Surface Plasmon Resonance Structure Research Articles

2D polymer/metal structures for surface plasmon resonance

In this paper, we present a new type of polymer/metal structures for surface plasmon resonance (SPR) applications. The polymer structures with different periods were fabricated by a single-step 3D laser lithography using direct laser writing process based on non-linear two-photon absorption. Prepared polymer structures were coated by a thin film of gold. The morphological properties of final SPR structures were analyzed using a scanning electron microscope and atomic force microscope. The reflection spectra of the SPR structure with a defined period were simulated using a finite-difference time-domain method. The prepared structures show a sharp spectral dip due to the SPR effect in reflection measurements in a visible spectral range. The effect of SPR was also investigated in the angular dependence and confirmed the strong effect of 2D grating on SPR dip formation.

Giant Goos-Hänchen Shifts in Au-ITO-TMDCs-Graphene Heterostructure and Its Potential for High Performance Sensor.

In order to improve the performance of surface plasmon resonance (SPR) biosensor, the structure based on two-dimensional (2D) of graphene and transition metal dichalcogenides (TMDCs) are proposed to greatly enhance the Goos-Hänchen (GH) shift. It is theoretically proved that GH shift can be significantly enhanced in SPR structure coated with gold (Au)-indium tin oxide (ITO)-TMDCs-graphene heterostructure. In order to realize high GH shifts, the number of TMDCs and graphene layer are optimized. The highest GH shift (−801.7 λ) is obtained by Au-ITO-MoSe2-graphene hybrid structure with MoSe2 monolayer and graphene bilayer, respectively. By analyzing the GH variation, the index sensitivity of such configuration can reach as high as 8.02 × 105 λ/RIU, which is 293.24 times of the Au-ITO structure and 177.43 times of the Au-ITO-graphene structure. The proposed SPR biosensor can be widely used in the precision metrology and optical sensing.

Simultaneous measurement of temperature and refractive index based on a hybrid surface plasmon resonance multimode interference fiber sensor.

We propose and demonstrate a hybrid fiber-based sensor combining a multimode interference (MMI) structure and a surface plasmon resonance (SPR) structure for simultaneous measurement of temperature and refractive index (RI) of a liquid sample. We configure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a single-mode fiber sequentially. We set up the SPR structure by coating a gold film with a thickness of 50 nm on the surface of the no-core fiber. We measure the sensitivity of RI and the temperature of the MMI and SPR structure, respectively. Then we obtain the coefficient matrix to simultaneously measure the temperature and RI of a liquid sample and obtain the highest RI sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9 pm/°C. We verify the feasibility of the sensor in liquid alcohol. The testing results indicate that the proposed sensor and testing method are feasible, accurate, and convenient.

Surface Plasmon Resonance Biosensor Performance Analysis on 2D Material Based on Graphene and Transition Metal Dichalcogenides

Since graphene was first isolated in 2004, research related to graphene-based 2D material for surface plasmon resonance (SPR) biosensor applications has increased. Recently, other types of 2D materials such as Transition Metal Dichalcogenides have also been investigated. This 2D material has exceptional optical and electronic properties and can be utilized to improve the performance of biosensors. The performance of SPR biosensors can be determined based on their sensitivity and detection accuracy (DA). To find out the sensitivity and DA, simulation approach can be done. In this paper, we perform SPR simulations on monolayer and multilayer structures of 2D material, namely graphene, graphene oxide, molibdenum disulfida, and tungsten diselenide. In addition, we also investigated the sensitivity and DA of SPR biosensors on hybrid structures. The results obtained indicate that the use of 2D material can increase the sensitivity of the SPR up to 5 times when compared to conventional structures. Of the four types of 2D material investigated, GO is the material with the best accuracy. If the SPR biosensor performance is determined based on the FOM value, the best performance is owned by the GO-based SPR structure with a sensitivity of 151.87 deg/RIU and DA 0.22 deg-1.

Gold-Film-Thickness Dependent SPR Refractive Index and Temperature Sensing with Hetero-Core Optical Fiber Structure

A simple hetero-core optical fiber (MMF-NCF-MMF) surface plasmon resonance (SPR) sensing structure was proposed. The SPR spectral sensitivity, full width of half peak (FWHM), valley depth (VD), and figure of merit (FOM) were defined to evaluate the sensing performance comprehensively. The effect of gold film thickness on the refractive index and temperature sensing performance was studied experimentally. The optimum gold film thickness was found. The maximum sensitivities for refractive index and temperature measurement were obtained to be 2933.25 nm/RIU and −0.91973 nm/°C, respectively. The experimental results are helpful to design the SPR structure with improved sensing performance. The proposed SPR sensing structure has the advantages of simple structure, easy implementation, and good robustness, which implies a broad application prospect.

Highly sensitive refractive index detection based on compact HSC-SPR structure in a microfluidic chip

In term of optic-fiber surface plasmon resonance (SPR) sensors, the achievement of both high sensitivity and structural compactness remains a challenge. Here, we propose a SPR optical fiber sensor based on hollow silica capillary (HSC) for highly sensitive refractive index (RI) measurement. A segment of HSC is tightly spliced between two multimode fibers (MMFs), which constitutes a compact hetero-core sensing structure. When RI of measured solution varies from 1.333 to 1.424, a 60 nm thickness gold coated HSC-SPR sensor is experimentally proved to reach a highest sensitivity of 7225.63 nm/RIU. Compared with the no-core fiber (NCF), the usage of HSC can improve spectra resolution and RI sensitivity of SPR sensors. Our research may pave the way for designing easy to fabricate, compact and low cost SPR structures and integrating them in micro-devices for biochemical analysis and medical diagnosis.

Phase sensitive measurement of the wavelength dependence of the complex permittivity of a thin gold film using surface plasmon resonance

We report on a new method for measuring the wavelength dependence of the complex permittivity of a thin gold film of a surface plasmon resonance (SPR) structure comprising a gold-coated SF10 slide with an adhesion film of chromium attached to an SF10 glass prism. The method is based on spectral interferometry and utilizes a setup with a birefringent crystal and the SPR structure in the Kretschmann configuration, in which channeled spectra are recorded and from them, the phase functions of the SPR for air at different angles of incidence are retrieved. The SPR phenomenon is manifested as an abrupt phase change with respect to the reference phase difference for the interference resolved with the SF10 glass prism alone. The phase changes for different angles of incidence are processed in the vicinity of the resonance wavelengths to obtain the real and imaginary parts of the complex permittivity in a wavelength range from 530 to 850 nm or equivalently, the parameters of a modified Drude-Lorentz model. This research, to the best of the authors’ knowledge, is the first demonstration of spectral interferometry-based measurement of the complex permittivity function of a thin metal film, which is important from the point of view of material characterization directly performed in the Kretschmann configuration.

Giant tunable Goos–Hänchen shifts based on surface plasmon resonance with Dirac semimetal films

Surface plasmon resonance (SPR) with bulk Dirac semimetals (BDS) has been proposed to enhance the Goos–Hänchen (GH) shift. It is found that a large positive and negative GH shift can be obtained by adjusting the thickness of the BDS film, and the GH shift is very sensitive to the Fermi level of the BDS film. By changing the Fermi level of the BDS film, we can get a controllable negative GH shift. The largest GH shift of the proposed configuration, as large as 361.4 times the incident wavelength, has been obtained. Compared with the traditional SPR structure, the GH shift is increased by nearly 13.4 times, which indicates that BDS is expected to replace the traditional SPR structure to excite the stronger SPR. Finally, the numerical simulation results further verify that the giant shift can be obtained by a finite width Gaussian beam in our proposed calculation. Based on this giant and controllable GH shift, it may have potential applications in tunable sensors, switches, filters, and other devices.

Surface Plasmon Resonance Based Measurement of the Dielectric Function of a Thin Metal Film.

A spectral method based on surface plasmon resonance (SPR) in air is used to measure the dielectric function of a thin metal film. The method utilizes the spectral dependence of the ratio of the reflectances of p- and s-polarized waves measured in the Kretschmann configuration at different angles of incidence. By processing these dependences in the vicinity of a dip, or equivalently near the resonance wavelength, and using the dispersion characteristics of a metal film according to a proposed physical model, the real and imaginary parts of the dielectric function of the metal can be determined. The corresponding dielectric function of the metal is obtained by a least squares method for such a thickness minimizing the difference between the measured and theoretical dependence of the resonance wavelength on the the angle of incidence. The feasibility of the method is demonstrated in measuring the dielectric function of a gold film of an SPR structure comprising an SF10 glass prism and a gold coated SF10 slide with an adhesion film of chromium. The dielectric function according to the Drude–Lorentz model with two additional Lorentzian terms was determined in a wavelength range from 534 to 908 nm, and the results show that the gold film is composed of homogenous and rough layers with thicknesses 42.8 nm and 2.0 nm, respectively. This method is particularly useful in measuring the thickness and dielectric function of a thin metal film of SPR structures, directly in the Kretschmann configuration.

Quantitative Cross-Platform Performance Comparison between Different Detection Mechanisms in Surface Plasmon Sensors for Voltage Sensing

Surface plasmon Resonance (SPR) has recently been of interest for label-free voltage sensing. Several SPR structures have been proposed. However, making a quantitative cross-platform comparison for these structures is not straightforward due to (1) different SPR measurement mechanisms; (2) different electrolytic solution and concentration in the measurement; and (3) different levels of external applied potential. Here, we propose a quantitative approach to make a direct quantitative comparison across different SPR structures, different electrolytic solutions and different SPR measurement mechanisms. There are two structures employed as example in this theoretical study including uniform plasmonic gold sensor and bimetallic layered structure consisting of uniform silver layer (Ag) coated by uniform gold layer (Ag). The cross-platform comparison was carried by several performance parameters including sensitivity (S), full width half maximum (FWHM) and figure of merit (FoM). We also discuss how the SPR measurement mechanisms enhance the performance parameters and how the bimetallic layer can be employed to enhance the FoM by a factor of 1.34 to 25 depending on the SPR detection mechanism.

Research on tunable distributed SPR sensor based on bimetal film.

In order to overcome the limitations in range of traditional prism structure surface plasmon resonance (SPR) single-point sensor measurement, a symmetric bimetallic film SPR multi-sensor structure is proposed. Based on this, the dual-channel sensing attenuation mechanism of SPR in gold and silver composite film and the improvement of sensing characteristics were studied. By optimizing the characteristics such as material and thickness, a wider range of dual-channel distributed sensing is realized. Using a He-Ne laser (632.8nm) as the reference light source, prism-excited symmetric SPR sensing was studied theoretically for a symmetrical metal-clad dielectric waveguide using thin-film optics theory. The influence of the angle of incidence of the light source and the thickness of the dielectric layer on the performance of SPR dual formant sensing is explained. The finite-difference time-domain method was used for the simulation calculation for various thicknesses and compositions of the symmetric combined layer, resulting in the choice of silver (30nm) and gold (10nm). When the incident angle was 78deg, the quality factor reached 5960, showing an excellent resonance sensing effect. The sensitivity reached a maximum of 5.25×10-5 RIU when testing the water content of an aqueous solution of honey, which proves the feasibility and practicality of the structure design. The structure improves the theoretical basis for designing an SPR multi-channel distributed sensing system, which can greatly reduce the cost of biochemical detection and significantly increase the detection efficiency.

Giant and controllable Goos-Hänchen shifts based on surface plasmon resonance with graphene-MoS2 heterostructure

Surface plasmon resonance (SPR) with two-dimensional (2D) materials has been proposed to enhance the sensitivity of biosensors. Here, we will apply SPR to greatly enhance and control the Goos-Hanchen (GH) shift. It is theoretically shown that the GH shifts can be significantly enhanced in the SPR structure coated with a graphene-MoS2 heterostructure. By changing the layer number of graphene or MoS2, the giant GH shifts can be obtained. Maximum GH shift (235.8λ) can be obtained when 2 layers of MoS2 and 3 layers of graphene are combined. Moreover, the GH shift can be positive or negative depending on the layer number of MoS2 and graphene. When the GH shift is used as the interrogation for the biosensor, it has a superior sensitivity. By comparing the sensitivity based on the SPR with only Au coating or Au-graphene coating, the sensitivity of the GH shift-interrogated biosensor can be enhanced by nearly 300 times, and hence paves the way for further applications in fundamental biological studies and environmental monitoring.

Finite-difference time-domain analysis of hydrogenated amorphous silicon and aluminum surface plasmon waveguides

The large majority of surface plasmon resonance (SPR)-based devices use noble metals, namely gold or silver, in their manufacturing process. These metals present low resistivity, which leads to low optical losses in the visible and near-infrared spectrum ranges. Gold shows high environmental stability, which is essential for long-term operation, and the lower stability of silver can be overcome through the deposition of an alumina layer, for instance. However, their high cost is a limiting factor if the intended target is large-scale manufacturing. This work considers a cost-effective approach through the selection of aluminum as the plasmonic material and hydrogenated amorphous silicon instead of its crystalline counterpart. This SPR structure relies on Fano resonance to improve its response to refractive index deviations of the surrounding environment. Fano resonance is highly sensitive to slight changes of the medium, hence the reason we incorporated this interference phenomenon in the proposed sensing structure. We report the results obtained when conducting finite-difference time-domain algorithm-based simulations on this metal–dielectric–metal structure when the active metal is aluminum, gold, and silver. Then, we evaluate their sensitivity, detection accuracy, and resolution. The obtained results for our proposed sensing structure show good linearity and similar parameter performance as the ones obtained when using gold or silver as plasmonic materials.

Giant Goos–Hänchen shifts of waveguide coupled long-range surface plasmon resonance mode**Project supported by the National Natural Science Foundation of China (Grant Nos. 61505111 and 11604216), the China Postdoctoral Science Foundation (Grant No. 2016M600667), the Science and Technology Planning Project of Guangdong Province, China (Grant No. 2016B050501005), the Fund from the Educational Commission of Guangdong Province, China (Grant No. 2016KCXTD006), and

A hybrid structure based on a planar waveguide (PWG) mode coupling a long-range surface plasmon resonance (LRSPR) mode is proposed to enhance the GH shift. Both the PWG mode and LRSPR mode can be in strong resonance, and these two modes can be coupled together due to the normal-mode splitting. The largest GH shift of PWG-coupled LRSPR structure is 4156 times that of the incident beam, which is 23 times and 3.6 times that of the surface plasmon resonance (SPR) structure and the LRSPR structure, respectively. As a GH shift sensor, the highest sensitivity of 4.68 × 107λ is realized in the coupled structure. Compared with the sensitivity of the traditional SPR structure, the sensitivity of our structure is increased by more than 2 orders, which theoretically indicates that the proposed configuration can be applied to the field of high-sensitivity sensors in the future.

Dispersion engineering with plasmonic nano structures for enhanced surface plasmon resonance sensing

We demonstrate numerically and experimentally the enhancement of Surface Plasmon Resonance (SPR) sensing via dispersion engineering of the plasmonic response using plasmonic nanograting. Following their design and optimization, the plasmonic nanograting structures are fabricated using e-beam lithography and lift-off process and integrated into conventional prism based Kretschmann configuration. The presence of absorptive nanograting near the metal film, provides strong field enhancement with localization and allows to control the dispersion relation which was originally dictated by a conventional SPR structure. This contributes to the enhancement in Q factor which is found to be 3–4 times higher as compared to the conventional Kretschmann configuration. The influence of the incident angle on resonance wavelength is also demonstrated both numerically and experimentally, where, only a negligible wavelength shift is observed with increasing the incident angles for plasmonic nanograting configuration. This surprising feature may be helpful for studying and utilizing light-matter interaction between plasmons and narrow linewidth media (e.g. Rb atom or molecule) having nonlocalities in their susceptibility-momentum relation. Finally, we analyze the role of plasmonic nanograting in enhancing the performance of an SPR sensor. Our results indicate that the integrated SPR-nanograting device shows a great promise as a sensor for various types of analytes.

Optical hysteresis in SPR structures with amorphous As2S3 film under low-power laser irradiation

Optical hysteresis is a fundamental phenomenon that can lead to optical bistability and high-speed signal processing. Here, we present a theoretical and experimental study of the optical hysteresis phenomenon in amorphous As2S3 chalcogenide based waveguide structures under surface plasmon resonance (SPR) conditions. The SPR structure is irradiated with low power CW Ar laser radiation at 514 nm wavelength, with photon energy near the optical band-gap of As2S3, in a Kretschmann–Raether configuration. First, we determined the incidence angle on the SPR structure for resonant coupling of the laser radiation within the waveguide structure. Subsequently, by setting the near resonance incidence angle, we analyzed the variation of the laser power reflected on the SPR structure with incident power. We demonstrated that, by setting the incidence angle at a value slightly smaller than the resonance angle, the increase followed by the decrease of the incident power lead to a wide (up to 60%) hysteresis loop of the reflected power. This behavior is related to the slow and persistent photo-induced modification of the complex refractive index of As2S3 under 514 nm laser irradiation. The experimental and theoretical results are in good agreement, demonstrating the validity of the theoretical model presented here.

Enhancing Photonic Spin Hall Effect in the Surface Plasmon Resonance Structure Covered by the Graphene–MoS2 Heterostructure

Due to the weak spin–orbit interaction, the photonic spin Hall effect (SHE) is generally very weak, which is disadvantageous for potential applications of nanophotonic devices. Surface plasmon resonance (SPR) has been proposed to enhance the photonic SHE. In this paper, by covering the heterostructure material composed of graphene and molybdenum sulfide (MoS2) on the SPR structure, it is demonstrated that the two-dimensional (2-D) material heterostructure can effectively enhance the photonic SHE. The transverse shift of light beam in the heterostructure is larger than those in SPR structure due to the larger refractive index gradient in the heterostructure. The maximum transverse shift can be up to 27.5 μ m under the optimized parameters, which is much larger than the previously reported values. This interesting phenomenon is attributed to the larger light absorption and hence the refractive index variation gradient of graphene/MoS2 heterostructure. These findings provide us with a new way to modulate the photonic SHE, and also establish foundation for developing nanophotonic devices based on 2-D nanomaterials heterostructure.

Enhanced Photonic Spin Hall Effect with a Bimetallic Film Surface Plasmon Resonance

The surface plasmon resonance (SPR) with the bimetallic film can achieve minimum reflectivity and higher evanescent field enhancement than the SPR configuration with the single gold film, which makes it to be a much better option for enhancing the photonic Spin Hall Effect (SHE). It is found that the thicknesses of silver and gold films have a major impact on the photonic SHE, and the maximal transverse shift of horizontal polarization state can reach to 5.34 μm near the SPR resonant angle by optimizing the thicknesses of silver and gold films in the bimetallic film. Our results are far greater than the previously reported transverse shifts in the conventional SPR structure. This novel phenomenon is attributed to the higher evanescent field enhancement and the larger refractive index variation gradient of silver/gold bimetallic film. These findings provide us an effective method to manipulate the photonic SHE and therefore establish a strong foundation for developing photonic devices based on photonic SHE.

A compact, flexible fiber-optic Surface Plasmon Resonance sensor with changeable sensor chips

We propose and demonstrate the concept of a novel compact, flexible fiber optic Surface Plasmon Resonance (SPR) sensor based on a double-pass Kretschmann-type configuration, where the SPR sensor chip can be replaced for various sensing applications. Simulation and experimental results demonstrate that the proposed fiber-optic SPR structure has a sensitivity to salt concentration of around 4.8μW/ppt.

Dispersion Curve Engineering of TiO₂/Silver Hybrid Substrates for Enhanced Surface Plasmon Resonance Detection.

As surface plasmon resonance (SPR)-based biosensors are well translated into biological, chemical, environmental, and clinical fields, it is critical to further realize stable and sustainable systems, avoiding oxidation susceptibility of metal films—in particular, silver substrates. We report an enhanced SPR detection performance by incorporating a TiO2 layer on top of a thin silver film. A uniform TiO2 film fabricated by electron beam evaporation at room temperature is an effective alternative in bypassing oxidation of a silver film. Based on our finding that the sensor sensitivity is strongly correlated with the slope of dispersion curves, SPR sensing results obtained by parylene film deposition shows that TiO2/silver hybrid substrates provide notable sensitivity improvement compared to a conventional bare silver film, which confirms the possibility of engineering the dispersion characteristic according to the incidence wavelength. The reported SPR structures with TiO2 films enhance the sensitivity significantly in water and air environments and its overall qualitative trend in sensitivity improvement is consistent with numerical simulations. Thus, we expect that our approach can extend the applicability of TiO2-mediated SPR biosensors to highly sensitive detection for biomolecular binding events of low concentrations, while serving a practical and reliable biosensing platform.