Experimental investigation on multiple breakdown in water induced by focused nanosecond laser

A mini-review on gene delivery technique using nanoparticles-mediated photoporation induced by nanosecond pulsed laser

Nanosecond pulsed laser induced photoporation has gained increasing attention from scholars as an effective method for delivering the membrane-impermeable extracellular materials into living cells. Compared with femtosecond laser, nanosecond laser has the advantage of high throughput and low costs. It also has a higher delivery efficiency than continuous wave laser. Here, we provide an extensive overview of current status of nanosecond pulsed laser induced photoporation, covering the photoporation mechanism as well as various factors that impact the delivery efficiency of photoporation. Additionally, we discuss various techniques for achieving photoporation, such as direct photoporation, nanoparticles-mediated photoporation and plasmonic substrates mediated photoporation. Among these techniques, nanoparticles-mediated photoporation is the most promising approach for potential clinical application. Studies have already been reported to safely destruct the vitreous opacities in vivo by nanosecond laser induced vapor nanobubble. Finally, we discuss the potential of nanosecond laser induced phototoporation for future clinical applications, particularly in the areas of skin and ophthalmic pathologies. We hope this review can inspire scientists to further improve nanosecond laser induced photoporation and facilitate its eventual clinical application.

Optical Effects of Focused Fractional Nanosecond 1064-nm Nd:YAG Laser: Techniques of Application on Human Skin

BACKGROUND AND OBJECTIVES

Considering the pulse widths of picosecond and nanosecond lasers used in cutaneous laser surgery differ by approximately one order of magnitude, can nanosecond lasers produce the optical effect in human skin similar to laser-induced optical breakdown (LIOB) caused by picosecond lasers?

METHODS

Cutaneous changes induced by a focused fractional nanosecond 1064-nm Nd:YAG laser were evaluated by VISIA-CR imaging, histological examination, and harmonic generation microscopy (HGM).

RESULTS

A focused fractional nanosecond 1064-nm Nd:YAG laser can generate epidermal vacuoles or dermal cavities similar to the phenomenon of LIOB produced by picosecond lasers. The location and extent of photodisruption can be controlled by the laser fluence and focus depth. Moreover, laser-induced shock wave propagation and thermal degeneration of papillary collagen can be observed by HGM imaging.

CONCLUSION

Focused fractional nanosecond lasers can produce an optical effect on human skin similar to LIOB caused by picosecond lasers. With techniques of application, the treatment can induce epidermal and dermal repair mechanisms in a tunable fashion to improve skin texture, wrinkles, scars, and dyspigmentation, without disrupting the epidermal surface.

Bubble dynamics and atomization of acoustically levitated diesel and biodiesel droplets using femtosecond laser pulses

This study focuses on the bubble dynamics and associated breakup of individual droplets of diesel and biodiesel under the influence of femtosecond laser pulses. The bubble dynamics were examined by suspending the droplets in the air through an acoustically levitated setup. The laser pulse energies ranged from 25 to 1050 µJ, and droplet diameters varied between 0.25 and 1.5 mm. High-speed shadowgraphy was employed to examine the influence of femtosecond laser intensity and multiple laser pulses on various spatial-temporal parameters. Four distinct sequences of regimes have been identified, depending on early and late times: bubble creation by individual laser pulses, coalescence, bubble rupture and expansion, and droplet fragmentation. At all laser intensities, early-time dynamics showed only bubble generation, while specifically at higher intensities, late-time dynamics revealed droplet breaking. The droplet breakup is further categorized into three mechanisms: steady sheet collapse, unstable sheet breakup, and catastrophic breakup, all following a well-known ligament and secondary breakup process. The study reveals that laser pulses with high repetition rates and moderate laser energy were the optimal choice for precise bubble control and cutting.

Secondary cavitation bubble dynamics during laser-induced bubble formation in a small container

We investigated secondary cavitation bubble dynamics during laser-induced bubble formation in a small container with a partially confined free surface and elastic thin walls. We employed high-speed photography to record the dynamics of sub-mm-sized laser-induced bubbles and small secondary bubble clouds. Simultaneous light scattering and acoustic measurements were used to detect the oscillation times of laser-induced bubbles. We observed that the appearance of secondary bubbles coincides with a prolonged collapse phase and with re-oscillations of the laser-induced bubble. We observed an asymmetric distribution of secondary bubbles with a preference for the upstream side of the focus, an absence of secondary bubbles in the immediate vicinity of the laser focus, and a migration of laser-induced bubble toward secondary bubbles at large pulse energies. We found that secondary bubbles are created through heating of impurities to form initial nanobubble nuclei, which are further expanded by rarefaction waves. The rarefaction waves originate from the vibration of the elastic thin walls, which are excited either directly by laser-induced bubble or by bubble-excited liquid-mass oscillations. The oscillation period of thin walls and liquid-mass were Twall = 116 µs and Tlm ≈ 160 µs, respectively. While the amplitude of the wall vibrations increases monotonically with the size of laser-induced bubbles, the amplitude of liquid-mass oscillation undulates with increasing bubble size. This can be attributed to a phase shift between the laser-induced bubble oscillation and the liquid-mass oscillator. Mutual interactions between the laser-induced bubble and secondary bubbles reveal a fast-changing pressure gradient in the liquid. Our study provides a better understanding of laser-induced bubble dynamics in a partially confined environment, which is of practical importance for microfluidics and intraluminal laser surgery.

Laser induced spherical bubble dynamics in partially confined geometry with acoustic feedback from container walls

We investigated laser-induced cavitation dynamics in a small container with elastic thin walls and free or partially confined surface both experimentally and by numerical investigations. The cuvette was only 8-25 times larger than the bubble in its center. The liquid surface was either free, or two thirds were confined by a piston-shaped pressure transducer. Different degrees of confinement were realized by filling the liquid up to the transducer surface or to the top of the cuvette. For reference, some experiments were performed in free liquid. We recorded the bubble dynamics simultaneously by high-speed photography, acoustic measurements, and detection of probe beam scattering. Simultaneous single-shot recording of radius-time curves and oscillation times enabled to perform detailed investigations of the bubble dynamics as a function of bubble size, acoustic feedback from the elastic walls, and degree of surface confinement. The bubble dynamics was numerically simulated using a Rayleigh-Plesset model extended by terms describing the acoustically mediated feedback from the bubble's environment. Bubble oscillations were approximately spherical as long as no secondary cavitation by tensile stress occurred. Bubble expansion was always similar to the dynamics in free liquid, and the environment influenced mainly the collapse phase and subsequent oscillations. For large bubbles, strong confinement led to a slight reduction of maximum bubble size and to a pronounced reduction of the oscillation time, and both effects increased with bubble size. The joint action of breakdown-induced shock wave and bubble expansion excites cuvette wall vibrations, which produce alternating pressure waves that are focused onto the bubble. This results in a prolongation of the collapse phase and an enlargement of the second oscillation, or in time-delayed re-oscillations. The details of the bubble dynamics depend in a complex manner on the degree of surface confinement and on bubble size. Numerical simulations of the first bubble oscillation agreed well with experimental data. They suggest that the alternating rarefaction/compression waves from breakdown-induced wall vibrations cause a prolongation of the first oscillation. By contrast, liquid mass movement in the cuvette corners result in wall vibrations causing late re-oscillations. The strong and rich interaction between the bubble and its surroundings may be relevant for a variety of applications such as intraluminal laser surgery and laser-induced cavitation in microfluidics.

Experimental investigation of nanosecond laser-induced shock waves in water using multiple excitation beams

Revealing the expansion and interaction dynamics of multiple shock waves induced by a nanosecond laser is important for controlling laser surgery. However, the dynamic evolution of shock waves is a complex and ultrafast process, making it difficult to determine the specific laws. In this study, we conducted an experimental investigation into the formation, propagation, and interaction of underwater shock waves that are induced by nanosecond laser pulses. The effective energy carried by the shock wave is quantified by the Sedov-Taylor model fitting with experimental results. Numerical simulations with an analytic model using the distance between adjacent breakdown locations as input and effective energy as fit parameters provide insights into experimentally not accessible shock wave emission and parameters. A semi-empirical model is used to describe the pressure and temperature behind the shock wave taking into account the effective energy. The results of our analysis demonstrate that shock waves exhibit asymmetry in both their transverse and longitudinal velocity and pressure distributions. In addition, we compared the effect of the distance between adjacent excitation positions on the shock wave emission process. Furthermore, utilizing multi-point excitation offers a flexible approach to delve deeper into the physical mechanisms that cause optical tissue damage in nanosecond laser surgery, leading to a better comprehension of the subject.

Characterization of nanosecond laser-induced underwater filamentation using acoustic measurements

This investigation aims to understand the temporal and spectral characteristics of underwater acoustic impulses due to ∼10n s laser-induced filamentation at various incident laser energies. In addition to the experimental study, finite element analysis (FEA) has been used in conjunction with experimentally acquired data to simulate and visualize acoustic impulse propagation and interaction across the water-rigid boundary. In the time domain, the increase in the underwater filament length is measured by the peak-to-peak (P _k-P _k) overpressures as a function of input laser energies. The P _k-P _k overpressures are maximum around the focal plane and are symmetrical on either side. This variation in the P _k-P _k overpressure is traced to characterize the spatial extent of the underwater filament. The arrival time of underwater acoustic impulse varies within the 8-9 µs for input laser energies. It was observed that with increasing laser beam power, the conversion efficiency of the acoustic impulse decreases, and the underwater filament length increases. In the frequency domain, with increasing input laser energy, the peak frequency of the underwater acoustic impulse shifts toward the lower frequencies. Using FEA, the short-time Fourier transform spectrogram simultaneously visualizes the arrival time and instantaneous frequency of the simulated acoustic impulse. The simulated acoustic impulse produced a single instantaneous peak frequency of around 76 kHz. Upon reflection from the water-rigid boundary, we observed that the reflected signal also has a single instantaneous peak frequency of around 76 kHz, while higher-frequency components are completely dissipated. Our results are promising for the development of remote laser-based acoustic generation and sensing applications.

Laser-Induced Plasmonic Nanobubbles and Microbubbles in Gold Nanorod Colloidal Solution

In this work, we studied the initiated plasmonic nanobubbles and the follow-up microbubble in gold nanorod (GNR) colloidal solution induced by a pulsed laser. Owing to the surface plasmon resonance (SPR)-enhanced photothermal effect of GNR, several nanobubbles are initiated at the beginning of illumination and then to trigger the optical breakdown of water at the focal spot of a laser beam. Consequently, microbubble generation is facilitated; the threshold of pulsed laser energy is significantly reduced for the generation of microbubbles in water with the aid of GNRs. We used a probing He-Ne laser with a photodetector and an ultrasonic transducer to measure and investigate the dynamic formations of nanobubbles and the follow-up microbubble in GNR colloids. Two wavelengths (700 nm and 980 nm) of pulsed laser beams are used to irradiate two kinds of dilute GNR colloids with different longitudinal SPRs (718 nm and 966 nm). By characterizing the optical and photoacoustic signals, three types of microbubbles are identified: a single microbubble, a coalesced microbubble of multiple microbubbles, and a splitting microbubble. The former is caused by a single breakdown, whereas the latter two are caused by discrete and series-connected multiple breakdowns, respectively. We found that the thresholds of pulsed energy to induce different types of microbubbles are reduced as the concentration of GNRs increases, particularly when the wavelength of the laser is in the near-infrared (NIR) region and close to the SPR of GNRs. This advantage of a dilute GNR colloid facilitating the laser-induced microbubble in the NIR range of the bio-optical window could make biomedical applications available. Our study may provide an insight into the relationship between plasmonic nanobubbles and the triggered microbubbles.

Control of the acoustic waves generated by intense laser filamentation in water

Experiments and simulations are performed to study filamentation and generation of acoustic waves in water by loosely focused multi-millijoules laser pulses. When the laser pulse duration is increased from femtosecond to nanosecond duration, a transition is observed from a filamentary propagation with extended and low energy density deposition to a localized breakdown, related to high energy density deposition. The transition suggests that Kerr self-focusing plays a major role in the beam propagation dynamics. As a result, the shape, the amplitude and the spectrum of the resulting pressure wave present a strong dependence on the laser pulse duration.

Influence of Parameters on the Death Pathway of Gastric Cells Induced by Gold Nanosphere Mediated Phototherapy

Gold nanosphere (AuS) is a nanosized particle with inert, biocompatible, easily modified surface functionalization and adequate cell penetration ability. Photothermal, photochemical, and vapor effects of AuS could be activated by irradiating with nanosecond laser to cause cell death. Hence, AuS-mediated phototherapy irradiated with nanosecond laser is a promising and minimally-invasive treatment method for cancer therapy. However, various effects require different parameters to be activated. At present, few studies have reported on the influence of parameters of AuS inducing cell death under nanosecond laser irradiation. This makes it very challenging to optimize gold-nanoparticle-mediated specific or synergistic anti-cancer therapy. In this study, we revealed the main parameters and threshold values for AuS-mediated gastric cancer phototherapy with nanosecond pulsed laser irradiation, evaluated the pathway of induced cell death, and discussed the roles of photothermal, photochemical and vapor effects which can induce the cell death. The results showed that AuS-mediated phototherapy activated with nanosecond pulsed laser is an effective method for gastric therapy, mainly based on the photochemical effect. Prolonging the incubation time could decrease the irradiation dose, increase ROS-mediated photothermal effect and vapor effect, and then quickly induce cell death to improve security.

Rescue of Failed XEN-45 Gel Implant by Nd:YAG Shock Wave to Anterior Chamber Tip to Dislodge Hidden Intraluminal Occlusion

PURPOSE

The purpose of this study was to inform ophthalmic surgeons in a timely manner of the hidden problem of clear intraluminal cellular debris as a cause for XEN-45 failure and to describe low energy neodymium-doped yttrium aluminum garnet (Nd:YAG) laser revision with periluminal anterior chamber tip shockwave treatment to improve flow to the bleb.

PATIENTS AND METHODS

Six patients with visibly patent stent lumen post XEN-45 surgery. These eyes developed rising intraocular pressure (IOP) with a history of excellent prior bleb formation and were treated successfully with Nd:YAG laser shockwave therapy to disperse assumed intraluminal cellular debris. The laser was aimed just anterior and axial to the intracameral tip of the gel stent through a gonioscopy lens.

RESULTS

Six patients with an average age of 75 years (60 to 90 y), preoperative IOP of 30 mm Hg (16 to 52 mm Hg) on an average of 2 antiglaucoma medications (0 to 4) underwent periluminal anterior chamber tip shock wave at an average of 12 months (1 to 38 mo) from XEN-45 surgery. The IOP was immediately reduced to an average of 15 mm Hg (8 to 23 mm Hg) and last IOP averaged 15 mm Hg (10 to 23 mm Hg) on 1.5 medications (0 to 4) at 4 months post periluminal anterior chamber tip shock wave.

CONCLUSION

Nd:YAG laser revision of hidden blockage of a XEN-45 gel implant with periluminal anterior chamber tip shockwave treatment can disperse invisible intraluminal cellular debris and improve flow in a failing XEN-45 microstent, especially when distal fibrosis is not excessive.

Input pulse duration effect on laser-induced underwater acoustic signals

The characteristics of laser-induced underwater acoustic signals (UASs) generated by focused 10 ns and 30 ps laser pulses of different energies under similar experimental conditions are compared. Time domain signals and time-frequency analysis of the UASs were used to understand the role of input pulse duration and energy on the evolution of UAS. In the time domain, the peak-to-peak (${{\rm P}_k} {-} {{\rm P}_k}$) overpressure of the UAS decreases, and the arrival time (${{\rm A}_t}$) increases as a function of propagation distance for both ns and ps laser-induced breakdown (LIB) of water. With increasing incident energy of both ns and ps laser pulses, the ${{\rm P}_k} {-} {{\rm P}_k}$ overpressures of acoustic signals increase almost linearly. In the time-frequency domain, the spectrogram obtained via short-time Fourier transform provides spectral information and ${{\rm A}_t}$ of both direct and reflected signals simultaneously. The spectrogram revealed that the transient UAS has broad acoustic spectra spanning from 10 to 800 kHz, perpendicular to the laser propagation direction. The initial acoustic impulse resulted in two major frequencies centered around 105 and 690 kHz with a standard error of 30 kHz. Upon reflection from the water-air interface, only the peak frequency corresponding to ${\sim}{105}\;{\rm kHz}$ was reflected, while the longer frequency was observed to dissipate. Our results demonstrate that the ns-LIB is more suitable for applications compared to the ps-LIB owing to stronger acoustic impulse of both direct and reflected signals.

Normalization of underwater laser-induced breakdown spectroscopy using acoustic signals measured by a hydrophone

Laser-induced breakdown spectroscopy (LIBS) signals in water always suffer strong pulse-to-pulse fluctuations that result in poor stability of the spectrum. In this work, a spectrum normalization method based on acoustic signals measured by a hydrophone immersed in water was developed and compared with laser energy normalization. The characteristics of the acoustic signals were studied first, and the correlations between the acoustic signals and LIBS spectra were analyzed. It showed that the spectral line intensity has a better linear relationship with the acoustic energy than with the laser energy. Consequently, the acoustic normalization exhibited better performance on the reduction of LIBS spectral fluctuation versus laser energy normalization. Calibration curves of Mn, Sr, and Li were then built to assess the analytical performance of the proposed acoustic normalization method. Compared with the original spectral data, the average RSD_C values of all analyte elements were significantly reduced from 5.00% to 3.18%, and the average RSD_P values were reduced from 5.09% to 3.28%, by using the acoustic normalization method. These results suggest that the stability of underwater LIBS can be clearly improved by using acoustic signals for normalization, and acoustic normalization works more efficiently than laser energy normalization. This work provides a simple and cost-effective external acoustic normalization method for underwater LIBS applications.

Laser-induced plasma in water at high pressures up to 40 MPa: A time-resolved study

The knowledge on the laser-induced plasma emission in water at high pressures is essential for the application of laser-induced breakdown spectroscopy (LIBS) in the deep-sea. In this work, we investigate the spectral features of ionic, atomic and molecular emissions for the plasma in water at different pressures from 1 to 40 MPa. By comparing between the time-resolved spectra and shadowgraph images, we demonstrate that the dynamics of the cavitation bubble at high pressures plays a key role on the characterization of plasma emission. The initial plasma emission depends weakly on the external pressure. As time evolves, the cavitation bubble is more compressed by the higher external pressure, leading to a positive confinement effect to maintain the plasma emission. However, at very high pressures, the bubble collapses extremely fast and even earlier than the cooling of the plasma. The plasma will gain energy from the bubble collapse phase, but quench immediately after the collapse, leading to a sharp reduction in the plasma persistence. These effects caused by bubble dynamics explain well the observed spectral features and are further proved by the temporal evolutions of the plasma temperature and electron density. This work gives not only some insights into the laser-induced plasma and bubble dynamics in high pressure liquids but also better understanding for the application of underwater LIBS in the deep-sea.