In this paper, we present a novel universal coupled theory for metamaterial bound states in the continuum (BIC) or quasi-bound states in the continuum (quasi-BIC) which provides ultra-high Q resonance for metamaterial devices. Our theory analytically calculates the coupling of two bright modes with phase information. Our method has much more accuracy for ultra-strong coupling comparing with the previous theories (the coupling of one bright mode and one dark mode and the two bright-mode coupling). Therefore, our theory is much more suitable for BIC or quasi-BIC and we can accurately predict the transmission spectrum of metamaterial BIC or quasi-BIC for the first time.

In this work, a simple side-polish plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and liquid level. The effects of side-polish depths on the sensing performance were studied. The experimental results show that the SPR peak wavelength will be changed as the RI changes, and the SPR peak intensity will be changed with the liquid level variation. By monitoring the changes in peak wavelength and intensity, the RI and liquid level can be detected simultaneously. Experimental results show that an RI sensitivity of 2008.58 nm/RIU can be reached at an RI of 1.39. This sensor has the advantages of simple structure and low cost, which has a good prospect in the field of biochemical sensing.

We proposed a high sensitivity long-period fiber grating strain and temperature sensor. The sensor is composed of input single mode fiber (SMF), helical capillary optical fiber (HCOF), and output SMF in cascade. By using a fusion junction structure between SMF and HCOF, almost all the light from input SMF can be coupled to the silica cladding of HCOF, and then modulated by HCOF. Because the light propagates in the same channel as the silica cladding of HCOF, the sensor is insensitive to refractive index (RI) change but has a high sensitivity to stress and temperature. The experiment results show that the strain sensitivity can reach -9.29 pm/μ ε when the hollow air capillary diameter and twisting pitch of HCOF are 10 μm and 1.2 mm, respectively. And the temperature sensitivity can reach 17.14 pm/°C in this condition. The sensing characteristics of the sensor are related to the diameter of the hollow air capillary and the twisting pitch of HCOF. Benefiting from eliminating the cross-influence of strain and temperature, the sensor may have the potential to realize stress detection in a harsh environment.

Governed by the Malus law, nanopolarizer-based metasurfaces can continuously modulate the intensity of incident linearly polarized (LP) light pixel-by-pixel, which has been applied to display metaimages with ultracompactness and high resolution. One unique characteristic of the Malus-assisted image display technique is that no matter the polarization direction of incident LP light, the image with different contrast always appears, which would affect the security of a metaimage. Here, inspired by the orientation degeneracy of the Malus law, we show that metaimage displays can be controlled in two fixed polarization directions, merely by reconfiguring the orientations of nanopolarizers. Specifically, for each metaimage pixel, there are two options for the orientation angle of the nanopolarizer corresponding to an equal grayscale value. By randomly selecting the orientation angle among two choices, the metaimage will be blurred or even hidden, except for the polarization direction of 0° for the original image and 90° for its contrast reversed partners. We experimentally demonstrate this concept by encoding a Lena image with pixels of 1000 × 1000 into a silver nanopolarizer-based metasurface and decoding/encrypting it in a polarization-controlled optical setup. The proposed metaimage display with the encryption functionality has advantages such as continuous grayscale modulation, ultrathinness, and high density, which can be applied in ultracompact image displays, high-end anti-counterfeiting, high-density optical information storage, information encryption, etc.

We propose a temperature sensor, which consists of a composite optical waveguide (COW) sandwiched between two single mode fibers (SMFs). The COW is formed by side polishing a capillary optical fiber (COF) and filling it with polydimethylsiloxane (PDMS). Unlike the conventional fixed sensing structure, the Mach-Zehnder interferometer (MZI) and multimode interferometer (MMI) can selectively be formed by adjusting different fusion splicing parameters and respectively introducing blunt and sharp tapered fusion structures between the SMF and COF. Because of the high thermo-optic coefficient of PDMS and the high-order modes excited by the tapered fusion structure, the sensor is very sensitive to temperature sensing. The experimental results show that within the temperature measurement range from 40~80C, several resonance wavelengths of MZI and MMI sensors exhibit a remarkable blue shift with the increase of temperature. The maximum temperature sensitivities of MZI and MMI sensors can reach -0.500 nm/ and -0.88 nm/, respectively. Due to the advantages of compact size, flexible structure, and high sensitivity, the sensor has great application potential in monitoring environmental and living temperature changes.

A high-sensitivity Mach–Zehnder interferometer (MZI) based on the cascaded core-offset and macrobending fiber structure is proposed for refractive index (RI) measurement. The core-offset structure makes the fiber core mode couple to the cladding modes, and some of them recouple back to the fiber core at the macrobending structure forming a model interference effect. The liquid RI can be measured by monitoring the spectral shift of the modal interference. The RI sensing performances for the interferometers with different macrobending radii and core offsets are investigated experimentally. Experimental results show that when the core offset is 2 μm and the macrobending radius is 5.5 mm, the sensitivity can reach 699.95 nm/RIU for the RI of 1.43. The temperature dependence for the proposed sensor is also tested, and a temperature sensitivity of 0.112 nm/°C is obtained.