Penetration Depth Of Evanescent Wave Research Articles

The evidences for the existence of evanescent wave-induced scattering in the transmission SAXS of high-density polyethylene

In an earlier study of ours (Li et al., IUCrJ,2019 6, 968–983), we conjectured that small-angle X-ray scattering (SAXS) in semicrystalline polymers might be from evanescence wave-induced scattering (Ievi) rather than claimed by classical scattering theory, the incident X-ray-induced scattering (Iin). In this study, we systematically compared Ievi and Iin under the influence of various factors, i.e., linear crystallinity, the number of lamellar crystals (N), the characteristic penetration depth of evanescent wave (dp*) and long period distribution, identifying the method to distinguish SAXS in semicrystalline polymers is from Iin or Ievi. Applying this method to SAXS of a high-density polyethylene (HDPE) sample, it was found that even excluding the influence of instrumental broadening and lamellar thickness/long period dispersity, the SAXS peak in HDPE is still greater than theoretically maximum peak width induced by Iin, indicating the SAXS in HDPE is not induced by incident X-ray directly. Excluding the interference of long period dispersity, it was found that dp* was around 5.9 nm, clearly demonstrating the existence of evanescent wave. Besides, results indicated that the lateral size determined with new theory is accurate, which in turn demonstrated that our assumption that Ievi is from a lamellar stack parallel to incident X-ray is reasonable. Taking all evidences together, it can be concluded safely that SAXS in HDPE is induced by evanescent wave.

Attenuated Total Reflection at THz Wavelengths: Prospective Use of Total Internal Reflection and Polariscopy

Capabilities of the attenuated total reflection (ATR) at THz wavelengths for increased sub-surface depth characterisation of (bio-)materials are presented. The penetration depth of a THz evanescent wave in biological samples is dependent on the wavelength and temperature and can reach 0.1–0.5 mm depth, due to the strong refractive index change ∼0.4 of the ice-water transition; this is quite significant and important when studying biological samples. Technical challenges are discussed when using ATR for uneven, heterogeneous, high refractive index samples with the possibility of frustrated total internal reflection (a breakdown of the ATR reflection mode into transmission mode). Local field enhancements at the interface are discussed with numerical/analytical examples. Maxwell’s scaling is used to model the behaviour of absorber–scatterer inside the materials at the interface with the ATR prism for realistic complex refractive indices of bio-materials. The modality of ATR with a polarisation analysis is proposed, and its principle is illustrated, opening an invitation for its experimental validation. The sensitivity of the polarised ATR mode to the refractive index between the sample and ATR prism is numerically modelled and experimentally verified for background (air) spectra. The design principles of polarisation active optical elements and spectral filters are outlined. The results and proposed concepts are based on experimental conditions at the THz beamline of the Australian Synchrotron.

Effect of the Geometries of Ge-Sb-Se Chalcogenide Glass Tapered Fiber on the Sensitivity of Evanescent Wave Sensors

A geometrical optimization study of Ge-Sb-Se chalcogenide glass tapered fiber sensors for concentration sensing of methanol is presented. The effects of the geometrical parameters such as taper waist diameter and transition region lengths on the evanescent wave penetration depth are discussed based on the ray-tracing theory. Numerical analysis results demonstrate that the performance of the tapered fiber sensor can be enhanced by decreasing the transition length of the down-taper combined with increasing the transition length of the up-taper. A homemade tapering platform enables controlling the taper waist diameter and transition region length of the tapered fiber accurately. An experimental study using tapered fiber sensors with different transition region lengths is carried out to detect the concentration of methanol aqueous solution. The highest sensitivity of 0.0120 a.u./% is obtained from the tapered fiber with a down-taper transition length of 5.4 mm and an up-taper transition length of 7.9 mm, which agrees with the trend presented by the simulation results. The effects of symmetric and asymmetric tapered fiber structure on the sensing characteristics are also investigated.

Characterisation of Biological Materials at THz Frequencies by Attenuated Total Reflection: Lard

The penetration depth of an evanescent wave in Attenuated Total Reflection (ATR) is dependent on the wavelength of the radiation utilised. At THz frequencies, the penetration depth into biological tissues is in the order of 0.1 to 0.5 mm; rendered pig lard was used as a model sample in this study. A method for the direct measurement of the evanescent wave penetration depth is presented which allows for the estimation of the dispersion of the complex refractive index by using the reflection of the evanescent wave from varying sample depths. The method employs frustrated total internal reflection, and has been demonstrated by using the THz/Far-IR beamline at the Australian synchrotron, and modelled using finite difference time domain (FDTD) simulations.

Optimization of Bending Angle of Fiber Optic Sensor for Non-Invasive Blood Glucose Measurement

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Graphene based chalcogenide fiber-optic evanescent wave sensor for detection of hemoglobin in human blood

Abstract Fiber optic evanescent wave sensor with graphene as an absorption-enhancing layer to measure hemoglobin concentration in human blood is proposed. Previous modal functions and experimental results describing the variation of optical constants of human blood with different hemoglobin concentrations in the near-infrared spectral region are considered for sensor design simulation. The sensor’s performance is closely analyzed in terms of its absorption coefficient, sensitivity, and detection limit. It is found that the proposed sensor should be operated at longer light wavelength to get more enhanced sensitivity and smaller detection limit. At 1000 nm wavelength, a detection limit of 18 μg/dL and sensitivity of 6.71 × 10−4 per g/dL is achievable with the proposed sensor. The sensitivity is found to be better for larger hemoglobin concentrations. The results are correlated with the evanescent wave penetration depth.

Theoretical analysis of metamaterial-gold auxiliary grating sensing structure for surface plasmon resonance sensing application based on polarization control method

A metamaterial-gold multilayer sensing structure designed using the particle swarm optimization (PSO) algorithm with an auxiliary grating is proposed for using in a surface plasmon resonance (SPR) sensor system based on the polarization control method. After numerical calculations and simulation analysis, it was found that the metamaterial sensing structure significantly improves the sensitivity of the SPR signal with intensity singularity. The metamaterial sensing structure also increases the penetration depth of evanescent wave, making it possible to detect low-molecular-weight biomolecules and larger cells such as bacteria. The auxiliary grating structure was designed to identify the refractive index of the sensing region on both sides of intensity singularity. The stability of recognition and the electric field intensity of the visible light band were also studied.

Light Humidity Sensor of Surface Plasmon Resonance by Symmetric Metal Film

The field of plasmonics has experienced a renaissance in recent years by providing a large variety of new physical effects and applications. Here, we demonstrate a light humidity sensor of surface plasmon resonance (SPR) by a symmetric metal film, which uses P polarised light of emergent He–Ne laser to stimulate SPR. Resonance angle change received by the spectrum detector can determine humidity via the relationship between humidity and effective refractive index. When the relatively short wavelengths are shown in the model, the evanescent wave penetration depth is shallow, resonance state is weak and energy loss is low. The opposite results are obtained, when the wavelength is long. Also, with increasing thickness, the resonance peak becomes acute, thereby indicating the improvement in detection accuracy. When the metal thickness of our model is 50 nm, the resonance peak of the reflection spectrum is narrower, accuracy is high and reflectivity is close to 0. By analysing the experimental results, the SPR resonance phenomenon is shown. The electromagnetic field energy is highly concentrated near the interface between the metal and SiO2, which appears highly localized. The resolution of the structure can reach 0.37% RH (relative humidity), which is significantly more than the resolution of capacitive humidity sensor, i.e., resolution is usually 1% RH to 2% RH. The optical sensor of our development can provide a key device for long-distance transmission sensing, in special conditions such as low temperature.

Vapor Sorption to Polyvinyl Alcohol Thin Film Observed Using a Hybrid Sensor of Quartz-Crystal-Microbalance and Surface-Plasmon-Resonance

A quartz-crystal-microbalance (QCM) and surface-plasmon-resonance (SPR) hybrid sensor was prepared, and the sorptions of water and ethanol vapors to polyvinyl alcohol (PVA) films were observed in situ. The shift of the SPR wavelength depended on the PVA film thickness as well as the sorbed vapor. It was estimated to be due to the film swelling induced by the vapor sorption and the relationship between the film thickness and the penetration depth of evanescent wave.

Spectroscopic and permeation studies of phospholipid bilayers supported by a soft hydrogel scaffold.

Polarized attenuated total reflection infrared (ATR-IR) spectroscopy, fluorescence microscopy, and fluorescence spectroscopy were used to characterize a lipid coating composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, supported on a spherical hydrogel scaffold. The fluorescence microscopy images show an association between the lipid coating and the hydrogel scaffold. Fluorescence permeability measurements revealed that the phospholipid coating acts as a permeability barrier, exhibiting characteristics of a lamellar bilayer coating structure. Variable evanescent wave penetration depth ATR-IR spectroscopy studies validated the determination of quantitative molecular orientation information for a lipid coating supported on a spherical scaffold. These polarized ATR-IR studies measured an average DMPC acyl chain tilt angle of ∼21-25°, with respect to the surface normal.

A double-taper optical fiber-based radiation wave other than evanescent wave in all-fiber immunofluorescence biosensor for quantitative detection of Escherichia coli O157:H7.

Cylindrical or taper-and-cylinder combination optical fiber probe based on evanescent wave has been widely used for immunofluorescence biosensor to detect various analytes. In this study, in contrast to the contradiction between penetration depth and analyte diameter of optical fiber probe-based evanescent wave, we demonstrate that double-taper optical fiber used in a radiation wave-based all-fiber immunofluorescence biosensor (RWAIB) can detect micron-scale analytes using Escherichia coli O157:H7 as representative target. Finite-difference time-domain method was used to compare the properties of evanescent wave and radiation wave (RW). Ray-tracing model was formulated to optimize the taper geometry of the probe. Based on a commercial multi-mode fiber, a double-taper probe was fabricated and connected with biosensor through a “ferrule connector” optical fiber connector. The RWAIB configuration was accomplished using commercial multi-mode fibers and fiber-based devices according to the “all-fiber” method. The standard sample tests revealed that the sensitivity of the proposed technique for E. coli O157:H7 detection was 103 cfu·mL−1. Quantitation could be achieved within the concentration range of 103 cfu·mL−1 to 107 cfu·mL−1. No non-specific recognition to ten kinds of food-borne pathogens was observed. The results demonstrated that based on the double-taper optical fiber RWAIB can be used for the quantitative detection of micron-scale targets, and RW sensing is an alternative for traditional evanescent wave sensing during the fabrication of fiber-optic biosensors.

Optimized Tapered Optical Fiber for Ethanol (C$_{\bf 2}$H$_{\bf 5}$OH) Concentration Sensing

An optimized study of biconical tapered multi-mode plastic optical fiber sensor for concentration sensing of ethanol (C2H5OH) is presented. The sensitivity is enhanced through V-number matching as well as by optimizing the taper radius and taper length. The ray-tracing method is used to analyze the evanescent wave penetration depth (EWPD). The theoretical analysis and experimental results are used to optimize the taper ratio and taper length for the achievement of high EWPD and high sensitivity. The analysis indicates that the sensitivity of tapered fiber sensor can be improved by decreasing the taper ratio with simultaneous increase in the taper length. The highest sensitivity of 1.527 mV/% is achieved from the tapered fiber with a taper ratio of 0.27 and taper length of 8 cm. The proposed parametric optimized tapered fiber sensor can detect the change in concentration of C2H5OH as small as 6.55 × 10-3.

Detection of recombinant growth hormone by evanescent cascaded waveguide coupler on silica‐on‐silicon

An evanescent wave based biosensor is developed on the silica-on-silicon (SOS) with a cascaded waveguide coupler for the detection of recombinant growth hormone. So far, U -bends and tapered waveguides are demonstrated for increasing the penetration depth and enhancing sensitivity of the evanescent wave sensor. In this work, a monolithically integrated sensor platform containing a cascaded waveguide coupler with optical power splitters and combiners designed with S -bends and tapper waveguides is demonstrated for an enhanced detection of recombinant growth hormone. In the cascaded waveguide coupler, a large surface area to bind the antibody with increased penetration depth of evanescent wave to excite the tagged-rbST is obtained by splitting the waveguide into multiple paths using Y splitters designed with s -bends and subsequently combining them back to a single waveguide through tapered waveguide and combiners. Hence a highly sensitive fluoroimmunoassay sensor is realized. Using the 2D FDTD (Finite-difference time-domain method) simulation of waveguide with a point source in Rsoft FullWAVE, the fluorescence coupling efficiency of straight and bend section of waveguide is analyzed. The sensor is demonstrated for the detection of fluorescently-tagged recombinant growth hormone with the detection limit as low as 25 ng/ml.

Nanopatterned submicron pores as a shield for nonspecific binding in surface plasmon resonance-based sensing

We present a novel approach to tackle the most common drawback of using surface plasmon resonance for analyte screening in complex biological matrices--the nonspecific binding to the sensor chip surface. By using a perforated membrane supported by a polymeric gel structure at the evanescent wave penetration depth, we have fabricated a non-fouling sieve above the sensing region. The sieve shields the evanescent wave from nonspecific interactions which interfere with SPR sensing by minimizing the fouled area of the polymeric gel and preventing the translocation of large particles, e.g. micelles or aggregates. The nanopatterned macropores were fabricated by means of colloidal lithography and plasma enhanced chemical vapor deposition of a polyethylene oxide-like film on top of a polymeric gel matrix commonly used in surface plasmon resonance analysis. The sieve was characterized using surface plasmon resonance imaging, contact angle, atomic force microscopy and scanning electron microscopy. The performance of the sieve was studied using an immunoassay for detection of antibiotic residues in full fat milk and porcine serum. The non-fouling membrane presented pores in the 92-138 nm range organized in a hexagonal crystal lattice with a clearance of about 5% of the total surface. Functionally, the membrane with the nanopatterned macropores showed significant improvements in immunoassay robustness and sensitivity in untreated complex samples. The utilization of the sensor built-in sieve for measurements in complex matrices offers reduction in pre-analytical sample preparation steps and thus shortens the total analysis time.

Optical Response of Magnetic Fluorescent Microspheres Used for Force Spectroscopy in the Evanescent Field

Force spectroscopy based on magnetic tweezers is a powerful technique for manipulating single biomolecules and studying their interactions. The resolution in magnetic probe displacement, however, needs to be commensurate with molecular sizes. To achieve the desirable sensitivity in tracking displacements of the magnetic probe, some recent approaches have combined magnetic tweezers with total internal reflection fluorescence microscopy. In this situation, a typical force probe is a polymer microsphere containing two types of optically active components: a pure absorber (magnetic nanoparticles for providing the pulling force) and a luminophore (semiconducting nanoparticles or organic dyes for fluorescent imaging). To assess the system's capability fully with regard to tracking the position of the force probe with subnanometer accuracy, we developed a body-of-revolution formulation of the method of auxiliary sources (BOR-MAS) to simulate the absorption, scattering, and fluorescence of microscopic spheres in an evanescent electromagnetic field. The theoretical formulation uses the axial symmetry of the system to reduce the dimensionality of the modeling problem and produces excellent agreement with the reported experimental data on forward scattering intensity. Using the BOR-MAS numerical model, we investigated the probe detection sensitivity for a high numerical aperture objective. The analysis of both backscattering and fluorescence observation modes shows that the total intensity of the bead image decays exponentially with the distance from the surface (or the length of a biomolecule). Our investigations demonstrate that the decay lengths of observable optical power are smaller than the penetration depth of the unperturbed excitation evanescent wave. In addition, our numerical modeling results illustrate that the expected sensitivity for the decay length changes with the angle of incidence, tracking the theoretical penetration depth for a two-media model, and is sensitive to the bead size. The BOR-MAS methodology developed in this work for near-field modeling of bead-tracking experiments fully describes the fundamental photonic response of microscopic BOR probes at the subwavelength level and can be used for future improvements in the design of these probes or in the setup of bead-tracking experiments.

Detection and Quantification of Trace Organic Contaminants in Water Using the FT-IR-Attenuated Total Reflectance Technique

The Fourier transform infrared-attenuated total reflectance (FT-IR-ATR) technique has been used to detect and quantify the following volatile organic compounds (VOCs) in water: 1,1,1,2-tetrachloroethane, 1,1,2,2-tetrachloroethane, styrene, and tetrachloroethylene, among which the first three compounds were investigated at parts per million levels for the first time. Enhancement of the detection was made by (1) coating the ATR crystal with a hydrophobic polymer membrane, (2) optimizing the flow rate of the sample solution, (3) varying polymer membrane thickness, and (4) increasing the number of reflection bounces within the ATR crystal. Our flow rate optimization confirmed a previous finding that turbulent flow is more favorable than laminar flow in detecting the VOCs in water. However, decreases of ATR signal intensity were observed at very high turbulency due to analytes flowing too quickly through and exiting the ATR cell to be adsorbed onto the polymer membrane. The optimal membrane thickness was found to be associated with the maximum overlap between the IR evanescent wave penetration depth and the analyte diffusion depth. Consequently, there is no universal optimal flow rate and optimal polymer membrane thickness for detection of all VOCs. Doubling the number of IR reflection bounces within the ATR crystal enhanced both detection and sensitivity by about a factor of 2. Finally, it was observed that the detection limit concentrations decrease with the water solubility of the VOCs.

Direct studies of liquid flows near solid surfaces by total internal reflection fluorescence cross-correlation spectroscopy

We present a new method to study flow of liquids near solid surface: Total internal reflection fluorescence cross-correlation spectroscopy (TIR-FCCS). Fluorescent tracers flowing with the liquid are excited by evanescent light, produced by epi-illumination through the periphery of a high numerical aperture oil-immersion objective. The time-resolved fluorescence intensity signals from two laterally shifted observation volumes, created by two confocal pinholes are independently measured. The cross-correlation of these signals provides information of the tracers' velocities. By changing the evanescent wave penetration depth, flow profiling at distances less than 200 nm from the interface can be performed. Due to the high sensitivity of the method fluorescent species with different size, down to single dye molecules can be used as tracers. We applied this method to study the flow of aqueous electrolyte solutions near a smooth hydrophilic surface and explored the effect of several important parameters, e.g. tracer size, ionic strength, and distance between the observation volumes.

Real time measurement of concentration ratio at an electrode/solution interface by calculating the inverse Laplace transform of SOWG spectroscopy

A novel technique for measuring the concentration rate of a redox compound at an electrode/solution interface was developed using slab optical waveguide (SOWG) spectroscopy. The electrolysis of 5 mmol dm −3 ferroin in 1 mol dm −3 potassium nitrate solution was carried out on an indium-tin oxide (ITO) electrode and plastic-formed carbon (PFC) electrode by cyclic voltammetry, using each spectroelectrochemical cell in SOWG spectroscopy. Their SOWG spectra were obtained every 0.05 mV during the potential sweep between +0.4 and +1.4 V for the ITO electrode and between +0.4 and +1.2 V for the PFC electrode. To obtain the concentration profiles near the electrode during the cyclic voltammetry of ferroin, the SOWG spectra were converted to concentration ratios at given locations, and these ratios were plotted against a penetration depth of evanescent wave. The theoretical derived concentration–distance profiles during CV were experimentally verified by the present technique.

Potential of Far-Ultraviolet Absorption Spectroscopy as a Highly Sensitive Analysis Method for Aqueous Solutions. Part II: Monitoring the Quality of Semiconductor Wafer Cleaning Solutions Using Attenuated Total Reflection

Far-ultraviolet (FUV) spectroscopy combined with attenuated total reflection (ATR) is employed for direct measurement of the concentrations of semiconductor wafer cleaning fluids such as SC-1 (aqueous solution of NH3 and H2O2) and SC-2 (aqueous solution of HCl and H2O2). FUV spectra of these aqueous solutions in the 170-200 nm region are highly sensitive to changes in both hydrogen bonding and hydration. Although ATR measurement results in lower absorptivity compared to transmittance measurement, it is possible to increase absorption with greater evanescent wave penetration depth using a low refractive index internal reflection element (IRE). We adopt quartz as an IRE material. Since the refractive index of quartz becomes lower than that of water in the low energy side of an intense absorption band due to the n-->sigma* transition of water, the quartz IRE yields non-total reflection wavelength regions. However, near 175 nm the effective absorptivity of the tail of water's absorption band can be successfully enlarged, making the FUV-ATR technique suitable for measuring the concentrations of the components in the semiconductor wafer cleaning fluids. In the present study we prepared the same cleaning fluids as those used in actual semiconductor fabrication and measured their FUV-ATR spectra in the 150-300 nm wavelength range. It was found that even with the quartz IRE one can measure FUV-ATR spectra under total reflection conditions at 175 nm or above. We created calibration models for predicting both NH3 and H2O2 in the concentration ranges of 0-10% in SC-1 using multiple linear regression (MLR). The standard deviations of the models were 0.033% and 0.265% for NH3 and H2O2, respectively. The same procedure was repeated under the same conditions for HCl and H2O2 in SC-2, yielding corresponding values of 0.018% for HCl and 0.178% for H2O2.

Estimates of scattering strength for buried cylindrical targets ensonified by evanescent waves

It is known that low-frequency subcritical sound waves can significantly scatter from targets buried in a seabed due to the significant penetration depth of the incident evanescent wave. Past computational work on scattering by buried spherical shells has been done, for example, using a T-matrix Method [R. Lim et al., J. Acoust. Soc. Am. 93, 1762-1783 (1993)], a Virtual Source Method [I. Lucifredi and H. Schmidt., J. Acoust. Soc. Am. 120, 3566-3583 (2006)], or finite-element methods [Zampolli et al., J. Acoust. Soc. Am., in press]. In addition to high-fidelity results which are expected from the preceding numerical methods, it desirable to have approximate analytical/asymptotic predictions of multistatic scattering strength for a variety of homogeneous or layered buried targets. Focusing on buried cylinders of infinite or finite-length, we first compute scattering using an approximate method that makes use of separation of variables and neglects multiple scattering between the interface and the target. Results are compared with those generated using the Axiscat/NURC/COMSOL finite-element method. Asymptotic estimates are then presented for scattering strength for objects completely buried in the seafloor for a flat interface (Work sponsored by ONR and NURC.)