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• We propose a highly-sensitive phase-interrogated RI sensor based on twin-core fiber with inherent noise suppression. • It is the first time that optical fiber sensing structure is combined with digital holography technology, which works as the phase interrogation method, for measuring environmental parameters. • The background signal is reduced to 1/25 of the original noise level by using phase subtraction method within a single measurement. • Experimental results show that a short response time of about 13 s and a high sensitivity of about 835 rad/refractive index unit (RIU) are obtained. • The achieved high sensitivity is about 47 times higher than the sensitivity of a refractive index plasmonic sensor realized by enabling its quadrupole resonance in phase interrogation (17.46 rad/RIU). In this work, we propose a highly-sensitive phase-interrogated RI sensor based on twin-core fiber with inherent noise suppression. This is the first time that optical fiber sensing structure is combined with digital holography technology, which works as the phase interrogation method, for measuring environmental parameters. The proposed system consists of a Mach–Zehnder interferometer formed by interference between the reference Gauss beam and the sensing dual-beam from the twin-core fiber (TCF). In the sensing optical path, a non-core fiber (NCF) and TCF offset splicing structure works as the differential fluid RI sensing element, which provides the capability of suppressing background signal by calculating the phase difference between the two cores of the TCF. This simple and practical phase subtraction method realizes direct background subtraction within a single measurement and reduces the background signal to 1/25 of the original noise level. The fluid is continuously introduced into a fluid tube in which the sensing fiber is installed to realize in-line measurement of fluid RI. Experimental results show that a short response time of about 13 s and a high sensitivity of about 835 rad/refractive index unit (RIU) are obtained. The high sensitivity benefits from direct interaction between light path and fluid, which is about 47 times higher than the sensitivity of a refractive index plasmonic sensor realized by enabling its quadrupole resonance in phase interrogation (17.46 rad/RIU). This scheme will find many applications in little sample consumption and high sensitivity required analytical measurement owing to its merits of high sensitivity, little sample consumption, fast response, and inherent background noise suppression. [ABSTRACT FROM AUTHOR]

Abstract Focusing by the designed negative Luneburg lens which is based on graded annular photonic crystals (GAPCs) are studied in this paper. The GAPCs are composed of ring-shaped silicon pillars in a hexagonal lattice. The gradient of refractive index is achieved by modifying the radii of inner holes. The effective negative refractive index is analyzed based on the photonic band structures and verified by the simulation of negative refraction in APCs flat lens. Numerical simulation results show that a 0.6λ focusing spot can be realized by our designed Luneburg lens and it has the potential to be utilized in photonic integrated circuits. Highlights • We designed a negative Luneburg lens based on the graded annular photonic crystals (GAPCs) by ring-shaped silicon pillars. • The gradient of refractive index is achieved by modifying radii of the inner hole that has been analyzed and verified. • A 0.6λ focusing and coupling by GAPCs Luneburg lens show the potential to be utilized in photonic integrated circuits. [ABSTRACT FROM AUTHOR]

• Electric field intensity of multilayer of GNPs coupled with gold film was studied. • The RI sensitivity of the double-layer gold nanospheres sensor was 15747 nm/RIU. • The RI sensitivity of the double-layer gold nanorods sensor was 25642 nm/RIU. • The LOD of double-layer gold nanospheres biosensor is 10 ng/mL. • The LOD of double-layer gold nanorods biosensor is 4.6 ng/mL. The main challenge for all biosensors is how to measure the biological samples at very low concentrations. In this paper, a surface plasmon resonance (SPR) fiber biosensor using multi-layer gold nanoparticles (Au NPs)/Au film coupling to enhance sensitivity was proposed. The coupling effect between such longitudinally aligned multilayer nanoparticles increases the intensity of the local electromagnetic field and the depth of propagation to the outside. We used the finite element method to analyze the local enhancement effect of multi-layer Au nanospheres and Au nanorods, and the fabricated double-layer Au NPs (Au nanospheres and Au nanorods)/Au film coupling enhancement sensors. These two sensors' refractive index sensitivities are 15,747 nm/RIU and 25,642 nm/RIU, respectively. By changing biomolecules immobilized on the sensor's surface, the highly sensitive double-layer Au nanoparticle/Au film fiber sensor enables high-precision measurement of different biomarkers at low concentration. For detection of human IgG, the detection limit of gold nanosphere sensor is 10 ng/mL and gold nanorod sensor is 4.6 ng/mL. [ABSTRACT FROM AUTHOR]

• Ultra-sensitive RI measurement in large range is realized by an optical fiber sensing system. • An ultra-high sensitivity of 44271 nm/RIU is achieved by the microfiber MZI with DTP. • Measurement range of the microfiber MZI is expanded by using a SMF-PCF-SMF MZI. • The sensing scheme solves the trade-off between high sensitivity and large measurement range. A dual-channel optical fiber sensing system for refractive index (RI) measurement with ultra-high sensitivity and large measurement range is proposed and demonstrated by combining a rough measurement sensor and a precise measurement sensor. One sensing channel consists of an optical microfiber Mach-Zehnder interferometer (MZI), which works as a RI precise measurement sensor. The microfiber MZI is fabricated on a single mode fiber (SMF) by using a two-step tapering method. The microfiber MZI operating around dispersion turning point (DTP) shows an ultra-high sensitivity of up to 44271 nm/RIU, however, its measurement range is limited to be only 0.0015 owing to the periodic interference spectrum. The other sensing channel contains an SMF-photonic crystal fiber (PCF)-SMF MZI, which works as a RI rough measurement sensor. RI sensitivity of the SMF-PCF-SMF MZI is 143.67 nm/RIU in 1.3317–1.3710. RI measurement range of the microfiber MZI is expanded by firstly read a rough RI value from the SMF-PCF-SMF MZI and then determine the observation window of the interference spectrum of the microfiber MZI. This proposed sensing scheme solves the trade-off between high sensitivity and large measurement range caused by the periodic interference spectrum, and it is promising in liquid RI-related sensing fields with high requirements of both sensitivity and measurement range. [ABSTRACT FROM AUTHOR]

Schematics of the bare and the PDMS-coated in-line microfiber MZI for RI and temperature sensing are shown in Fig. 1 (a) and (b), respectively. The microfiber MZI which is fabricated by non-adiabatically tapering a single mode fiber is composed of two abrupt tapered regions and a uniform waist region. When the fundamental HE 11 mode is propagating through the first abrupt tapered region, only symmetrical high order modes (HE 1m) can be excited owing to the symmetry of the abrupt tapered regions. Besides the fundamental HE 11 mode, only the high order HE 12 mode is excited by the fabricated microfiber whose taper waist diameter is 4.8 μm. The mode profiles of HE 11 mode and HE 12 mode are shown in Fig. 1 (c) and (d). These two modes interfere with each other at the second abrupt tapered region, forming dual-path interference. • An in-line microfiber MZI is demonstrated for ultrasensitive surrounding RI and temperature measurement. • An ultrahigh RI sensitivity of 24,209 nm/RIU and a high temperature sensitivity of -2.47 nm/℃ are experimentally achieved. • By embedding this microfiber MZI into PDMS, the temperature sensitivity is improved from -2.47 nm/℃ to about 8.33 nm/℃. An in-line microfiber Mach-Zehnder interferometer (MZI) is demonstrated for ultrasensitive refractive index (RI) and temperature measurement by operating at two sides of the dispersion turning point (DTP), respectively. An ultrahigh RI sensitivity of 24,209 nm/RIU (RI unit) around 1.3320 and a high temperature sensitivity of -2.47 nm/℃ are experimentally achieved by a microfiber MZI with taper waist diameter of about 4.8 μm. By embedding this microfiber MZI into polydimethylsiloxane (PDMS), the temperature sensitivity is improved from -2.47 nm/℃ to about 8.33 nm/℃. The positive temperature sensitivity of this PDMS-coated microfiber MZI and the negative thermo-optic coefficient of PDMS show that this microfiber MZI has a negative RI sensitivity around the material RI of PDMS (about 1.405), which indicates that the microfiber MZI works at two sides of the DTP for RI measurement and temperature measurement, respectively. The PDMS-coated microfiber MZI could achieve a temperature measurement resolution of about 0.0024 ℃ if the resolution of the optical spectrum analyzer were 0.02 nm, making it suitable for temperature measurement applications with high precision requirement. The bare microfiber MZI is promising for liquid analytical measurement in fields like biochemistry for its advantages of ultrahigh sensitivity, small size, and little sample consumption. [ABSTRACT FROM AUTHOR]

A novel refractive index (RI) sensor based on an asymmetrical Mach–Zehnder interferometer (MZI) with two different step-like tapers is proposed. The step-like taper is fabricated by fusion splicing two half tapers with an appropriate offset. By further applying offset and discharging to the last fabricated step-like taper of MZI, influence of taper parameters on interference spectrum is investigated using only one device. This simple technique provides an on-line method to sweep parameters of step-like tapers and speeds up the optimization process of interference spectrum, meanwhile. In RI sensing experiment, the sensor has a high sensitivity of −185.79 nm/RIU (refractive index unit) in the RI range of 1.3333–1.3673. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, superlens realized by a two-dimensional (2D) triangular lattices graded photonic crystal (GPC) was studied. The graded photonic crystal (GPC) was obtained by varying the radius in each row so that its effective refractive index changes along the transverse direction. Plane wave expansion (PWE) method and finite-difference time-domain (FDTD) were adopted to show that negative refraction and superlens can be realized in the designed graded photonic crystal. [Copyright &y& Elsevier]

[Display omitted] • A kind of all-dielectric metasurface has been designed. • The physical mechanism of the toroidal dipole bound state in the continuum is studied in detail. • High quality-factor (54757) can be obtained with the designed metasurface. • The designed metasurface sensor can measure refractive index and temperature simultaneously. In this numerical study, an all-dielectric metasurface consisting of silicon disks with tilted split gap, which can measure the refractive index and temperature is proposed. It is demonstrated that the double resonances in the near-infrared wavelength are excited by toroidal dipole (TD) resonance in different directions. Further analysis proves that the resonance at the short wavelength corresponds to the TD resonance governed by the symmetry-protected bound state in the continuum (BIC), and it can be transformed to the TD quasi-BIC resonance with a high quality-factor (Q-factor) by breaking symmetry. The spectral responses to different incidence angles are evaluated. The high Q-factor of the TD quasi-BIC makes the designed metasurface a promising sensing candidate; the results show that the sensitivity and figure of merit for refractive index sensing are 746 nm/RIU and 18650 RIU−1, respectively. For temperature sensing, the sensitivity can be as high as 54 pm/°C. This study provides a new approach for the excitation of strong TD resonance quasi-BIC, which facilitates the design of high-performance sensing applications. [ABSTRACT FROM AUTHOR]

The paper proposed and studied a Mach-Zehnder mode interferometric refractive index sensor, which is based on splicing points tapered SMF-PCF-SMF (SMF, single-mode fiber; PCF, photonic crystal fiber) structure. For the reason that the effective refractive index of photonic crystal fiber cladding high-order modes near fiber core are more sensitive to surrounding refractive index changes, the refractive index measurement sensitivity of splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer can be enhanced further through tapering the splicing points. Relations between refractive index measurement sensitivity and photonic crystal fiber length and taper waist diameter are studied through numerical simulations and experiments. Simulation and experimental results show that sensitivity will be increased with the increase of photonic crystal fiber length and the decrease of taper waist diameter. In the refractive range of 1.3333–1.3737, splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm and taper waist diameter of 60.4 μm has refractive index measurement sensitivity of 260.8 nm/RIU, compared with sensitivity of 224.2 nm/RIU of direct splicing SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm, the sensitivity increased by 16.3%. The research shows that the sensing structure is with good linearity and repeatability. [ABSTRACT FROM AUTHOR]

Abstract: An ultrasensitive two-dimensional photonic crystal biosensor is theoretically demonstrated in this paper. Such device consists of a waveguide and high Q-value microcavities which are realized by introducing line and point defects into the photonic crystal respectively. The band structures and the transmission spectra are obtained from the Finite-Difference –Time-Domain (FDTD) method. The simulation results showed that such device is strongly sensitive to the refractive index of the analyte injected into the point defect. The designed device can be applied for measurements of the refractive index and detection of protein-concentrations. [Copyright &y& Elsevier]