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We present a bifunctional polarization converter based on Dirac semimetals (DSMs) and vanadium dioxide (VO2), which consists of two layers of DSMs on both sides, a metal grating and a VO2 board. The polarization converter frequency is dynamically tuned by changing the Fermi energy level of the DSMs. The result suggests that when VO2 is in an insulated state, the device behaves as a transmissive polarization converter. The dual-band transmissive polarization conversion and asymmetric transmission (AT) function of circularly polarized (CP) waves are realized at 1.99 THz and 3.46 THz, with the polarization converter ratio (PCR) reached 97.6% and 95.8%, respectively. In addition, when VO2 is in the metal state, the designed polarization converter is a reflective device, which can maintain the chirality of the reflected CP wave to the incident wave in a wide band. The polarization-maintaining ratio (PMR) in the range of 2 THz to 3.55 THz is higher than 88%. When the angle of incidence is less than 60°, the frequency band of the PMR is narrowed and the amplitude reaches more than 90%.

In this paper, we propose an approach to improve the sensitivity of an optical fiber surface plasmon resonance (SPR) sensor with a pure higher-order mode excited by a designed mode selective coupler (MSC). We calculate the proportion of the power of the higher-order mode in the cladding. Compared to the LP01 mode, the power proportion of the LP11 mode (LP21 mode) in the cladding theoretically improves by 100% (150%). To generate a relatively pure LP11 mode or LP21 mode, a mode selective coupler (MSC, 430–580 nm) is designed. The coupling efficiency of the LP01–LP11 mode coupler is over 80%, and that of the LP01–LP21 mode coupler is over 50%. The simulation results show that the sensitivity of the LP11  mode and the LP21 mode increases by approximately 330% and 360%, respectively, using the intensity modulation (n = 1.33–1.38, 430–580 nm); the resolution of the refractive indices of our sensor, using the LP11 mode (LP21 mode), is 2.6×10−4 RIU (2.4×10−4 RIU). The higher sensitivity and resolution of our presented fiber SPR sensor containing a visible MSC make it a promising candidate for the measurement of refractive indices.

A nanoparticle-based few-mode multi-core fiber (FM-MCF) localized surface plasmon resonance (LSPR) biosensor is proposed and analyzed using the finite element method (FEM). It’s critical to narrow the loss spectrum and improve the coupling efficiency, which makes it have high resolution and high sensitivity. With the aid of open air holes, the gold nanoparticles are easily assembled on the surface of this FM-MCF LSPR biosensor. Through multiple investigations, the performance of the sensor can be improved by properly setting gold nanoparticle configurations, such as radius, positions, shapes, and nanoparticle arrays. The simulation results show that when three circular gold nanoparticles with a radius of 150 nm are placed symmetrically in the open air hole and the angle between adjacent nanoparticles is 5°, the maximum sensitivity of 7,351.6 nm/RIU (LP02y mode na = 1.38) can be obtained in the sensing range of 1.33–1.38, which covers the refractive index (RI) of biological fluids, such as bovine serum albumin (BSA) solution and human Immunoglobulin G.