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In this methods paper, we explore the capabilities of high-speed ultrasound imaging (USI) to study fast varying and complex multi-phase structures in liquids and soft materials. Specifically, we assess the advantages and the limitations of this imaging technique through three distinct experiments involving rapid dynamics: the fl ow induced by a liquid jet, the dissolution of sub-micron bubbles in water, and the propagation of shear waves in a soft elastic material. The phenomena were simultaneously characterized using optical microscopy and USI with bubbles as contrast agents. In water, we use compounded high-speed USI for tracking a multi-phase flow produced by a jetting bubble diving into a liquid pool at speeds around 20 m/s. These types of jets are produced by focusing a single laser pulse below the liquid surface. Upon breakup, they create a bubbly fl ow that exhibits high reflectivity to the ultrasound signal, enabling the visualization of the subsequent complex turbulent flow. In a second experiment, we demonstrate the potential of USI for recording the stability and diffusive shrinkage of micro- and nanobubbles in water that could not be optically resolved. Puncturing an elastic material with a liquid jet creates shear waves that can be utilized for elastography measurements. We analysed the shape and speed of shear waves produced by different types of jetting bubbles in industrial gelatin. The wave characteristics were simultaneously determined by implementing particle velocimetry in optical and ultrasound measurements. For the latter, we employed a novel method to create homogeneously distributed micro- and nanobubbles in gelatin by illuminating it with a collimated laser beam.

Lewis hunting reaction refers to the alternating cold-induced vasoconstriction and dilation in extremities, whose underlying mechanism is complex. While numerous studies reported this intriguing phenomenon by measuring cutaneous temperature fluctuation under cold exposure, few of them tracked peripheral vascular responses in real-time, lacking a non-invasive and quantitative imaging tool. To better monitor hunting reaction and diagnose relevant diseases, we developed a hybrid photoacoustic ultrasound (PAUS) tomography system to monitor finger vessels’ dynamic response to cold, together with simultaneous temperature measurement. We also came out a standard workflow for image analysis with self-defined indices. In the small cohort observational study, vascular changes in the first cycle of hunting reaction were successfully captured by the image series and quantified. Time difference between vasodilation and temperature recovery was noticed and reported for the first time, thanks to the unique capability of the PAUS imaging system in real-time and continuous vascular monitoring. The developed imaging system and indices enabled more objective and quantitative monitoring of peripheral vascular activities, indicating its great potential in numerous clinical applications.

Cavitation bubbles collapsing near boundaries create liquid flow through their center of mass movement, the formation of liquid jets, and long living vorticities. Here, we demonstrate robust pumping of the liquid with a compact and simple geometry, the open end of a thin-walled circular capillary tube filled with liquid. We study the dynamics of the cavitation bubbles and report on the resultant microjet formation through experiments and simulations. In the experiments, the dynamics of laser-induced cavitation bubbles are captured with high-speed microscopy. Simulations show excellent agreement with the experiments. The jet flow pumps liquid flow toward the capillary opening. The simulation reveals that, in the current study range, both the non-dimensional inner diameter of the capillary and the non-dimensional stand-off distance show influences on the jet width, and only the non-dimensional stand-off distance affects the maximum jet velocities. The results demonstrate that the confinement of the bubble within the capillary alters the anisotropic pressure field around the bubble, leading to a more mild collapse.

Utilisation of high-power ultrafast laser for ablation-based industrial processes such as milling, drilling or cutting requires high production rates and superior quality. In this paper, we demonstrate highly efficient, rapid and high-quality laser micro-machining of three industrial metals (aluminium, copper, and stainless steel). Our proposed optimisation methods of pulse energy division in time result in simultaneous enhancement of ablation efficiency (volume per energy) and ablation rate (volume per time) while maintaining a focused laser beam on the target surface and high resolution. A high-tech femtosecond burst laser, producing laser pulses of τ = 350 fs duration and intra-burst repetition rates of f_P = 50 MHz, was employed in the experiments. Due to the utilisation of bursts, material removal efficiency and removal rate were increased by 18.0 %, 44.5 %, and 37.0 % for aluminium, copper, and stainless steel if compared with the best performance of single-pulses. In addition to the high processing rate, processing by burst mode resulted in lower surface roughness. This technique is believed to be a solution enabling extremely high femtosecond laser powers for precise microfabrication.

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Laser polishing offers numerous advantages, one of which is the convenience of using the same system for the
whole manufacturing process. In this work, an ultrashort pulse laser operating in a GHz burst regime was used to polish
stainless steel. The aim was to minimise surface roughness, characterised by the average roughness parameter R_a. Different
laser processing parameters (average laser power, number of pulses per burst, scanning speed, hatch size) were varied to polish
samples that were covered in laser-induced periodic surface structures (LIPSS). Thermal effects, such as melt layer formation,
were noticed and discussed. It was demonstrated that LIPSS can be erased and the initial surface roughness of 73 nm was
reduced to 41 nm using 100 pulses per burst and burst fluence of F_B = 0.15 J/cm².

SignificanceEndocavity ultrasound (US) imaging is a frequently employed diagnostic technique in gynecology and urology for the assessment of male and female genital diseases that present challenges for conventional transabdominal imaging. The integration of photoacoustic (PA) imaging with clinical US imaging has displayed promising outcomes in clinical research. Nonetheless, its application has been constrained due to size limitations, restricting it to spatially confined locations such as vaginal or rectal canals.AimThis study presents the development of a video-rate (20 Hz) endocavity PA/harmonic US imaging (EPAUSI) system.ApproachThe approach incorporates a commercially available endocavity US probe with a miniaturized laser delivery unit, comprised of a single large-core fiber and a line beamshaping engineered diffuser. The system facilitates real-time image display and subsequent processing, including angular energy density correction and spectral unmixing, in offline mode.ResultsThe spatial resolutions of the concurrently acquired PA and harmonic US images were measured at 318 μm and 291 μm in the radial direction, respectively, and 1.22 deg and 1.50 deg in the angular direction, respectively. Furthermore, the system demonstrated its capability in multispectral PA imaging by successfully distinguishing two clinical dyes in a tissue-mimicking phantom. Its rapid temporal resolution enabled the capture of kinetic dye perfusion into an ex vivo porcine ovary through the depth of porcine uterine tissue. EPAUSI proved its clinical viability by detecting pulsating hemodynamics in the male rat’s prostate in vivo and accurately classifying human blood vessels into arteries and veins based on sO₂ measurements.ConclusionsOur proposed EPAUSI system holds the potential to unveil previously overlooked indicators of vascular alterations in genital cancers or endometriosis, addressing pressing requirements in the fields of gynecology and urology.

The interaction of surface acoustic waves (SAWs) with liquids enables the production of aerosols with adjustable droplet sizes in the micrometer range expelled from a very compact source. Understanding the nonlinear acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is essential for acousto-microfluidics platforms, particularly aerosol generators. In this study, we demonstrate the presence of micro-cavitation in an MHz-frequency SAW aerosol generation platform, which is touted as a leap in aerosol technology with versatile application fields including biomolecule inhalation therapy, micro-chromatography and spectroscopy, olfactory displays, and material deposition. Using analysis methods with high temporal and spatial resolution, we demonstrate that SAWs stabilize spatially arranged liquid micro-domes atop the generator's surface. Our experiments show that these liquid domes become acoustic resonators with highly fluctuating pressure amplitudes that can even nucleate cavitation bubbles, as supported by analytical modeling. The observed fragmentation of liquid domes indicates the participation of three droplet generation mechanisms, including cavitation and capillary-wave instabilities. During aerosol generation, the cavitation bubbles contribute to the ejection of droplets from the liquid domes and also explain observed microstructural damage patterns on the chip surface eventually caused by cavitation-based erosion.

Photoacoustic tomography (PAT) is increasingly being used for high-resolution biological imaging at depth. Signal-to-noise ratios and resolution are the main factors that determine image quality. Various reconstruction algorithms have been proposed and applied to reduce noise and enhance resolution, but the efficacy of signal preprocessing methods which also affect image quality, are seldom discussed. We, therefore, compared common preprocessing techniques, namely bandpass filters, wavelet denoising, empirical mode decomposition, and singular value decomposition. Each was compared with and without accounting for sensor directivity. The denoising performance was evaluated with the contrast-to-noise ratio (CNR), and the resolution was calculated as the full width at half maximum (FWHM) in both the lateral and axial directions. In the phantom experiment, counting in directivity was found to significantly reduce noise, outperforming other methods. Irrespective of directivity, the best performing methods for denoising were bandpass, unfiltered, SVD, wavelet, and EMD, in that order. Only bandpass filtering consistently yielded improvements. Significant improvements in the lateral resolution were observed using directivity in two out of three acquisitions. This study investigated the advantages and disadvantages of different preprocessing methods and may help to determine better practices in PAT reconstruction.

Although spherical gold (Au) nanoparticles have remarkable photothermal conversion efficiency and photostability, their weak absorption in the near-infrared (NIR) region and poor penetration into deep tissues have limited further applications to NIR light-mediated photoacoustic (PA) imaging and noninvasive photothermal cancer therapy. Here, we developed bimetallic hyaluronate-modified Au–platinum (HA-Au@Pt) nanoparticles for noninvasive cancer theranostics by NIR light-mediated PA imaging and photothermal therapy (PTT). The growth of Pt nanodots on the surface of spherical Au nanoparticles enhanced the absorbance in the NIR region and broadened the absorption bandwidth of HA-Au@Pt nanoparticles by the surface plasmon resonance (SPR) coupling effect. In addition, HA facilitated the transdermal delivery of HA-Au@Pt nanoparticles through the skin barrier and enabled clear tumor-targeted PA imaging. Compared to conventional PTT via injection, HA-Au@Pt nanoparticles were noninvasively delivered into deep tumor tissues and completely ablated the targeted tumor tissues by NIR light irradiation. Taken together, we could confirm the feasibility of HA-Au@Pt nanoparticles as a NIR light-mediated biophotonic agent for noninvasive skin cancer theranostics.