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We present the design of a Single Polarization Single Mode (SPSM) Terahertz Hollow-Core Anti-Resonant Waveguide (THz HC-ARW) by using multi-size semi-elliptical tubes (SETs) and high-resistance silicon layer. High single mode performance can be achieved by optimizing the structural parameters in the structure of the THz HC-ARW with multi-size SETs in the cladding. Numerical results indicate that the single mode bandwidth with High Order Mode Extinction Ratio (HOMER) above 100 reaches up to 420 GHz covering the frequency range from 0.82 THz to 1.24 THz, in which the loss of Fundamental Mode (FM) remains below 0.05 dB/m. To the best of our knowledge, this is the first instance of achieving low loss of FM under 0.05 dB/m and HOMER exceeding 100 within an exceptionally wide frequency bandwidth for a THz HC-ARW. On the basis of the above structure, a modified structure is proposed to further obtain single polarization characteristics by selectively coating a high-resistance silicon layer on the inner surface of one SET. When a high-resistance silicon layer with the thickness of 0.258 mm is coating on the inner surface of the biggest SET, a high polarization extinction ratio of 371 and a HOMER of 106 can be attained simultaneously at a frequency of 1 THz, thus only the X-polarization FM with low loss of 0.04 dB/m can propagate through the waveguide. This means SPSM can be achieved in the proposed modified structure.

We propose a concept of surface plasmon resonance sensor based on double-sided polished microstructured optical fiber coated with graphene-on-silver layers. The silver films on the polishing planes are covered with multilayer graphene which can prevent the oxidation of silver and enhance the adsorption of biomolecules. The sensing performance of the proposed sensors with polishing planes angle of 0°, 60° and 120°, are investigated by using both wavelength and amplitude interrogations. Our numerical results demonstrate that increasing the size of the air hole in the core area can effectively improve the wavelength and maximum amplitude sensitivities. Moreover, growing number layer of graphene can increase wavelength sensitivity, whereas decrease the maximum amplitude sensitivity.

We demonstrate a sensitivity-tunable temperature SPR sensor based on side-opening grapefruit fiber. The opening sections of the fiber are coated with gold layer and then filled with a liquid mixture of ethanol and chloroform as the sensing medium. The variation of the temperature will lead to different coupling efficiencies between the core-guided mode and the plasmonic mode, and then resulting in the change of loss spectra of the fiber that will be detected. Simulation results indicate that the temperature sensitivity of the sensor is 19.9 nm/°C at 10°C when the volume fraction of ethanol in the mixture is 0.1. Moreover, by adjusting the volume fraction, the sensitivity of proposed sensor can achieve the upper limit for a certain temperature range without the excitation of higher order plasmonic modes.

We propose a concept of orthogonal-side polished microstructured optical fiber (MOF)-based surface plasmon resonance (SPR) sensor to implement the simultaneous sensing for two parameters. Two feasible structures, L-shaped MOF based and rectangular MOF based SPR sensors are investigated theoretically, which can support x- and y-polarized resonance peaks that can be used to measure and separate variations of the temperature and refractive index (RI). Our results show that the temperature coefficients (KTx and KTy) are 8 pm/°C and 6 pm/°C, and the RI coefficients (Knx and Kny) are 1470 nm/RIU and 1570 nm/RIU, for the L-shaped MOF-SPR sensor, while the KTx and the KTy are 10 pm/°C and 8 pm/°C, the Knx and Kny are 1460 nm/RIU and 1500 nm/RIU, for the rectangular MOF-based SPR sensor. Moreover, these coefficients can be further improved by choosing the appropriate structure parameters. The proposed SPR sensors with the advantages of simultaneously measuring two parameters and no need to be filled with the sensing media are expected to be more competitive in the MOF-based SPR sensor field.

A surface plasmon resonance sensor based on double-sided polished microstructured optical fiber with hollow core is put forward for refractive index sensing. Two gold films parallel to each other attached to the polished surface act as microfluidic sensing channels for the analyte. The artificially introduced air hole can facilitate the phase matching between the core mode and the plasmon mode. The sensitivities of the proposed sensor are investigated by the wavelength, amplitude and phase interrogation methods when the analyte refractive index increases from 1.33 to 1.34. In contrast to the D-shaped design, the double-sided polished structure demonstrates narrower resonance spectral width and greater phase sensitivity. Moreover, the numerical results indicate that the proposed sensor shows a good stability in the fabrication tolerances of ±5% of the thickness of gold film and the depth of polishing, respectively.

A surface plasmon resonance (SPR) sensor based on a dual-side polished microstructured optical fiber (MOF) with a dual core is proposed for a large analyte refractive index (RI; na) detection range. Gold is used as a plasmonic material coated on the polished surface, and analytes can be directly contacted with the gold film. The special structure not only facilitates the fabrication of the sensor, but also can work in the na range of 1.42–1.46 when the background material RI is 1.45, which is beyond the reach of other traditional MOF-SPR sensors. The sensing performance of the sensor was investigated by the wavelength and amplitude interrogation methods. The detailed numerical results showed that the proposed sensor can work effectively in the na range of 1.35–1.47 and exhibits higher sensitivity in the na range of 1.42–1.43.

An H-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor is proposed for detecting large refractive index (RI) range which can either be higher or lower than the RI of the fiber material used. The grooves of the H-shaped PCF as the sensing channels are coated with gold film and then brought into direct contact with the analyte, which not only reduces the complexity of the fabrication but also provides reusable capacity compared with other designs. The sensing performance of the proposed sensor is investigated by using the finite element method. Numerical results show that the sensor can work normally in the large analyte RI (na) range from 1.33 to 1.49, and reach the maximum sensitivity of 25,900 nm/RIU (RI units) at the na range 1.47−1.48. Moreover, the sensor shows good stability in the tolerances of ±10% of the gold-film thickness.