Kaiwei Li

Ultrasensitive optical microfiber coupler based sensors operating near the turning point of effective group index difference

We propose and study an optical microfiber coupler (OMC) sensor working near the turning point of effective group index difference between the even supermode and odd supermode to achieve high refractive index (RI) sensitivity. Theoretical calculations reveal that infinite sensitivity can be obtained when the measured RI is close to the turning point value. This diameter-dependent turning point corresponds to the condition that the effective group index difference equals zero. To validate our proposed sensing mechanism, we experimentally demonstrate an ultrahigh sensitivity of 39541.7 nm/RIU at a low ambient RI of 1.3334 based on an OMC with the diameter of 1.4 μm. An even higher sensitivity can be achieved by carrying out the measurements at RI closer to the turning point. The resulting ultrasensitive RI sensing platform offers a substantial impact on a variety of applications from high performance trace analyte detection to small molecule sensing.

Design and analysis of surface plasmon resonance sensor based on high-birefringent microstructured optical fiber

Optical fiber based surface plasmon resonance (SPR) sensors are favored by their high sensitivity, compactness, remote and in situ sensing capabilities. Microstructured optical fibers (MOFs) possess microfluidic channels extended along the entire length right next to the fiber core, thereby enabling the infiltrated biochemical analyte to access the evanescent field of guided light. Since SPR can only be excited by the polarization vertical to metal surface, external perturbation could induce the polarization crosstalk in fiber core, thus leading to the instability of sensor output. Therefore for the first time we analyze how the large birefringence suppresses the impact of polarization crosstalk. We propose a high-birefringent MOF based SPR sensor with birefringence larger than 4 × 10−4 as well as easy infiltration of microfluidic analyte, while maintaining sensitivity as high as 3100 nm/RIU.

In-line optofluidic refractive index sensing in a side-channel photonic crystal fiber

An in-line optofluidic refractive index (RI) sensing platform is constructed by splicing a side-channel photonic crystal fiber (SC-PCF) with side-polished single mode fibers. A long-period grating (LPG) combined with an intermodal interference between LP01 and LP11 core modes is used for sensing the RI of the liquid in the side channel. The resonant dip shows a nonlinear wavelength shift with increasing RI over the measured range from 1.3330 to 1.3961. The RI response of this sensing platform for a low RI range of 1.3330-1.3780 is approximately linear, and exhibits a sensitivity of 1145 nm/RIU. Besides, the detection limit of our sensing scheme is improved by around one order of magnitude by introducing the intermodal interference.

Graphene enhanced surface plasmon resonance fiber-optic biosensor

We experimentally demonstrate a side-polished optical fiber based graphene-on-gold biosensor. Single layer of graphene is deposited to improve the sensitivity in single-stranded DNA detection. Our proposed biosensor provides a detection limit lower than 1 pM.

Optical Micro/Nanofiber-Based Localized Surface Plasmon Resonance Biosensors: Fiber Diameter Dependence

Integration of functional nanomaterials with optical micro/nanofibers (OMNFs) can bring about novel optical properties and provide a versatile platform for various sensing applications. OMNFs as the key element, however, have seldom been investigated. Here, we focus on the optimization of fiber diameter by taking micro/nanofiber-based localized surface plasmon resonance sensors as a model. We systematically study the dependence of fiber diameter on the sensing performance of such sensors. Both theoretical and experimental results show that, by reducing fiber diameter, the refractive index sensitivity can be significantly increased. Then, we demonstrate the biosensing capability of the optimized sensor for streptavidin detection and achieve a detection limit of 1 pg/mL. Furthermore, the proposed theoretical model is applicable to other nanomaterials and OMNF-based sensing schemes for performance optimization.

Highly sensitive gas refractometers based on optical microfiber modal interferometers operating at dispersion turning point

In most fiber-optic gas sensing applications where the interested refractive index (RI) is ~1.0, the sensitivities are greatly constrained by the large mismatch between the effective RI of the guided mode and the RI of the surrounding gaseous medium. This fundamental challenge necessitates the development of a promising fiber-optic sensing mechanism with the outstanding RI sensitivity to achieve reliable remote gas sensors. In this work, we report a highly sensitive gas refractometer based on a tapered optical microfiber modal interferometer working at the dispersion turning point (DTP). First, we theoretically analyze the essential conditions to achieve the DTP, the spectral characteristics, and the sensing performance at the DTP. Results show that nonadiabatic tapered optical microfibers with diameters of 1.8-2.4 µm possess the DTPs in the near-infrared range and the RI sensitivities can be improved significantly around the DTPs. Second, we experimentally verify the ultrahigh RI sensitivity around the DTP using a nonadiabatic tapered optical microfiber with a waist diameter of ~2 μm. The experimental observations match well with the simulation results and our proposed gas refractometer provides an exceptional sensitivity as high as -69984.3 ± 2363.3 nm/RIU.

Large-scale synthesis of single-crystalline self-standing SnSe2 nanoplate arrays for wearable gas sensors

Advances in two-dimensional semiconducting thin films enable the realization of wearable electronic devices in the form factor of flexible substrate/thin films that can be seamlessly adapted in our daily lives. For wearable gas sensing, two-dimensional materials, such as SnSe2, are particularly favorable because of their high surface-to-volume ratio and strong adsorption of gas molecules. Chemical vapor deposition and liquid/mechanical exfoliation are the widely applied techniques to obtain SnSe2 thin films. However, these methods normally result in non-uniform and isolated flakes which cannot apply to the practical industrial-scale wearable electronic devices. Here, we demonstrate large-scale (10 cm × 10 cm), uniform, and self-standing SnSe2 nanoplate arrays by co-evaporation process on flexible polyimide substrates. Both structural and morphological properties of the resulting SnSe2 nanoplates are systematically investigated. Particularly, the single-crystalline SnSe2 nanoplates are achieved. Furthermore, we explore the application of the polyimide/SnSe2 nanoplate arrays as wearable gas sensors for detecting methane. The wearable gas sensors show high sensitivity, fast response and recovery, and good uniformity. Our approach not only provides an efficient technique to obtain large-area, uniform and high-quality single-crystalline SnSe2 nanoplates, but also impacts on the future developments of layered metal dichalcogenides-based wearable devices.

Formation of ultra-flexible, conformal, and nano-patterned photonic surfaces via polymer cold-drawing

We report a lithography-free method for large-area plasmonic nano-patterning on ultrathin plastic films through a polymer cold-drawing process. We further transfer the ultra-flexible nano-patterned films onto the curved surfaces of plant leaves and apples to work as conformal SERS sensors.

High-Q silicon microsphere whispering gallery mode resonator fabricated by laser induced in-fiber capillary instability

A method of fabricating silicon microsphere resonators feathered high quality factor whispering gallery modes of ∼ 0.8 × 10⁶ is demonstrated. We fabricate the silicon microspheres from silica-cladding/silicon-core fiber based on the CO<inf>2</inf> laser induced in-fiber capillary instability phenomenon.

Ordered and Atomically Perfect Fragmentation of Layered Transition Metal Dichalcogenides via Mechanical Instabilities

Thermoplastic polymers subjected to a continuous tensile stress experience a state of mechanical instabilities, resulting in neck formation and propagation. The necking process with strong localized strain enables the transformation of initially brittle polymeric materials into robust, flexible, and oriented forms. Here we harness the polymer-based mechanical instabilities to control the fragmentation of atomically thin transition metal dichalcogenides (TMDs). We develop a simple and versatile nanofabrication tool to precisely fragment atom-thin TMDs sandwiched between thermoplastic polymers into ordered and atomically perfect TMD nanoribbons in arbitrary directions regardless of the crystal structures, defect content, and original geometries. This method works for a very broad spectrum of semiconducting TMDs with thicknesses ranging from monolayers to bulk crystals. We also explore the electrical properties of the fabricated monolayer nanoribbon arrays, obtaining an on/off ratio of ∼106 for such MoS2 arrays based field-effect transistors. Furthermore, we demonstrate an improved hydrogen evolution reaction with the resulting monolayer MoS2 nanoribbons, thanks to the largely increased catalytic edge sites formed by this physical fragmentation method. This capability not only enriches the fundamental study of TMD extreme and fragmentation mechanics, but also impacts on future developments of TMD-based devices.