High Sensitivity Refractive Index Sensor Research Articles

High-sensitivity refractive index sensor based on 3 × 3 microfiber coupler structure

In this study, a 3 × 3 microfiber coupler sensing structure is fabricated by the fusion taper technology, and the mode coupling distribution of its light energy is numerically simulated by the beam propagation method. Results confirmed that the sensing structure produces regular and stable mode coupling within an 11–25 µm effective coupling region. The 3 × 3 microfiber coupler is used as a light splitter. In addition, it can also perform refractive index (RI) sensing. The RI characteristics of the proposed sensor are further verified experimentally. Experimental findings reveal that the RI sensitivity is inversely proportional to the effective coupling region diameter, and the sensitivity is 5223.83 nm/RIU when the effective coupling region diameter is 11 µm.

A High-Sensitivity Refractive Index Sensor with Period-Doubling Plasmonic Metasurfaces to Engineer the Radiation Losses

Plasmonic nanostructures with strong on-resonance local electric fields at the metal interfaces provide an excellent means for sensing applications. However, most plasmonic resonances suffer from relatively broad bandwidths, mainly due to the intrinsic material dissipation, restricting the optical performances. Moreover, little attention is paid on the radiation loss. In this work, we utilize quasi-guided modes supported by a metallic disk array and demonstrate the manipulation of radiation losses to further improve the resonance property. When a relative angle is introduced between every two adjacent metallic disks in a subwavelength-period array to form a zigzag metasurface, the period doubles leading to a transition of the true guided modes to be flipped above the light cone into the quasi-guided modes with finite yet relatively high-quality factors. We demonstrate the application of these quasi-guided modes with reduced bandwidths as refractive index sensors. Our experiment demonstrates that a refractive index sensitivity as high as 655 nm/RIU can be achieved with this structure, which has a good agreement with the numerical results. This sensitivity outperforms many other plasmonic sensors, e.g., those based on localized surface plasmon resonances or detuned plasmonic resonators. The proposed scheme of using quasi-guided modes as the refractive index sensor provides a prospective platform for biological and chemical sensing.

Highly sensitive short-range mode resonance sensor with multilayer structured hyperbolic metamaterials.

Hyperbolic metamaterial (HMM) based sensors can achieve superior sensing performance than conventional surface plasmon resonance sensors. In this work, the operator approach to effective medium approximation (OEMA) is used to characterize the HMM dielectric constant properties of metal-dielectric multilayer structures, which are classified into short-range (SR) mode and long-range (LR) mode according to the propagation length of the bulk high K waves in HMM. The dispersion relations of SR modes are derived, and a high-sensitivity refractive index sensor is designed for the near-infrared SR mode resonance. The effects of the number of periods, cell thickness, metal fill rate and incidence angle on the SR mode resonance were analyzed for the multilayer structured HMM. Our designed sensing structure achieves a maximum sensitivity of 330 µm/RIU in the near-infrared band with a quality factor of 492 RIU-1. In addition, the simulations show that the SR mode resonance wavelength is flexible and tunable. We believe that the study of HMM-based SR mode resonance sensors offers potential applications for high-sensitivity biochemical detection.

High-sensitivity Refractive Index Sensor of Arc-shape Photonic Crystal Fiber Based on Surface Plasmon Resonance

High-sensitivity Refractive Index Sensor of Arc-shape Photonic Crystal Fiber Based on Surface Plasmon Resonance

High-sensitivity refractive index sensor based on FMF-CLF-FMF optical fiber structure

光纤折射率传感器广泛应用于各种复杂环境的监测。设计了一种基于少模光纤（few-mode fiber，FMF）–无芯光纤（coreless fiber，CLF）–FMF结构的高灵敏度折射率传感器。该传感器由2小段FMF之间熔接1段减薄的CLF组成马赫-增德尔干涉仪（Mach–Zehnder interference，MZI），测量外界折射率，利用光纤布拉格光栅（fiber Bragg grating，FBG）进行温度补偿。MZI干涉光谱中的谐振波谷同时受折射率和温度影响，FBG只受温度的影响。利用MZI和FBG的折射率和温度灵敏度系数构建灵敏度矩阵，实现折射率和温度的同步测量。实验结果表明，MZI折射率灵敏度为345.66 nm/RIU，温度灵敏度为0.0134 nm/℃；FBG的温度灵敏度为0.0104 nm/℃。

Research on sensing characteristics of third-order runway series symmetric microring resonator based on hybrid plasma waveguide and metal insulator metal.

In this paper, a high-sensitivity refractive index sensor based on a hybrid plasma waveguide and metal-insulator-metal waveguide combined third-order runway series mosaic microring resonator is proposed. In this structure, a GaAs waveguide ring surrounds a gold waveguide ring in the middle, and the innermost layer is a disk made of gold material. The outer groove waveguide is composed of GaAs-air-alloy, and the inner groove waveguide is made of the Gold-Air-Gold material disc. By filling different substances in the groove, the change of refractive index will affect the optical signal strength of the output spectrum. The finite element method simulates the transmission spectrum and electric field distribution of the sensor structure. The amplitude coupling coefficient and attenuation factor affecting the resonator's performance are analyzed, and the structural parameters of the slot waveguide are optimized. The numerical simulation results show that the sensor quality factor of this structure is 1.54 × 104 , the sensitivity is 1.2 × 103 nm/RIU which is about 1.5 times higher than that of the Si ring with the same structure, the detection limit can reach 8.1892 × 10-7 RIU, and the free spectral range can reach 109 nm. Compared with the traditional microring structure, this microring has higher design freedom and free spectral range and is more suitable for producing biosensors with high sensitivity, low detection limit, and multi-parameter measurement.

All-dielectric metasurfaces with high Q-factor Fano resonances enabling multi-scenario sensing.

We propose and numerically demonstrate high Q-factor sensors based on all-dielectric metasurfaces, which are very sensitive to the change of the refractive index of the surrounding media and the incident angle. By using the light incident angular scanning method, the all-dielectric metasurface based on symmetric tetramer can act as an excellent sensing platform for trace-amount molecules such as protein A/G, 2, 4-DNT, and 2D material graphene with huge absorbance enhancement in the mid-infrared broadband spectrums. The results reveal that envelope of absorbance amplitudes is in good agreement with the vibrational mode of molecules, and absorbance enhancement factors reach as high as 10 dB in the mid-infrared wavelength range from 5.75 to 6.80μm. To further increase the Q-factor of the resonances, the all-dielectric metasurface based on asymmetric tetramer is investigated. This asymmetric structure can induce toroidal and magnetic dipoles governed by quasi-BIC to produce multi-extremely narrow linewidth Fano resonances, and the maximum sensitivity reaches up to 1.43 μm/RIU. Therefore, the proposed all-dielectric metasurface demonstrates highly enhanced performance in refractive index and chemical information sensing for trace-amount biomolecules, which inspires the development of new high-sensitivity refractive index sensors for the nondestructive identification in the mid-infrared regime.

Tunable transmission of a normal incidence through a double-slab structure

In this work, a double-slab structure is proposed, in which one slab is made of near-zero-real-part-of-impedance metamaterial (NZRPIM), and the other slab is made of ordinary transparent material. We shall demonstrate that, when a beam of electromagnetic wave is normally incident into the double-slab structure, the transmission may be enhanced significantly due to existence of a cross term induced by coherent superposition between forward and backward waves arising in the NZRPIM slab, thus the corresponding transmission peak may be strongly affected by the other slab and fluid arising between the two slabs, which is instructive significance for the design and application of high sensitivity refractive index sensor based on electromagnetic wave intensity detection. Our work may provide a new possible way to design and manufacture sensors by using NZRPIM.

High sensitivity refractive index sensor with wide detection range and high linearity based on LSPR in hollow-core anti-resonance fiber

A high sensitivity refractive index sensor based on local surface plasmon resonance in hollow-core anti-resonance fiber with wide detection range and high linearity is proposed. Gold nanowires are filled in two symmetric cladding tubes of the hollow-core anti-resonance fiber, and analyte is filled in the fiber core area. A full vectorial finite element method is used to design and analyze the hollow-core anti-resonance fiber sensor based on local surface plasmon resonance. High average wavelength sensitivity around 14996.81 nm/RIU is achieved in a wide refractive index detection range of 1.28–1.44 with high linearity of 0.99991. The maximum figure of merits of the sensor reaches 139.78 RIU−1. This separate filling of analyte and sensitive material opens up convenience applications for microfluidic detection. Due to its high sensitivity, wide detection range and high linearity of response, the proposed refractive index sensor also has potential applications in environmental protection, food safety and biosensing.

All-Dielectric Refractive Index Sensor Based on Multiple Fano Resonance with High Sensitivity in the Long-Wave Infrared Region

In this paper, we propose an all-dielectric metamaterials structure which contains four asymmetric square holes in the unit cell to design a high-sensitivity refractive index sensor in the long-wave infrared region. Theoretical analysis of the electromagnetic field distributions shows that the four transmission dips originate from magnetic dipole, electric quadrupole and Toroidal dipole. And its position can be tuned by adjusting different geometric parameters, which can optimize the structure to obtain a narrower linewidth to improve the performance of the sensor. Finally, we evaluate the performance of the structure as refractive index sensor by changing the refractive index of the tested substrate. The results show the refractive index sensor has high sensitivity in the long-wave infrared region: the highest sensitivity is 2803 nm/RIU and the figure of merit will reach up to 350.

High-sensitivity refractive index sensors based on Fano resonance in a metal-insulator-metal based arc-shaped resonator coupled with a rectangular stub

A metal-insulator-metal (MIM)-based arc-shaped resonator coupled with a rectangular stub (MARS) structure is proposed. This structure can generate two tunable Fano resonances originating from two different mechanisms. The structure has the advantage of being sensitive to the refractive index, and this feature makes it favorable for application in various microsensors. The relationship between the structural parameters and Fano resonance is researched using the finite element method (FEM) based on the software COMSOL Multiphysics 5.4. The simulation reveals that the sensitivity reaches 1900 nm/refractive index unit (RIU), and the figure of merit (FOM) is 23.75.

Design of high sensitivity refractive index sensor based on small chirp coefficient LPFG

Design of high sensitivity refractive index sensor based on small chirp coefficient LPFG

High-Q refractive index sensors based on all-dielectric metasurfaces.

Possessing fantastic abilities to freely manipulate electromagnetic waves on an ultrathin platform, metasurfaces have aroused intense interest in the academic circle. In this work, we present a high-sensitivity refractive index sensor excited by the guided mode of a two-dimensional periodic TiO2 dielectric grating structure. Numerical simulation results show that the optimized nanosensor can excite guided-mode resonance with an ultra-narrow linewidth of 0.19 nm. When the thickness of the biological layer is 20 nm, the sensitivity, Q factor, and FOM values of the nanosensor can reach 82.29 nm RIU−1, 3207.9, and 433.1, respectively. In addition, the device shows insensitivity to polarization and good tolerance to the angle of incident light. This demonstrates that the utilization of low-loss all-dielectric metasurfaces is an effective way to achieve ultra-sensitive biosensor detection.

High-sensitivity refractive index sensor with cascaded dual-core photonic crystal fiber based on vernier effect

A cascaded dual-core photonic crystal fiber (DC-PCF) refractive index sensor is proposed. The sensor is composed by cascading a single-mode fiber with a DC-PCF, thus realizing the vernier effect. The central air hole of the DC-PCF is filled with different refractive indices. The refractive index sensitivity is investigated using the full vector finite element method (FEM). The simulation results show that the cascade structure of the DC-PCF with three different air hole diameter parameters has high sensitivity up to −303376 nm/RIU in the refractive index range of 1.362–1.366, and the highest sensitivity of the cascade corresponds to a resolution of 3.30 × 10−7. The sensitivity is increased by 37 times compared to the DC-PCF without vernier effect.By replacing the central air hole material, the sensor can be used for temperature sensing, magnetic field sensing and water quality analysis.

High sensitivity refractive index sensor based on cascaded core-offset splicing NCF-HCF-NCF structure

In this paper, a high sensitivity refractive index sensor based on cascaded core-offset splicing structure with no-core fiber (NCF)-hollow core fiber (HCF)-no-core fiber (NCF) is proposed. The sensor consists of single mode fiber (SMF) cascaded with no-core fiber, and then the no-core fiber and hollow core fiber are cascaded, which is in core-offset splicing NCF-HCF-NCF structure. The simulation results show that with the increase of environmental refractive index, the transmission spectrum is red shifted. The experimental results show that the refractive index sensitivity is up to 245.67149 nm/RIU during the range of 1.3355 ∼ 1.3625. The sensor has the advantages of compact structure, simple fabrication and high sensitivity, so it can be widely used in the field of refractive index sensing.

Highly sensitive refractive index sensing based on PCF modal interferometer

A high sensitive optical fiber refractive index (RI) sensor based on Mach-Zehnder interferometer (MZI) is proposed. The sensor consists of single mode fiber-multimode fiber-photonic crystal fiber-single mode fiber (SMPS) structure using fusion technology. The experimental results have shown that the RI sensitivity of the SMPS structure is as high as 342.78 nm/RIU in the range of 1.3330–1.3775. Meanwhile, the effects of different length of PCF on the RI are also investigated. It was found that the shorter the length of PCF, the higher the sensitivity. In addition, a temperature sensitivity of 16.37 pm/℃ for the sensor is achieved. The sensor has advantages of high fringe visibility over 20 dB and simple fabrication, which has displayed great potential in the field of RI measurement.

High contrast and sensitive near-infrared refractive index sensors based on metal-dielectric-metal plasmonic metasurfaces

We report a numerical study of metal-dielectric-metal (MDM) plasmonic metasurface-based refractive index sensors that have high contrast and sensitivity in the near-infrared region. The MDM plasmonic metasurface is formed by stacking up silver (Ag) subwavelength disk array on thin silica (SiO2) spacer-layer and Ag film (acting as a reflector) on a silicon substrate. The MDM plasmonic metasurfaces with various heights, Ag disk diameters and SiO2 spacer-layer thicknesses are designated and numerically investigated using a finite-difference time-domain method. As results have shown, the optical properties of the device are weakly dependent on the height of Ag disks, but strongly dependent on the size of Ag disks and SiO2 spacer-layer thickness. By looking at the field distribution at the resonance, the strong confinement of incident light localized inside the SiO2 spacer-layer, thus extremely low reflection closed to 0%, corresponding to the absorption of up to 100% and high quality-factor (Q-factor) of ∼180 have been achieved. Because of operating in the near-infrared range, the proposed MDM plasmonic metasurface has low Ohmic loss, and then the refractive index biosensor based on that shows the figure-of-merit (FOM) to have the contrast and selectivity higher than that of other established MDM plasmonic biosensors. The sensitivities of 1109 nm/RIU and 1290 nm/RIU according to FOMs of 38.8 and 95.3, which correspond to the vapor and liquid sensors, respectively, have been achieved. Our systematic investigation provides useful guidelines for designing MDM plasmonic metasurface sensing devices.

Helical Intermediate-Period Fiber Grating for Refractive Index Measurements With Low-Sensitive Temperature and Torsion Response

We inscribe and characterize a series of helical intermediate-period fiber gratings (HIPFGs) on single-mode fiber with pitches from 24 μm to 48 μm. The fabricated HIPFGs have low temperature sensitivity, low torsion sensitivity, and high refractive index sensitivity. A series of separated peaks with low insertion losses and high coupling strengths are observed in the transmission spectra of the HIPFGs. For the HIPFG with grating pitch of 36 μm (HIPFG-3), the peaks located at around 1596 nm and 1603 nm are chosen to evaluate the sensor performance of the fiber gratings. The temperature sensitivities are 6.86 pm/°C and 7.53 pm/°C for peaks around 1596 nm and 1603 nm, respectively. The corresponding values of the torsion sensitivities are 8.67 nm/(rad/mm) and 9.49 nm/(rad/mm) for those two peaks, respectively. In addition, the average refractive index sensitivity of the grating reaches 393.9 nm/RIU in the range of 1.310-1.408. The sensitivity of the HIPFG-3 at the RI of 1.408 is calculated to be 1291 nm/RIU. Therefore, we conclude that the HIPFGs proposed in this paper can be used as a high-sensitivity refractive index sensor which is not sensitive to temperature and torsion.

High-sensitivity refractive index sensor based on Ge–Sb–Se chalcogenide microring resonator

In this article, we report a liquid sensing device based on a Ge–Sb–Se microring resonator (MRR) operating at a wavelength of 1550 nm. The transmission loss of the waveguide is 4.3 dB/cm, and the intrinsic Q factor of the MRR is 7.74 × 104, with an extinction ratio of approximately 40 dB. By detecting the different concentrations of NaCl solutions, we observe that the sensitivity of our proposed device is approximately 123 nm/RIU, and the intrinsic limit of detection is approximately 3.24 × 10−4 RIU. Compared with the other materials, such as silicon, Si3N4, or polymer, the moderate refractive index (RI) of Ge28Sb12Se60 glass allows it to obtain a high sensitivity and a high Q factor, while maintaining a strong confinement on the light field to avoid suffering from bending loss under a small bending radius, with a compact structure.

Dual-demodulation large-scope high-sensitivity refractive index sensor based on twin-core PCF.

In this paper, a refractive index (RI) sensor based on the twin-core photonic crystal fiber (TC-PCF) is presented. Introducing the rectangular array in the core area makes the PCF possible to obtain high birefringence and low confinement loss over the wavelength range from 0.6 µm to 1.7 µm. Therefore, the core region can enhance the interaction between the core mode and the filling material. We studied theoretically the evolution characteristics of the birefringence and operating wavelength corresponding to the strongest polarization point under the condition of filling the rectangular array with RI matching fluid range from 1.33 to 1.41. Simulation results reveal that the proposed TC-PCF has opposite evolutions of change rates between the B and wavelength, and the maximum RI sensing sensitivities of 1.809×10−2 B/RIU and 8 700 nm/RIU at low and high RI infill are obtained respectively, which means that the TC-PCF features of dual-parameter demodulation for the RI sensing can maintain a high refractive index sensing sensitivity within a large scope of RI ranging from 1.33 to 1.41. Compared with the results of single-parameter demodulation, it is an optimized method to improve the sensitivity of low refractive index sensors, which has great application potency in the field of biochemical sensing and detection.