Capillary Waveguide Research Articles

A microfluidic analyzer based on liquid waveguide capillary cells for the high-sensitivity determination of phosphate in seawater and its applications

BackgroundOptical detection is frequently performed on microfluidic chips for colorimetric analysis. Integrating liquid waveguide capillaries with total internal reflection with the microfluidic chip requires less procedures, which is suitable in the optical detection of microfluidic systems and is a practical alternative to increase the optical path length in the colorimetric assay of microfluidic devices for higher sensitivities and lower detection limit. However, this alternative has not been applied to the connection of PMMA chips or the microfluidic devices for the detection of phosphate in seawater. ReslutsHere, a lab-on-a-chip system integrating a microfluidic chip and an external liquid waveguide capillary cell was presented to detect the phosphate in seawater. The detachable total internal reflection capillary made of Teflon AF 2400 connected to the chip transports sample and transmits light, greatly reducing detection limit, eliminating the interference from stray light and widening the dynamic range of the system without specific surface treatment of the microchannel. By utilizing an internal 5-cm absorption cell and an external 20-cm liquid waveguide capillary cell, the system reaches detection limits of 59 nM and 8 nM, respectively, and can detect phosphate concentration from 0 to 23 μM. An online analyzer was developed based on the high-sensitivity system and was applied to shipboard underway analysis for two scientific cruises and to laboratory measurements for seawater samples from Xisha sea area. SignificanceCorrelation analyses between the shipboard and laboratory phosphate measurements and other physical and biochemical elements revealed the marine ecological characteristics of the corresponding areas, demonstrating the high-sensitivity of this method over slight variations and narrow ranges of phosphate and the ability to provide microfluidic systems for high spatiotemporal resolution phosphate determination a practical and cost-effective alternative.

Measurement of light absorption by chromophoric dissolved organic matter using a type-II liquid capillary waveguide: assessment of an achievable accuracy

Light absorption by chromophoric dissolved organic matter (CDOM) in the ocean is often measured using liquid waveguide capillary cells coupled to spectral array detectors. This type of optical setup is affected by several sources of uncertainties related to the waveguide and the detector. Uncertainties from the waveguide arise from errors in the effective path length and the effects of water salinity, while errors related to the detector are due to the non-linearity in the response, internal stray light, and wavelength accuracy. Here, uncertainties in the measurements of the spectral absorption coefficient of CDOM due to the optical setup itself were investigated in detail. The related systematic errors were very often significant (2–15%) and larger than expected from simple measurement uncertainty (±1%). However, they can be corrected by characterizing the detector’s response for non-linearity and stray light, regularly performing calibrations for the detector’s wavelength response, and routinely measuring the waveguide’s effective path length. Including such corrections and timely calibrations reduces the uncertainties related to the spectrophotometric measurements to about ±2%. Uncertainties related to the necessary handling of samples are not included here.

A photometric-turbidimetric fusion sensor for measuring the water content in diesel with sensitivity increase by the extended infrared path capillary

Petroleum-based diesel is the main energy supply for heavy-duty vehicles and industrial equipment. Excessive water in the diesel would affect engine performance directly and even lead to serious accidents. In this paper, a new on-board compact sensor for detecting water content in diesel is proposed, which is composed of LED, photodetector, special metal waveguide capillary and ultrasonic cavity. Especially, the metal waveguide capillary with extended optical path can significantly improves the measurement accuracy meanwhile decrease the size. The small ultrasonic oscillation cavity is connected with the metal waveguide capillary to ensure the saturated water droplets to be evenly mixed with petroleum-based diesel before being detected. Then, the photometric - turbidimetric fusion method is proposed to measure the total water contents in different states (i.e., dissolved water, emulsified water and free water) in the diesel. The designed metal waveguide capillary with extended optical path improves sensor accuracy by the way of multiple reflections of light. In addition, the sensor is installed in the line between the fuel tank and the engine, then, the actual water pumped into the engine can be accurately measured. The experimental results show that the root mean square of the prediction error (RMSEP) is only 7.34 ppm, which is much lower than the optical sensor based on non-dispersive infrared spectroscopy of 18.6 ppm. The sensor fabricated in this paper finds application in detecting water content in diesel, which has the advantages of high sensitivity, small volume, and are suitable for on-board detection.

Bimetallic nanozyme triple-emission fluorescence intelligent sensing platform-integrated molecular imprinting for ultrasensitive visual detection of triclosan

Triclosan (TCS) is an endocrine disruptor, which has been widely used in daily chemicals, resulting in the potential risk to the ecosystem and human health. Herein, a smartphone-integrated bimetallic nanozyme triple-emission fluorescence capillary imprinted sensing system was developed for ultrasensitive and intelligent visual microanalysis of TCS. Carbon dots (CDs) and bimetallic organic framework (MOF-(Fe/Co)-NH2) were used as fluorescence sources to synthesize nanozyme fluorescence molecularly imprinted polymer (MOF-(Fe/Co)-NH2@CDs@NMIP), which oxidized o-phenylenediamine to 2,3-diaminophenazine (OPDox), resulting in the derivation of a new fluorescence peak at 556 nm. In the existence of TCS, the fluorescence of MOF-(Fe/Co)-NH2 at 450 nm was restored, the fluorescence of OPDox at 556 nm was suppressed, and the CDs fluorescence of at 686 nm remained constant. The color of triple-emission fluorescence imprinted sensor varied from yellow to pink to purple to blue. The response efficiency (F450/F556/F686) of this sensing platform based on the capillary waveguide effect demonstrated a significant linear relationship toward the concentration of TCS ranged from 1.0 × 10−12 to 1.5 × 10−10 M with the LOD of 8.0 × 10−13 M. Compared with dual-emission capillary fluorescence sensor, this sensing system has higher sensitivity and richer visual color. Combined with the smartphone-integrated portable sensing platform, the color of fluorescence was transformed into an RGB value to calculate TCS concentration with the LOD of 9.6 × 10−13 M, providing a novel method for intelligent visual microanalysis (18 μL/time) of environmental pollutants.

Accurate modeling and measurement of pressure-induced group velocity dispersion variations in anti-resonant hollow-core fibers.

Precise control of group velocity dispersion (GVD) by pressure in a gas-filled hollow-core fiber (HCF) is of essential importance for many gas-based nonlinear optical applications. To accurately calculate the pressure-induced dispersion variations (∂β2/∂p) in anti-resonant types of HCF, an analytical model combining the contribution of the gas material, capillary waveguide, and cladding resonances is developed, with an insightful physical picture. Broadband (∼1000 nm) GVD measurements in a single-shot manner realize accuracy and precision as low as 0.1 ps2/km and 2 × 10-3 ps2/km, respectively, and validate our model. Consistent with our model, a pronounced negative ∂β2/∂p is observed experimentally for the first time, to our knowledge. Our model can also be extended to other HCFs with cladding resonances in predicting ∂β2/∂p, such as in photonic bandgap types of HCF.

Impact of electron transport models on capillary discharge plasmas

Magnetohydrodynamics (MHD) can be used to model capillary discharge waveguides in laser-wakefield accelerators. However, the predictive capability of MHD can suffer due to poor microscopic closure models. Here, we study the impact of electron heating and thermal conduction on the capillary waveguide performance as part of an effort to understand and quantify uncertainties in modeling and designing next-generation plasma accelerators. To do so, we perform two-dimensional high-resolution MHD simulations using an argon-filled capillary discharge waveguide with three different electron transport coefficients models. The models tested include (i) Davies et al., (ii) Spitzer, and (iii) Epperlein–Haines (EH). We found that the EH model overestimates the electron temperature inside the channel by over 20% while predicting a lower azimuthal magnetic field. Moreover, the Spitzer model, often used in MHD simulations for plasma-based accelerators, predicts a significantly higher electron temperature than the other models suggest.

Ferrozine colorimetry and reverse flow injection analysis (rFIA) based method for the determination of total iron in aqueous solutions at nanomolar concentrations

Iron is one of the most microbiologically and chemically important metals in natural waters. The biogeochemical cycling of iron is significantly influenced by the redox cycling of Fe(II) and Fe(III). Because of the unique chemistry of iron, it is often needed to analyze iron at nano-molar concentrations. This article describes a reverse flow injection analysis (rFIA) based method with ferrozine spectrophotometric detection to quantify total iron concentration in stream water at nanomolar concentrations. The rFIA system has a 0.65 nM detection limit and a linear dynamic range up to 1.40 μM for the total iron analysis. The detection limit was achieved using a 1.0 m long liquid waveguide capillary flow cell, 1.50 m long knotted reaction coil, 87.50 μL injection loop and a miniature fiber optics spectrophotometer. The optimized colorimetric reagent has 1.0 mM ferrozine, 0.1 M ascorbic acid, 1.0 mM citric acid and 0.10 M acetate buffer adjusted to pH 4.0. The best sample flow rate is 2.1 mL min−1 providing a sample throughput of more than 15 samples h−1. The linear dynamic range of the method can be adjusted by changing the volume of the injection loop. The rFIA manifold was assembled exclusively from commercially available components.

Deep Penetration of UV Radiation into PMMA and Electron Acceleration in Long Plasma Channels Produced by 100 ns KrF Laser Pulses

Long (~1 mm), narrow (30−40 μm in diameter) corrugated capillary-like channels were produced in the axially symmetric 2D interaction regime of 100 ns KrF laser pulses with polymethylmethacrylate (PMMA) at intensities of up to 5 × 1012 W/cm2. The channels extended from the top of a deep (~1 mm) conical ablative crater and terminated in a 0.5 mm size crown-like pattern. The modeling experiments with preliminary drilled capillaries in PMMA targets and Monte Carlo simulations evidenced that the crown origin might be caused by high-energy (0.1–0.25 MeV) electrons, which are much higher than the electron temperature of the plasma corona ~100 eV. This indicates the presence of an unusual direct electron acceleration regime. Firstly, fast electrons are generated due to laser plasma instabilities favored by a long-length interaction of a narrow-band radiation with plasma in the crater. Then, the electrons are accelerated by an axial component of the electrical field in a plasma-filled corrugated capillary waveguide enhanced by radiation self-focusing and specular reflection at the radial plasma gradient, while channel ripples serve the slowing down of the electromagnetic wave in the phase with electrons.

Generation of high-quality GeV-class electron beams utilizing attosecond ionization injection

Acceleration of electrons in laser-driven plasma wakefields has been extended up to the 10 GeV energy within a distance of 10s of centimeters. However, in applications, requiring small energy spread within the electron bunch, only a small portion of the bunch can be used and often the low-energy electrons represent undesired background in the spectrum. We present a compact and tunable scheme providing clean and mono-energetic electron bunches with less than one percent energy spread and with central energy on the GeV level. It is a two-step process consisting of ionization injection with attosecond pulses and acceleration in a capillary plasma wave-guide. Semi-analytical theory and particle-in-cell simulations are used to accurately model the injection and acceleration steps.

Radial density profile and stability of capillary discharge plasma waveguides of lengths up to 40 cm

Abstract We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 $\mathrm{\mu} \mathrm{m}$ to 2 mm and lengths of 9 to 40 cm. To the best of the authors’ knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for $\ge$ 10 GeV electron energy gain in a single laser-driven plasma wakefield acceleration stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to $<0.2$ % and their average on-axis plasma electron density to $<1$ %. These variations explain only a small fraction of laser-driven plasma wakefield acceleration electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and is in excellent agreement with magnetohydrodynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.

On Peculiarities of Betatron Oscillations of Accelerated Electron Bunches in Capillary Waveguides

It is shown that the dynamics of electrons accelerated in narrow capillary waveguides is significantly influenced by the parametric excitation of their betatron oscillations. On the one hand, this excitation can irreversibly spoil the emittance of an accelerated electron bunch that limits the possibilities of their practical use. On the other hand, controlled parametric excitation of betatron oscillations can be used to generate short-pulse sources of synchrotron radiation. The article analyzes the regions of parametric instabilities, their dependence on the parameters of accelerated electron bunches and guiding structures, and their influence on the dynamics of accelerated electrons. The parameters of the generated synchrotron radiation are also estimated. Measurements of the spectral parameters of synchrotron radiation can serve as a tool for diagnostics of betatron oscillations and their excitation in the case of parametric resonances.

A Novel Visible Light-Excited Interference Behavior Occurred in Capillary Waveguide

In this article we demonstrated a novel interference behavior excited by visible light in the capillary waveguide (CWG). We investigated the sensing principle of CWG with filling liquid and optical characteristics of interference systematically. This CWG-based sensing element is theoretically verified to possess continuously tunable thermal sensitivity by changing the thermo-optic coefficient of the filling liquid. As expected, experimental results show that the proposed sensor possesses sharp interference fringes and the tunable sensitivity of temperature. Unlike most optical fiber interferometers, the optical path difference of our sensing element is generated in the wall of CWG instead of air cavity. Besides, interference occurs in the visible light band, which greatly improves dynamic range and reduces the cost of system setup. The novel optical behavior we investigated offers a way to the utility of the waveguide as an optical interferometer.

Modeling of capillary discharge plasmas for wakefield acceleration and beam transport

Next generation accelerators demand sophisticated beam sources to produce ultra-low emittances with large gradients. The subsequent beamline optics are equally critical to transporting these beams between accelerating stages or to interaction points. Capillary discharge plasmas may address each of these challenges. Capillaries have been demonstrated as sources capable of increasing the peak energy and beam quality of laser wakefield accelerators, and as active plasma lenses featuring orders-of-magnitude increases in peak magnetic field. These systems are sensitive to energy deposition, heat transfer, ionization dynamics, and magnetic field penetration; therefore, improved modeling will enable advances in capillary design. We present simulations of capillary discharge waveguides and active plasma lenses in using FLASH, a publicly-available multi-physics code in development at the University of Chicago. We report on the implementation of a 2D, cylindrically symmetric capillary model for capturing plasma density and temperature evolution with realistic conductivities and magnetic fields. We then illustrate the use of laser energy deposition to model low density channel formation for the matching and guiding of intense laser pulses. Lastly, we discuss simulations of active capillary plasmas with different fill species, which show agreement with experimental observations of nonlinearities in the current density profile and magnetic field.

Laser-Based Trace Gas Detection inside Hollow-Core Fibers: A Review.

Thanks to the guidance of an optical wave in air, hollow-core fibers may serve as sampling cells in an optical spectroscopic system. This paper reviews applications of hollow-core optical fibers to laser-based gas sensing. Three types of hollow-core fibers are discussed: Hollow capillary waveguides, photonic band-gap fibers, and negative curvature fibers. Their advantages and drawbacks when used for laser-based trace gas detection are analyzed. Various examples of experimental sensing systems demonstrated in the literature over the past 20 years are discussed.

Laser-heated capillary discharge waveguides as tunable structures for laser-plasma acceleration

Laser-heated capillary discharge waveguides are novel, low plasma density guiding structures able to guide intense laser pulses over many diffraction lengths and have recently enabled the acceleration of electrons to 7.8 GeV by using a laser-plasma accelerator (LPA). These devices represent an improvement over conventional capillary discharge waveguides, as the channel matched spot size and plasma density can be tuned independently of the capillary radius. This has allowed the guiding of petawatt-scale pulses focused to small spot sizes within large diameter capillaries, preventing laser damage of the capillary structure. High performance channel-guided LPAs require control of matched spot size and density, which experiments and simulations reported here show can be tuned over a wide range via initial discharge and laser parameters. In this paper, measurements of the matched spot size and plasma density in laser-heated capillary discharges are presented, which are found to be in excellent agreement with simulations performed using the MHD code MARPLE. Strategies for optimizing accelerator performance are identified based on these results.

Single-Photon Ionization Mass Spectrometry Using a Vacuum Ultraviolet Femtosecond Laser.

The wavelength of a femtosecond Ti:sapphire laser (TS, 800 nm) was converted into the ultraviolet (UV, 200 nm) using three β-barium borate crystals (β-BaB2O4) for frequency doubling and subsequent mixing. The UV pulse was further converted into the vacuum ultraviolet (VUV, 185 nm) based on four-wave Raman mixing, in which a two-color pump beam consisting of the fundamental beam (800 nm) of the TS and the signal beam of an optical parametric amplifier (1200 nm) pumped by the TS was focused onto a capillary waveguide filled with hydrogen gas for molecular phase modulation and the single-color UV probe beam (200 nm) was then focused onto the waveguide for frequency modulation to generate anti-Stokes and high-order Stokes Raman sidebands at wavelengths of 185 and 218-267 nm, respectively. The efficiency of conversion from the UV (200 nm) to the VUV (185 nm) was 6%. The ionization energy was calculated for 13 amino polycyclic aromatic hydrocarbons using density functional theory, since they are associated with the development of occupational bladder cancers. The values calculated by the B3LYP/cc-pVDZ and ωB97Xd/cc-pVTZ methods were 6.24-7.14 eV (199-174 nm) and 6.41-7.35 eV (194-169 nm), respectively. A sample containing a mixture of 9-aminoanthracene, 3-aminofluoranthene, and 1-aminopyrene was separated by gas chromatography (GC), and the eluents were ionized with the VUV pulse (0.015 μJ) in mass spectrometry (MS). The analytes were observed on a two-dimensional display of GC/MS, and the detection limit obtained by single-photon ionization of 3-aminofluoranthene was 1 ng/μL.

Development of an In Situ Analyzer Based on Sequential Injection Analysis and Liquid Waveguide Capillary Flow Cell for the Determination of Dissolved Reactive Phosphorus in Natural Waters.

This study presents an innovative technique for the in situ analysis of aquatic biochemical elements detected through wet chemical processes. A new compact in situ phosphate analyzer based on sequential injection analysis, liquid waveguide capillary flow cell and spectrophotometry was developed, and a safe and modular electronics-chemical separation mechanical structure was designed. The sequential injection system of this analyzer was optimized, and the major functions of this analyzer were studied and estimated. With a 10 cm liquid waveguide capillary flow cell and a 6.3 min time cost of detection, the analyzer reaches a detection limit of 1.4 μg·L−1 (≈14.7 nM, [PO43−]) and a consumption of 23 μL at most for each reagent. This analyzer was operated in situ and online during two scientific research cruises in the Pearl River Estuary and northern South China Sea. The advantages of this analyzer include its simple versatile manifold, full automation, low chemical consumption and electronics-chemical separate safe structure. Long-term in situ performance of this analyzer will be validated in the future.

Laser-heated capillary discharge plasma waveguides for electron acceleration to 8 GeV

A plasma channel created by the combination of a capillary discharge and inverse Bremsstrahlung laser heating enabled the generation of electron bunches with energy up to 7.8 GeV in a laser-driven plasma accelerator. The capillary discharge created an initial plasma channel and was used to tune the plasma temperature, which optimized laser heating. Although optimized colder initial plasma temperatures reduced the ionization degree, subsequent ionization from the heater pulse created a fully ionized plasma on-axis. The heater pulse duration was chosen to be longer than the hydrodynamic timescale of ≈1 ns, such that later temporal slices were more efficiently guided by the channel created by the front of the pulse. Simulations are presented which show that this thermal self-guiding of the heater pulse enabled channel formation over 20 cm. The post-heated channel had lower on-axis density and increased focusing strength compared to relying on the discharge alone, which allowed for guiding of relativistically intense laser pulses with a peak power of 0.85 PW and wakefield acceleration over 15 diffraction lengths. Electrons were injected into the wake in multiple buckets and times, leading to several electron bunches with different peak energies. To create single electron bunches with low energy spread, experiments using localized ionization injection inside a capillary discharge waveguide were performed. A single injected bunch with energy 1.6 GeV, charge 38 pC, divergence 1 mrad, and relative energy spread below 2% full-width half-maximum was produced in a 3.3 cm-long capillary discharge waveguide. This development shows promise for mitigation of energy spread and future high efficiency staged acceleration experiments.

Field Ion Emission in an Atom Probe Microscope Triggered by Femtosecond-Pulsed Coherent Extreme Ultraviolet Light.

This paper describes initial experimental results from an extreme ultraviolet (EUV) radiation-pulsed atom probe microscope. Femtosecond-pulsed coherent EUV radiation of 29.6 nm wavelength (41.85 eV photon energy), obtained through high harmonic generation in an Ar-filled hollow capillary waveguide, successfully triggered controlled field ion emission from the apex of amorphous SiO2 specimens. The calculated composition is stoichiometric within the error of the measurement and effectively invariant of the specimen base temperature in the range of 25 K to 150 K. Photon energies available in the EUV band are significantly higher than those currently used in the state-of-the-art near-ultraviolet laser-pulsed atom probe, which enables the possibility of additional ionization and desorption pathways. Pulsed coherent EUV light is a new and potential alternative to near-ultraviolet radiation for atom probe tomography.

Theoretical study of wakefield acceleration of electrons in capillary Z-pinch plasma waveguide

The theoretical investigation of wakefield acceleration of electrons by CO2-laser pulse with central wavelength of 10.6 μm and input peak intensity of ∼1017 W/cm2 in transient hydrogen-plasma waveguide has been conducted. The plasma waveguide is produced by the fast Z-pinch discharge inside a 3 mm inner diameter and 50 mm long capillary. The waveguide properties of the capillary Z-pinch plasma were obtained from the space and frequency dependent wave equation that combines the attenuated charged particle inertia and the light wave effects in the plasma for the ideal Gaussian laser beam. For simulation of temporal and spatial evolution of electron density and other plasma variables during the capillary discharge, we used a standard one-fluid, two-temperature one-dimensional magnetohydrodynamic (MHD) model complemented with atomic data of hydrogen. Simulations showed that the guiding channel occurs far from capillary wall and exists for a few nanoseconds. In terms of laser driven plasma wakefield accelerators (LWFA), the Z-pinch waveguide is able to extend the acceleration length over the whole capillary, assuming that a single-mode transmission takes place. To quantify such ability of the channel, a correlation coefficient was introduced and computed at different input beam spot sizes. An optimal beam spot size was determined by taking maximum of time average of the coefficient over the channel lifetime. At the end of the guiding channel existence, the repetitive focusing and defocusing patterns were observed with intensity increase at the focal points. For simulation of the wakefield acceleration of electrons in the different waveguiding regimes we used the particle-in-cell (PIC) combined with the aforementioned MHD model. Influence of the waveguiding regimes on the electron acceleration was demonstrated.