Photoacoustic Method Research Articles

A comparative study on laser induced shock cleaning of radioactive contaminants in air and water

Efficient removal of Uranium-di-oxide (UO2) particulates from stainless steel surface was effected by Nd-YAG laser induced plasma shock waves in air as well as in water environment. The propagation velocity of the generated shock wave was measured by employing the photo-acoustic probe deflection method. Monitoring of the alpha activity of the sample with a ZnS (Ag) scintillation detector before and after the laser exposure allowed the estimation of decontamination efficiency defined as the percentage removal of the initial activity. Experiments were carried out to study the effect of laser pulse energy, number of laser exposures, orientation of the sample, the separation between the substrate surface and the onset point of the shock wave on the de-contamination efficiency. The most optimised cleaning was found to occur when the laser beam impinged normally on the sample that was immersed in water and placed at a distance of ∼0.7 mm from the laser focal spot. Analysis of the cleaned surface by optical microscopes established that laser induced shock cleaning in no way altered the surface property. The shock force generated in both air and water has been estimated theoretically and has been found to exceed the Van der Waal’s binding force for spherical contaminant particulate.

Application of laser ultrasound technique to evaluate wave velocity in bovine meniscus

In vivo elasticity evaluation of meniscus and cartilage is necessary to understand the condition of the joint. Ishihara et al. have used the photoacoustic method with an endoscope.1 Another possible technique is the laser ultrasound. Making use of the thermoelastic effect, we can stimulate a short longitudinal wave by irradiating the pulsed laser beam to the material. This technique enables noncontact measurements. As an initial study, we have measured the longitudinal wave velocities in a bovine meniscus sample using a pulsed laser (COHERENT Helios 1064-5-50). The meniscus samples were obtained from the knee joint of a bovine femur. The sample size was 4.00×6.50×6.00 mm3. We could excite short longitudinal waves around 30 MHz and measure wave velocities propagating in three directions (bone axis, posterior-anterior, and medial-lateral). The excited ultrasonic wave propagated through the meniscus sample and was received by a piezoelectric sensor. The velocity range was about 2950 m/s–3450 m/s, showing the highest velocity in the anterior-posterior direction. Velocities also depended on the water content and showed small heterogeneity. [1] M.Ishihara et al., Jpn. J. Appl. Phys. 42, 556–558 (2003).

Determination of UV–visible–NIR absorption coefficient of graphite bulk using direct and indirect methods

Absorption coefficient of graphite bulk pressed from 1 to 5 μm-sized crystalline grains was measured in UV–Vis–NIR range with three different methods: (i) determination of pulsed laser ablation rate as the function of laser fluence for different wavelengths (248, 337, 532, and 1064 nm, respectively); (ii) production of aerosol particles by UV laser ablation of the bulk graphite in inert atmosphere and determination of the mass-specific absorption coefficient with a four-wavelength (266, 355, 532, and 1064 nm, respectively) photoacoustic spectrometer, and (iii) spectroscopic ellipsometry in 250–1000 nm range. Taking into account the wide range of the absorption coefficients of different carbon structures, an overall relatively good agreement was observed for the three methods. The ellipsometric results fit well with the ablation rate measurement, and the data obtained with photoacoustic method are also similar in the UV and NIR region; however, the values were somewhat higher in visible and near-UV range. Taking into account the limitations of the methods, they can be promising candidates for the determination of absorption coefficient when the samples are strongly scattering and there is no possibility to perform transmissivity measurements.

Observation of surface plasmon resonance of gold nanoparticles in energy-related material: pentaerythritol tetranitrate

The surface plasmon resonance of gold nanoparticles in a polycrystalline energy-related material – pentaerythritol tetranitrate (PETN) – has been observed for the first time. The extinction spectra of this material are recorded, and its absorption characteristics are determined by the photoacoustic method using pulsed laser irradiation. Using the data obtained, it is concluded that gold nanoparticles are promising inclusions in explosive materials when designing an optical detonator.

Melt-processed P3HT and PE Polymer Nanofiber Thermal Conductivity

ABSTRACT Thermal management is a growing challenge for electronics packaging because of increased heat fluxes and device miniaturization. Thermal interface materials (TIMs) are used in electronic devices to transfer heat between two adjacent surfaces. TIMs need to exhibit high thermal conductivity and must be soft to minimize thermal contact resistance. Polymers, despite their relative softness, suffer from low thermal conductivity (∼0.2 W/m-K). To overcome this challenge, we infiltrate nanoporous anodic aluminum oxide (AAO) templates with molten polymer to fabricate large area arrays of vertically aligned polymer nanofibers. Nanoscale confinement effects and flow induced chain elongation improve polymer chain alignment (measured using polarized Raman spectroscopy) and thermal conductivity (measured using the photoacoustic method) along the fiber’s long axis. Conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) and non- conjugated polyethylene (PE) of various molecular weights are explored to establish a relationship between polymer structure, nanofiber diameter, and the resulting thermal conductivity. In general, thermal conductivity improves with decreasing fiber diameter and increasing polymer molecular weight. Thermal conductivity of approximately 7 W/m-K was achieved for both the ∼200 nm diameter HDPE fibers and the 100 nm diameter P3HT fibers. These results pave the way for optimization of the processing conditions to produce high thermal conductivity fiber arrays using different polymers, which could potentially be used in thermal interface applications.

Phase-domain photoacoustics eliminating acoustic detection variations.

As one of the fastest-growing imaging modalities in recent years, photoacoustic imaging has attracted tremendous research interest for various applications including anatomical, functional and molecular imaging. Majority of the photoacoustic imaging systems are based on time-domain pulsed photoacoustic method, which utilizes pulsed laser source to induce wideband photoacoustic signal revealing optical absorption contrast. An alternative way is frequency-domain photoacoustic method utilizing chirping modulation of laser intensity to achieve lower system cost. In this paper, we report another way of photoacoustic method, called phase-domain photoacoustic sensing, which explores the phase difference between two consequent intensity-modulated laser pulses induced photoacoustic measurements to reveal the optical property. The basic principle is introduced, modelled and experimentally validated in this paper, which opens another potential pathway to perform photoacoustic sensing and imaging eliminating acoustic detection variations beyond the conventional time-domain and frequency-domain photoacoustic methods.

Glutathione-Capped Gold Nanoparticles-Based Photoacoustic Sensor for Label-Free Detection of Lead Ions

The photoacoustic signal generated by laser-induced nanobubbles (PA-LINB) proved to be a sensitive tool to monitor the aggregation of gold nanoparticles. Here, a simple and label-free photoacoustic method for the rapid detection of Pb2+ in the aqueous phase was developed. Due to the high affinity of Pb2+ ions to glutathione, the presence of Pb2+ led to the aggregation of glutathione-conjugated gold nanoparticles (GSH-GNPs). Hence, by measuring the variation of the PA-LINB signal after the aggregation of GSH-GNPs, Pb2+ can be quantified. A low detection limit for Pb2+ (42 nM) and a wide linear working range (~42–1000 nM) were achieved. Furthermore, the proposed method showed good selectivity against other metal ions.

Structural and piezo-ferroelectric properties of K0.5Na0.5NbO3 thin films grown by pulsed laser deposition and tested as sensors

We have grown K0.5Na0.5NbO3 (KNN) films on Pt/TiO2/SiO2/Si substrates by using pulsed laser deposition. Thin films with pure KNN crystal phase and homogenous morphology were successfully prepared by optimizing the deposition conditions, mainly the oxygen partial pressure and deposition temperature. For oxygen pressure as low as 8Pa a tungsten-bronze phase is formed, while for higher values i.e. 16 and 32Pa, the desired KNN perovskite structure is obtained. To prepare pure KNN thin films, a deposition temperature as low as 675°C is required, however, increasing the temperature to 725°C promotes better homogeneity and densification of the films. The piezoelectric and ferroelectric behavior, confirmed by piezoresponse force microscopy, revealed a local piezoelectric coefficient of d33=37pm/V and a coercive voltage of 3.5V. Finally, we measured the frequency response of this material prepared as a sensor, using the photoacoustic method. This measurement shows that the resonant frequency of a KNN thin film of 230nm thickness is centered at 3MHz.

A photoacoustic imaging reconstruction method based on directional total variation with adaptive directivity

BackgroundIn photoacoustic tomography (PAT), total variation (TV) based iteration algorithm is reported to have a good performance in PAT image reconstruction. However, classical TV based algorithm fails to preserve the edges and texture details of the image because it is not sensitive to the direction of the image. Therefore, it is of great significance to develop a new PAT reconstruction algorithm to effectively solve the drawback of TV.MethodsIn this paper, a directional total variation with adaptive directivity (DDTV) model-based PAT image reconstruction algorithm, which weightedly sums the image gradients based on the spatially varying directivity pattern of the image is proposed to overcome the shortcomings of TV. The orientation field of the image is adaptively estimated through a gradient-based approach. The image gradients are weighted at every pixel based on both its anisotropic direction and another parameter, which evaluates the estimated orientation field reliability. An efficient algorithm is derived to solve the iteration problem associated with DDTV and possessing directivity of the image adaptively updated for each iteration step.Results and conclusionSeveral texture images with various directivity patterns are chosen as the phantoms for the numerical simulations. The 180-, 90- and 30-view circular scans are conducted. Results obtained show that the DDTV-based PAT reconstructed algorithm outperforms the filtered back-projection method (FBP) and TV algorithms in the quality of reconstructed images with the peak signal-to-noise rations (PSNR) exceeding those of TV and FBP by about 10 and 18 dB, respectively, for all cases. The Shepp–Logan phantom is studied with further discussion of multimode scanning, convergence speed, robustness and universality aspects. In-vitro experiments are performed for both the sparse-view circular scanning and linear scanning. The results further prove the effectiveness of the DDTV, which shows better results than that of the TV with sharper image edges and clearer texture details. Both numerical simulation and in vitro experiments confirm that the DDTV provides a significant quality improvement of PAT reconstructed images for various directivity patterns.

Photoacoustic viscoelasticity imaging dedicated to mechanical characterization of biological tissues

Since changes in mechanical properties of biological tissues are often closely related to pathology, the viscoelastic properties are important physical parameters for medical diagnosis. A photoacoustic (PA) phase-resolved method for noninvasively characterizing the biological tissue viscoelasticity has been proposed by Gao et al. [G. Gao, S. Yang, D. Xing, “Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement,” Opt. Lett. 36, 3341–3343 (2011)]. The mathematical relationship between the PA phase delay and the viscosity–elasticity ratio has been theoretically deduced. Moreover, systems of PA viscoelasticity (PAVE) imaging including PAVE microscopy and PAVE endoscopy were developed, and high-PA-phase contrast images reflecting the tissue viscoelasticity information have been successfully achieved. The PAVE method has been developed in tumor detection, atherosclerosis characterization and related vascular endoscopy. We reviewed the development of the PAVE technique and its applications in biomedical fields. It is believed that PAVE imaging is of great potential in both biomedical applications and clinical studies.

In vivo photoacoustic imaging of blood vessels using a homodyne interferometer with zero-crossing triggering.

We demonstrate a quasinoncontact photoacoustic imaging method using a homodyne interferometer with a long coherence length laser. The generated photoacoustic signal is detected by a system that is locked at its maximum sensitivity through the use of balanced detection and zero-crossing triggering. The balanced detector is substantially equalized, so its output is zero when the system reaches the maximum sensitivity. The synchronization approach is used to trigger the excitation and detection of the photoacoustic signal. The system is immune to ambient vibrations. A thin water layer on the sample surface is used to reduce the effect of the rough tissue surface. The performance of the system is demonstrated by in vivo imaging of the microvasculature in mouse ears.

A non-invasive photoacoustic and ultrasonic method for the measurement of glucose solution concentration

Diabetes mellitus (DM) is a chronic disease affecting nearly 400 million people worldwide. In order to manage the disease, patients need to monitor the blood glucose level by puncturing the finger several times a day, which is uncomfortable and inconvenient. We present here a potential non-invasive monitoring method based on the velocity of ultrasonic waves generated in glucose solution by the photoacoustic principal, which can recognize the glucose concentration down to 20mg/dL. In order to apply this method to warm bodies, we carefully designed the experiment and performed measurements from 30 °C to 50 °C to generate a set of calibration curves, which may be used by engineers to build devices. Most importantly, we have theoretically explained the relationship between the compressibility and the glucose concentration. Our results show that the compressibility of solution decreases with the glucose concentration, which clarified the controversy between theory and experiment results in the literature. The derived formula is generally validity, which can be used to nondestructively measure solution concentration for other types of solutions using photoacoustic principle.

Phase-domain photoacoustic sensing

As one of the fastest-growing imaging modalities in recent years, photoacoustic imaging has attracted tremendous research interest for various applications including anatomical, functional, and molecular imaging. The majority of the photoacoustic imaging systems are based on the time-domain pulsed photoacoustic method, which utilizes a pulsed laser source to induce a wideband photoacoustic signal, revealing optical absorption contrast. An alternative way is the frequency-domain photoacoustic method utilizing the chirping modulation of laser intensity to achieve lower system cost. In this paper, we report another way of the photoacoustic method, called phase-domain photoacoustic sensing, which explores the phase difference between two consequent intensity-modulated laser pulse induced photoacoustic measurements to reveal the optical properties. The basic principle is introduced, modeled, and experimentally validated in this paper, which opens another potential pathway to perform photoacoustic sensing and imaging, eliminating acoustic detection variations beyond the conventional time-domain and frequency-domain photoacoustic methods.

Study of Silicon Cantilevers by the Photoacoustic Elastic Bending Method

Rectangular silicon cantilevers are studied by the photoacoustic (PA) elastic bending method. Experimental signals versus modulation frequency of the excitation optical beam are measured and analyzed in a frequency range from 20 Hz to 50 000 Hz. The procedure for experimental signal correction to eliminate the frequency characteristics of the measuring system is given. The corrected experimental signal shows a good correlation with theoretically calculated PA signal at frequencies below 32 000 Hz. The corrected experimental PA elastic bending signals for cantilevers with different thicknesses are analyzed. The experimental results allow identifying the resonant frequency (the first resonant mode) of the cantilever vibrations. These values are in good agreement with the theoretically computed values. A theoretical model of the optically excited Si cantilever is derived, taking into account plasmaelastic, thermoelastic, and thermodiffusion mechanisms. Dynamic relations for the amplitude and phase of electronic and thermal elastic vibrations in optically excited cantilevers are derived. The theoretical model is compared to the experimental results.

15th Anglo-French Physical Acoustics Conference (AFPAC 2016)

The 15th Anglo-French Physical Acoustics Conference (AFPAC) was held at Selsdon Park Hotel, near London, United Kingdom, on 13-15 January 2016. The venue was an excellent location to exchange ideas, regardless whether this took place in the conference room, over lunch, at the drinks reception, or in the bar after the conference dinner. A total of 65 papers were presented at the conference. There were over 80 delegates from institutions covering five countries.On the first day of AFPAC, the Institute of Physics joined forces with the National Physical Laboratory (UK) to host a special session on cavitation. The Cavitation User Forum, a bi-annual event specifically dedicated to applications of high power ultrasound, brought together experts from academia and from the cleaning, processing and medical industries. This session was kicked off with an invited talk by Dr David Fernandez Rivas (University of Twente, The Netherlands), on the reproducibility of sonochemistry and ultrasonic cleaning.The Cavitation User Forum was followed by a special session on biomedical ultrasound, co-sponsored by the Medical Physics Group of the Institute of Physics, which featured a keynote talk by Prof Robin Cleveland (University of Oxford) on ultrasonic surgery. The session included talks on acoustic microscopy of live cells, histotripsy, phase-insensitive ultrasound computed tomography for the diagnosis of breast cancer, high-intensity focused ultrasound and the biomedical applications of solitary wave impulses generated by granular chainsThe second day featured an invited presentation by Prof Tim Leighton (University of Southampton, UK) on the acoustic bubble, which discussed ocean, cetacean and extra-terrestrial acoustics, and cold water cleaning. Prof Christ Glorieux (KU Leuven, Belgium) discussed the applications of photothermal and photoacoustic methods using different spatiotemporal excitation patterns. A broad range of physical acoustics topics was reviewed that day. Work was presented on phononic crystals, high resolution imaging of in-depth rough interfaces, shear wave reconversion in nano-fluids and the prediction of radiation by finite-sized sources in semi-infinite isotropic plates. The field of non-destructive evaluation was well represented, with a presence from institutions such as Imperial College London, University of Nottingham and Université de Montpellier.During the Conference Dinner, the 2015 Bob Chivers prize was awarded to Dr Bo Lan for his paper entitled “Experimental and computational studies of ultrasound wave propagation in hexagonal close-packed polycrystals for texture detection”.On the final day of the conference, Prof Olivier Dazel (Université du Maine, Le Mans, France) gave an invited talk discussing the past present and future of the modelling of sound absorbing materials.The UK organising committee was particularly happy to welcome the many French contributors that travelled to Selsdon Park and would like to thank Alain Lhémery and the Société Française d’Acoustique (SFA) for publicising the event in France. We are happy to announce that many of the presented papers will be published in the conference proceedings in Journal of Physics: Conference Series. We would also like to draw the reader’s attention to the upcoming 16th AFPAC conference that is scheduled to take place on 23-25 of January 2017 at the Laboratoire de Mécanique et d’Acoustique, Marseille, France.Pierre Gélat (UCL, United Kingdom), Theodosia Stratoudaki (University of Nottingham, United Kingdom), Rachel Edwards (University of Warwick, United Kingdom), Robert Malkin (University of Bristol), Nader Saffari (UCL, United Kingdom) and Alain Lhémery (CEA, France)

Nondestructive Detection of Simulated Defect under Painted Surface by Photoacoustic Method

Nondestructive Detection of Simulated Defect under Painted Surface by Photoacoustic Method

軸対称座標系モデルを適用した光音響法による接触熱抵抗の評価

軸対称座標系モデルを適用した光音響法による接触熱抵抗の評価

Photoacoustic temperature measurement based on dual-wavelength method

Photoacoustic temperature measurement is a novel technique in which photoacoustic effect is used to measure temperature. It has the advantages of non-invasiveness, high sensitivity and deep penetration depth, which is suitable for monitoring the temperature distribution for the safe deposition of heat energy and efficient destruction of tumor cells during thermotherapy or cryotherapy. However, the present reported methods usually use one single wavelength for photoacoustic temperature measuring and are vulnerable to systematic and environmental influence, including the instability of system caused by fluctuation of laser energy, position displacement of transducer, and tissue complexity, which could reduce the measuring accuracy and stability. To solve this problem, a new photoacoustic temperature measuring method by employing two laser wavelengths is proposed in this paper. Firstly a brief theoretical analysis of dual-wavelengths photoacoustic temperature method is performed based on the linear relationship between photoacoustic signal and tissue temperature under two different wavelengths. Then two different samples including phantom of graphite and ex vivo pig blood are experimented respectively. The experimental temperature is set to be in a range of 26 ℃-48 ℃, which is controlled by a precise hot plate. And for improving the detection accuracy, the dual-wavelengths are selected as 760 and 900 nm for graphite phantom, 820 nm and 860 nm for ex vivo pig blood according to their absorption spectrum repetitively. The obtained results reveal that the temperature measuring correlation coefficients by dual-wavelength method can reach to 0.98 in graphite phantom and 0.99 in ex vivo tissue, respectively. And the average measurement deviation decreases to 0.88 ℃ in dual-wavelength method from 1.31 ℃ for the traditional single wavelength method for graphite phantom. While in ex vivo tissue, the measurement deviation decreases to 0.90 ℃ in dual-wavelength method from the average value 1.45 ℃ for the single wavelength method. Furthermore, the standard deviations of error are respectively reduced by an average of 38% in graphite phantom and an average of 30% in ex vivo tissue, respectively. These results indicate that the dual-wavelength method of photoacoustic temperature measurement can improve both the measuring accuracy and stability, and has a potential to be applied to medical therapy and other biomedical fields.

Accuracy assessment of aerosol source apportionment by dual wavelength photoacoustic measurements

Aerosol source apportionment is a major challenge for air quality monitoring. Due to the frequently observed correlation between the chemical properties of aerosol particles and the wavelength dependence of their optical absorption coefficient, the photoacoustic (PA) method (similarly to alternative optical absorption measurement techniques) offers the unique possibility of real time source apportionment under certain circumstances. However, so far, the selection of lasers into a multi-wavelength PA system for source apportionment was based only on rule of thumb type estimations (like “one should use the highest available light powers with the widest possible wavelength separation”). Here we introduce a simulation method to estimate the accuracy of source apportionment and to be used for the optimised selection of the light sources of a dual wavelength PA system in cases, when light absorbing carbonaceous aerosol (LAC) originates from two distinct sources and dominates the atmospheric aerosol load. The method has two phases and it can be applied either on actual measurement results or on simulated datasets. In the first “calibration” phase the parameters of a non-linear calibration curve are determined, which describes the correlation between the ratio of optical absorption coefficients (OACs) measured photoacoustically at two wavelengths and the ratio of the source specific segregated mass concentrations (mass concentration ratio, MCR) measured by an independent calibration method. During the second “measurement” phase these calibration parameters are used to estimate the MCRs solely from the photoacoustically measured OAC ratios. The relative uncertainty of this estimation is used to grade dual wavelength PA systems. Executions of this simulation on different MCR, OAC datasets have proved that with a fairly simple PA system (which consists of a 405 and a 1064nm diode laser having modulated light power of 150 and 300mW, respectively) MCR determination is feasible with at least 10% relative accuracy whenever the mass concentration of LAC exceeds 1µg/m3.

Laser initiation of PETN-based composites with additives of ultrafine aluminium particles

This paper describes the study on the dependence of the critical energy density of the explosive decomposition of PETN on the mass concentration of the additives of ultrafine Al particles (100–120 nm) in the range of 0.025–1% under the influence of the first and second harmonics of a neodymium laser (12 ns, 1064 and 532 nm). It is shown that the critical energy of the explosion initiation by the first and second harmonics of the laser radiation reaches a minimum value at the same absorption rates, but different concentrations of additives. The photoacoustic method is used to show that, as the laser energy radiation is absorbed, the pressure amplitude in the heated layer reaches a maximum value when the concentration of additives corresponds to the minimum value of the critical energy density.