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The rate constant and branching ratios of ethyl reaction with hydroperoxyl radical, C2H5 + HO2 (1), a key radical-radical reaction for intermediate temperature combustion chemistry, were measured in situ for the first time in a photolysis Herriott cell by using mid-IR Faraday rotation spectroscopy (FRS) and UV-IR direct absorption spectroscopy (DAS). The microsecond time-resolved diagnostic technique in this work enabled the direct rate measurements of the target reaction at 40 and 80 mbar and reduced the experimental uncertainty considerably. C2H5 and HO2 radicals were generated by the photolysis of (COCl)2/C2H5I/CH3OH/O2/He mixture at 266 nm. By direct measurements of the transient profiles of C2H5, HO2 and OH concentrations, the overall rate constant for this reaction at 297 K was determined as k1(40 mbar) = (3.8 ± 0.8) × 10 − 11 cm3 molecule − 1 s − 1 and k1(80 mbar) = (4.1 ± 1.0) × 10 − 11 cm3 molecule − 1 s − 1 . The direct observation of hydroxyl radical (OH) indicated that OH formation channel was the major channel with a branching ratio of 0.8 ± 0.1.

Layered nickel-rich materials are promising next-generation cathode materials for lithium ion batteries due to their high capacity and low cost. However, the poor thermal stability and longtime cycling performance hinders the commercial applications of high nickel materials. Doping with heteroatoms has been an effective approach for improving electrochemical performance of cathode materials. Controlling doping concentration and geometrical distribution is desired for optimal electrochemical performance, but it is challenging in traditional co-precipitation methods. In this work, controlled dysprosium (Dy) doping to NCM811 was studied in an aerosol synthesis method by controlling the precursor concentrations and heating parameters. The obtained materials were characterized by SEM, XRD, and XPS, and their electrochemical properties and thermal stability were evaluated. By controlling the doping concentration (1.5%), Dy-doped NCM811 was improved simultaneously in long-term cycling and high-rate performance. The thermal-chemical stability of the Dy-doped cathode materials was examined in a microflow reactor with a mass spectrometer. The results showed that Dy-doping shifted the O2 onset temperature to a higher temperature and reduced O2 release by 80%, thus dramatically increasing the thermal-chemical stability and improving the fire safety of cathode materials. Since high temperature aerosol synthesis is a low-cost and scalable method, the findings in this work have broad implications for commercial synthesis of novel materials with controlled doping modification to achieve high electrochemical performance and safety in lithium ion batteries.

The present study investigates the kinetics of low-temperature pyrolysis and oxidation of n-dodecane/O2/N2 mixtures in a repetitively-pulsed nanosecond discharge experimentally and numerically. Time-resolved TDLAS measurements, steady-state gas chromatography (GC) sampling, and mid-IR dual-modulation Faraday rotation spectroscopy (DM-FRS) measurements are conducted to quantify temperature as well as species formation and evolution. A plasma-assisted n-dodecane pyrolysis and oxidation kinetic model incorporating the reactions involving electronically excited species and NOx chemistry is developed and validated. The results show that a nanosecond discharge can dramatically accelerate n-dodecane pyrolysis and oxidation at low temperatures. The numerical model has a good agreement with experimental data for the major intermediate species. From the pathway analysis, electronically excited N 2 * plays an important role in n-dodecane pyrolysis and oxidation. The results also show that with addition of n-dodecane, NO concentration is reduced considerably, which suggests that there is a strong NO kinetic effect on plasma-assisted low-temperature combustion via NO-RO2 and NO2-fuel radical reaction pathways. This work advances the understandings of the kinetics of plasma-assisted low-temperature fuel oxidation in N2/O2 mixtures.

Photoacoustic imaging (PAI), an emerging imaging technique, exploits the merits of both optical and ultrasound imaging, equipped with optical contrast and deep penetration. Typical linear PAI relies on a nanosecond laser pulse to induce photoacoustic signals. To construct a multi-wavelength PAI system, a multi-wavelength nano-second laser source is required, which greatly increases the cost of the PAI system. However, according to the nonlinear photoacoustic effect, the amplitude of the photoacoustic signals will vary with different base temperatures of the tissue. Therefore, using continuous-wave lasers with different wavelengths to induce different temperature variations at the same point of the tissue, and then using a single-wavelength pulsed laser to induce photoacoustic signals has been an alternative method to achieve multi-wavelength PAI. In this paper, based on the nonlinear photoacoustic effect, we developed a continuous-wave multi-wavelength laser source to cut down the cost of the conventional multi-wavelength PAI system. The principle will be introduced firstly, followed by qualitative and quantitative experiments.

Photoacoustic imaging (PAI) is a fast-developing non-invasive imaging technique, which provides optical contrast in deep scattering tissue with high spatial resolution. Conventional photoacoustic (PA) imaging has been widely studied based on linear PA effect, which is induced by single short laser pulse. Recently, nonlinear photoacoustic effect has been proposed by using two consecutive laser pulses with two laser sources. In this paper, we report a dual-contrast nonlinear photoacoustic (dNPA) sensing and imaging technique by single high-repetition-rate laser induced nonlinear PA effect. Using the high-repetition-rate pulsed laser, two different kinds of imaging contrasts can be extracted: optical and thermal contrasts. Unlike conventional PA imaging that exhibits optical absorption only, we could extract thermal contrast that is reflected by the rising slope/trend of temperature and consequent PA signals. In this paper, the high-repetition-rate nonlinear PA effect and dual-contrast mechanism were proposed in theory with detailed mathematical modeling. The dual-contrast PA sensing and imaging were demonstrated by experimental results with both in vitro phantom and ex vivo porcine tissues. The results indicate both optical absorption and thermal accumulation properties of different samples, which could enhance the image contrast significantly.

Photoacoustic tomography is a technique to reconstruct the image of light energy absorption distribution in tissues based on the detected photoacoustic signals. In recent years, this research field has been greatly developed, and its application range is wide, including anatomy, functionality, and molecular imaging. However, the conversion efficiency of photoacoustic effect from light to sound is quite low, which leads to the low signal-to-noise ratio of photoacoustic signal and the poor quality of reconstructed photoacoustic image. The traditional method to improve the signal-to-noise ratio of photoacoustic signals is data averaging method, but it seriously limits the imaging speed due to multiple acquisition. Without sacrificing signal fidelity and imaging speed, an empirical mode decomposition (EMD) combined with conditional mutual information de-noising algorithm for photoacoustic tomography is proposed in this paper. The simulation results and experimental results of photoacoustic signal de-noising achieve significant improvement of signal-to-noise ratio of photoacoustic signal and the enhancement of contrast of the reconstructed image. The simulation results and experimental results show that EMD combined with mutual information method improves at least 2 dB and 3 dB, respectively, more than traditional wavelet threshold method and band-pass filter. The improvement of contrast-to-noise ratio is more than 2 dB and 3 dB, respectively, more than traditional wavelet threshold method and band-pass filter.

Photoacoustic imaging has been intensively studied in recent years and many of the achievements have already been applied in clinical diagnosis and therapy, e.g., using spectroscopic photoacoustic imaging to extract functional and molecular information. However, spectroscopic photoacoustic imaging requires expensive and bulky tunable laser source, which severely hinders its further development toward a portable device. In this paper, we propose a novel imaging method, named optical spectroscopic ultrasound displacement (OSUD) imaging, which enables optical spectroscopic imaging in deep scattering tissue using multiple low-cost continuous-wave laser sources and ultrasound imaging equipment. The principle of the OSUD imaging method will be introduced, followed by preliminary experimental results. The OSUD imaging may exploit the deep penetration merit of ultrasound and provide another pathway to provide spectroscopic optical absorption contrast in deep scattering tissue beyond commonly used photoacoustic imaging, which will be further validated in the future work. Accepted version

We perform HO2 detection in a photolysis reactor by means of conventional laser absorption spectroscopy and Faraday rotation spectroscopy. The advantages and disadvantages of the two methods are evaluated for this application.

Photoacoustic imaging (PAI) combines the deep penetration of ultrasound imaging and the high absorption contrast of optical imaging, which has attracted increasing research interest in recent years. The PAI based on multi-wavelength laser source is conducive to detecting different components in the blood such as oxygen saturation. The generation of PA signals could rely on the relative low-cost pulsed laser diode. However, the multi-wavelength PAI system requires multiple pulsed laser diodes, which will multiply the cost of the imaging system. On the other hand, the amplitude of the PA signals will change when inducing the Gruneisen relaxation effect due to the heat accumulation. In this paper, we proposed a continuous multi-wavelength photoacoustic imaging (CMPAI) system based on the combination of a single-wavelength pulsed laser diode and a multi-wavelength continuous laser diode module. The continuous wave (CW) laser is adapted for heating the sample. The PA signals are induced by a single-wavelength pulsed laser. By controlling the laser irradiation sequence (pulse-CW-pulse), the images before and after heating will be attained. By proper differentiating operations, the images with different-wavelengths’ light absorption could be revealed. Furthermore, using this proposed system to detect the concentration of ink that is mixed by different portion of green and red ink would be demonstrated in the following.

Photoacoustic imaging has attracted increasing research interest in recent years due to its unique merit of combining light and sound. Enabling deep tissue imaging with high ultrasound spatial resolution and optical absorption contrast, photoacoustic imaging has been applied in various application scenarios including anatomical, functional and molecular imaging. However, the bulky and expensive laser source is one of the key bottlenecks that need to address for further compact system development. Photoacoustic imaging system based on low-cost laser diode (LD) is one of the promising solutions. In this paper, we report a custom-made fingertip laser diode system enabling both pulsed and continuous modulation modes with shortest pulse-width of 40 ns, largest driving current of 13 A, and highest modulation frequency of 3 MHz, which are suitable for both time and frequency domain photoacoustic imaging. To the best of our knowledge, this may be the most compact laser source reported for photoacoustic imaging. Owing to its super-compact size, the proposed LD system could pave the pathway to low-cost photoacoustic sensing and imaging device, even wearable photoacoustic biomedical sensors.

Diffusive light scattering in biological tissue causes a heterogeneous and variable spectrum of light fluence, which is a significant challenge to predict due to unknown optical properties of tissues. Then it is difficult to accurately and quantitatively image blood oxygen saturation (sO 2 ) in photoacoustic imaging because of unknown fluence distribution. To tackle this problem, we develop a deep residual, and recurrent neural network, i.e., DR2U-net for the quantitative estimation of blood oxygenation of photoacoustic imaging. The fine-tuned DR2U-net we developed can extract fluence distribution information from optical absorption images only using two wavelengths of light in Monte Carlo simulation, and afterward generate a quantitative image of sO 2 . The measurement results of sO 2 show a very high accuracy by testing in simulated biological tissue, and its error is as low as 1.27% compared with conventional linear mixing method (48.76%). Besides, our model is high-speed, about 18. 4ms is achieved for every quantitative sO2 image.

The stability of the weakly ionized plasma and the transition from a stable homogeneous discharge to unstable filaments play an important role in gas laser physics, plasma-assisted combustion, chemical reforming, and material synthesis. Theoretical stability analysis and thermal-chemical mode analysis were performed to understand the mechanism of plasma thermal-chemical instability by using a zero-dimensional plasma system with both simplified and detailed chemical kinetics of H2/O2/N2 mixtures. The plasma dynamic and kinetic models accounted for multiple physical mechanisms in the chemically-reactive weakly ionized plasma, including ionization, attachment/detachment, recombination, vibrational and electronic energy relaxation, convective and diffusive species/heat removal, Joule heating, and detailed chemical kinetics. An analytical criterion and the explosive mode species/temperature pointers were formulated while the representative active species were identified for different thermal-chemical modes. The results showed that in addition to the classical thermal-ionization mechanism, various chemical modes from chemical heat imbalance and elementary kinetics significantly modified the time dynamics and the stability of the weakly ionized plasma. The present analysis provides insights and guidance to control plasma instability using chemical kinetics.

Faraday rotation spectroscopy (FRS) employs the Faraday effect to detect Zeeman splitting in the presence of a magnetic field. In this article, we present system design and implementation of radical sensing in a photolysis reactor using FRS. High sensitivity (100 ppb) and time resolved in situ HO2 detection is enabled with a digitally balanced acquisition scheme. Specific advantages of employing FRS for sensing in such dynamic environments are examined and rigorously compared to the more established conventional laser absorption spectroscopy (LAS). Experimental results show that FRS enables HO2 detection when LAS is deficient, and FRS compares favorably in terms of precision when LAS is applicable. The immunity of FRS to spectral interferences such as absorption of hydrocarbons and other diamagnetic species absorption and optical fringing are highlighted in comparison to LAS.

Photoacoustic imaging has been intensively studied in recent years, and many of the achievements have already been applied in important biomedical and clinical applications, e.g. spectroscopic photoacoustic imaging to extract functional and molecular information. However, spectroscopic photoacoustic imaging requires expensive and bulky tunable laser source, which severely hinder its further development towards portable device. In this paper, we propose a novel imaging method, named optical spectroscopic ultrasound displacement (OSUD) imaging, which enables optical spectroscopic imaging in deep scattering tissue using multiple low-cost continuous-wave laser sources and ultrasound imaging equipment. The principle of the OSUD imaging method will be introduced, and followed by preliminary experimental results. The OSUD imaging may provide another pathway to provide spectroscopic optical absorption contrast in deep scattering tissue beyond commonly used photoacoustic imaging.

Photoacoustic tomography (PAT) is a non-invasive imaging technique which provided high lateral resolution and axial resolution. Conventional linear photoacoustic (PA) imaging has been widely applied to state-of-art PA system, which excited by short pulse laser. Recently, nonlinear photoacoustic effect has been excavated and utilized, which indicates the different character with linear PA by dual-pulse laser. In this paper, we report a dual-contrast photoacoustic sensing by quasi-CW nonlinear PA effect. Using the high-repetition pulsed laser, and two different contrast imaging were extracted. Unless conventional contrast that using short laser pulse by detected tissue light absorption, we extracted another contrast that indicated the rising of temperature. The quasi-CW nonlinear PA effect could be illustrated by a simple mathematical derivation. The dual-contrast PA imaging have been demonstrated by experiment in vitro and ex vivo imaging. The results indicate different heat absorbency of different materials, which could be utilized to distinguish materials and intensify imaging contrast. Compared with conventional PA imaging, the proposed method has more scope of works and potential applications.

Multi-wavelength photoacoustic (PA) imaging has been studied extensively to explore the spectroscopic absorption contrast of biological tissues. To generate strong PA signals, a high-power wavelength-tunable pulsed laser source has to be employed, which is bulky and quite expensive. In this paper, we propose a hybrid multi-wavelength PA imaging (hPAI) method based on combination of single-wavelength pulsed and multi-wavelength continuous-wave (CW) laser sources. By carefully controlling laser illumination sequence (pulse-CW-pulse), and extracting the PA signals' difference before and after heating of CW lasers, the optical absorption property of multi-wavelength CW lasers could be obtained. Compared with conventional PA imaging, the proposed hPAI shows much lower system cost due to the usage of single-wavelength pulsed laser and cheap CW lasers. Theoretical analysis and analytical model are presented in this paper, followed by proof-of-concept experimental results.

Photoacoustic tomography (PAT), a promising medical imaging method that combines optical and ultrasound techniques, has been developing for decades mostly in preclinical application. A recent trend is to utilize the economical laser source to develop a low-cost sensing and imaging system, which aims at an affordable solution in clinical application. These low-cost laser sources have different modulation modes such as pulsed modulation, continuous modulation and coded modulation to generate different profiles of PA signals in photoacoustic (PA) imaging. In this paper, we review the recent development of the photoacoustic sensing and imaging based on the economical laser sources such as laser diode (LD) and light-emitting diode (LED) in different kinds of modulation types, and discuss several representative methods to improve the performance of such imaging systems based on low-cost laser sources. Finally, some perspectives regarding the future development of portable PAT systems are discussed, followed by the conclusion.

We demonstrate the use of laser induced incandescence (LII) of submicron tungsten carbide (WC) particles as a method for particle image velocimetry (PIV). The technique allows a single laser to be used for separate measurements of velocity of two phases in a droplet-laden flow. Submicron tungsten carbide (WC) particles are intentionally seeded into a two-phase flow, and heated by a light sheet generated by a double-pulsed PIV laser operating at sufficiently high pulse energy. The small size and large absorption cross-section allows particles to be heated up to several thousand degrees Kelvin to emit strong incandescence signals, whilst the laser-induced temperature increase in liquid droplets/large particles is negligible. The incandescence signal from WC and Mie scattering from droplets/large particles are separately captured by deploying different filters to a PIV camera. The consecutive images of the laser-induced incandescence (LII) are used to determine the velocity field of the gas-phase flow, and those of Mie scatter are used to extract the velocity of droplets/large particles. The proposed technique is demonstrated in an air jet first and compared with the result given by a normal PIV test, which shows that submicron WC particles can accurately follow the gas flow, and that the LII images can be used to perform cross-correlations. We then apply this technique on an ethanol droplet/air jet (non-reacting), demonstrating the resulting slip velocity between two phases. The proposed technique combining PIV and LII with a single laser requires little additional equipment, and is applicable to a much higher droplet/particle density than previously feasible. Finally the possibility of applying this technique to a flame is demonstrated and discussed.

We report measurements of HO2 radicals using wavelength modulated Faraday Rotation Spectroscopy in a multi-pass photolysis reactor. The current setup enables line-locked measurements to improve sensitivity and time resolution compared to the line-scanning system.

The control of plasma instability of weakly ionized plasma in a reactive flow is of great importance in plasma-assisted combustion, catalysis, fuel reforming and material synthesis. In this work, we propose a new concept of plasma chemical instability and analyze the mechanisms and impact of plasma chemical instability on the transition from a uniform discharge to a contracted state of a self-sustained glow discharge in a reactive Halt;subagt;2alt;/subagt;-Oalt;subagt;2alt;/subagt;-Nalt;subagt;2alt;/subagt; mixture. A one-dimensional numerical model for plasma chemical instability was developed which accounted for convective heat loss, Joule heating of plasma, and major non-equilibrium plasma-assisted combustion kinetic pathways including electron-impact ionization, vibrational energy transfer, electron attachment, combustion reactions, and heat release. The results showed that plasma chemical instability significantly modified the onset plasma current of plasma thermal instability. Specifically, the critical condition of the instability transition was strongly influenced by electron-impact reactant ionization, electron attachment to oxygen, endothermic/exothermic chemical reactions, and the formation of reaction products.

Photoacoustic imaging has attracted increasing research interest in recent years, due to its unique merit of combining light and sound. Enabling deep tissue imaging with high ultrasound spatial resolution and optical absorption contrast, photoacoustic imaging has been applied in various application scenarios including anatomical, functional and molecular imaging. However, the bulky and expensive laser source is one of the key bottlenecks that needs to be addressed for further compact system development. A photoacoustic imaging system based on a low-cost laser diode (LD) is one of the promising solutions. In this paper, we report a custom-made fingertip laser diode system enabling both pulsed and continuous modulation modes with shortest pulse-width of 40 ns, largest driving current of 13 A, and highest modulation frequency of 3 MHz, which is suitable for both time and frequency domain photoacoustic imaging. To the best of our knowledge, this may be the most compact laser source reported for photoacoustic imaging enabling both two modulation modes. Owing to its super-compact size, the proposed LD system could pave the pathway to a low-cost photoacoustic sensing and imaging device, even wearable photoacoustic biomedical sensors.

Multi-wavelength photoacoustic (PA) imaging has been studied extensively to explore the spectroscopic absorption contrast of biological tissues. To generate strong PA signals, a high-power wavelength tunable pulsed laser source has to be employed, which is bulky and quite expensive. In this Letter, we propose a hybrid multi-wavelength PA imaging (hPAI) method based on the combination of a single-wavelength pulsed laser source and multi-wavelength continuous-wave (CW) laser sources. By carefully controlling the laser illumination sequence (pulse-CW-pulse) and extracting the PA signal difference before and after the heating of CW lasers, the optical absorption property of multi-wavelength light illumination could be obtained. Compared with conventional PA imaging, the proposed hPAI shows a much lower system cost due to the usage of single-wavelength pulsed lasers and multiple inexpensive CW lasers. As the preliminary results show in this Letter, hPAI imaging has the potential to provide another pathway for high spectroscopic optical absorption contrast in PA imaging.