Evanescent Field Research Articles

Dissolved oxygen monitoring using coating-free, surface-activated, intensity-modulated coreless termination fibre sensors and transducers

The study on the structural change of the coreless termination fibre (CTF) sensing region as a result of surface modification through wet cleaning under three conditions (organic, alkaline, and acidic solutions) resulted in excellent DO sensing and monitoring performance at zero salinity and ambient pressure and temperature. The modulation of light and detection of DO by absorbance-evanescent field sensing mechanism is also disclosed. Coating-free with improved surface-activated, intensity-modulated evanescent field sensor and transducer for DO sensing and monitoring is presented. The experiment showed that the optimised sample, CTF treated with 1 M of sulphuric acid, H2SO4 had produced a good sensitivity of 1.93 × 10−3 a.u./% (with an R-squared value of 0.99 and achieved more than 120% sensitivity improvement as compared to the untreated sensing region of the CTF), stability with variance and standard deviation values of 2.92 × 10−5 and 5.41 × 10−3 respectively and a detection limit of 8.40 %, equivalent to 0.69 mg/L, The optimised sample provided a good selectivity in DO sensing and monitoring in comparison with common colourless and odourless soluble gasses in deionised water, in this study. The sensor also offered a good relative response time compared to a commercial galvanic-based DO transducer used as a reference with the value of 2 µs. Owing to the advantages of structural change of the CTF sensing region with improved optical responses, surface activation, wettability, surface morphology, and surface roughness, the proposed design of the sensor and transducer provides a powerful choice for DO sensing and monitoring.

Impact of Mass-Gap on the Dispersion Interaction of Nanoparticles with Graphene out of Thermal Equilibrium

We consider the nonequilibrium dispersion force acting on nanoparticles on the source side of a gapped graphene sheet. Nanoparticles are kept at the environmental temperature, whereas the graphene sheet may be either cooler or hotter than the environment. Calculation of the dispersion force as a function of separation at different values of the mass-gap parameter is performed using the generalization of the fundamental Lifshitz theory to out-of-thermal-equilibrium conditions. The response of the gapped graphene to quantum and thermal fluctuations in the electromagnetic field is described by the polarization tensor in (2+1)-dimensional space–time in the framework of the Dirac model. The explicit expressions for the components of this tensor in the area of evanescent waves are presented. The nontrivial impact of the mass-gap parameter of graphene on the nonequilibrium dispersion force, as compared to the equilibrium one, is determined. It is shown that, unlike the case of pristine graphene, the nonequilibrium force preserves an attractive character. The possibilities of using the obtained results in the design of micro- and nanodevices, incorporating nanoparticles and graphene sheets for their functionality, is discussed.

Microfiber evanescent-field photothermal gas detection using acoustic-induced mode-dependent frequency shift

Abstract In this study, we experimentally showcase the microfiber evanescent-field photothermal gas detection by exploiting all-fiber MHz-level frequency shift scheme. Based on the acousto-optic interaction effect, the low-frequency shifts of 0.9 MHz and 1.83 MHz can be obtained through the cyclic conversion between the transverse core modes LP01 and LP11 in the few-mode fiber. Our proposed all-fiber frequency shifters show flexible MHz-level up(down) frequency shifts with superior sideband rejection ratio (over 40 dB) and low insertion loss (less than 1 dB). Furthermore, an all-fiber heterodyne interferometric detection system is implemented by leveraging the above low-frequency shifters, in which around 1-μm-diameter microfiber is investigated for photothermal gas detection. A pump-probe configuration is employed to obtain the photothermal effect induced by the gas absorption of the modulated evanescent field. By demodulating the phase of the beat signal output by the interferometer, an equivalent detection limit (1σ) of 32 ppm and a response time of 22 s are achieved for ammonia, as well as 0.24 % instability within 48 pump cycles. Given its compact all-fiber configuration and high sensitivity with fast response, the experimental results can pave the way for widespread applications like heterodyne detection, fiber optical sensors, and interplanetary coherent communications.

An arbitrary Lagrangian–Eulerian method for analyzing finite-amplitude viscous acoustic waves radiated from vibrational solid boundaries: An implicit method

This paper is concerned with the development of an arbitrary Lagrangian–Eulerian (ALE) method for predicting nonlinear acoustic waves radiated from large-amplitude vibrational solid boundaries immersed in viscous fluids of infinite extent. The method is formulated based on a nonlinear acoustic model characterized by velocity potential and acoustic pressure. Unsplit implementations of three perfectly matched layer (PML) techniques are presented, namely conventional PML, convolution PML, and complex frequency-shifted PML (CFS-PML), to enable bounded-domain modeling of nonlinear acoustic waves propagating in an unbounded fluid medium. The long-time stability characteristics of the three distinct PML techniques are examined through theoretical analysis and numerical verification of their ability to absorb evanescent waves. Our results reveal that the CFS-PML technique exhibits superior long-time stability performance compared to the other two techniques. The CFS-PML is also compared to two different absorbing layer techniques: the Kosloff Absorbing Layer (KAL) technique and the Acoustic Black Hole (ABH) technique. The finite element method is adopted for spatial discretization of the finite-amplitude acoustic wave model, and an implicit predictor–corrector algorithm is proposed for time advancement. Several benchmark problems, including acoustic waves produced by a vibrational piston, and transversely oscillating and pulsating cylinders, are analyzed by the proposed method. It is found that the proposed method attains a second-order rate of convergence for spatial discretization, and the convergence rate in terms of the time step size is about first-order. The application of the method to acoustic levitation problems is also demonstrated, and the nonlinear phenomenon of acoustic waves in a single-axis acoustic levitator is discussed.

Evanescent wave in multiple slit diffraction and n-array antennas in metamaterial using Cesàro convergence

In this paper, we study Multiple slit diffraction and n- array linear antennae in negative refractive index material. We show that the evanescent wave plays a vital role in the near-field term. The evanescent wave grows significantly, unlike in conventional materials, and satisfies a novel kind of convergence known as Cesàro convergence. We calculate the intensity of multiple slits and the antenna’s amplification factor (AF) in terms of the Riemann zeta function. We further demonstrate that the Riemann zeta function gives rise to additional nulls. We deduce that all the diffraction scenarios in which the traveling wave satisfies the geometric series in the medium of the positive refractive index will enhance the evanescent wave, which satisfies Cesàro convergence in the medium of the negative refractive index.

Topological enhancement of evanescent field localization in all-dielectric metasurfaces

Bound states in a continuum in periodic photonic structures can be described as topological defects in the k-space. These vortices can migrate within the Brillouin zone when the geometrical parameters of the system vary. In this study, we focus on the migration of the polarization vortex across the light line, from the radiation continuum to the waveguide mode domain. We found that during this transition, the far-field polarization vortex transforms into a near-field vortex. This near-field vortex is characterized by an enhanced localization of the evanescent field due to the vanishing fundamental Fourier harmonic of the Bloch amplitude. This mechanism can be used to improve polaritonic devices and optical sensors based on all-dielectric metasurfaces by increasing the overlap between optical modes and 2D excitons or thin layers of bio-analytes.

Rapid detection of SARS-CoV-2 genetic targets using nanoporous waveguide based competitive displacement assay

Rapid detection of unlabeled SARS-CoV-2 genetic target was demonstrated using a competitive displacement hybridization assay made by a nanostructured anodized alumina oxide (AAO) membrane. The assay applied the toehold-mediated strand displacement reaction. The nanoporous surface of the membrane was functionalized with a complementary pair consisting of Cy3-labeled probe and quencher-labeled nucleic acids through a chemical immobilization process. In the presence of the unlabeled SARS-CoV-2 target, the quencher-tagged strand of the immobilized probe-quencher duplex was separated from the Cy3-modifed strand. A stable probe-target duplex formed and regained a strong fluorescence signal, thus enabling real-time and label-free SARS-CoV-2 detection. Assay designs with different numbers of base pair (bp) matches were synthesized to compare their affinities. Because of the large surface of a free-standing nanoporous membrane, two orders enhancement of the fluorescence was observed, where the detection limit of the unlabeled concentration can be improved to 1 nM. The assay was miniaturized by integrating a nanoporous AAO layer onto an optical waveguide device. The detection mechanism and the sensitivity improvement of the AAO-waveguide device were illustrated from the finite difference method (FDM) simulation and the experimental results. Light-analyte interaction was further improved due to the presence of the AAO layer, which created an intermediate refractive index and enhanced the waveguide's evanescent field. Our competitive hybridization sensor is an accurate and label-free testing platform applicable to the deployment of compact and sensitive virus detection strategies.

Fiber Optic SERS Sensor with Silver Nanocubes Attached Based on Evanescent Wave for Detecting Pesticide Residues.

Surface-enhanced Raman scattering (SERS) has great potential in the field of rapid detection of pesticide residues in food. In this paper, a fiber optic SERS sensor excited by evanescent waves was proposed for efficient detection of thiram. Silver nanocubes (Ag NCs) were prepared as SERS active substrates, which had much stronger electromagnetic field intensity than nanospheres under laser excitation due to much more "hot spots". By using the method of electrostatic adsorption and laser induction, Ag NCs were uniformly assembled at the fiber taper waist (FTW) for enhancing the Raman signal. Different from the traditional way of stimulation, evanescent wave excitation greatly increased the interaction area between the excitation and analyte, while reducing the damage of the excited light to the metal nanostructures. The methods proposed in this work have been successfully used to detect the pesticide residues of thiram and showed good detection performance. The detection limits for 4-Mercaptobenzoic acid (4-MBA) and thiram were determined to be 10-9 and 10-8 M, the corresponding enhancement factor could be 1.64 × 105 and 6.38 × 104. Low concentration of thiram was detected in the peels of tomatoes and cucumbers, indicating its feasibility in actual sample detection. The combination of evanescent waves and SERS provides a new direction for the application of SERS sensors, which had great application potential in the field of pesticide residue detection.

Tunable Au@SiO2/Au Film Metasurface as Surface Plasmon Resonance Enhancer for Direct and Ultrasensitive Detection of Exosomes.

Surface plasmon resonance (SPR) spectroscopy with non-labelling, sensitive, and real-time properties is critical for clinical diagnosis applications. However, conventional SPR sensors face the challenge of lower sensitivity and selectivity for trace exosomes assay in complex serum. We proposed a core-shell Au@SiO2-Au film (Au@SiO2-Au film) metasurface to enhance SPR signal based on systematic study on the relationship between gap modes and SPR enhancement. The self-assembled multifunctional peptide was designed as recognition layer with antifouling properties for ultrasensitive and selective detection of PD-L1+ exosomes in serum. The tuning electromagnetic (EM) field model by manipulating the gap was established to guide the preparation of Au@SiO2-Au film metasurface. The in-plane and out-of-plane coupling of Au@SiO2 nanoparticles (NPs) could greatly enlarge and enhance three-dimensional EM field to meet the size of exosomes located in the evanescent field. At the structural level, we achieved high sensitivity (0.16 particles/mL) and a broad response range (10-5 × 103 particles/mL) through optimizing the thickness of SiO2 and surface coverage of Au@SiO2. Furthermore, clinical sample assay achieved the optimal diagnostic accuracy (AUC = 0.97) for differentiating cancer patients from healthy controls. This work provides an opportunity for the construction of a tunable gap mode as SPR enhancer in a total internal reflection architecture. The systematic study on the relationship between gap modes and SPR sensitivity provides a broad scope for promoting direct, efficient, highly selective, and sensitive detection of SPR sensors for clinical application.

Recent Achievements in Development of Chalcogenide Optical Fibers for Mid-IR Sensing

Recent results of research of passive and active optical waveguides made of high-purity chalcogenide glasses for middle infrared fiberoptic evanescent wave spectroscopy of liquid and gaseous substances are presented. On the basis of selenide and telluride glass fibers, novel types of highly sensitive fiber probes are developed. On the basis of Pr(3+)- and Tb(3+)-doped Ga(In)-Ge-As-Se and Ga-Ge-Sb-Se glass fibers, the 4.2–6 μm wavelength radiation sources are created for all-fiber sensor systems. Successful testing of chalcogenide glass fiber sensors for the analysis of some liquid and gaseous mixtures was carried out.

Sensitive and reproducible on-chip SERS detection by side-polished fiber probes integrated with microfluidic chips

The combination of surface-enhanced Raman scattering (SERS) and microfluidic technology offers a promising solution to improve the reproducibility and reliability of SERS detection. Optical fiber SERS probes, as an important class of SERS substrates, provide a large SERS interaction area and flexible integration with microfluidic chips. In this article, a side-polished multimode fiber (SPMF) SERS probe is prepared by depositing Au nanorods on a planar polished fiber surface and integrating it into a microfluidic channel to form a glass-based microfluidic-SERS chip. The impact of SPMF residual thickness on SERS performance is thoroughly investigated, and the optimized residual thickness of 62 μm is determined by considering the evanescent wave intensity, transmission loss, and experimental feasibility. The optimized microfluidic-SERS chip shows low detection limits (as low as 10-8 M for MG) and good spectral reproducibility (RSD less than 10%) due to the large SERS interaction area provided by the SPMF SERS probe and the stable liquid flow in the microfluidic channel. The chip is furtherly used to detect pesticide and antibiotic residues in tap water, with the detection limits of 10-9 M for thiram and 10-6 M for levofloxacin. This work provides an effective way to realize highly sensitive and reproducible SERS detection, and may find important applications in environmental science, food safety, and biomedicine.

Portable biosensor for the detection of Enrofloxacin and Ciprofloxacin antibiotic residues in food, body fluids, environmental and wastewater samples

Enrofloxacin (ENR) and its metabolite Ciprofloxacin (CIP) are both a class of fluoroquinolone antibiotics effective against a broad-spectrum microbial infection. Recent surge in the consumption of CIP and ENR has been linked to increased cases of drug-resistant pathogens. This is due to the fact that the antibiotic residues remain in milk, meat, soil and environmental water for a prolonged duration. Although gold standard methods such as LC-MS are sensitive, they suffer from expensive operation and maintenance cost, and would need dedicated facilities and tedious sample preparation steps. Such limitations make on site detection impossible for regulatory bodies in developing countries. To address this issue, we developed a portable device that can detect the presence of CIP and ENR antibiotics in the range of parts per billion (ppb) concentrations accurately. It consists of a polyaniline (PAni) coated U-bent optical fiber with anti-ENR/CIP antibody immobilized on the polymer surface. The sensor relies on the principle of evanescent wave absorbance by antigen-antibody complex. The sensor showed limit of detection (LOD) of 1 ppb with a linear range of operation from 1 ppb to 500 ppb (R2 = 0.96-0.99) in lake water, waste water treatment plant effluent, urine, blood serum, milk and meat samples. The recovery of the sensor ranges from 88% to 120% indicating reasonable accuracy. The sensor has excellent selectivity towards CIP and ENR and showed stability for four weeks indicating its field deployability and robustness. The portable sensor is scalable and contract has been given to an industry partner to mass manufacture the device.

Harnessing Evanescent Waves in UV-Irradiated TiO2-Coated Quartz Optical Fibers Improves Pollutant Degradation in Water

Coupling TiO2-coated quartz optical fibers (TiO2-QOFs) with LEDs shows potential in degrading organics in water. Conventional TiO2-QOFs with thick and compact coatings are inefficient to degrade organic pollutants due to refraction losses. The research focuses on manipulating coating strategies to increase the generation of evanescent waves, which react more efficiently with the coated TiO2, which shows a crystal structure at anatase/rutile of 85/15. Through the use of dip-coating methods, TiO2 can be effectively deposited on quartz optical fibers, leveraging the principle of electrostatic attraction. By decreasing TiO2 dip-coating concentrations from 200 to 10 mg/L, dip-coating duration from 2 to 0.5 h, and coating cycles from 3 to 1, TiO2 patchiness on quartz fibers was reduced by 50–60%, enhancing the formation of more interspaces between the fiber surfaces and the coatings, which in turn allowed over 90% of evanescent waves in the TiO2-QOFs. The evanescent wave-dominated TiO2-QOFs irradiated by 275 nm UV-LED exhibited methylene blue degradation rate constants at around 0.03 h−1 and quantum yields greater than unity, indicating a highly efficient interaction between the evanescent wave and the surface coatings. These findings offer significant insights into optimizing the utilization of evanescent waves in TiO2-QOFs for pollutant degradation by regulating the coating structures.

Homemade low-cost fabrication technique andstability analysis of a U-shaped fiber sensorstructure.

In this work, the fabrication method of a U-shaped optical fiber (UOF) structure using single-mode fiber is proposed. Few UOF sensors have been developed to date, but the fabrication process has not been described in detail. Here, its subsequent homemade fabrication, optimization strategies, and analysis are thoroughly explored. Further, the influence of transmission on U-shaped diameter is explored. The transmitted intensity is mainly used to assess the strength of the evanescent field. For this purpose, three different diameters of 2mm, 4mm, and 6mm UOFs are fabricated. The results show that the transmission of the U-shaped structure is dependent on the diameter of the UOF. Thereafter, different concentrations of glucose solutions are detected using the optimized stable UOF structure to showcase the sensing properties. Overall, this work is essential for beginners who want to conduct research on optical fiber sensors with a curved shape.

Antibody- and aptamer-free SERS substrate for ultrasensitive and anti-interference detection of SARS-CoV-2 spike protein in untreated saliva

Sensitive and anti-interference detection of targeted signal(s) in body fluids is one of the paramount tasks in biosensing. Overcoming the complication and high cost of antibody/aptamer-modification, surface-enhanced Raman spectroscopy (SERS) based on antibody/aptamer-free (AAF) substrates has shown great promise, yet with rather limited detection sensitivity. Herein, we report ultrasensitive and anti-interference detection of SARS-CoV-2 spike protein in untreated saliva by an AAF SERS substrate, applying the evanescent field induced by the high-order waveguide modes of well-defined nanorods for SERS for the first time. A detection limit of 3.6 × 10−17 M and 1.6 × 10−16 M are obtained in phosphate buffered saline and untreated saliva, respectively; the detection limits are three orders of magnitude improved than the best records from AAF substrates. This work unlocks an exciting path to design AAF SERS substrates for ultrasensitive biosensing, not limited to detection of viral antigens.

Real-Time Probing of Melamine-Induced Gold Nanoparticle Aggregation Kinetics via Evanescent-Wave Coupled Cavity Ring-down Spectroscopy.

Insight into the aggregation kinetics of gold nanoparticles (GNPs) is critical for developing a colorimetric assay extensively used in chemical and biomolecular sensing. The aggregation of NPs plays a significant role in many natural and industrial processes, demanding comprehensive perceptions of the aggregation kinetics at a solid-liquid interface. However, the direct observation of the melamine-induced aggregation process of GNPs in the time-domain still remains a challenge. There is little to no information on the fundamental mechanisms of such kinetics using evanescent waves. Total internal reflection (TIR) has been applied to generate the evanescent field (EF), exploring aggregation kinetics near the solid-liquid interface. Here, we utilized a precise optical cavity-based method, an evanescent-wave coupled cavity ring-down spectroscopy (EW-CRDS), that can probe the melamine-induced aggregation kinetics of GNPs. The key feature of the present method is that the evanescent field generated by TIR illumination harnesses the power of CRDS to study 2D fractals via the collision and attachment of the GNPs and their melamine-induced aggregates at the interfacial region in real-time. This kinetic study reveals a critical point for diffusion-limited aggregation and provides insights into the design and optimization of colorimetric sensors that exploit the aggregation of GNPs. Furthermore, the EW-CRDS is a unique analytical approach that helps to deepen our understanding in probing the real-time aggregation process, detecting the presence of aggregator as compared to UV-vis and dynamic light scattering (DLS) spectroscopy.

An Implementation Method of the Complex Frequency-Shifted Uniaxial/Multi-Axial PML Technique for Viscoelastic Seismic Wave Propagation

ABSTRACT A numerical implementation method of the complex frequency-shifted uniaxial/multi-axial PML (CFS UPML/MPML) is developed for modeling half-space viscoelastic wave propagation. A mixed CFS PML combining CFS UPML and CFS MPML is designed to improve PML’s performances in avoiding evanescent waves at grazing incidence and stabilizing long-time simulation of low-frequency viscoelastic wave propagation. Two numerical examples’ results showed that the developed method for the mixed CFS PML can implement elimination of evanescent waves at grazing incidence near the PML interface, long-time accurate simulation of low-frequency viscoelastic wave propagation and efficient absorption of the near-fault outgoing seismic waves in homogeneous earth medium.

Sensing Performance of Ethanol Microfiber Probe Augmented by ZnO Nanosheet and UV Glue Film

A microfiber reflective ethanol gas sensing probe was designed and fabricated. The single-mode fiber was heated and stretched to prepare a microfiber taper, on which a mixed material of ZnO nanosheets and UV glue was built by the dip-coating method. The influencing factors on its sensing performance for ethanol have been discussed, including the dozen ratio of ZnO nanosheets, UV glue materials, and end-face morphology. As the concentration of ethanol gas increased, the intensity of the reflection spectrum increased with the responding sensitivity of 7.28 × 10−4 dBm/ppm. The exchanging efficiency of the optical signal is enhanced by the strong evanescent field of the microfiber taper. This sensing probe is convenient for high-density integration and working in a small space and is capable of high-performance monitoring for ethanol at room temperature.

Ag/Ag–Cu doped ZnO/graphene coated with STMS/SDTMS for UV detection: Theoretical and experimental studies

The impact of Ag and Ag–Cu codoping on the structure and optical performance of ZnO composite graphene (ZnO/G) was investigated. Moreover, the photoelectric properties of G, ZnO, and Ag–Cu co-doped ZnO/G (different numbers of doped atoms) were analyzed based on first-principles calculations. ZnO/G, Ag-doped ZnO/G (1%–4%), and Ag–Cu co-doped ZnO/G (1%–4%) were prepared using the hydrothermal method. Energy-dispersive X-ray spectroscopy and scanning electron microscopy were performed to characterize their structures and morphology. Single-mode–tapered-multimode–single-mode (STMS) and single-mode–double-tapered-multimode–single-mode (SDTMS) sensor units were designed based on evanescent field theory. RSOFT simulated their structural properties and then experimentally validated them. The prepared materials were coated with STMS and SDTMS sensor units for UV detection experiments. When 1% Ag–Cu co-doped ZnO/G was coated with SDTMS, the highest sensitivity obtained was − 1.367 × 10−1 mW/mW·cm−2. Ag–Cu codoping affected the free-carrier concentration of the ZnO composite under UV illumination, which altered the refractive index of the materials and resulted in a change in the transmission spectrum of the sensor. The results show that the Ag–Cu co-doped ZnO modification coated with SDTMS can effectively improve the UV light–absorption properties of the sensors. This method will play a significant role in future applications of UV detection.

Anisotropic leaky-like perturbation with subwavelength gratings enables zero crosstalk

Electromagnetic coupling via an evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer in a photonic circuit but limiting integration density. A leaky mode, which combines both evanescent field and radiative wave, causes stronger coupling and is thus considered not ideal for dense integration. Here we show that a leaky oscillation with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by ≈40 dB compared to conventional strip waveguides, corresponding to ≈100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse–magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices.