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α-RuCl3, a narrow-band Mott insulator with a large work function, offers intriguing potential as a quantum material or as a charge acceptor for electrical contacts in van der Waals devices. In this work, we perform a systematic study of the optical reflection contrast of α-RuCl3 nanoflakes on oxidized silicon wafers and estimate the accuracy of this imaging technique to assess the crystal thickness. Via spectroscopic micro-ellipsometry measurements, we characterize the wavelength-dependent complex refractive index of α-RuCl3 nanoflakes of varying thickness in the visible and near-infrared. Building on these results, we simulate the optical contrast of α-RuCl3 nanoflakes with thicknesses below 100 nm on SiO2/Si substrates under different illumination conditions. We compare the simulated optical contrast with experimental values extracted from optical microscopy images and obtain good agreement. Finally, we show that optical contrast imaging allows us to retrieve the thickness of the RuCl3 nanoflakes exfoliated on an oxidized silicon substrate with a mean deviation of −0.2 nm for thicknesses below 100 nm with a standard deviation of only 1 nm. Our results demonstrate that optical contrast can be used as a non-invasive, fast, and reliable technique to estimate the α-RuCl3 thickness. [ABSTRACT FROM AUTHOR]

A highly sensitivity balloon-like fiber interferometer based on ethanol coating is presented in this paper. The Mach–Zehnder interferometer is formed by bending a single-mode fiber to a balloon-like structure and nested in the Teflon tube. Then, an ethanol solution was filled into the tube of the balloon-like fiber interferometer by the capillary effect. Due to the high sensitivity of the refractive index (RI) of ethanol solutions to temperature, when the external temperature varies, the optical path difference changes. The change in temperature can be detected by the shift in the interference spectrum. Limited by the size of the balloon-like structure, three kinds of these structures with different sensitive lengths were prepared to select the best parameters. The sensitive lengths were 10, 15 and 20 mm, respectively, and the RI detection performance of each structure in 10~26% NaCl solutions was investigated experimentally. The results show that when the sensitive length is 20 mm, the RI sensitivity of the sensor is the highest, which is 212.88 nm/RIU. Ultimately, the sensitive length filled with ethanol is 20 mm. The experimental results show that the temperature sensitivity of the structure is 1.145 nm/°C in the range of 28.1 °C~35 °C, which is 10.3 times higher than that of an unfilled balloon-like structure (0.111 nm/°C). The system has the advantages of low cost and easy fabrication, which can potentially be used in high-precision temperature monitoring processes. [ABSTRACT FROM AUTHOR]

A new type of human immunoglobulin G (IgG) sensors based on the surface plasmon resonance (SPR) in the low refractive index (RI) plastic optical fiber (POF) and an antibody immobilization method is presented. A 50-nm-thick gold film was formed on the polished D-shaped fiber surface by magnetron sputtering. The RI response of the POF sensor is 30 049.61 nm/RIU, which is 26.5 times higher than that of single mode fiber (SMF) SPR sensors. The proposed SPR biosensor can be developed by simple and rapid modification of the gold film with 11-mercapto undecanoic acid (MUA). Upon immobilization of the goat anti-human IgG antibody, the resonance wavelength shifts by 11.2 nm. The sensor can be used to specifically detect and quantify the human IgG at concentrations down to 245.4 ng/mL with the sensitivity of 1.327 7 nm per µg/mL, which offers an enhancement of 12.5-fold compared to that of the conventional SMF based SPR sensors. The proposed device may find the potential applications in the case of use at the point of care. [ABSTRACT FROM AUTHOR]

A distributed optical fiber refractive index sensor based on etched Ge-doped SMF in optical frequency domain reflection (OFDR) was proposed and demonstrated. The etched Ge-doped SMF was obtained by only using wet-etching, i.e., hydrofluoric acid solution. The distributed refractive index sensing is achieved by measuring the spectral shift of the local RBS spectra using OFDR. The sensing length of 10 cm and the spatial resolution of 5.25 mm are achieved in the experiment. The refractive index sensing range is as wide as 1.33–1.44 refractive index units (RIU), where the average sensitivity was about 757 GHz/RIU. Moreover, the maximum sensitivity of 2396.9 GHZ/RIU is obtained between 1.43 and 1.44 RIU. [ABSTRACT FROM AUTHOR]

An all-fiber glucose sensor is proposed and demonstrated based on a helical intermediate-period fiber grating (HIPFG) produced by using a hydrogen/oxygen flame heating method. The HIPFG, with a grating length of 1.7 cm and a period of 35 μm, presents four sets of double dips with low insertion losses and strong coupling strengths in the transmission spectrum. The HIPFG possesses an averaged refractive index (RI) sensitivity of 213.6 nm/RIU nm/RIU in the RI range of 1.33–1.36 and a highest RI sensitivity of 472 nm/RIU at RI of 1.395. In addition, the HIPFG is demonstrated with a low-temperature sensitivity of 3.67 pm/°C, which promises a self-temperature compensation in glucose detection. In the glucose-sensing test, the HIPFG sensor manifests a detection sensitivity of 0.026 nm/(mg/mL) and a limit of detection (LOD) of 1 mg/mL. Moreover, the HIPFG sensor exhibits good stability in 2 h, indicating its capacity for long-time detection. The properties of easy fabrication, high flexibility, insensitivity to temperature, and good stability of the proposed HIPFG endow it with a promising potential for long-term and compact biosensors. [ABSTRACT FROM AUTHOR]

Beam splitters are vital components in several optical systems. It is highly desirable, and compact beam splitters with ultra-broadband performances, high efficiencies, and large split angles are still being sought. In this paper, we demonstrate and numerically investigate an ultra-broadband and highly efficient optical beam splitter based on a quasi-continuous metasurface. The proposed design is constructed of quasi-continuous triangle-shaped gallium phosphide nanoantennas on a silica substrate. The simple structure can achieve a conversion efficiency and an anomalous transmission intensity above 90% and 0.8 covering the wavelength range of 1537–1826 nm, respectively. The maximum beam split angle in the operating bandwidth reaches 131.84° at the wavelength of 1826 nm. Particularly, the operating bandwidth is still as high as 125 nm with the anomalous transmission intensity above 0.92 and the conversion efficiency exceeding 99%. Moreover, the results show that the performance of the metasurface-based optical beam splitter can be further enhanced by optimizing structural parameters. We also demonstrate the adjustability of the beam splitter by adding refractive index (RI) materials on the surface of the device. The results show that the incident plane wave can be divided into three beams with intensity adjustability. The presented metasurface is very promising in the fields of multiplexers, interferometers, and optical communications, owing to its advantages of ultra-broadband, highly efficient, and large split angle simultaneously. [ABSTRACT FROM AUTHOR]

Blood glucose concentration is important for metabolic homeostasis in humans and animals. Many diabetic patients need to detect blood glucose daily which burdens community hospitals and family healthcare. Optical fiber sensors are widely used in biomedical detection because of their compact structure, fast response, high sensitivity, low cost, and ease of operation. In this work, we constructed a Fabry-Perot (FP) cavity biosensor for the fast detection of glucose concentration in serum. The femtosecond laser micromachining was applied to fabricate the FP cavity by printing the fiber-tip fixed-supported bridge at the end face of the optical fiber. An additional hemisphere was printed at the center of the outer surface of the bridge to avoid multi-beam interference. The results demonstrated that the proposed biosensor had high refractive index (RI) detection sensitivity, roughly 1039 nm/RIU at a wavelength of 1590 nm, and the detection sensitivity for glucose was around 0.185 nm/ (mg/mL) at a wavelength of 1590 nm. Due to its high sensitivity, compact structure, and fast response, the FP cavity biosensor has great potential to be applied in family healthcare for glucose concentration detection of diabetic patients. [ABSTRACT FROM AUTHOR]

We propose and numerically investigate a multi-parameter integrated sensor based on a selectively filled D-shaped photonic crystal fiber (PCF). The simple structure can be used to comprehensively detect refractive index, magnetic field, temperature, and voltage. According to the surface plasmon resonance and directional coupling effect, the PCF is coated with a gold nano-film to detect the refractive index of the external environment. In addition, magnetic fluid (water-based Fe3O4), toluene, and nematic liquid crystal (NLC E7) are selectively filled into different cladding air holes of the D-shaped PCF to realize the different sensing of the magnetic field, temperature, and voltage. The measurement of refractive index, magnetic field, temperature, and voltage are independent of each other, so these four parameters can be measured simultaneously. The sensing characteristics of the proposed structure are investigated systematically by the finite element method. The results show that the sensitivities of refractive index, magnetic field, temperature, and voltage are 4600 nm/RIU, 1.375 nm/Oe, 15.143 nm/°C, and 0.971 nm/V, respectively. The presented design based on materials selectively filled with D-shaped PCF might enable promising application in multi-parameter optical sensing. [ABSTRACT FROM AUTHOR]

Fiber Bragg gratings (FBGs) inscribed by using femtosecond laser point-by-point (PbP) technology typically have high birefringence due to the elliptical cross-sectional pattern of refractive index modulations (RIMs) created in the fiber core. Additionally, a highly reflective type II PbP FBG, which has a large coupling coefficient, also exhibits large insertion loss due to the limited RIM area induced by a single femtosecond laser pulse. Here we demonstrate a slit beam shaping method for femtosecond laser PbP inscription of high-quality FBGs, featuring by high reflectivity, low insertion loss, and low birefringence. The slit beam shaping method could reduce the ellipticity in the cross-sectional pattern of RIMs without reducing the cross-sectional area, leading to low birefringence and low insertion loss. The experimental results agree well with numerical calculations. Hence, a high-quality type II PbP FBG, which has high reflectivity of 99.12% (i.e., Bragg resonance attenuation of 20.52 dB), low insertion loss of 0.30 dB, and low birefringence of 1.86 × 10−6, was successfully created by use of a slit width of 0.8 mm. Moreover, an enlarged cross-sectional area was created by use of a slit width of 0.2 mm, resulting in a high ratio of 172.46 of the coupling strength coefficient to the scattering loss coefficient in the fabricated PbP FBG, which exhibits very high reflectivity of 99.99% (i.e., a strong Bragg resonance attenuation of 46.65 dB) and low insertion loss of 0.27 dB. Such high-quality FBGs will be promising in many applications, such as optical fiber communications, sensors, and lasers. [ABSTRACT FROM AUTHOR]

This study is intended to develop long period fibre grating sensors for potential applications in environmental and durability monitoring of coastal structures. High-quality helical long period fibre gratings (HLPFGs) are inscribed in different types of small-core single mode fibre (SMF) by use of hydrogen-oxygen flame heating technique. A detailed investigation of the effect of core diameter on their transmission spectrum and optimum length of the HLPFG has been pursued. A longer length is required to achieve the same coupling attenuation in a smaller-core SMF than that of a larger-core fibre. The strain, torsion and refractive index (RI) properties of the HLPFG is investigated experimentally to develop a high-sensitivity sensor. The experimental results show that the strain sensitivity could be enhanced by means of employing a larger-core diameter SMF. Moreover, the HLFPGs are also sensitive to the torsion and external RI. Hence, such HLFPGs have great potential for sensing applications. [ABSTRACT FROM AUTHOR]

Discriminating edible oils from gutter oils has significance in food safety, as illegal gutter oils cannot meet a variety of criteria such as the acid value, peroxide value and quality. To discriminate these illegal cooking oils, we propose an ultrasensitive optofluidic detection method based on a hybrid-waveguide coupler. Prior to the straight waveguide inscription in the cladding of the silica tube using a femtosecond laser, a section of coreless fibre is firstly spliced with the ST to supply a platform for the inscription of an S-band waveguide. Then a pair of microfluidic channels are ablated on the ST using the fs laser to enable liquid analytes to flow in and out of the air channel. In the transmission spectrum, a unique resonant loss dip can be observed, which is produced by coupling the light from the laser inscribed waveguide to the liquid core when the phase-matching condition is met. This hybrid-waveguide coupler with a simplified structure realizes dynamic optofluidic refractive index sensing with an ultrahigh sensitivity of −112 743 nm RIU−1, a detection limit of 2.08 × 10−5 RIU and a refractive index detection range from 1.4591 to 1.4622. This novel method can be used for food safety detection, specifically, for the discrimination of gutter oils. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate an automatic arc discharge technology for inscribing high-quality helically twisted long period fiber gratings (H-LPFGs) with greatly improved inscription efficiency for single mode fibers. The proposed technology has been developed by implementing an embedded program in a commercial fusion splicer, which employs an ultraprecision motorized translation stage while the tensioning mass required by conventional inscribing technology is eliminated. More significantly, the arc-induced H-LPFGs have been reported to have potential usage as sensors in temperature, refractive index, twist stress, and strain. [ABSTRACT FROM PUBLISHER]

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor. [ABSTRACT FROM AUTHOR]

An integrated surface plasmon resonance (SPR) based on an Au-film-coated, side-opened suspended core fiber is proposed and simulated to develop a highly sensitive, real-time refractive index sensor. The inner surface of the side-opened hole is supposed to be uniformly deposited with a thin Au film for realizing the SPR operation. Such a sensor not only ensures a fast response time, but also brings up a designing flexibility for SPR biosensing applications. Two potential sensing mechanisms, i.e., monitoring the transmission loss spectrum peak shift and measuring the transmission power change at a fixed wavelength are utilized and analyzed to achieve a better sensitivity. Moreover, our calculation results show that this novel sensor could cover a wide index detection range with an optimized resolution of up to 2.3e−5 refractive index unit. [ABSTRACT FROM PUBLISHER]

We proposed and demonstrated a novel scheme for simultaneous measurement of temperature and refractive index (RI) by cascading a liquid-filled photonics crystal fiber (PCF) to a long period fiber grating (LPFG) written in a single-mode fiber. The liquid-filled PCF showed a high-temperature sensitivity of 1.695 nm/°C, which is two orders of magnitude higher than that of the LPFG (0.0642 nm/°C). The LPFG exhibited a blue shift with the increased RI. In contrast, liquid-filled PCF showed a total immunity to the surrounding RI. So, the temperature was solely measured by the liquid-filled PCF, and the RI information was extracted from the total response of the LPFG with the temperature effect having been compensated by the liquid-filled PCF. [ABSTRACT FROM PUBLISHER]

Highlights: [•] We fabricated a refractive index sensor based on in-fiber Michelson interferometer. [•] The sensor has the advantages of short size (less than 2mm) and easy fabrication. [•] We use the method of intensity-modulated to solve cross sensitivity. [•] The sensor has the sensitivity of −208.24dB/RIU at the refractive index of 1.440. [•] The sensor could apply in the fields of chemical and biomedical sensing. [Copyright &y& Elsevier]

Laser direct writing (LDW) is versatile in structuring fibers with micro‐sized functional elements such as fiber Bragg grating (FBG) and backscattering centers by finely manipulating back and side scattering from laser‐induced refractive index modified (RIM) points. However, the side‐scattering is a lesser‐explored property in laser‐structured fibers. In this work, a concise physical model is established to understand the side‐scattering as a combined effect of microstructure and geometry of RIM points. Based on a single‐pulsed LDW method, the parametric decoupling between scattering loss (α) and coupling strength (κ) coefficients of FBGs is reported, whose cross‐section is customized to have a flattened ellipse with thoroughly positive RIM, enabling controllable reflectivity from −21.33 dB to −0.0018 dB while maintaining narrow bandwidth and low loss. Exemplarily, a designed FBG realizes ultra‐low loss of 0.008 dB with a resonance attenuation of 10.81 dB, exhibiting a record‐breaking κ/α of 2083. Using this FBG as the high‐reflective mirror of a home‐made bismuth‐doped fiber laser, narrow‐band lasing with a high optical signal‐to‐noise ratio of ≈43 dB is achieved, demonstrating flexibilities of the proposed approach in customizing both back‐ and side‐scattering in fibers and opening up wide opportunities for combining multifunctional components into optical fibers and realizing all‐fiber networks. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a fiber Bragg grating (FBG) with a wide range and a comb with continuous cladding mode resonances inscribed in non-photosensitive single mode fibers using a femtosecond laser and a phase mask. The FBG is inscribed in the core and cladding, exciting a series of cladding modes in transmission. The birefringence induced by this FBG structure offers significant polarization-dependence for cladding modes, thus allowing the vector fiber twist to be perceived. By measuring the peak-to-peak differential intensity of orthogonally polarized cladding mode resonances, the proposed sensor presents totally opposite intensity response in the anticlockwise direction for the torsion angle ranging from −45° to 45°. The cladding mode comb approximately covers wavelengths over the O-, E-, S-, and C-bands in transmission. The cutoff cladding mode of air can be observed in the spectrum. Thus, the sensible refractive index range is estimated to be from 1.00 to 1.44. Temperature responsivity of the grating is also characterized. The proposed device potentially provides new solutions to the various challenges of physical vector and bio-chemical parameters sensing. [ABSTRACT FROM AUTHOR]

A high sensitivity fiber-optic sensor based on self-imaging effect in a hollow-core capillary waveguide (HCCW) is presented for sensing applications. The sensor is composed of a section of HCCW fusion spliced between single mode fibers (SMFs). The self-imaging effect in the HCCW is investigated with different fiber lengths and arc-fusion parameters. By infiltrating the hollow core with index matching liquids, the peak wavelength of the proposed device shifts towards longer wavelengths. The temperature and refractive index (RI) responses of the sensor are studied systematically. When temperature is increased from 25 °C to 75 °C, the temperature sensitivity of the device can be improved significantly with the infiltrated structure, and reaches −0.49 nm/°C, compared with that of the un-filled device, which is 9.8 pm/°C. For the RI response, the liquid-filled structure achieves sensitivity of 12,005 nm/RIU in the range between 1.448 and 1.450, slightly higher than the 11,920 nm/RIU achieved by the un-filled one. The proposed sensor exhibits the advantages of simple structure, high sensitivity and low cost, which may find potential applications in physical and chemical sensing. [ABSTRACT FROM AUTHOR]

A highly sensitive temperature sensor based on surface plasmon resonance (SPR) of a side-polished single mode fiber is demonstrated. The sensor consists of a gold film coated side-polished fiber covered by a layer of UV-curable adhesive. Before introducing the UV-curable adhesive, the gold-coated fiber exhibits refractive index (RI) sensitivity of 1691.6 nm/RIU to 8800 nm/RIU in the range of 1.32 to 1.43. The resonant wavelength of the SPR sensor shifts to 650 nm when the adhesive is coated on the gold film, and is fixed at about 725 nm when the adhesive is cured. Due to the high thermo-optic and thermal expansion coefficient of the adhesive, the sensor structure achieves a temperature sensitivity of −0.978 nm/°C between 25 °C and 100 °C. The proposed optical fiber SPR sensor is simple, highly sensitive and cost effective, which may find potential applications for temperature measurements in the biomedical and environmental industries. [ABSTRACT FROM AUTHOR]

Optical manipulation is a non-contact and non-destructive method to capture and manipulate micro/nano particles, biological macromolecules and cells with application of optical forces. With the rapid development of optical manipulation, various kinds of optical fiber tips have been presented and demonstrated in recent years. In this letter, a novel structure of optical fiber tip with an imbedded hollow cavity is presented. The electric field distribution of the fiber tip and the influence of cavity geometry are simulated with finite-difference time-domain method. The unique advantage is that the electric field intensity will be significantly enhanced when the fiber tip is filled with high refractive index medium in the cavity, which brings the potential to generate a three-dimensional optical trap for particles. Through experiments, we also demonstrate that the velocity of the particles driven by the optical force has a quadratic relationship with the laser power. The relationship can be used in microfluidic technology to measure the velocity of micro fluid. And the balance between the scattering force and the fluid force can be used to study non-invasive migration of particles. [ABSTRACT FROM AUTHOR]

Abstract: Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing is studied in this paper. The modes propagation properties and coupling properties in cladding index modulated FBG are studied based on the classical coupling mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Evolution of cladding mode resonance spectra of cladding index modulated FBG with the decreasing of fiber diameter is simulated and compared with the core grating. Sensing characteristics of cladding modes resonance of cladding index modulated FBG to the variation of ambient refractive index is detailed and emphasized for cladding modes resonances has a much narrower bandwidth. Results show that cladding index modulated fiber grating has better spectrum for ambient refractive index sensing applications under the same sensitivity with core index modulated fiber grating and may be applied to many field including biochemical sensing and environmental monitoring. [Copyright &y& Elsevier]

We demonstrate a surface plasmon resonance refractive sensor based on the side-polished single mode optical fiber and investigated the sensor performance decay induced by silver coating degradation. The sensor showed fast response to the refractive index liquid in the range from 1.32 to 1.40 RIU. The highest sensitivity up to 4365.5 nm/RIU and a FOM of 51.61 RIU −1 were achieved, which were comparable to the reported polished-fiber-based SPR sensors. Characteristics of the silver-coated sensor after exposure to the air as well as to the liquid environment were investigated. The sensor always stored in dry air could respond stably two weeks later with sensitivity comparable with the fresh one and lower resolution due to the broadened resonant dip. But the sensor immersed in the refractive liquid for 30 min firstly and then exposed to ambient air for hours decayed dramatically due to silver degradation. The presented silver-coated SPR sensor could be used as a simple, cost-effective, disposable device in bio-chemical, environment, and food safety detection. [ABSTRACT FROM AUTHOR]

Breath, as an important health monitoring indicator, provides valuable diagnostic information for cardiovascular disease and pulmonary function. Humidity can act as a bridge between breath and sensing signals. Current monitoring methods depend on humidity-sensitive material characteristics. In this work, an all fiber-optic flexible humidity sensor for wearable breath monitoring is reported. An eccentric fiber Bragg grating (EFBG) is inscribed in a single mode fiber to excite a stable core mode and sensitive cladding modes. The core mode is shown to maintain stable spectral features under a high-humidity atmosphere and can be used to calibrate the wavelength and power of the system. Importantly, the interface evanescent field of the cladding mode is highly sensitive to the ambient refractive index (RI) and even humidity-induced RI variation. Without combining any sensitized material, EFBG can directly perceive humidity fluctuations during breath with fast response (92 ms) and recovery times (100 ms). Different breathing patterns can be recognized, and breathing frequency can be extracted by sensor responses. The EFBG humidity sensor demonstrates great reproducibility, fast response, high flexibility, excellent robustness, and self-compensation capability, showing promising potential for wearable breath monitoring. • A novel all-fiber humidity sensor without any additional sensitive material is proposed for human breath monitoring. • The proposed sensor is successfully applied to monitor human breathing with different breathing pattern. • The proposed sensor offers self-compensation for source power fluctuation and temperature effect. • The long-term stability, reliability and reproducibility of proposed sensor are superior to most sensitized material based sensors. [ABSTRACT FROM AUTHOR]