Ultrahigh Refractive Index Sensitivity Research Articles

Ultra-high sensitive refractive index sensor based on etched SNS fiber structure and self-imaging

An ultra-high sensitive refractive index (RI) sensor based on etched singlemode-nocore-singlemode (SNS) fiber structure and self-imaging effect is proposed. For a short nocore fiber (NCF) of 20 mm, the fourth self-imaging point (SIP) is obtained in the desired wavelength range by chemically reducing the NCF diameter to be 61.5 μm. Compared to the conventional SNS sensor, the sensitivity is significantly enhanced to1836.39 nm/RIU, which is dependent on a comprehensive effect of diameter reduction and the utilization of the fourth SIP. The shortening of NCF in SNS structure leads to the miniaturization of the sensor configuration. Furthermore, an FFT analysis reveals that the sensitivity difference between the SIP and other dips/peaks is generated by the superposition of interference of different modes. In addition, a thermal test shows an ultra-low response to temperature in the range from 30 °C to 80 °C, which results in the single-parameter response to RI. The small-size, ultra-high RI sensitivity, and low temperature sensitivity make this sensor more competitive in subsequent biochemical sensing applications.

In-situ monitoring of charge and discharge process in supercapacitor with a micro-cavity Mach-Zehnder interferometer and a fiber Bragg grating

Supercapacitors are promising energy storage devices. However, efficient monitoring of charge and discharge process remains challenging. Current methods are limited by large size, high cost and complex manufacturing processes, restricting their practical application. To address this issue, a low-cost and easily fabricated micro-cavity Mach-Zehnder interferometer (MZI) combined with a Fiber Bragg grating (FBG) is proposed for real-time, in-situ monitoring of charge and discharge process in supercapacitor. The MZI sensor has a ultrahigh refractive index sensitivity of −21833 nm/RIU. In cyclic voltammetry(CV) test, a charge sensitivity of 11.49 nm/C is obtained. Furthermore, the temperature sensitivities of MZI and FBG are 1.64 nm/℃ and 0.011 nm/℃ respectively, measured in an electrolyte environment. This innovative approach enables precise and instantaneous measurement of critical parameters like capacitance and temperature throughout energy storage and release cycles. The proposed compact, flexible and low-cost fiber MZI sensor offers a highly promising solution for in-situ supercapacitor monitoring.

Ultrahigh Refractive Index Sensitivity With Lossy Mode Resonance in a Side-Polished Low-Index Polymer Optical Fiber

Ultrahigh Refractive Index Sensitivity With Lossy Mode Resonance in a Side-Polished Low-Index Polymer Optical Fiber

Ultrasensitive, dynamic, and online monitoring photonic sensors for protein conformation

The monitoring of molecular dynamic transition and protein conformation changes is important for some neurodegenerative diseases (NDs) and has attracted much attention due to its ability to reveal the pathological mechanism of several NDs. Here, a label-free optical detection method with an ultrahigh refractive index sensitivity (S) and dynamical S tuning range was developed to provide a novel technology roadmap for the online monitoring of protein conformation. The real-time conformation variation process was transduced into a visualization optical signal, allowing it to be monitored. The biosensor prototype was constructed by the cross-overlapping of two rows of square capillaries on a gold mirror to form two microcavities with different free spectral ranges (denoted as the sensing and reference cavities). The coupling of the resonance mode between the two microcavities induced the generation of the Vernier effect and the magnifying of S, presented in the form of a modulated envelope in the spectrum. S values ranged from −17,256 nm/RIU to 93,937 nm/RIU, indicating that the biosensor could not only realize large dynamic tuning ranges but also an ultrahigh S. The conformation changes of silk proteins from the random coil to the β–sheet were sensitively monitored by the output spectrum after it was stimulated with ethanol solution. This ultrasensitive biosensor (which is both label-free and optical) provides a novel and supplementary technique to monitor the structural and conformation changes process of proteins. The results reveal its significant potential for clinical diagnosis, healthcare, pharmaceutical screening, and food security.

Ultrasensitive refractive index fiber sensor based on high-order harmonic Vernier effect and a cascaded FPI.

This paper proposes and demonstrates an ultrasensitive refractive index (RI) sensor based on harmonic Vernier effect (HEV) and a cascaded Fabry-Perot interferometer (FPI). The sensor is fabricated by sandwiching a hollow-core fiber (HCF) segment between a lead-in single-mode fiber (SMF) pigtail and a reflection SMF segment with an offset of 37 µm between two fiber centers to form a cascaded FPI structure, where the HCF is the sensing FPI, and the reflection SMF is the reference FPI. To excite the HEV, the optical path of the reference FPI must be multiple times (>1) that of the sensing FPI. Several sensors have been made to conduct RI measurements of gas and liquid. The sensor's ultrahigh RI sensitivity of up to ∼378000 nm/RIU can be achieved by reducing the detuning ratio of the optical path and increasing the harmonic order. This paper also proved that the proposed sensor with a harmonic order of up to 12 can increase the fabricated tolerances while achieving high sensitivity. The large fabrication tolerances greatly increase the manufacturing repeatability, reduce production costs, and make it easier to achieve high sensitivity. In addition, the proposed RI sensor has advantages of ultrahigh sensitivity, compactness, low production cost (large fabrication tolerances), and capability to detect gas and liquid samples. This sensor has promising potentials for biochemical sensing, gas or liquid concentration sensing, and environmental monitoring.

Ultrasensitive optofluidic coupled Fabry-Perot capillary sensors.

Refractive index (RI) measurements are pertinent in concentration and biomolecular detection. Accordingly, an ultrasensitive optofluidic coupled Fabry-Perot (FP) capillary sensor based on the Vernier effect for RI sensing is proposed. Square capillaries integrated with the coupled FP microcavity provide multiple microfluidic channels while reducing the complexity of the fabrication process. The incoherent light source and spectrometer used during measurement facilitate the development of a low-cost sensing system. An ultrahigh RI sensitivity of 51709.0 nm/RIU and detection limit of 2.84 × 10-5 RIU are experimentally demonstrated, indicating acceptable RI sensing performance. The proposed sensor has significant potential for practical and low-cost applications such as RI, concentration, or biomolecular sensing.

Ultrasensitive refractometer based on helical long-period fiber grating near the dispersion turning point.

Precise and accurate measurements of the optical refractive index (RI) for liquids are increasingly finding applications in biochemistry and biomedicine. Here, we demonstrate a dual-resonance helical long-period fiber grating (HLPFG) near the dispersion turning point (DTP), which exhibits an ultrahigh RI sensitivity (∼25546 nm/RIU at ∼1.440). The achieved RI sensitivity is, to the best of our knowledge, more than one order of magnitude higher than a conventional HLPFG. The ultrahigh RI sensitivity can improve the RI measurement precision and accuracy significantly. Furthermore, ultralow wavelength shifts (nearly zero) with temperature and strain ranging from 20 to 100°C and 0 to 2226 µε, respectively, are also demonstrated for the proposed HLPFG, which may be a good candidate for developing new low-cross-talk sensors.

Femtosecond laser processing for a high sensitivity fiber MZI microcavity.

An ultra-compact fiber inline Mach-Zehnder interferometer sensor based on femtosecond laser micromachining technology is demonstrated. It is found that the microstructure has an ultra-high refractive index sensitivity of 16660 nm/RIU when a femtosecond pulsed laser is used to remove the upper cladding and part of the core of a standard single-mode fiber. However, its temperature sensitivity is not much different from that of most pure quartz fibers and can be as high as 7.934 nm/°C when the microcavity is coated with a low-refractive-index ultraviolet adhesive, which was originally used for bonding glass. With this coating, however, it demonstrates excellent robustness.

Ultrasensitive and Broadband Optical Toroidal Modes in all‐Dielectric Nanostructures (Laser Photonics Rev. 16(3)/2022)

Toroidal Metamaterials In article number 2100404, Hui-Hsin Hsiao, and Ai-Yin Liu demonstrate optical toroidal dipole and anapole metamaterials through the designs of periodic amorphous silicon nano-pillars embedded in a spin-on glass layer. Upon normal incidence of plane wave, the transverse toroidal moment, stemming from a complete head-to-tail configuration of the magnetic dipole moments within each individual nanopillar, are excited in optical regime. The toroidal response can be enhanced and spectrally tuned by elongating cylindrical axis perpendicular to the light polarization and exhibit ultrahigh refractive index sensitivity compared to other multipoles.

Ultra compact and sensitive optical fiber interferometric refractive index sensor

An ultra-compact and sensitive Fabry-Perot interferometer liquid refractive index sensor is presented. The device consists of an inner reflection mirror in single-mode fiber written by femtosecond laser and a hollow-core fiber. Two micro-holes are drilled on the side wall of the hollow-core fiber by femtosecond laser micromachining, which enables the liquid freely enters or leaves the hollow-core cavity. An enhanced and precisely controllable amplification factor can be obtained for the Vernier effect used, which provides an ultra-high refractive index sensitivity of − 73417.9 nm/RIU (refractive index unit). The device has ultra-compact size, simple structure, high sensitivity, and is featured with easy fabrication and good reproducing capability, thus exhibiting high potential in many sensing applications in biomedicine and chemical industry.

Ultrasensitive and Broadband Optical Toroidal Modes in all‐Dielectric Nanostructures

AbstractDynamic toroidal dipole (TD) with its peculiar characteristic of broken space‐inversion and time‐reversal symmetries plays an important role in the fundamental physics of light–matter interaction. Here, TD metamaterials comprised of amorphous silicon nanopillar arrays embedded in spin‐on‐glass layer are experimentally demonstrated. Upon normal incidence of plane wave, the transverse toroidal moment and the associated anapole‐like state are excited in optical regime. The strong TD response stems from a complete head‐to‐tail configuration of the magnetic dipole moments within each individual nanopillar. Both the experimental and simulation results show that such TD mode sustains a large structural tolerance and can be spectrally tuned by elongating the cylindrical axis perpendicular to the light polarization, corresponding to a cross‐sectional variation from circular to elliptical shapes. The excited TD mode is found to exhibit ultrahigh refractive index sensitivity compared to other multipoles, resulting in a sensitivity of 459 nm (470 nm) per external refractive index change in the experiment (calculation). This approach provides a simple and straightforward path in realizing toroidal metamaterials and establishes a new flat‐optics platform for realizing active metadevices, sensors, and nonlinear nanophotonics.

Novel high-quality Fano resonance based on metal-insulator-metal waveguide with L-shaped resonators* *Project supported by the National Natural Science Foundation of China (Grant No. 51965007) and the “One thousand Young and Middle-Aged College and University Backbone Teachers Cultivation Program” of Guangxi, China (Grant No. 2019).

Developing a convenient method that can be routinely applied for ascertaining proportions of different vegetable oils employed in commercial blended edible oils remains a significant challenge. We address this issue by proposing a novel method for detecting volume fraction of different oils based on the fact that these oils are optically transparent and have slightly different indices of refraction at a given temperature and wavelength. Accordingly, we develop a highly sensitive sensor for measuring the index of refraction of oil blends based on Fano resonance spectra obtained using a metal-insulator-metal (MIM) waveguide structure comprising a gapped straight waveguide coupled with two L-shaped resonators. The index of refraction sensitivity and figure of merit of the structure are calculated based on modeling using the finite element method, and the waveguide structure is accordingly optimized by adjusting the different geometric parameters to achieve a high-quality Fano resonance spectrum. The optimized structure achieves an ultra-high refractive index sensitivity of 770 nm/RIU in terms of a refractive index unit (RIU) of 1. Moreover, a highly stable linear relationship is obtained between the refractive index of mixed edible oils and the resonance wavelength. Experimental results demonstrate that the proposed structure can detect slight changes in the volume fractions of the components in blended oils.

Ultrasensitive enhanced fabrication-tolerance refractometer based on PANDA-air-hole microfiber at the birefringent dispersion turning point.

We present an ultrasensitive enhanced fabrication-tolerance refractometer utilizing the polarimetric interference of a tapered PANDA-air-hole fiber (PAHF). To obtain high birefringence and unique group birefringence, the PAHF is specially designed by introducing double air holes into the cladding. Ultrahigh sensitivity can be achieved by reducing the group birefringence difference to zero, defined as birefringent dispersion turning point (BDTP). By modifying the diameter of PAHF, the birefringent dispersion can be effectively manipulated to reduce the group birefringence difference. In this way, the workable diameter range for realizing the ultrahigh sensitivity is twice as large as that of conventional microfibers. Additionally, the ultrasensitive wavelength band is dramatically expanded by at least 600 nm, enabling a compact structure and a flexible fiber-length design. Due to the tunable dispersion optimization, the distinctive properties of ultrahigh sensitivity, enhanced fabrication tolerance, and broadband operation can be achieved. We experimentally verified the ultrahigh refractive index sensitivity of 47223 nm/RIU around the BDTP, and the experimental results matched well with the simulations.

Enhancing the Visibility of Vernier Effect in a Tri-Microfiber Coupler Fiber Loop Interferometer for Ultrasensitive Refractive Index and Temperature Sensing

In this article a Vernier effect based sensor is analyzed and demonstrated experimentally in a tri-microfiber coupler (Tri-MFC) and polarization-maintaining fiber (PMF) loop interferometer (Tri-MFC-PMF) to provide ultrasensitive refractive index and temperature sensing. The main novelty of this work is an analysis of parameters of the proposed Tri-MFC-PMF with the objective of determining the conditions leading to a “strong” Vernier effect. It has been identified by simulation that the Vernier effect is a primary factor in the design of Tri-MFC-PMF loop sensing structure for sensitivity enhancement. It is furthermore demonstrated experimentally that enhancing the visibility of the Vernier spectrum in the Tri-MFC-PMF allows to achieve an ultrahigh refractive index and temperature sensitivity with improved measurement accuracy. Specifically it is shown that small values of the total phase difference ( <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>π</i></b> / <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</b> + <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>N</i><i>π</i></b> )∼( <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>π</i></b> / <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</b> + <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>N</i><i>π</i></b> ), where N is an integer, accumulated over the PMF loop and Tri-MFC loop result in a “strong” Vernier effect. Experimentally an ultrahigh refractive index sensitivity of -20588 nm/RIU and temperature sensitivity of 0.019 nm/°C are demonstrated by utilizing the stronger Vernier effect with “clear” Vernier spectrum. This analysis of the parameters may be useful to future researchers seeking to increase the measurement accuracy of sensors by enhancing the spectral visibility of the Vernier effect in other types of fiber optic interferometers.

Ultrasensitive refractive index sensor based on enhanced Vernier effect through cascaded fiber core-offset pairs.

An ultrasensitive refractive index (RI) sensor based on enhanced Vernier effect is proposed, which consists of two cascaded fiber core-offset pairs. One pair functions as a Mach-Zehnder interferometer (MZI), the other with larger core offset as a low-finesse Fabry-Perot interferometer (FPI). In traditional Vernier-effect based sensors, an interferometer insensitive to environment change is used as sensing reference. Here in the proposed sensor, interference fringes of the MZI and the FPI shift to opposite directions as ambient RI varies, and to the same direction as surrounding temperature changes. Thus, the envelope of superimposed fringe manifests enhanced Vernier effect for RI sensing while reduced Vernier effect for temperature change. As a result, an ultra-high RI sensitivity of -87261.06 nm/RIU is obtained near the RI of 1.33 with good linearity, while the temperature sensitivity is as low as 204.7 pm/ °C. The proposed structure is robust and of low cost. Furthermore, the proposed scheme of enhanced Vernier effect provides a new perspective and idea in other sensing field.

Lithography‐Free Random Bismuth Nanostructures for Full Solar Spectrum Harvesting and Mid‐Infrared Sensing

AbstractA lithography‐free, double‐functional single bismuth (Bi) metal nanostructure is designed, fabricated, and characterized for ultrabroadband absorption in the visible (vis) and near‐infrared (NIR) ranges, and for a narrowband response with ultrahigh refractive index sensitivity in the mid‐infrared (MIR) range. To achieve a large‐scale fabrication of the design in a lithography‐free route, the oblique‐angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It is shown that this fabrication technique can provide a bottom‐up approach to controlling the length and spacing of the design. The characterization findings reveal a broadband absorbance above 0.8 in vis and NIR, and a narrowband absorbance centered around 6.54 µm. Dense architecture and extraordinary permittivity of Bi provide strong field confinement in ultrasmall gaps between nanostructures, and this can be utilized for a sensing application. An ultrahigh sensitivity of 2151 nm refractive‐index unit (RIU–1) is acquired, which is, as far as it is known, the experimentally highest sensitivity attained so far. The simple and large‐scale compatible fabrication route of the design together with the extraordinary optical response of Bi coating makes this design promising for many optoelectronic and sensing applications.

In-line microfiber MZI operating at two sides of the dispersion turning point for ultrasensitive RI and temperature measurement

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.

Ultra-high sensitivity of dual dispersion turning point taper-based Mach-Zehnder interferometer.

We present here a detailed investigation into the sensitivity of the taper-based Mach-Zehnder interferometer as a function of external refractive index, with particular attention to the dispersion turning point (DTP) and possibilities for ultra-sensitive sensors. Our numerical simulation revealed that two DTPs exist with a decrease in the microfiber waist diameter; given this relationship, it is possible to obtain an ultra-sensitive operation. We then conducted experiments with fabricated devices with different waist diameters to achieve both positive and negative sensitivities at two DTPs. In particular, we achieved an ultrahigh refractive index sensitivity of approximately 95,832 nm/RIU at the second DTP when working with a diameter of 1.87 µm around the RI of air. These results show its potential for use in acoustic sensing and biochemical detection.

Tapered Fiber-Optic Mach-Zehnder Interferometer for Ultra-High Sensitivity Measurement of Refractive Index.

A Mach-Zehnder interferometer (MZI) based fiberoptic refractive index (RI) sensor is constructed by uniformly tapering standard single mode fiber (SMF) for RI measurement. A custom flame-based tapering machine is used to fabricate microfiber MZI sensors directly from SMFs. The fabricated MZI device does not require any splicing of fibers and shows excellent RI sensitivity. The sensor with a cladding diameter of 35.5 µm and length of 20 mm exhibits RI sensitivity of 415 nm/RIU for RI range of 1.332 to 1.384, 1103 nm/RIU for RI range of 1.384 to 1.4204 and 4234 nm/RIU for RI range of 1.4204 to 1.4408, respectively. The sensor reveals a temperature sensitivity of 0.0097 nm/°C, which is relatively low in comparison to its ultra-high RI sensitivity. The proposed inexpensive and highly sensitive optical fiber RI sensors have numerous applications in chemical and biochemical sensing fields.

Live E. coli bacteria label-free sensing using a microcavity in-line Mach-Zehnder interferometer

The paper presents the first study to date on selective label-free biosensing with a microcavity in-line Mach-Zehnder interferometer induced in an optical fiber. The sensing structures were fabricated in a single-mode fiber by femtosecond laser micromachining. In contrast to other studies of this sensing scheme, where only the sensitivity to refractive index changes in the cavity was investigated, this research used chemical surface treatment of the sensor to ensure detection specificity. Immobilized MS2 bacteriophages were applied as recognition elements specifically targeting live E. coli C3000 bacteria. It is shown that the sensor allows for real-time monitoring of biological phenomena taking place on the surface of the microcavity. The developed biosensor exhibits ultrahigh refractive index sensitivity of 15,000 nm/RIU and is capable of detecting live E. coli bacteria concentrations as low as 100 colony forming units (CFU)/mL in liquid volume as low as picoliters.