Time resolved studies reveal the origin of the unparalleled high efficiency of one nanosecond laser ablation in liquids

Laser ablation in liquid is a scalable nanoparticle production method with applications in areas like catalysis and biomedicine.Due to laser-liquid interactions,different energy dissipation channels such as absorption by the liquid and scattering at the ablation plume and cavitation bubble lead to reduced laser energy available for nanoparticle production.Ultrashort pulse durations cause unwanted nonlinear effects in the liquid,and for ns pulses,intra-pulse energy deposition attenuation effects are to be expected.However,intermediate pulse durations ranging from hundreds of picoseconds up to one nanosecond have rarely been studied in particular in single-pulse settings.In this study,we explore the pico-to nanosecond pulse duration regimes to find the pulse duration with the highest ablation efficiency.We find that pulse durations around 1-2 ns enable the most efficient laser ablation in liquid since the laser beam shielding by the ablation plume and cavitation bubble sets in only at longer pulse durations.Furthermore,pump-probe microscopy imaging reveals that the plume dynamics in liquids start to differ from plume dynamics in air at about 2 ns after pulse impact.

Correlation ofⅢ/Ⅴsemiconductor etch results with physical parameters of high-density reactive plasmas excited by electron cyclotron resonance

Reactive ion etching is the interaction of reactive plasmas with surfaces. To obtain a detailed understanding of this process, significant properties of reactive composite low-pressure plasmas driven by electron cyclotron resonance（ECR） were investigated and compared with the radial uniformity of the etch rate. The determination of the electronic properties of chlorine-and hydrogen-containing plasmas enabled the understanding of the pressure-dependent behavior of the plasma density and provided better insights into the electronic parameters of reactive etch gases. From the electrical evaluation of I（V） characteristics obtained using a Langmuir probe,plasmas of different compositions were investigated. The standard method of Druyvesteyn to derive the electron energy distribution functions by the second derivative of the I（V）characteristics was replaced by a mathematical model which has been evolved to be more robust against noise, mainly, because the first derivative of the I（V） characteristics is used. Special attention was given to the power of the energy dependence in the exponent. In particular, for plasmas that are generated by ECR with EM modes, the existence of Maxwellian distribution functions is not to be taken as a self-evident fact, but the bi-Maxwellian distribution was proven for Ar-and Kr-stabilized plasmas. In addition to the electron temperature, the global uniform discharge model has been shown to be useful for calculating the neutral gas temperature. To what extent the invasive method of using a Langmuir probe could be replaced with the noninvasive optical method of emission spectroscopy, particularly actinometry, was investigated,and the resulting data exhibited the same relative behavior as the Langmuir data. The correlation with etchrate data reveals the large chemical part of the removal process—most striking when the data is compared with etching in pure argon. Although the relative amount of the radial variation of plasma density and etch rate is approximately ？5%, the etch rate shows a slightly concave shape in contrast to the plasma density.

Recording Spatially Resolved Plasma Parameters in Microwave-Driven Plasmas

In an almost cubical reactor 90 1 in volume which is intended to deposit organic polymers by plasma-enhanced chemical vapor deposition （PECVD）, microwave power is coupled into the volume via a quartz window which extends to approximately 1/10 of the sidewall area. Since the plasma is excited locally, plasma parameters like electron temperature and plasma density are expected to exhibit a spatial variation. The compilation of these plasma quantities has been accomplished with a bendable single Langmuir probe. To isolate the tungsten wire against its grounded housing tube, it was coated with polyparylene. After having compared this construction with our Langmuir probe, which has been now in use for more than a decade, we have taken data of more than half the volume of the reactor with argon and have found a definitive radial inhomogenity for all plasma parameters. To investigate whether this conduct can be determined applying optical emission spectroscopy, we improved our spectrometer which had been used for endpoint detection purposes and plasma diagnostics in chlorine-containing ambients where we could detect also a spatial dependence. This behavior is discussed in terms of Lieberman＇s global model.

Topology-Optimized 4D Printing of a Soft Actuator

Soft robots and actuators are emerging devices providing more capabilities in the field of robotics.More flexibility and compliance attributing to soft functional materials used in the fabrication of these devices make them ideal for delivering delicate tasks in fragile environments,such as food and biomedical sectors.Yet,the intuitive nonlinearity of soft functional materials and their anisotropic actuation in compliant mechanisms constitute an existent challenge in improving their performance.Topology optimization(TO)along with four-dimensional(4D)printing is a powerful digital tool that can be used to obtain optimal internal architectures for the efficient performance of porous soft actuators.This paper employs TO analysis for achieving high bending deflection of a 3D printed polyelectrolyte actuator,which shows bending deformations in response to electrical stimuli in an electrolyte solution.The performance of the actuator is studied in terms of maximum bending and actuation rate compared with a solid,uniformly 3D printed and topology-optimized actuator.The experimental results proved the effectiveness of TO on achieving higher bending deformation and actuation rate against a uniformly 3D printed actuator.

Comparison of ultrashort pulse ablation of gold in air and water by time-resolved experiments

Laser ablation in liquids is a highly interdisciplinary method at the intersection of physics and chemistry that offers the unique opportunity to generate surfaaant-free and stable nanoparticles from virtually any material.Over the last decades,numerous experimental and computational studies aimed to reveal the transient processes governing laser ablation in liquids.Most experimental studies investigated the involved processes on timescales ranging from nanoseconds to microseconds.However,the ablation dynamics occurring on a sub-nanosecond timescale are of fundamental importance,as the conditions under which nanoparticles are generated are established within this timeframe.Furthermore,experimental investigations of the early timescales are required to test computational predictions.We visualize the complete spatiotemporal picosecond laser-induced ablation dynamics of gold immersed in air and water using ultrafast pump-probe microscopy.Transient reflectivity measurements reveal that the water confinement layer significantly influences the ablation dynamics on the entire investigated timescale from picoseconds to microseconds.The influence of the water confinement layer includes the electron injection and subsequent formation of a dense plasma on a picosecond timescale,the confinement of ablation products within hundreds of picoseconds,and the generation of a cavitation bubble on a nanosecond timescale.Moreover,we are able to locate the temporal appearance of secondary nanoparticles at about 600 ps after pulse impact.The results support computational predictions and provide valuable insight into the early-stage ablation dynamics governing laser ablation in liquids.

Unexpectedly large energy variations from dopant interactions in ferroelectric HfO_(2) from high-throughput ab initio calculations

Insight into the origin of process-related properties like small-scale inhomogeneities is key for material optimization.Here,we analyze DFT calculations of randomly doped HfO_(2) structures with Si,La,and VO and relate them to the kind of production process.Total energies of the relevant ferroelectric Pbc2_(1) phase are compared with the competing crystallographic phases under the influence of the arising local inhomogeneities in a coarse-grained approach.The interaction among dopants adds to the statistical effect from the random positioning of the dopants.In anneals after atomic layer or chemical solution deposition processes,which are short compared to ceramic process tempering,the large energy variations remain because the dopants do not diffuse.Since the energy difference is the criterion for the phase stability,the large variation suggests the possibility of nanoregions and diffuse phase transitions because these local doping effects may move the system over the paraelectric-ferroelectric phase boundary.

Recent advances in optical fiber high-temperature sensors and encapsulation technique[Invited]

In the aerospace field,for aerospace engines and other high-end manufacturing equipment working in extreme environments,like ultrahigh temperatures,high pressure,and high-speed airflow,in situ temperature measurement is of great importance for improving the structure design and achieving the health monitoring and the fault diagnosis of critical parts.Optical fiber sensors have the advantages of small size,easy design,corrosion resistance,anti-electromagnetic interference,and the ability to achieve distributed or quasi-distributed sensing and have broad application prospects for temperature sensing in extreme environments.In this review,first,we introduce the current research status of fiber Bragg grating-type and Fabry–Perot interferometer-type high-temperature sensors.Then we review the optical fiber hightemperature sensor encapsulation techniques,including tubular encapsulation,substrate encapsulation,and metalembedded encapsulation,and discuss the extreme environmental adaptability of different encapsulation structures.Finally,the critical technological issues that need to be solved for the application of optical fiber sensors in extreme environments are discussed.