Photoacoustic Conversion Efficiency Research Articles

Mediating coherent acoustic phonon oscillation of a 2D semiconductor/3D dielectric heterostructure by interfacial engineering

Abstract Coherent acoustic phonon (CAP) oscillation of 2D layered semiconductor/3D dielectric heterostructure generated by femtosecond laser pulse excitation can realize ultrafast photoacoustic conversion, while the photoacoustic conversion efficiency suffers from interfacial phonon scatterings of simultaneously laser-induced lattice heat. Here, taking advantage of graphene’s high thermal conductivity and large acoustic impedance, we demonstrate that phonon scatterings can be markedly mediated in a MoS$_2$/graphene/glass heterostructure via femtosecond laser pump-probe measurements. Equilibrium temperatures of MoS$_2$ lattice have been cooled down by about 45 %. As a benefit, both lifetime of CAP oscillations and pump pulse-picosecond acoustic pulse energy conversion efficiency have been enhanced by a factor of about 2. Our results constitute insights into CAP manipulations via interfacial engineering, that are fundamentally important for ultrafast photoacoustics based on 2D layered semiconductors.

Imaging localisation of damages on switch rail foot based on laser ultrasonic testing

ABSTRACT The corrosion on the switch rail’s foot may easily lead to rail breakage. Laser ultrasound detection is often used to assess the integrity of rails. Nevertheless, the inadequate photoacoustic conversion efficiency and vulnerability to surface interference of the entirely non-contact laser ultrasound result in a diminished signal-to-noise ratio (SNR), hence impeding the efficient detection and analysis of minor damages in the rail. This work demonstrates piezoceramic material lead zirconate titanate (PZT) wafers in conjunction with a pulsed laser to create a sensor array capable of accurate array image inspections. The PZT sensor arrays are manufactured in a flexible format that may be easily installed and altered to complicated geometries. Pulsed lasers are used to achieve dense and high-frequency array excitation. This study proposes a novel probabilistic picture location identification strategy by merging the variable mode decomposition algorithm based on parameter optimization (PVMD) and the principle component analysis (PCA) algorithm. The aim is to enhance the SNR and increase the accuracy of damage findings. Simulation and experimental tests have demonstrated the accuracy of the suggested mechanisms and algorithms in identifying damaged regions.

Multipoint Energy-Balanced Laser-Ultrasonic Transducer Based on a Thin-Cladding Fiber.

This study proposes a novel multipoint transducer system by utilizing the single-mode-multimode-thin-cladding fiber (SMTC) structure. This structure leverages the disparity in mode field diameter between the multimode fiber (MMF) and thin-cladding fiber (TCF) to generate high-amplitude ultrasonic signals safely and efficiently. The fabricated transducer exhibits signal amplitudes 2-3-fold higher compared to conventional laser-ultrasonic transducers. Simulation analysis investigates the impact of the length of the MMF and the diameter of the TCF on coupling efficiency. The coupling efficiency of individual transducer units can be accurately controlled by adjusting the length of the MMF. A three-point energy-balanced laser-ultrasonic transducer system was achieved, with improved energy conversion efficiencies, and the optimal thickness of candle soot nanoparticles (CSNPs) is experimentally determined. Additionally, we carried out experiments to compare the performance of the proposed SMTC-based transducer system under different material conditions using two different photoacoustic materials: graphite-epoxy resin and candle soot nanoparticle-polydimethylsiloxane (CSNP-PDMS) composite. CSNPs, as a cost-effective and easy-to-prepare composite material, exhibit higher photoacoustic conversion efficiency compared to graphite-epoxy resin. The proposed system demonstrates the potential for applications in non-destructive testing techniques.

Ligand‐dependent aggregation‐enhanced photoacoustic of atomically precise metal nanocluster

AbstractAtomically precise metal nanoclusters (MNCs), as a potential type of photoacoustic (PA) contrast agent, are limited in application due to their low PA conversion efficiency (PACE). Here, with hydrophilic Au25SR18 (SR = thiolate) as model NCs, we present a result that weakly polar solvent induces aggregation, which effectively enhances PA intensity and PACE. The PA intensity and PACE are highly dependent on the degree of aggregation, while the aggregation‐enhanced PA intensity (AEPA) positively correlates to the protected ligands. Such an AEPA phenomenon indicates that aggregation actually accelerates the intramolecular motion of Au NCs, and enlarges the proportion of excited state energy dissipated through vibrational relaxation. This result conflicts with the restriction of intramolecular motion mechanism of aggregation‐induced emission. Further experiments show that the increased energy of AEPA originates from the aggregation inhibiting the intermolecular energy transfer from excited Au NCs to their surrounding medium molecules, including solvent molecule and dissolved oxygen, rather than restricting radiative relaxations. This study develops a new strategy for enhancing the PA intensity of Au NCs, and contributes to a deeper understanding of the origin of the PA signal and the excited state energy dissipation processes for MNCs.

High-consistent optical fiber photoacoustic generator with carbon nanoparticles-PDMS composite

Optical fiber photoacoustic (PA) probe is considered to have great prospects in high resolution invasive imaging due to the advantages of flexible, miniaturized broadband ultrasonic excitation. Unfortunately, more attentions are currently focused on the fabrication of the PA probe with a high PA conversion efficiency, instead of the consistency of the developed probe that is of great importance for practical applications. In this paper, in consideration of the coating thickness and flatness closely concerned with the consistency, a number of PA probes with 15 wt.% carbon nanoparticles (CNPs) and polydimethylsiloxane (PDMS) composite were fabricated by a spin-transfer method with the help of transferring uniformly spin-coated film on the endface of an optical fiber. Underwater ultrasonic detection showed that the fabricated probes exhibited a good consistency with the corresponding lower coefficients of variation of 1.7%, 1.4% and 1.1% for the sound pressure, center frequency and bandwidth, sharply reduced by 89%, 72% and 81% in comparison with the commonly used dip-coating probes tested in this work. Also, due to the thinner film obtained through the presented fabrication method, the maximum PA pressure value of 1.04 MPa with a -6 dB bandwidth of 54.2 MHz was achieved, which further demonstrates a great potential in high-resolution ultrasound imaging utilizing multiple probes with high consistency.

Quantifying the shape effect of plasmonic gold nanoparticles on photoacoustic conversion efficiency.

Gold nanoparticles with strong localized plasmonic effects have found wide applications in photoacoustic imaging, which are ascribed to their unique microscopic mechanism of converting photons to ultrasound. In this report, we quantitatively model the time-resolved temperature field, thermal expansion, and pressure distribution based on the finite element analysis method, and two-dimensional gold nanoparticles spanning from the triangle, square, pentagon, and hexagon to the circle have been systematically studied. Results show that the shape of gold nanoparticles has a nontrivial effect on photoacoustic conversion efficiency, and the square-shaped gold structure exhibits the best performance. Our findings could shed light on the shape design of high-performance photoacoustic agents in the future.

Laser ultrasonic improvement and its application in defect detection based on the composite coating method.

Herein, we studied the increasing tendency of photoacoustic (PA) conversion efficiency of the Au/polydimethylsiloxane (PDMS) composite. The thickness of the Au layer was optimized by modeling the PA process based on the Drude-Lorentz model and finite element analysis method, and corresponding results were verified. The results showed that the optimal Au thickness of the Au/PDMS composite was 35 nm. Finally, the Au/PDMS composites were coated onto the surface of aluminum alloys, which improved the thermoelastic laser ultrasonic (LU) signals to near 100 times. Besides, the defect mapping was performed by thermoelastic LU signals with Au/PDMS coating and ablation LU signals without coating; the Pearson correlation coefficient was higher than 0.95. The application in the defect detection in metal could provide guides for nondestructive detection on metals by laser ultrasound.

High-efficient photoacoustic generation with an ultrathin metallic multilayer broadband absorber.

Metal nanomaterials have been widely used to generate photoacoustic (PA) signals because of their high optical absorption characteristics. However, the PA conversion efficiency of metal nanomaterials is limited by the single-wavelength absorption at the resonant peak. To mitigate this issue, a three-layer ultrathin film containing a thin PDMS layer sandwiched between two ultrathin chromium films is proposed. This kind of film structure can attain high optical absorbance (>80%) through the visible light range (450-850 nm). The optical absorption characteristics can be easily modulated by varying the thickness of the PDMS layer. Under the same excitation condition, the PA signal generated by this film structure is twice that of an only Cr film and three times that of an only Au film. This film structure is easily fabricated and can operate with lasers having different central wavelengths or even white light sources, leading to its applications in many fields, including photoacoustic communications and audio transducers.

Enhancement characteristics of laser ultrasonic basin-type insulator based on carbon nanocomposites

Laser ultrasonic technology has been widely used in the field of NDT due to its advantages of non-contact and high resolution. The sensitivity of laser ultrasonic detection signal is not only related to laser excitation parameters, but also to the thermal absorption of the target. Therefore, once the measured target is determined, the sensitivity of laser ultrasonic detection based on thermoelastic effect is limited by the damage threshold of the measured target. In order to improve the sensitivity of the detection signal under the damage threshold, it is a good choice to add a layer of enhanced medium with high photoacoustic conversion efficiency on the surface of the laser target material. Based on this, a method based on CB(carbon black)-PDMS composite film was proposed to enhance the sensitivity of detection signal.

Polydimethylsiloxane/multi-walled carbon nanotube nanocomposite film prepared by ultrasonic-assisted forced impregnation with a superior photoacoustic conversion efficiency of 9.98 × 10−4

Polydimethylsiloxane/multi-walled carbon nanotube (PDMS/MWCNT) composite had high photoacoustic transduction efficiency owing to its high light absorption coefficient and low interfacial thermal resistance. The high expansion coefficient and light absorption ability of PDMS/MWCNT nanocomposite films have significant positive effects when it is used as a laser ultrasonic transducer. We introduce an ultrasonic-assisted forced impregnation method for the preparation of PDMS/MWCNT nanocomposite films. PDMS was used as the matrix phase, and MWCNTs were used as the nano-supporting framework. PDMS matrix was introduced into the MWCNT framework by the forced impregnation method. The morphology, thermal, and mechanical properties of the nanocomposite film were analyzed systematically. The photoacoustic conversion efficiency was researched through both experiment and simulation methods. The output sound pressure of our PDMS/MWCNT nanocomposite film reached 7.59 MPa when applied as an ultrasonic transducer under the pulsed laser excitation of 1 mJ / pulse energy. The photoacoustic conversion efficiency of our sample was 9.98 × 10 − 4, three times higher than similar products reported before, which offered the possibility to greatly enhance the photoacoustic signals.

Impact of Kapitza resistance on the stability and efficiency of photoacoustic conversion from gold nanorods

Plasmonic particles have been proposed for a broad variety of optical and hybrid applications, including the photothermal ablation and photoacoustic imaging of cancer, or their integration in photonic sensors. Here, we address the effect of thermal resistance at the gold-water interface, or Kapitza resistance, on the performance of photoacoustic conversion of gold nanorods. Our findings point to possible strategies for the optimization of plasmonic particles as contrast agents for imaging, or even as transducers for biosensing. We perform numerical simulations that project a simultaneous increase of efficiency and stability of photoacoustic conversion with a decrease of Kapitza resistance. We suggest an effective approach to modulate Kapitza resistance by including underresolved features as roughness or the presence of adsorbates. Inspired by this idea, we synthesize a rough variant of gold nanorods by the deposition and galvanic replacement of a silver shell, where roughness provides higher photoacoustic signals by about 70% and damage thresholds by 120%. In addition, we coat our particles with a protein corona and find a decrease of photoacoustic signals with shell thickness, which may inspire new solutions for biosensors based on a mechanism of photoacoustic transduction. Both our findings are consistent with an effective modulation of Kapitza resistance, which decreases upon roughening, due to an underlying increase of specific surface area, and increases upon coating with a protein shell that may act as a thermal insulation. We discuss possible directions to gain more advantage of our concept for topical applications at the crossroads of plasmonics, biomedical optics and biosensing.

A flexible laser ultrasound transducer for Lamb wave-based structural health monitoring

Lamb waves are widely used in structural health monitoring (SHM) for plate-like structures. In this paper, a new and flexible ultrasonic transducer with a high photo-acoustic conversion efficiency was proposed by using candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). Experimental results demonstrate that the developed transducer can generate a longitudinal wave with a short duration of 0.28 μs under the illumination of a nanosecond laser pulse. The amplitude of the excited longitudinal wave is 10 times that of the signal generated by the traditional laser ultrasound technique. Further, wedge-shape transducers were developed to excite Lamb waves in a 1.5-mm thick aluminum substrate by the oblique incidence method. The specific dA0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminum plate. Based on the synthetic aperture focusing imaging technique, a delay-and-sum signal processing method was adopted for damage location in the plate by using the A0 mode Lamb. A 3.5-mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a good alternative method for SHM.

Fiber-optic photoacoustic gas sensor with temperature self-compensation.

A high-precision fiber-optic photoacoustic (PA) gas sensor with temperature self-compensation is reported. The target gas diffuses into a micro-chamber and absorbs the laser energy to generate a PA signal, which is detected by a Fabry-Perot interferometric cantilever. The temperature affects not only the acoustic sensitivity of the cantilever, but also the PA conversion efficiency. The test result of the PA frequency response demonstrates that there is a temperature-insensitive operating frequency of 1208.4 Hz in the range of 0-80°C. The temperature self-compensated measurement was realized by setting the laser modulation frequency to 604.2 Hz and using the second-harmonic detection technique.

Characteristics of fiber-optic photoacoustic transducers with monolayer of metal nanoparticles for systems of technical diagnostics

The work is devoted to the experimental study of the microstructural and morphological properties of nanostructures as part of a prototype of fiber-optic photoacoustic transducer. The transducer has been created to confirm the theoretical investigations previously obtained by the authors during the study the conditions of the most effective photoacoustic gene ration. To solve the main problem that arises when creating photoacoustic transducers, namely reducing the thickness of the absorbing layer, we used a nanostructure based on a monolayer of silver nanoparticles with size gamma-distribution, the average diameter of 35 nm with RMS-size of 12 nm. The method of simultaneous measuring both efficiency of photoacoustic conversion and frequency response of a photoacoustic transducer is proposed for the first time. The method allows experimental investigation of transduces output parameters versus the modulation mode of the optical signal. The proposed method is based on the usage of the main measurement channel for irradiating the photoacoustic transducer and a reference channel based on fiber optical coupler and photodiode. The experiment shows the reliable generation of ultrasound at frequencies of 10 –18 MHz with a prototype of photoacoustic transducer. During one hour irradiation, degradation of two-dimensional surface nanostructures has not been observed. This allows such type of photoacoustic transducer to be used as part of a new generation of technical diagnostics systems.

Photoacoustic nanoprobe for β-galactosidase activity detection and imaging in vivo

Detection and visualization of [Formula: see text]-galactosidase ([Formula: see text]-gal) is essential to reflect its physiological and pathological effects on human health and disease, but it is still challenging to precisely track [Formula: see text]-gal in vivo owing to the limitation of current analytical methods. In our work, we reported a photoacoustic (PA) nanoprobe for selective imaging of the endogenous [Formula: see text]-gal in vivo. Our nanoprobe Cy7-[Formula: see text]-gal-LP was constructed by encapsulation of a near-infra red (NIR) dye Cy7-[Formula: see text]-gal within a liposome (LP, DSPE-PEG2000-COOH). The dye Cy7-[Formula: see text]-gal was synthesized based on a dye Cy-OH where the hydroxyl group was replaced by a [Formula: see text]-D-galactopyranoside residue, which can be recognized by [Formula: see text]-gal as an enzyme hydrolytic site. With the addition of [Formula: see text]-gal, the absorbance of Cy7-[Formula: see text]-gal exhibited a significant red shift with the absorption peak moved from 600[Formula: see text]nm to 680[Formula: see text]nm, which should generate a switch-on PA signal at 680[Formula: see text]nm in the presence of [Formula: see text]-gal. In addition, as the fluorescence of the dye was totally quenched due to aggregation within the liposome, Cy7-[Formula: see text]-gal-LP exhibited high PA conversion efficiency. With the nanoprobe, we achieved the selective PA detection and imaging of [Formula: see text]-gal in the tumor-bearing mice.

Simulation Studies of Photoacoustic Response from Gold-Silica Core-Shell Nanoparticles

Metal nanoparticles especially of noble metals are used as an exogenous contrast agent for biomedical photoacoustic (PA) imaging in the tissue transmission window extending from visible to near infrared 700–1100 nm band. Different geometrical configurations of gold and silver nanoparticles like spherical core-shell, nanorod, and nanocages are promising candidates for thermoplasmonics, photothermal therapy, photothermal imaging, and photoacoustic imaging. In the current study, we simulated the photoacoustic response of gold and silica core-shell nanoparticle in water medium. Finite element simulations were carried out to study the spectral absorption response and effect of nanosecond laser pulse excitation on the spatial/temporal temperature as well as photoacoustic pressure variations of different core-shell geometry of nanoparticle. We have optimized the dimensions of gold nanosphere, gold-silica, and silica-gold core-shell geometries for optimum photoacoustic conversion efficiency. Further, the effect of shell thickness on the pulse photoacoustic signals for core-shell gold-silica and silica-gold nanoparticle has been studied. We concluded that silica-gold core-shell nanoparticles possess better photoacoustic conversion efficiency in comparison to gold nanosphere and gold-silica core-shell geometries. The prime aim of this study is to design efficient nano-probes for photoacoustic imaging, photoacoustic tomography, photothermal therapy, and drug delivery.

Tunable photoacoustic properties of gold nanoshells with near-infrared optical responses

Photoacoustic (PA) properties of liquid-immersed gold nanoshells (GNSs) with near-infrared optical responses are investigated using the finite element method. We focus on the dependence of the PA signal of the GNS on the geometry, surrounding medium, laser fluence, and laser pulse width. It is found that the PA signal of the GNS is strongly sensitive to the optical absorption of the GNS and can be greatly modulated by changing the geometry. At the wavelength of 800 nm, the maximal PA signal could be obtained for the GNS with the optimized size of the inner radius (r1 = 48.5 nm) and outer radius (r2 = 57 nm). The increased laser fluence enhances the optical absorption and PA signal. The decreased laser pulse width induces the decreased PA pulse width, the increased bandwidth of the PA signal, and the increased PA conversion efficiency, resulting in the enhanced PA signal. In addition, we find that the larger Gruneisen parameter of the embedding medium leads to a stronger PA signal.

New insight into photoacoustic conversion efficiency by plasmon-mediated nanocavitation: Implications for precision theranostics

The probe-assisted integration of imaging and therapy into a single modality offers significant advantages in bio-applications. As a newly developed photoacoustic (PA) mechanism, plasmon-mediated nanocavitation, whereby photons are effectively converted into PA shockwaves, has excellent advantages for image-guided therapy. In this study, by simulating the laser absorption, temperature field, and nanobubble dynamics using both finite-element analysis and computational fluid dynamics, we quantified the cavitation-induced PA conversion efficiency of a water-immersed gold nanosphere, revealing new insights. Interestingly, sequential multi-bubble emission accompanied by high PA signal production occur under a single high-dose pulse of laser irradiation, enabling a cavitation-induced PA conversion efficiency up to 2%, which is ~50 times higher than that due to thermal expansion. The cavitation-induced PA signal has unique frequency characteristics, which may be useful for a new approach for in vivo nanoparticle tracking. Our work offers theoretical guidance for accurate diagnosis and controllable therapy based on plasmon-mediated nanocavitation.

Quantifying the Plasmonic Nanoparticle Size Effect on Photoacoustic Conversion Efficiency

Efficient contrast agents such as plasmonic nanoparticles (PNPs) are highly desirable for good-performance photoacoustic (PA) imaging. Owing to the small size effect, PNPs have a unique microscopic mechanism of PA conversion from photons to ultrasound. Here, by quantitatively modeling the optical absorption, the time-resolved temperature field, and thermal expansion based on the analytical and finite element analysis (FEA) method, we obtained the quantitative size-dependent PA conversion efficiency of nanospheres/nanorods spanning a comprehensive range of particle sizes, which also provides a deep understanding of the microscopic PA conversion mechanism for nanoprobes. Results show that both the plasmon-mediated absorption and energy conversion from absorbed laser energy to ultrasound are strongly dependent on the PNPs’ size, which comes from their sharply increased surface-to-volume ratios. The gold nanospheres and nanorods possess peak size values for maximizing the PA conversion efficiency. Our work gives theoretical guidelines for constructing highly sensitive PA nanoprobes through rational optimization of the PNPs’ size.

A PIID-DTBT based semi-conducting polymer dots with broad and strong optical absorption in the visible-light region: Highly effective contrast agents for multiscale and multi-spectral photoacoustic imaging

As a hybrid imaging technique, photoacoustic imaging (PAI) can provide multiscale morphological information of tissues, and the use of multi-spectral PAI (MSPAI) can recover the spatial distribution of chromophores of interest, such as hemoglobin within tissues. Herein, we developed a contrast agent that can very effectively combine multiscale PAI with MSPAI for a more comprehensive characterization of complex biological tissues. Specifically, we developed novel PIID-DTBT based semi-conducting polymer dots (Pdots) that show broad and strong optical absorption in the visible-light region (500–700 nm). The performances of gold nanoparticles (GNPs) and gold nanorods (GNRs), which have been verified as excellent photoacoustic contrast agents, were compared with that of the Pdots based on the multiscale PAI system. Both ex vivo and in vivo experiments demonstrated that the Pdots have better photoacoustic conversion efficiency at 532 nm than GNPs and showed similar photoacoustic performance with GNRs at 700 nm at the same mass concentration. Photostability and toxicity tests demonstrated that the Pdots are photostable and biocompatible. More importantly, an in vivo MSPAI experiment indicated that the Pdots have better photoacoustic performance than the blood and therefore the signals can be accurately extracted from the background of vascular-rich tissues. Our work demonstrates the great potential of Pdots as highly effective contrast agents for the precise localization of lesions relative to the blood vessels based on multiscale PAI and MSPAI.