Fiber-optic surface plasmon resonance (FOSPR) sensing technology has become an appealing candidate in biochemical sensing applications due to its distinguished capability of remote and point-of-care detection. However, FOSPR sensing devices with a flat plasmonic film on the optical fiber tip are seldom proposed with most reports concentrating on fiber sidewalls. In this paper, we propose and experimentally demonstrate the plasmonic coupled structure of a gold (Au) nanodisk array and a thin film integrated into the fiber facet, enabling the excitation of the plasmon mode on the planar gold film by strong coupling. This plasmonic fiber sensor is fabricated by the ultraviolet (UV) curing adhesive transferring technology from a planar substrate to a fiber facet. The experimental results demonstrate that the fabricated sensing probe has a bulk refractive index sensitivity of 137.28 nm/RIU and exhibits moderate surface sensitivity by measuring the spatial localization of its excited plasmon mode on Au film by layer-by-layer self-assembly technology. Furthermore, the fabricated plasmonic sensing probe enables the detection of bovine serum albumin (BSA) biomolecule with a detection limit of 19.35 μM. The demonstrated fiber probe here provides a potential strategy to integrate plasmonic nanostructure on the fiber facet with excellent sensing performance, which has a unique application prospect in the detection of remote, in situ, and in vivo invasion.

Simultaneous observation of Tamm plasmon (TP) and surface plasmon (SP) resonances has attracted much attention to improve the sensing performance and broaden the application field of plasmonic biosensors. Currently, enormous efforts are being devoted to simultaneously excite both SP and TP modes by exploiting total internal reflection. However, this method requires a bulky and complicated prism or microscopy setup to fulfill the phase-matching condition, limiting the miniaturization and integration of sensors. Herein, we propose and theoretically demonstrate a nanograting-coupled plasmonic-photonic multilayer structure potentially integrated on the optical fiber tip, comprising a multilayer of photonic crystal (PC) inserted between a flat gold film and a gold nanograting. The proposed plasmonic-photonic multilayer structure enables the simultaneous excitation of TP and SP modes at the normal incidence by using the zeroth and first-order diffraction light of the nanograting, respectively. The resonant positions of both SP and TP modes can be independently adjusted by changing the nanograting period and the center wavelength of the PC bandgap. Due to the insensitivity of the TP mode to ambient surroundings, the proposed structure can be served as a self-reference SP resonance sensor for robust measurement where the sensitivity of the SP resonance sensor is determined by the wavelength shift of SP mode relative to TP mode. Moreover, the spectral overlapping of both TP and SP modes results in the abrupt change of reflection intensity, which enables a rapid judgment to the refractive index of ambient surroundings by observing the red or blue shifts of SP resonant position relative to the case of TP. The simultaneous excitation of both TP and SP modes in the proposed structure opens a new path for improving detection accuracy and broadening application scenarios of fiber-optic SP resonance sensors.

Fiber-optic surface plasmon resonance (FOSPR) technology has drawn considerable attention to the label-free biosensing field. Currently, all FOSPR sensing devices capitalizing on flat plasmonic film are only performed on the fiber sidewalls, and demonstrated in a limited wavelength range, which is unfavorable to broaden their practical sensing application. Here, we develop and theoretically demonstrate an effective alternative for the realization of SPR on a flat gold film achieved on the end facet of optical fiber through the diffracting coupling of nanograting. This enables the consecutively adjustment of operating wavelength over a broad spectral range from visible (650 nm) to near-infrared light (1550 nm) through modulating the nanograting period. The sensing performances of the proposed nanograting-coupled FOSPR sensors with different specified resonance wavelengths are numerically investigated from two aspects of bulk refractive index and surface sensitivities. As a result, the highest bulk refractive index sensitivity of 1163 nm/RIU is obtained at the operating wavelength of 1550 nm and the optimal surface sensitivity is dependent on the location away from top surface of structure. The dependence of incident polarization direction on the sensing performances of the proposed structure are also explored. Due to the ease of adjustment of operating wavelength and compact integration on the end facet, this sensing platform can further extend the utility of FOSPR sensing technology in the practical application fields of disease diagnosis, environment monitoring, etc.

Plasmonic chirality, which has garnered significant attention in recent years due to its ability to generate strong near-field enhancement and giant circular dichroism (CD). Currently, various theories have been proposed to explain plasmonic extrinsic chirality, however, a comprehensively quantitative explanation for the high-order optical response of extrinsic metamolecule has yet to be established. Herein, we present a concise and quantitative explanation of the giant high-order extrinsic CD of a plasmonic nanocrescent, which origins from multipole decomposition in combination with the optical theorem. Our findings indicate that the high-order resonance modes exhibit giant CD comparable to dipolar modes and can be conveniently applied to the chiral recognition of metamolecules. Furthermore, the nonradiative electric quadrupole resonance exhibits enormous electric field enhancement near metamolecule, which has great application potential in the fields of molecular recognition and sensing in the visible region.

The coupling between surface plasmon polaritons (SPPs) and waveguide (WG) modes has been widely investigated by using prism-coupled structures and has demonstrated a large number of interesting physical phenomena. However, these conventional structures mainly rely on the angle-dependent total internal reflection excitation. This is not conducive to their further development due to the large volume and the requirement of oblique incidence. In this paper, we theoretically propose a three-layer nanograting-dielectric-metal (NDM) plasmonic structure. Within this structure, a thickness-dependent plasmonic WG (PWG) mode in the middle dielectric cavity strongly couples with SPPs on the top surface, resulting in two new hybridized PWG-SPPs modes. This hybridization coupling phenomenon is analyzed in detail by using plasmonic hybridization and two coupled oscillator models. Besides, a thorough investigation is conducted on the sensing performance of these two PWG-SPPs hybridized modes. The difference in sensing characteristics between these two hybridized modes can be well explained by their coupling strength variation. This NDM plasmonic nanostructure owns unparalleled advantages in the generation and modulation of a variety of new modes, effectively promoting the development of miniaturized optoelectronic devices.

In the current energy transition procedure, the application prospect of hydrogen as a clean energy material has attracted much attention. However, the widespread use of hydrogen is also accompanied by safety hazards, and how to detect hydrogen safely and efficiently has become a research focus. In this paper, we propose a fiber-optic hydrogen sensor based on the thermo-optic effect and nanomaterials, which combines the unique advantages of fiber-optic grating and platinum-loaded tungsten trioxide and is capable of detecting hydrogen concentration with high sensitivity. The principle of this sensor is to absorb hydrogen molecules by nanomaterials and trigger the exothermic effect, which leads to grating period change and refractive index change in the fiber, thus modulating the resonant wavelength of grating. By monitoring the wavelength drift in real time, the hydrogen concentration can be accurately detected. The experimental results show that the sensor can provide high sensitivity, fast response, wide detection range, and miniaturized design, which are suitable for hydrogen detection in complex environments. In addition, its dual-channel operational method further improves detection accuracy and environmental adaptability. This work provides technical support for safe hydrogen detection, which is suitable for hydrogen production, storage, industrial safety and environmental monitoring.