Long-range Surface Plasmon Research Articles

Mid-Infrared Sensor Based on Dirac Semimetal Coupling Structure.

A multilayer structure based on Dirac semimetals is investigated, where long-range surface plasmon resonance (LRSPR) of a dielectric layer/Dirac semimetal/dielectric layer are coupled with surface plasmon polaritons (SPPs) on graphene to substantially improve the Goos–Hänchen (GH) shift of Dirac semimetals in the mid-infrared band. This has important implications for the study of mid-infrared sensors. We studied the reflection coefficient and phase of this multilayer structure using a generalized transport matrix. We established that subtle changes in the refractive index of the sensing medium and the Fermi energy of the Dirac semimetal significantly affected the GH shift. Our numerical simulations show that the sensitivity of the coupling structure is more than , which can be used as a potential new sensor application. The novelty of this work is the design of a tunable, highly sensitive, and simple structured mid-infrared sensor that takes advantage of the excellent properties of Dirac semimetals.

A Ti3C2-MXene-functionalized LRSPR biosensor based on sandwich amplification for human IgG detection.

Long-range surface plasmon resonance (LRSPR) has demonstrated excellent performance in sensing and detection, due to its higher accuracy and sensitivity compared with conventional surface plasmon resonance (cSPR). In this work, we establish an LRSPR biosensor which employs PDA/Ti3C2-MXene/PDA-gold film as a sensing substrate and gold nanoparticles (AuNPs) as enhancers. Ti3C2-MXene is an emerging two-dimensional (2D) layered material which is used extensively in immunoassay and biosensing. The sensing substrate comprises two polydopamine (PDA) films between which is sandwiched a Ti3C2-MXene film based on a gold film, which provides a large surface area and abundant binding sites to rabbit anti-human IgG (Ab1). Sandwich amplification is adopted to enhance the sensitivity of the LRSPR biosensor, and AuNPs/staphylococcal protein A (SPA)/mouse anti-human IgG (Ab2) composites are introduced into the flow cell as enhancers after the immune binding of human IgG to Ab1. The antigen (human IgG) detection range is 0.075μgmL-1 to 40μgmL-1, and the limit of detection is almost 20 times lower than that for cSPR biosensors. This novel LRSPR biosensor demonstrates excellent performance in immune sensing over a broad detection range and a low limit of detection. Subsequent modification of the LRSPR sensing platform could be made for extensive application in various biological detection fields.

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.

High-Sensitivity Biosensor With Optical Tunneling Effect Excited by Long-Range Surface Plasmon Resonance

Optical tunneling is an effect extremely sensitive to wavelength changes. In this article, we use this effect to design a new type of biosensor. The tunneling effect is excited by long range surface plasmon resonance, the energy is concentrated in the sensing medium in the form of standing waves, which lead a very high wavelength sensitivity and quality factor. By using COMSOL for finite element analysis, for the sensor structure of (BK7/Cytop /Al2O3/Ag/Al2O3/sensing media/air), a quality factor of 10296 RIU-1 is achieved under angle modulation,a sensitivity of 170,000 nm/ RIUand a quality factor of 11333 RIU-1 are obtainedunder wavelength modulation. At the same time, the advantage of this structure is that there is no excessive requirement on the refractive index of the material, and a good sensing effect can be achieved even if the material is cheap and easy to prepare. After optimization, this structure is suitable for sensing media with a refractive index of 1.335-1.375, which can be applied to most liquid solutions.

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.

Smartphone integrated handheld Long Range Surface Plasmon Resonance based fiber-optic biosensor with tunable SiO2 sensing matrix

In the present work, a novel smartphone assisted fiber optic (FO)-Long range surface plasmon resonance (LRSPR) based biosensor is proposed. In the developed biosensor, the inbuilt color sensitive property of the digital camera present in the smartphone is used for the monitoring of blue and red color channel intensities. This will replace the most exploited diffraction gratings or narrow band filters used for analyzing the spectral data in reported smartphone based SPR sensors. The proposed technique helps in improving the sensitivity and reduces the chances of wrong detection. For the first time, SiO2 nanostructured film is employed as the dielectric sensing layer to excite the Long range surface plasmons (LRSPs) in the dielectric-metal-dielectric configuration. The proposed FO-LRSPR biosensor possess limit of detection of 0.02 mM and sensitivity of 0.9/mM and, for uric acid detection in the 0.1 mM–1 mM concentration range. The novel fabricated sensor which is found to be stable up to 24 weeks can be effectively utilized in health sector and environment monitoring and it possess the ability of point-of-care detection, even in rural and remote areas.

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.

Long-range surface plasmon resonance sensor based on side-polished D-shaped hexagonal structure photonic crystal fiber with the buffer layer of magnesium fluoride

In this paper, we demonstrate a long-range surface plasmon resonance (LRSPR) sensor based on D-shaped hexagonal structure photonic crystal fiber (PCF) with buffer layer of magnesium fluoride (MgF2). The theoretical analysis is first introduced to reveal surface plasmon polariton (SPP) mode excitation by the y-polarized core-guided mode. The influences exerted by different thicknesses of Au film and MgF2 buffer layer are theoretically investigated with the full vectorial finite element method for optimizing structure parameters. A novel status of electric field boundary (SEFB) technique is proposed to analyze the reciprocal sensing performance of symmetric LRSPR (sLRSPR) sensor. Theoretical simulation reveals that the LRSPR with PCF/MgF2 (100 nm)/Au (50 nm) architecture exhibits the significant performance and has an intensity sensitivity improvement of 160.13% compared with conventional SPR (cSPR) sensor. With the simulation providing the theoretical basis, a proof-of-concept experiment is performed with the D-shaped PCF fabricated by a wheel polishing system. The experimental results show good agreement with the theoretical calculations, and both of them consistently indicate that the LRSPR-based sensor has an improved intensity sensitivity (−490.625% RIU−1) and an increased penetration depth. Furthermore, focusing on the sensing principle, the proposed SEFB method can be applied in the analysis of other types of multilayer SPR sensor. The optimal LRSPR sensor with improved detection accuracy shows great promise for applications in the field of biochemical measurement.

Sensitivity Enhanced Refractive Index Fiber Sensor Based on Long-Range Surface Plasmon Resonance in SiO2-Au-TiO2 Heterostructure

Long-range surface plasmon resonance (LRSPR), generated from a coupled plasmon polariton in a thin metal slab sandwiched by two dielectrics, has attracted more and more attention due to its merits, such as longer propagation and deeper penetration than conventional single-interface surface plasmon resonance. Many useful applications related to light–medium interaction have been demonstrated based on the LRSPR effect, especially in the sensing area. Here, we propose and demonstrate an LRSPR-based refractive index sensor by using a SiO2-Au-TiO2 heterostructure, in which a D-shaped honeycomb-microstructure optical fiber (MOF) is designed as the silica substrate and then deposited with a gold film and thin-layer titanium dioxide (TiO2). By using the full-vector finite-element method (FEM), this heterostructure is numerically investigated and demonstrated to excite LRSPR without a buffer layer, which is usually necessary in previous LRSPR devices. Through comprehensive discussion about the influence of structural parameters on the resonant wavelength, the excitation of the LRSPR in the proposed heterostructure is revealed to be highly related to the effective refractive index of MOF’s fundamental core mode, which is mainly determined by the MOF’s pitch, the thicknesses of the silica web and the planar-layer silica. Moreover, the thin-layer TiO2 plays an important role in significantly enhancing the resonance and the sensitivity to analyte’s refractive index as well, when it is coated on the top of the Au film rather than between the metal and waveguide. Finally, the proposed LRSPR sensor based on SiO2-Au-TiO2 heterostructure shows an ultra-high wavelength sensitivity of 20,100 nm/RIU and the corresponding minimum resolution is as low as 4.98×10−7 RIU. Thus, the proposed LRSPR device offers considerable potential for sensing applications in biomedical and biochemical areas.

Double-Antibody Sandwich Immunoassay and Plasmonic Coupling Synergistically Improved Long-Range SPR Biosensor with Low Detection Limit.

A long-range surface plasmonic resonance (LR-SPR) biosensor modified with double-antibody sandwich immunoassay and plasmonic coupling is demonstrated for human-immunoglobulin G detection with a low limit of detection (LOD). The double-antibody sandwich immunoassay dramatically changes the average refractive index of the medium layer on the sensor surface. The near-field electron coupling between the localized surface plasmon and the long-range surface plasmon leads to a significant perturbation of the evanescent field. The large penetration depth and the long propagation distance of the long-range surface plasmonic waves facilitate the LR-SPR sensor in the detection of biological macromolecules. The unique light absorption characteristic of the nanocomposite material in the sensor provides the in situ self-compensation for the disturbance. Therefore, besides the inherent advantages of optical fiber sensors, the developed biosensor can realize the detection of biomolecules with high sensitivity, low LOD and high accuracy and reliability. Experimental results demonstrate that the LOD of the biosensor is as low as 0.11 μg/mL in the detection of the phosphate-buffered saline sample, and the spike-and-repetition rate is 105.56% in the detection of the real serum sample, which partly shows the practicability of the biosensor. This indicates that the LR-SPR biosensor provides better response compared with existing similar sensors and can be regarded as a valuable method for biochemical analysis and disease detection.

Long-Range Surface Plasmon Resonance–Based Sensitivity Study on D-shaped Ag-MgF2-Coated Models with Analyte Variations

Long-Range Surface Plasmon Resonance–Based Sensitivity Study on D-shaped Ag-MgF2-Coated Models with Analyte Variations

Photonic spin Hall effect and terahertz gas sensor via InSb-supported long-range surface plasmon resonance

The photonic spin Hall effect (SHE), featured by a spin-dependent transverse shift of left- and right-handed circularly polarized light, holds great potential for applications in optical sensors, precise metrology and nanophotonic devices. In this paper, we present the significant enhancement of photonic SHE in the terahertz range by considering the InSb-supported long-range surface plasmon resonance (LRSPR) effect. The influences of the InSb/ENZ layer thickness and temperature on the photonic SHE were investigated. With the optimal structural parameters and temperature, the maximal spin shift of the horizontal polarization light can reach up to 2.68 mm. Moreover, the spin shift is very sensitive to the refractive index change of gas, and thus a terahertz gas sensing device with a superior intensity sensitivity of 2.5 × 105 μm/RIU is proposed. These findings provide an effective method to enhance the photonic SHE in the terahertz range and therefore offer the opportunity for developing the terahertz optical sensors based on photonic SHE.

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.

Near-infrared long-range surface plasmon resonance in a D-shaped honeycomb microstructured optical fiber coated with Au film.

Long-range surface plasmon resonances (LRSPRs) are featured with longer propagation and deeper penetration, compared with conventional surface plasmon resonances (SPRs). Thus, LRSPR-based fiber sensors are considered to have great potential for highly sensitive detection in chemistry or biomedicine areas. Here, we propose and demonstrate a near-infrared LRSPR sensor based on a D-shaped honeycomb microstructured optical fiber (MOF) directly coated with gold film. Although there is no additional heterogeneous buffer layer, the optical field of the long-range surface plasmon polariton (LRSPP) mode penetrates strongly into the analyte region. Thus the effective refractive index of the LRSPP mode depends highly on the analyte's material refractive index and an abnormal dispersion relationship between the LRSPP mode and MOF's y-polarized core mode is observed. The mechanism of the LRSPR excitation in the coupling zone is attributed to an avoided crossing effect between these two modes. It also results in the generation of a narrow-bandwidth peak in the loss spectrum of the core mode. Further discussion shows that the resonance wavelength is mainly determined by the core size that is contributed by the MOF's cladding pitch, silica-web thickness and planar-layer-silica thickness together. It indicates that the operation wavelength of the proposed LRSPR device can be flexibly tuned in a broadband wavelength range, even longer than 2 µm, through appropriately designing the MOF's structural parameters. Finally, the proposed LRSPR sensor shows the highest wavelength sensitivity of 14700 nm/RIU and highest figure of merit of 475 RIU-1 for the analyte refractive index range from 1.33 to 1.39.

Figure of merit enhancement of Ti3C2Tx-graphene based long-range surface plasmon sensor at telecommunication wavelength

In this work, the long-range surface plasmon resonance (LRSPR) sensor based on dielectric Ti3C2Tx-graphene layers is demonstrated. Here, MXene (Ti3C2Tx) is used as a metal for strong surface plasmon’s generation at 1550 nm excitation wavelength. Graphene is used to attach the biomolecules having carbon–carbon structure with pi stacking interactions. The detection accuracy (DA) and figure of merit (FoM) for the proposed LRSPR sensor is theoretically investigated at telecommunication wavelength. The highest FoM of (559.64 RIU−1) is obtained at 3100 nm dielectric thickness and 14 nm Ti3C2Tx thickness of the proposed sensor. The maximum improvement in FoM is 273% to the traditional LRSPR sensors. Propagation depths are also evaluated for Au-graphene and proposed LRSPR sensor at 14 and 27 nm thicknesses of Ti3C2Tx We believe that the Ti3C2Tx can be used in place of metals for strong plasmon generation in future sensor designs at telecommunication wavelengths.

Boosting Long-Range Surface-Enhanced Raman Scattering on Plasmonic Nanohole Arrays for Ultrasensitive Detection of MiRNA

A fundamental challenge, particularly, in surface-enhanced Raman scattering (SERS) analysis is the detection of analytes that are distant from the sensing surface. To tackle this challenge, we herein report a long-range SERS (LR-SERS) substrate supporting an extension of electric field afforded by long-range surface plasmon resonance (LRSPR) excited in symmetrical dielectric environments. The LR-SERS substrate has a sandwich configuration with a triangle-shaped gold nanohole array embedded between two dielectrics with similar refractive indices (i.e., MgF2 and water). The finite-difference time-domain simulation was applied to guide the design of the LR-SERS substrate, which was engineered to have a wavelength-matched LRSPR with 785 nm excitation. The simulations predict that the LR-SERS substrate exhibits great SERS enhancement at distances of more than 10 nm beyond its top surface, and the enhancement factor (EF) has been improved by three orders of magnitude on LR-SERS substrates compared to that on conventional substrates. The experimental results show good agreement with the simulations, an EF of 4.1 × 105 remains available at 22 nm above the LR-SERS substrate surface. The LR-SERS substrate was further applied as a sensing platform to detect microRNA (miRNA) let-7a coupled with a hybridization chain reaction (HCR) strategy. The developed sensor displays a wide linear range from 10 aM to 1 nM and an ultralow detection limit of 8.5 aM, making it the most sensitive among the current detection strategies for miRNAs based on the SERS-HCR combination to the best of our knowledge.

Rotating Angle Modulation Method for Improving the Measurement Performance of LRSPR Sensor

Long-range surface plasmon resonance (LRSPR) sensor has attracted extensive attention from research institutions and commercial companies due to its label-free detection, real-time analysis, high detection accuracy (DA) and high sensitivity. In this paper, a novel, simple, and effective rotating angle modulation (RAM) method for the metal-graphene structure LRSPR sensor is proposed. Compared with the general SPR sensor, the metal-graphene structure LRSPR sensor has a narrower full width at half maximum (FWHM) and a very high DA, especially the metal material is copper (Cu). The theoretical analysis for the Cu-graphene structure LRSPR sensor using RAM method shows that the FWHM, sensitivity and figure of merit (FOM) of the proposed sensor can be 0.74°, 320.4°/RIU and 498.24/RIU, respectively. In addition, this method enables the LRSPR sensor to obtain good measurement results even with the low-resolution rotating device and effectively reduces the dependence of the sensor on the high-resolution expensive rotating device. A preliminary comparative experiment for the SPR sensor has proved this new method. Finally, the concept of combining the RAM and general incident angle methods is proposed to provide the sensor with a large measuring range and low optical losses. We believe that this method can provide a favorable reference for solving the shortcomings of LRSPR sensors, such as complex structure, limited sensitivity and high cost, so that it can be widely promoted and applied in biological detection, medical assay, chemical investigation, etc.

All-epitaxial long-range surface plasmon polariton structures with integrated active materials

We demonstrate all-epitaxial structures capable of supporting short- and long-range surface plasmon polariton (SRSPP and LRSPP) modes in the long-wave infrared region of the electromagnetic spectrum. The SRSPP and LRSPP modes are bound to the interfaces of a buried heavily doped (n++) semiconductor layer and surrounding quantum-engineered type-II superlattice (T2SL) materials. The surrounding T2SLs are designed to allow optical transitions across the frequency dispersion of the SPP modes. We map the SPP dispersion in our structure using grating-coupled angle- and polarization-dependent reflection and photoluminescence spectroscopy. The epitaxial structures are analytically described using a simplified three-layer system (T2SL/n++/T2SL) and modeled using rigorous coupled wave analysis with excellent agreement to our experimental results. The presented structures offer the potential to serve as long-range interconnects or waveguides in all-epitaxial plasmonic/optoelectronic systems operating in the long-wave infrared.