We theoretically propose a simple ultra-narrow multi-band perfect absorber for sensing applications. The perfect absorber consists of periodically arranged metallic nanodisks etched with regular prismatic holes standing on the dielectric-metal bi-layer films. Multiple ultra-narrow perfect absorption bands are obtained in the near-infrared region with the maximum bandwidth less than 21 nm and the intensity as high as 99.86%. The ultra-narrow multi-band perfect absorption originates from the synergy of localized surface plasmons, propagating surface plasmons and lattice resonances. The perfect absorber also presents other significant advantages, e.g. polarization insensitivity and high sensitivity of surrounding environments. Moreover, the prominent sensing performance for detecting the trace amounts of glucose in water is demonstrated. These features make it a promising candidate with great potential in the fields of perfect absorbers, plasmonic sensors, filters and multiplexing binding bio-molecular detection.

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The cladding mode reorganization in high refractive-index (HRI)-coated long period gratings (LPGs) is theoretically analyzed and experimentally observed with the aim of exploring the sensitivity of the resonance wavelength to the change of the refractive index in a nanoscale overlay. Experimental results show that the transition between cladding modes and overlay modes occurs when the refractive index of the liquid crystal (LC) overlay is changed from 1.477 to 1.515 by increasing its temperature from 20 degrees C to 65 degrees C. The spectral tuning ability of LPGs coated with a HRI LC layer by electro-optic modulation on a LC layer is also demonstrated, and the maximum tuning range can reach approximately 10 nm by choosing a highly sensitive operating point in the transition region.

In this paper, the tuning characteristics of locally bent microfiber taper covered with a nanosized high-refractive-index liquid crystal (LC) layer under different temperatures and electric field intensities have been theoretically analyzed and experimentally investigated. A locally bent microfiber taper interferometer with a waist diameter of ∼3.72  μm is fabricated by using the flame brushing technique, followed by bending the transition region of the taper to form a modal interferometer and later by placing a ∼200  nm LC layer over the uniform taper waist region. Experimental results indicate that a high-efficiency thermal or electric tuning of an LC-coated locally bent microfiber taper interferometer could be achieved. This suggests a potential application of this device as tunable all-fiber photonic devices, such as filters, modulators, and sensing elements.

The ultrathin fiber taper fabricated by adiabatically stretching a heated single-mode optical fiber (SMF) is made in a C-shape bent to form a modal interferometer. The interference fringes due to the mode beating in the multimode taper waist are dependent on the bending radius. Under an optimized bending radius, a maximum 18 dB interference depth can be obtained. The influence of the fiber taper geometry on the interference fringes is discussed. The proposed modal interferometer has a high refractive index (RI) sensitivity of ∼658 nm/RIU (refractive index unit) for RI= 1.333–1.353, which is expected to be useful for precision bio/chemical sensing applications.