long-range surface plasmon polariton (LRSPP) is theoretically designed. The waveguide sensor consists of a gold film to enable the plasmonic resonance on top of a Cytop polymer layer. A proper finite element method was used to design and optimize the geometric parameters at the optical wavelength of 633 nm. In addition, the spectral performance was evaluated using the transfer matrix method from 580 to 680 nm. The redshifted interference spectrum results from an increasing analyte refractive index. The sensitivities of 6313 dB/cm/RIU and 251.82 nm/RIU can be obtained with a 400 nm wide and 25 nm thick Au layer. The proposed sensor has the potential for point-of-care applications considering its compactness and simplicity of construction.

Water-Immersion Laser-Scanning Annealing for Improving Polycrystalline Au Films.

A water-immersion laser-scanning annealing (WILSA) method was developed for the heat treatment of a deposited polycrystalline Au film on a glass. The material characterization using X-ray diffraction, field-emission scanning electron microscopy, and electron backscatter diffraction shows improved crystallinity with a more uniform crystallographic orientation of (111) and the grain growth of the annealed Au film. Additionally, the optical constants of the Au film before and after annealing were characterized by spectroscopic ellipsometry in the visible to near-infrared (NIR) regime, and the corresponding optical densities (ODs) were measured by transmittance spectroscopy. Our results show that the extinction coefficient and the OD of the annealed film are significantly reduced, particularly in the NIR regime. This is because the grain growth caused by the annealing reduces the density of grain boundaries, leading to the decrease of the loss of free electrons' scattering at grain boundaries. Hence, the damping effect of the surface plasmon is reduced. Additionally, the integrity of the WILSA-treated thin film is kept intact without pinholes, usually produced by the conventional thermal annealing. Based on the improved optical property of the WILSA-treated Au film, two performances of an insulator-metal-insulator (IMI) layered structure of biosensors are theoretically analyzed. Numerical results show that the propagation length of a long-range surface plasmon polariton along an IMI structure with an annealed Au film is significantly increased, compared to an unannealed film, particular in the NIR region. For the other application of using an IMI sensor to detect the shift of the surface-plasmon-resonance dip in the total internal reflection spectrum for the measurement of a change of the medium's refractive index, the sensitivity is also profoundly improved by the WILSA method. It is worth mentioning that the optimal heating conditions (laser wavelength, fluence, exposure time, and scanning step) depend on the thickness of the Au film. Our study provides a postprocess of WILSA to improve the optical properties of a deposited polycrystalline Au film for raising the sensitivity of the related biosensors.

Excitation of "forbidden" guided-wave plasmon polariton modes via direct reflectance using a low refractive index polymer coupling layer.

A surface plasmon polariton (SPP) is an excitation resulting from the coupling of light to a surface charge oscillation at a metal-dielectric interface. The excitation and detection of SPPs is foundational to the operating mechanism of a number of important technologies, most of which require SPP excitation via direct reflectance, commonly achieved via Attenuated Total Reflection (ATR) using the Kretschmann configuration. As a result, the accessible modes are fundamentally high-loss “leaky modes,” presenting a critical performance barrier. Recently, our group provided the first demonstration of “forbidden,” or guided-wave plasmon polariton modes (GW-PPMs), collective modes of a MIM structure with oscillatory electric field amplitude in the central insulator layer with up to an order of magnitude larger propagation lengths than those of traditional SPPs. However, in that work, GW-PPMs were accessed by indirect reflectance using Otto configuration ATR, making them of limited applied relevance. In this paper, we demonstrate a technique for direct reflectance excitation and detection of GW-PPMs. Specifically, we replace the air gap used in traditional Otto ATR with a low refractive index polymer coupling layer, mirroring a technique previously demonstrated to access Long-Range Surface Plasmon Polariton modes. We fit experimental ATR data using a robust theoretical model to confirm the character of the modes, as well as to explore the potential of this approach to enable advantageous propagation lengths. The ability to excite GW-PPMs using a device configuration that does not require an air gap could potentially enable transformative performance enhancements in a number of critical technologies.

Rigorous Analysis and Systematical Design of Double-Layer Metal Superlens for Improved Subwavelength Imaging Mediated by Surface Plasmon Polaritons.

A double-layer metal superlens was rigorously analyzed and systematically designed to improve subwavelength imaging ability. It was revealed that transmission properties of the imaging system could be accurately interpreted by the five-layer waveguide mode theory—each amplification peak among the spatial frequency range of evanescent waves was associated with a corresponding surface plasmon polariton (SPP) mode of an insulator-metal-insulator-metal-insulator (IMIMI) structure. On the basis of such physical insight, evanescent waves of higher spatial frequency were effectively amplified via increasing propagation constants of symmetrically coupled short-range SPP (s-SRSPP) and antisymmetrically coupled short-range SPP (a-SRSPP), and evanescent waves of lower spatial frequency were appropriately diminished by approaching to cut off symmetrically coupled long-range SPP (s-LRSPP). A flat and broad optical transfer function of the imaging system was then achieved, and improved subwavelength imaging performance was validated by imaging an ideal thin object of two slits with a 20-nm width distanced by a 20-nm spacer, under 193-nm illumination. The resolution limit of the designed imaging system with double-layer superlens was further demonstrated to be at least ~λ/16 for an isolated two-slit object model. This work provided sound theoretical analysis and a systematic design approach of double-layer metal superlens for near-field subwavelength imaging, such as fluorescent micro/nanoscopy or plasmonic nanolithography.

Topological edge states of a long-range surface plasmon polariton at the telecommunication wavelength.

Confining light by plasmonic waveguides is promising for miniaturizing optical components, while topological photonics has been explored for robust light localization. Here we propose combining the two approaches into a simple periodically perforated plasmonic waveguide (PPW) design exhibiting robust localization of long-range surface plasmon polaritons. We predict the existence of a topological edge state originating from a quantized topological invariant, and numerically demonstrate the viability of its excitation at telecommunication wavelength using near-field and waveguide-based approaches. Strong modification of the radiative lifetime of dipole emitters by the edge state, and its robustness to disorder, are demonstrated.

Tunable Narrow-Band Filter Based on Long-Range Surface Plasmon Polariton Waveguide Bragg Grating

A narrow-band Bragg grating filter based on a long-range surface plasmon polariton (LRSPP) waveguide is theoretically demonstrated. The three-dimensional Au stripe that is embedded in polymer SU-8 acts as both the waveguide and the heating electrode. With the eigen mode expansion and finite element method optimizations, the proposed filter shows a reflectivity of 0.578 and a 3 dB bandwidth of 1.1 nm. The central wavelength can be tuned from 1549.9 nm to 1544.3 nm by varying temperature from 25 °C to 75 °C, while maintaining the optical return loss at −2.5 dB. This proposed tunable filter has potential in on-chip light signal processing.

Excitation of Surface Plasmon Polariton Modes with Double-Layer Gratings of Graphene.

A long-range surface plasmon polariton (SPP) waveguide, composed of double-layer graphene, can be pivotal in transferring and handling mid-infrared electromagnetic waves. However, one of the key challenges for this type of waveguide is how to excite the SPP modes through an incident light beam. In this study, our proposed design of a novel grating, consisting of a graphene-based cylindrical long-range SPP waveguide array, successfully addresses this issue using finite-difference time-domain simulations. The results show that two types of symmetric coupling modes (SCMs) are excited through a normal incident light. The transmission characteristics of the two SCMs can be manipulated by changing the interaction of the double-layer gratings of graphene as well as by varying various parameters of the device. Similarly, four SCMs can be excited and controlled by an oblique incident light because this light source is equivalent to two orthogonal beams of light. Furthermore, this grating can be utilized in the fabrication of mid-infrared optical devices, such as filters and refractive index sensors. This grating, with double-layer graphene arrays, has the potential to excite and manipulate the mid-infrared electromagnetic waves in future photonic integrated circuits.

Fano Resonance Hybrid Waveguide-Coupled Plasmonic Sensor Using Transparent Conductive Oxide in the Near-Infrared Range

We proposed an ultra-sensitive refractive index sensor by using indium-doped cadmium oxide as a plasmonic material operating in near-infrared based on Fano resonance. The proposed sensor has a hybrid multilayer waveguide structure that supports both a long-range surface plasmon polariton (LRSPP) mode and a dielectric waveguide (DWG) mode. The design strategy of the structure parameters of the inner layers is elaborated in detail through the numerical analysis of the two modes. By suitably tailoring the thickness of the coupling layer, a strong mode coupling between the two modes could be achieved, leading to a sharp asymmetric Fano resonance. With the designed optimal physical parameters, our proposed sensor could achieve a maximum intensity sensitivity of 19,909 RIU−1, a 193-fold enhancement than that of a conventional long-range SPR (LRSPR) based scheme. The proposed design can be a promising platform for biochemical sensing in the near-infrared region.

Directional coupling with parity-time symmetric Bragg gratings.

Parity-time symmetric Bragg gratings produce unidirectional reflection around the exceptional point. We propose and explore directional coupling of gain and loss modulated waveguide Bragg gratings operating at around 880 nm with long-range surface plasmon polaritons. Step-in-width modulation of a Ag stripe supporting long-range plasmons combined with a periodic modulation of the cladding were used to balance the real and imaginary index perturbation of the gratings. IR140 dye molecules in solvent forms a portion of the uppercladding, providing gain under optical pumping. We investigate directional coupling between a pair of parity-time symmetric waveguide Bragg gratings operating near their exceptional point, arranged in various configurations - duplicate, duplicate-shifted and duplicate-flipped. We also investigate coupling to a bus waveguide and to a conventional waveguide Bragg grating. Unidirectional multi-wavelength reflection and coupled supermode conversion are predicted.

Integration of on-chip perovskite nanocrystal laser and long-range surface plasmon polariton waveguide with etching-free process.

Perovskite materials prepared in the form of solution-processed nanocrystals and used in top-down fabrication techniques are very attractive to develop low-cost and high-quality integrated optoelectronic circuits. Particularly, integrated miniaturized coherent light sources that can be connected to light-guiding structures on a chip are highly desired. To control light propagating on a small footprint with low-loss optical modes, long-range surface plasmon polariton (LRSPP) waveguides are employed. Herein, we demonstrate an on-chip fabricated photonic-plasmonic hybrid system consisting of a perovskite lasing structure coupled to an LRSPP waveguide achieving a low lasing threshold and a propagation length over 100 μm. Preventing perovskite material degradation and the formation of surface roughness of the laser cavity during fabrication is made possible by designing a fabrication technique without any etching step.

Antimonene mediated long-range SPR imaging sensor with ultrahigh imaging sensitivity and figure of merit

We proposed a long-range surface plasmon (LR-SPR) resonance imaging sensor based on antimonene and magnesium fluoride (MgF2) polymer with ultra-high imaging sensitivity (Simg) and figure of merit (FOM). The effect of dielectric buffer layers (DBL) (such as Cytop, Teflon, MgF2, and LiF) and plasmonic metals (Au, Ag, and Al) on the stimulating long-range surface plasmon polaritons (LR-SPPs) is explored in this work. We numerically demonstrated that the proposed LR-SPR imaging sensor configuration (antimonene-aluminum-MgF2 on 2S2G chalcogenide glass prism) could achieve much higher imaging sensitivity (7028.1 RIU−1) and FOM (335 RIU−1) than the reported papers by taking advantage of antimonene's sensing ability and aluminum's steeper reflectance curve. As a result, the proposed LR-SPR imaging sensor in the field of biosensing is a promising tool for detecting biomolecules. Finally, a fair comparison between the most recently published research publications on prism coupled LR-SPR sensor and the proposed work is made.

Fabrication of Bloch Long Range Surface Plasmon Waveguides Integrating Counter Electrodes and Microfluidic Channels for Multimodal Biosensing

We report the fabrication of a novel multimodal biosensor combining plasmonic and electrochemical detection. The plasmonic sensors are based on monitoring the propagation of Bloch long-range surface plasmon polaritons (LRSPPs) along thin narrow Au stripes, integrating grating couplers as input/output means. The electrochemical sensors use the same Au stripes as working electrodes with nearby Pt stripes integrated on-chip as counter electrodes. The structures are fabricated on a truncated 1D photonic crystal comprised of a 15-period stack of alternating layers of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> . The Au and Pt stripes are fabricated using bilayer lift-off photolithography, and the gratings are fabricated using e-beam lithography. The structures are arranged into arrays targeting multichannel biosensing. The wafer is covered with CYTOP as the upper cladding with etched microfluidic channels providing access to the sensing surfaces and is wafer-bonded to a Borofloat silica wafer to encapsulate the fluidic channels and enable edge (in-plane) fluidic interfacing. The wavelength response of the grating-coupled plasmonic waveguide sensors is presented along with surface sensing results. Cyclic voltammetry measurements using the Au and Pt stripes as the working and counter electrodes are also presented. [2021-0103]

Suppressed Transmission of Long-Range Surface Plasmon Polariton by TE-Induced Edge Plasmon.

Work on controlling the propagation of surface plasmon polaritons (SPPs) through the use of external stimuli has attracted much attention due to the potential use of SPPs in nanoplasmonic integrated circuits. We report that the excitation of edge plasmon by TE-polarized light passing across gapped-SPP waveguides (G-SPPWs) leads to the suppressed transmission of long-range SPPs (LRSPPs) propagating along G-SPPWs. The induced current density by highly confined edge plasmon is numerically investigated to characterize the extended radiation length of decoupled LRSPPs by the TE-induced edge plasmon. The suppressed transmission of LRSPPs is confirmed using the measured extinction ratio of the plasmonic signals which are generated from the modulated optical signals, when compared to the extended radiation length calculated for a wide range of the input power. It is also shown that LRSPP transmission is sensitive to the excited power of edge plasmon in the gap through the permittivity change near the gap. Such a control of SPPs through the use of light could be boosted by the hybridized edge plasmon mode and a huge field enhancement using nanogap, gratings or metasurfaces, and could provide opportunities for ultrafast nano-plasmonic signal generation that is compatible with pervasive optical communication systems.

Compact resonant plasmonic hybrid waveguide refractive index sensors for aqueous medium

Ultra-compact and high sensitivity resonant plasmonic refractive index sensors for aqueous medium are proposed. The sensors are based on the resonant coupling between the waveguide mode and the surface plasmon mode supported by a thin metal layer. For optimized layer thicknesses, the periodic coupling between the two hybrid modes guided by the structure takes place and a resonant absorption peak appears for a particular sensor length. The sensors are designed to operate in the spectral mode at visible wavelengths centred about 0.6328 μm. The sensors have been designed for the long range surface plasmon polaritons (LRSPP) as well as the short range surface plasmon polaritons (SRSPP). This study provides a detailed understanding of designing an ultra-short and efficient sensor for the aqueous medium with high spectral sensitivity (S), ultrahigh resolution (R) and figure of merit (FOM). The attained numerical values show that for optimized layer thicknesses, a sensitivity of 9200 nm/RIU for LRSPP mode and 1800 nm/RIU for SRSPP mode can be achieved for the aqueous medium with the sensor lengths of 83.5 μm and 15 μm (shortest to the best of our knowledge) respectively. Also, the values of R and FOM are 1.52 × 10−7 RIU and 7.7 × 103 RIU−1 for the LRSPP based sensor and 6.7 × 10−7 RIU and 4.5 × 103 RIU−1 for the SRSPP mode based sensor.

Continuous quantum walk in a 1-dimensional plasmonic lattice structure based on metal strip waveguides.

We experimentally studied a continuous time evolution of a "plasmonic" walker in a 1-dimensional lattice structure based on long-range surface plasmon polariton waveguides. The plasmonic walker exhibited a typical time evolution of a 1-dimensional quantum walk, which indicates that the plasmonic system is a potential platform to construct quantum walk simulators. By comparing experimental results to numerical simulations, the fidelity of the plasmonic quantum walk simulator is estimated to be > 0.96, which demonstrates that the plasmonic system can be a feasible platform for large-scale and high dimensional quantum walk simulators.