Highly sensitive refractive index sensor based on hybrid plasmonic waveguide microring resonator

We propose, to the best of our knowledge, a novel kind of tunable liquid crystal core refractive index (RI) sensor based on photonic crystal fiber (PCF) covered with a nanoring gold film. The finite element method is used to discuss and analyze the sensing performance of the RI sensor. Gold is used as the excitation material for surface plasmon resonance, and a gold nanoring is embedded around the first cladding of the PCF. The liquid analytes penetrate the outermost layer of the cladding, and the central hole is filled with liquid crystal E7. Complete coupling and incomplete coupling are excited as the analyte RI increases, and the resonance strength of complete coupling is stronger than that of incomplete coupling. It can be proved by calculation that at different wavelengths, resonant coupling of fifth-order and sixth-order surface plasmon polaritons is obtained. The RI of liquid analytes ranges from 1.405 to 1.445. The diameters of the liquid crystal cores are 0.2, 0.4, 0.6, and 0.8 µm; their average sensitivities are 10700, 10566, 10167, and 9166 nm/RIU; and the linear fitting constants are 0.98025, 0.97387, 0.96597, and 0.95507, respectively. In short, the RI sensor has the advantages of tunability, wide range, and high sensitivity, and is expected to be applied in various fields.

A liquid level optical fiber sensor with refractive index compensation through cascading double multi-mode interferences

A D-shaped surface plasmon resonance temperature and refractive index (RI) sensor based on photonic crystal fiber is proposed. The fiber core is composed of five air holes arranged in a pentagonal shape, which effectively improves the sensitivity. The gold film is used as a plasmonic material, and the mixture of ethanol and chloroform is used as a temperature sensitive liquid. A finite element method with higher accuracy and stronger applicability is used to study the performance of the sensor. The results indicate that when filling the mixture, the temperature sensor can detect the temperature in the range of 0 °C–60 °C. A maximum temperature sensitivity of 11.0 nm °C−1 is obtained by filling the mixture. The RI sensor can detect RI in the range of 1.13–1.40, with the wavelength range of 1.2–2.4 μm, and the maximum wavelength sensitivity reaches up to 21 000 nm RIU−1. The sensor can be used in biomedicine, environmental monitoring, food detection, temperature detection and other related fields, and has certain competitiveness and commercial value.

Graphene-based metamaterial absorber for refractive index sensing applications in terahertz band

A refractive index (RI) sensor based on plastic optical fiber (POF) with narrow groove structures coated with a gold layer (POF-NGS (Au)) is proposed and demonstrated. The proposed sensor was characterized using intensity interrogation, where the change in transmission power is induced by light filed interaction and Surface Plasmon Resonance (SPR). Narrow groove structures with lengths of 5 mm were fabricated using an ultraviolet laser, and a gold layer was sputtered to the surface of the whole fiber. The effect of the machining pitch on the RI response was studied. In this paper, dB and $\mu \text{W}$ were used to represent the change of transmission power. Experiments have shown that the POF-NGS (Au) RI sensor has liner response between 1.340 and 1.356. The curvature of the output is largest when the machining pitch is $400~\mu \text{m}$ with sensitivity 12.5 dB/RIU ($126~\mu \text{W}$ /RIU). The repeatability and temperature sensitivity characteristics were also characterized. Furthermore, the proposed sensor offers improved stability and signal to noise ratio.

Due to their small footprint and high sensitivity to biological molecule binding, planar optical microring resonators gained high interest for use as optical biosensors. Typically, microring resonators are made of semiconductor based materials, and are manufactured by time-consuming lithography and etching steps. Semiconductor based waveguides have high refractive indices, and thus, a high refractive index contrast between core and cladding. In this case, due to strong mode confinement, bending loss is a comparably minor issue and becomes relevant only at small bending radii of less than 5 μm. The main loss is determined by surface scattering, and thus, semiconductor based curved waveguides need to be designed and manufactured to have very smooth sidewalls. If polymer materials are used, microring resonators can be cost-efficiently manufactured by nanoimprint lithography. The resulting larger polymer waveguide dimensions facilitate in- and out-coupling, and polymer surfaces allow using established surface biofunctionalization techniques. For polymer waveguides, due to the small refractive index contrast, surface scattering loss is a minor issue, but bending loss becomes dominant for radii of less than 80 μm due to the low mode confinement to the core. In this work, design guidelines for polymer microring resonator waveguides are given and compared to semiconductor based waveguides. Waveguide losses due to bending and surface roughness are determined analytically or numerically by finite element methods. Coupling coefficients are calculated by finite element methods and coupled-mode theory. Resulting conclusions for designing polymer waveguides and semiconductor waveguides are derived.

We propose a refractive index (RI) and temperature sensor design based on hybrid mechanisms in a D-shaped all-solid photonic crystal fiber. The D-shaped flat surface is coated with a silver film to construct surface plasmon resonance for the RI sensing, while a single hole in the all-solid structure is filled with benzene to form a directional resonance coupling for the temperature sensing. With a strong coupling between the core mode and the resonance modes, the maximum sensitivities of the RI and the temperature are as high as 12900 nm/RIU and 23.85 nm/°C. Since two resonance designs are used, this sensor can avoid the need for metal coating in the fiber holes and use a high RI sensing medium for temperature sensing.

An ultra high-sensitivity temperature and refractive index (RI) sensor based on surface plasmon resonance (SPR) effect in D-shape high birefringence photonic crystal fiber (PCF) is presented and discussed by the finite element method (FEM). The designed PCF is polished to enhance the interaction of the evanescent field with the external medium, ultimately improving the sensing performance. The influence of three different plasma materials, gold, silver and aluminum, on the sensing performance was investigated. Finally, the chemically stable plasma material gold is selected to coat on flat surface of D-shape PCF for SPR sensing, realizing the sensitive real-time monitoring of RI and temperature. The sensing performance was greatly improved, with a wide RI (1.43 to 1.50) and temperature (36 °C to 86 °C) detection range, and the corresponding maximum sensitivity was 44850 nm/RIU and -16.875 nm/°C, respectively. Furthermore, the sensitivity is not particularly sensitive to the sizes of the designed air holes, which leads to a sensor with high structure tolerance. Numerical results show that the coating thickness not only affects the sensitivity but also the detection range of the sensor, so we can tune the RI and temperature detection range by adjusting the gold film thickness to meet specific detection requirements. The sensor with enhanced sensitivity, tunable detection range, and high fabrication tolerance has a wide application prospect in the field of life science research, chemical production, and environmental monitoring.

A plasmonic nanostructure (PNS) which integrates metallic and dielectric media within a single structure has been shown to exhibit specific plasmonic properties which are considered useful in refractive index (RI) sensor applications. In this paper, the simultaneous realization of sensitivity and tunability of the optical properties of PNSs consisting of alternative Ag and dielectric of nanosphere/nanorod array have been proposed and compared by using three-dimensional finite element method. The proposed system can support plasmonic hybrid modes and the localized surface plasmonic resonances and cavity plasmonic resonances within the individual PNS can be excited by the incident light. The proposed PNSs can be operated as RI sensor with a sensitivity of 500 nm/RIU (RIU = refractive index unit) ranging from UV to the near-infrared. In addition, a narrow bandwidth and nearly zero transmittance along with a high absorptance can be achieved by a denser PNSs configuration in the unit cell of PNS arrays. We have demonstrated the number of modes sustained in the PNS system, as well as, the near-field distribution can be tailored by the dielectric media in PNSs.

We discuss a first-order design tool for waveguide-coupled microring resonators based on an approximate solution of the wave propagation in a microring waveguide with micron-size radius of curvature and a large lateral index contrast. The model makes use of the conformal transformation method and a linear approximation of the refractive index profile, and takes into account the effect of waveguide thickness, dispersion, and diffraction. Based on this model, we develop general design rules for the major physical characteristics of a waveguide-coupled microring resonator, including the resonance wavelength, the free spectral range, the coupling ratio, the bending radiation loss and the substrate leakage loss. In addition, the physical model provides leads to alternative coupling designs. We present two examples, one using a phase-matching parallel waveguide with a smaller width than the ring waveguide, and the other using a vertical coupling structure. Both these designs significantly increase the coupling length and reduce or eliminate the dependence on a narrow air gap in a waveguide coupled microring resonator.

Single-mode eccentric-core D-shaped photonic crystal fiber surface plasmon resonance sensor

Surface plasmon resonance refractive index sensor based on photonic crystal fiber covering nano-ring gold film

Design of novel Penta core PCF SPR RI sensor based on fusion of IMD and EMD techniques for analysis of water and transformer oil

A novel surface plasmon resonance (SPR) refractive index (RI) sensor based on the D-type dual-mode photonic crystal fiber (PCF) is proposed. The sensor employs a side-polished few-mode PCF that facilitates the transmission of the fundamental and second-order modes, with an integrated microfluidic channel positioned directly above the fiber core. This design minimizes the distance to the analyte and maximizes the interaction between the optical field and the analyte, thereby enhancing the SPR effect and resonance loss for improved sensing performance. Au-TiO2 dual-layer material was coated on the surface of a microfluidic channel to enhance the penetration depth of the core evanescent field and tune the resonance wavelength to the near-infrared band, meeting the special needs of chemical and biomedical detection fields. The finite element method was utilized to systematically investigate the coupling characteristics between various modes and surface plasmon polariton (SPP) modes, as well as the impact of structural parameters on the sensor performance. The results indicate that the LP11b_y mode exhibits greater wavelength sensitivity than the HE11_y mode, with a maximum sensitivity of 33,000 nm/RIU and an average sensitivity of 8272.7 nm/RIU in the RI sensing range of 1.25-1.36, which is higher than the maximum sensitivity of 16,000 nm/RIU and average sensitivity of 5666.7 nm/RIU for the HE11b_y mode. It is believed that the proposed PCF-SPR sensor features both high sensitivity and high resolution, which will become a critical device for wide RI detection in mid-infrared fields.

With the development of aerospace, deep-sea exploration and other technologies, the demand for anti-electromagnetic, high-sensitivity and miniaturized sensors is increasingly urgent. In this paper, a model of a quasi-photonic crystal fiber (Q-PCF) refractive index (RI) sensor enhanced with surface plasmon resonance (SPR) is proposed. A stable gold film with a significant SPR effect is applied to the two identically sized and oppositely positioned air holes of the proposed sensor, and all air holes are filled with analyte. A detailed analysis of the mode characteristics, structural parameters and RI sensing performance of the sensor has been carried out using the finite element method. It has been shown that the maximum sensitivity (S) is 4977.59 nm/RIU in the RI range of 1.35–1.40, corresponding to a resolution (R) of 2.01 × 10−5 RIU and a figure of merit (FOM) of 160.36 RIU−1. The proposed Q-PCF sensor has unique fabrication advantages and outstanding sensing properties, providing a new idea for biosensing, complex environment monitoring and long-range measurement, and is of great practical value in the field of highly integrated sensing.