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As femtosecond (fs) laser machining advances from micro/nanoscale to macroscale, approaches capable of machining macroscale geometries that sustain micro/nanoscale precisions are in great demand. In this research, an fs laser sharp shaping approach was developed to address two key challenges in macroscale machining (i.e. defects on edges and tapered sidewalls). The evolution of edge sharpness (edge transition width) and sidewall tapers were systematically investigated through which the dilemma of simultaneously achieving sharp edges and vertical sidewalls were addressed. Through decreasing the angle of incidence (AOI) from 0° to −5°, the edge transition width could be reduced to below 10 µm but at the cost of increased sidewall tapers. Furthermore, by analyzing lateral and vertical ablation behaviors, a parameter-compensation strategy was developed by gradually decreasing the scanning diameters along depth and using optimal laser powers to produce non-tapered sidewalls. The fs laser ablation behaviors were precisely controlled and coordinated to optimize the parameter compensations in general manufacturing applications. The AOI control together with the parameter compensation provides a versatile solution to simultaneously achieve vertical sidewalls as well as sharp edges of entrances and exits for geometries of different shapes and dimensions. Both mm-scale diameters and depths were realized with dimensional precisions below 10 µm and surface roughness below 1 µm. This research establishes a novel strategy to finely control the fs laser machining process, enabling the fs laser applications in macroscale machining with micro/nanoscale precisions.

The controllable and facile fabrication of surface micro/nanostructures with the required dimensions and morphologies is the key to achieving surface superhydrophobicity. With the advantages of being a noncontact, maskless, programmable, and one-step process, ultrafast laser irradiation is a very flexible and adaptive technique for fabricating various microscale, nanoscale, and micro/nanomultiscale surface structures on diverse solids, thus realizing superhydrophobicity on their surfaces. In this feature article, a comprehensive review of our recent research advances on versatile superhydrophobic surfaces enabled by ultrafast lasers is presented from the perspectives of materials, methodologies, and functionalization. The realization of superhydrophobicity and even superamphiphobicity on varied solid surfaces through ultrafast laser treatment and the underlying mechanisms for the wettability transition of ultrafast-laser-processed surfaces from superhydrophilicity to superhydrophobicity will be discussed. For the sake of practical applications, the ultrafast-laser-based strategies for the large-scale and cost-effective fabrication of superhydrophobic surface micro/nanostructures will be introduced. A special focus will be devoted to the enhancement of structural durability and the Cassie-Baxter stability of ultrafast-laser-enabled superhydrophobic surfaces. Beyond that, the achievement of integrated surface functions including remarkable wetting functions such as the directional collection of water droplets and superhydrophobic surfaces simultaneously with unique optical properties will also be presented.

In this paper, we propose and experimentally demonstrate a new laser processing method for low-temperature formation of effective aluminum oxide passivation layers on crystalline silicon. An aluminum film with a thickness of only several nm is evaporated on the silicon surface, after which a long pulse laser is utilized to convert the metal film to oxide. Laser processing parameters, especially laser energy fluence, are systematically optimized to get the critical condition to realize carrier lifetime similar to that of the bulk wafer, which we consider good passivation, while causing no damage to its surface. An effective surface recombination velocity below 25 cm s−1 is obtained right after the proper laser treatment of the Al metal film. After further annealing in forming gas, a 1.02 millisecond effective carrier lifetime as well as an effective surface recombination velocity below 13 cm s−1 is achieved for 1.4 Ω·cm n-type Si wafer.

The chemical and physical processes that take place in the sub-μm3 volume of the tightly focused laser beam during TPP writing are complex. Variations in photon flux, exposure time, heat dissipation kinetics, and concentration of photoinitiator and monomer can give rise to differences in the properties of the polymer material, such as the degree of conversion, density, and mechanical properties. Quality control of the TPP process, especially in terms of the chemical conversion during and immediately after laser illumination, is highly desirable. Alongside developments in TPP writing techniques, the detailed chemical in situ characterization of TPP structures plays an equally important role in driving the technology toward maturation.

Surface micro-nano structures play an important role in realizing various functions at solid surfaces and interfaces. Ultrafast lasers, especially the new-generation high-repetition rate high-power ultrafast lasers, have been proven to be powerful and versatile tools in enabling the fabrication of surface micro-nano structures. Taking the antireflection property as the key and cut-in point, this chapter presents a brief overview on the state-of-the-art and current challenges on the fabrication of surface micro-nano structures. Special focus will be placed on our achievements in the recent years in developing ultrafast laser-based approaches for the general, scalable, and controllable construction of surface micro-nano functional structures, including nanoscale structures, micro-nano dual-scale structures, and macro-micronano-nanowire multiscale structures, and the unique antireflection performances realized by these surface micro-nano structures. This chapter intends to offer readers an integrated tour of ultrafast laser fabrication techniques for surface micro-nano structures and an outlook on their great application potentials.

Molybdenum disulfide (MoS2), a promising non-precious electrocatalyst for the hydrogen evolution reaction with two-dimensional layered structure, has received increasing attention in recent years. Its electrocatalytic performance has been limited by the low active site content and poor conductivity. Herein, we report a facile and general ultrafast laser ablation method to synthesize MoS2 quantum dots (MS-QDs) for electrocatalytic HER with fully exposed active sites and highly enhanced conductivity. The MS-QDs were prepared by ultrafast laser ablation of the corresponding bulk material in aqueous solution, during which they were partially oxidized and formed defective structures. The as-prepared MS-QDs demonstrated high activity and stability in the electrocatalytic HER, owing to their very large surface area, defective structure, abundance of active sites, and high conductivity. The present MS-QDs can also find application in optics, sensing, energy storage, and conversion technologies.

W/Cu joining is key for the fabrication of plasma facing components of fusion reactors, which however is very challenging due to the low bonding strength and high W/Cu interface thermal stress as a result of the immiscible nature of W-Cu binary system and thermal expansion coefficient mismatch between W and Cu. In this paper, we proposed a method for comprehensive enhancement of the mechanical and thermo-mechanical properties of W/Cu joints based on femtosecond (fs) laser fabricated micro/nano interface structures. Four kinds of surface structures, namely pristine structure, nano ripples, micro-cubes and micro-pits were fabricated on W by fs laser ablation and introduced into W/Cu joining interface by hot pressing joining at 1000 °C, 80 MPa. The micro/nano interface structures contribute to significant enhancement of the tensile strength, shearing strength and thermal fatigue life of the W/Cu joints, which reach 101.58 MPa, 186.47 MPa and >800 thermal cycles, increased by about 150%, 320% and 400% compared with W/Cu joints with pristine interface structure. This research provides an approach for enhanced joining between dissimilar materials, including but not limited to W and Cu.

Hydrogen production from water based on photoelectrochemical (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO3 micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidation. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO3 nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent density of 1.2 mA cm–2 at 1.0 V vs Ag/AgCl (1.23 V vs RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidation conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective, and controllable for large-scale fabrication, which...

There is a growing interest in oxide nanocrystal based electrocatalysts for overall water splitting. Despite tremendous efforts, large-scale fabrication of highly-active and durable oxide electrocatalytic electrodes remains as a great challenge. Herein, we report a fast and general strategy for manufacturing a series of hierarchical nanostructured metal oxides (MO x , M= Ti, Mn, Fe, Co, Ni, Cu, Mo, Ag, Sn, W and NiFe) as electrocatalysts by laser ablation on corresponding metal substrates. Particularly, the NiO nanocrystal electrocatalysts (~3 nm) grown on Ni plates have been directly employed as highly active and stable bifunctional electrodes for both hydrogen evolution and oxygen evolution reactions, by taking advantage of its large surface area, rich defects, high hydrophilicity and aerophobicity. The facile laser treatments to construct nanoporous electrodes should facilitate the development of low-cost and high-performance electrocatalytic overall water splitting with large-scale.

Micro/nano surface structures show great significance in both fundamental research and practical applications. Imprinting technique is widely used to fabricate micro/nano surface structures on polymers at low temperature, but barely implemented on metals. In this paper, femtosecond (fs) laser was used to fabricate micro/nano surface structures on tungsten (W), which served as molds for subsequent solid state imprinting process to replicate the counter structures onto copper (Cu) surface at temperature up to 1000 °C. Nano-ripples superimposed on micro-bumps array are fabricated without surface oxidation, and the height of the micro-bumps is adjustable by varying the laser energy input for W molds preparation. The as-prepared Cu surface structures are hydrophobic and turn to superhydrophobic after chemical modification. To demonstrate the feasibility and capability of this technique, similar surface structures were fabricated on A356 aluminum alloy (A356 AA) by liquid state imprinting at 740 °C using M2 steel molds prepared by picosecond (ps) laser ablation. This approach combines the advantages of ultrafast laser ablation to fabricate molds with micro/nano surface structures and high-temperature imprinting process to replicate the micro/nano structures onto target metallic surfaces. It opens new possibility for the mass fabrication of functional micro/nano surface structures on metals.

Metal oxide nanowires are one of the most promising material systems utilized in energy conversion and storage devices. Directly synthesizing oxide nanowires on conductive substrates and assembling them into 3D self-supporting spatial architectures are of great importance for improving their performances. We propose and experimentally demonstrate a top-down and bottom-up combined manufacturing strategy, which can facilely fabricate various 3D self-supporting macro–micronano-nanowire multiscale architectures directly on bulk metal substrates. By employing micro–nano precursor structures, the gap between the commonly micrometer scale top-down and nanometer scale bottom-up approaches is bridged seamlessly. The presented strategy combines the advantages of the ultrafast laser direct writing approach in creating versatile micro patterns with those of the oxidation growth approach in creating elaborate nanowires. Both the precursor structures and the nanowires can be flexibly tuned in order to achieve arbitrary desired 3D architectures. Successfully pairing the scalable top-down laser direct writing technique with the high throughput bottom-up oxidation growth technique greatly improves the potential of our strategy in mass production and widespread adoption. Our approach opens a general and convenient route for integrating low dimensional oxide nanostructures with macroscopic metal substrates for various applications.

Efficient solar energy harvesting and photothermal conversion have essential importance for many practical applications. Here, we present a laser-induced cauliflower-shaped hierarchical surface nanostructure on a copper surface, which exhibits extremely high omnidirectional absorption efficiency over a broad electromagnetic spectral range from the UV to the near-infrared region. The measured average hemispherical absorptance is as high as 98% within the wavelength range of 200-800 nm, and the angle dependent specular reflectance stays below 0.1% within the 0-60° incident angle. Such a structured copper surface can exhibit an apparent heating up effect under the sunlight illumination. In the experiment of evaporating water, the structured surface yields an overall photothermal conversion efficiency over 60% under an illuminating solar power density of ∼1 kW m(-2). The presented technology provides a cost-effective, reliable, and simple way for realizing broadband omnidirectional light absorptive metal surfaces for efficient solar energy harvesting and utilization, which is highly demanded in various light harvesting, anti-reflection, and photothermal conversion applications. Since the structure is directly formed by femtosecond laser writing, it is quite suitable for mass production and can be easily extended to a large surface area.

Optical functional surfaces are key components of nearly every optical device and they have become a special focus in both academia and industry. The no contact, one step, direct, and maskless laser surface texturing technique is one of the most encouraging approaches for realizing the surface functions. We use a high power and high repetition rate ultrafast laser system to produce micro–nano structures on metal surfaces. We demonstrate that metal surface micro–nano structures and correspondingly their optical responses can be facilely tailored by simple controlling the ultrafast laser processing parameters. Nano particles of tens to hundreds nm, sub-micro particles of 0.5–1 μm, fine-micro particles of 1–10 μm, micro particles of 10–50 μm, and coarse-micro particles larger than 50 μm have been fabricated on Cu surfaces. And surface reflection of copper surfaces has been tuned from 10% to 90% in spectra level and from UV to MIR in spectrum range, with unique optical properties like visible selective reflection, linear changing reflection, band reflection, and broadband absorption being achieved. The formation processes of those particle structures as well as the underlying mechanisms for their optical responses are discussed.

We report here a simple and efficient hot embossing process capable of mass fabricating superhydrophobic and self-cleaning polytetrafluoroethylene (PTFE) film surfaces. Adding superhydrophobicity to hydrophobic polytetrafluoroethylene (PTFE) significantly enhances their application potential in industry as well as in daily life. We applied a picosecond laser to fabricate regular array of micro-holes companied with submicron structures on high strength steel substrate to form a lotus-leaf-like template. Then the hot embossing process was performed on flat PTFE films to introduce a dual-scale structure composed of the micro-scale protrusions and nano-scale fibril structures on the top of protrusions. The hot embossing parameters such as the embossing pressure and time were optimized to achieve required micro- and nano-scale dual structure on PTFE film very closed to the similar dual structure of the lotus leaf surface. The PTFE films then exhibited superhydrophobicity with contact angle up to 154.6° and sliding angle of as low as 5.5°. The thermal stability of the superhydrophobic PTFE films was investigated from room temperature up to 430 °C. We demonstrate that the micro–nano dual structure on PTFE films and their superhydrophobicity is thermally stable up to 340 °C. The micro-scale protrusions will collapse and the superhydrophobicity will lose when the temperate is over 370 °C.

Functional metal surfaces with minimum optical reflection over a broadband spectrum have essential importance for optical and optoelectronic devices. However, the intrinsically large optical impedance mismatch between metals and the free space causes a huge obstacle in achieving such a purpose. We propose and experimentally demonstrate a general pulse injection controlled ultrafast laser direct writing strategy for fabricating highly effective antireflection structures on metal surfaces. The presented strategy can implement separate and flexible modifications on both microscale frame structures and nanoscale particles, a benefit from which is that optimized geometrical light trapping and enhanced effective medium effect reducing the surface reflection can be simultaneously achieved within one hybrid structure. Thus, comprehensively improved antireflection performances can be realized. Hybrid structures with substantial nanoparticles hierarchically attached on regularly arrayed microcones are generally constructed on different metal surfaces, achieving highly efficient light absorption over ultraviolet to near-infrared broadband spectrum regions. Reflectance minimums of 1.4%, 0.29%, and 2.5% are reached on Cu, Ti, and W surfaces, respectively. The presented strategy is simple in process, adaptable for different kinds of metals, reproduceable in dual-scale structural features, and feasible for large-area production. All these advantages make the strategy as well as the prepared antireflection structures excellent candidates for practical applications.

In this study, functional copper surfaces combined with vivid structural colors and superhydrophobicity were fabricated by picosecond laser. Laser-induced periodic surface structures (LIPSS), i.e. ripples, were fabricated by picosecond laser nanostructuring to induce rainbow-like structural colors which are uniquely caused by the grating – type structure. The effects of laser processing parameters on the formation of ripples were investigated. We also discussed the formation mechanism of ripples. With different combinations of the laser processing parameters, ripples with various morphologies were fabricated. After the modification with triethoxyoctylsilane, different types of ripples exhibited different levels of wettability. The fine ripples with minimal redeposited nanoparticles exhibited high adhesive force to water. The increased amount of nanoscale structures decreased the adhesive force to water and increased the contact angle simultaneously. In particular, a specific type of ripples exhibited superhydrophobicity with a large contact angle of 153.9 ± 3.2° and a low sliding angle of 11 ± 3°.

W/Cu joining is key for the fabrication of high heat load components for fusion reactors, which however suffers from the low W/Cu bonding strength due to the immiscible nature of W-Cu system. In this study, we proposed a method for strengthened W/Cu joining based on femtosecond (fs) laser induced micron-scale W/Cu interface structure. W surfaces were irradiated by fs laser to form micron-scale cubes array, and then joined to Cu by hot pressing at 1000 °C, 80 MPa for 2.5 hours. The tensile strength of the W/Cu joining samples was investigated. The results show that micron-scale cubes array was successfully introduced into W/Cu interface without any cracks or pores. The interface structure helps to increase the W/Cu bonding strength to as high as 59.61 MPa, increased by about 50% as compared to W/Cu joining with a flat interface (bonding strength 40.11 MPa). The W/Cu bonding strength shows positive correlation with the W/Cu interface area, indicating the possibility to control the W/Cu bonding strength by simply adjusting the fs laser ablation parameters for the fabrication of cubes array on W surface. Our research provides a method for strengthened joining between intrinsically immiscible materials, including but not limited to W and Cu.

Realizing superhydrophobicity, high transparency on polydimethylsiloxane (PDMS) surface enlarges its application fields. We applied a femtosecond laser to fabricate well-designed structures combining microgrooves with microholes array on mirror finished stainless steel to form a template. Then liquid PDMS was charged for the duplicating process to introduce a particular structure composed of a microwalls array with a certain distance between each other and a microprotrusion positioned at the center of a plate surrounded by microwalls. The parameters such as the side length of microwalls and the height of a microcone were optimized to achieve required superhydrophobicity at the same time as high-transparency properties. The PDMS surfaces show superhydrophobicity with a static contact angle of up to 154.5 ± 1.7° and sliding angle lower to 6 ± 0.5°, also with a transparency over 91%, a loss less than 1% compared with plat PDMS by the measured light wavelength in the visible light scale. The friction robust over 100 cycles by sandpaper, strong light stability by 8 times density treatment, and thermal stability up to 325 °C of superhydrophobic PDMS surface was investigated. We report here a convenient and efficient duplicating method, being capable to form a transparent PDMS surface with superhydrophobicity in mass production, which shows extensive application potentials.

The Cassie-state stability plays a vital role in the applications of metallic superhydrophobic surfaces. Although a large number of papers have reported the superhydrophobic performance of various surface micro/nanostructures, the knowledge of which kind of micro/nanostructure contributes significantly to the Cassie-state stability especially under low temperature and pressure is still very limited. In this article, we fabricated six kinds of typical micro/nanostructures with different topography features on metal surfaces by a femtosecond laser, and these surfaces were modified by fluoroalkylsilane to generate superhydrophobicity. We then systematically studied the Cassie-state stability of these surfaces by means of condensation and evaporation experiments. The results show that some superhydrophobic surfaces, even with high contact angles and low sliding angles under normal conditions, are unstable under low temperature or external pressure. The Cassie state readily transits to a metastable state or even a Wenzel state under these conditions, which deteriorates their superhydrophobicity. Among the six micro/nanostructures, the densely distributed nanoscale structure is important for a stable Cassie state, and the closely packed micrometer-scale structure can further improve the stability. The dependence of the Cassie-state stability on the fabricated micro/nanostructures and the laser-processing parameters is also discussed. This article clarifies optimized micro/nanostructures for stable and thus more practical metallic superhydrophobic surfaces.

Infrared antireflection is an essential issue in many fields such as thermal imaging, sensors, thermoelectrics, and stealth. However, a limited antireflection capability, narrow effective band, and complexity as well as high cost in implementation represent the main unconquered problems, especially on metal surfaces. By introducing precursor micro/nano structures via ultrafast laser beforehand, we present a novel approach for facile and uniform growth of high-quality oxide semiconductor nanowires on a Cu surface via thermal oxidation. Through the enhanced optical phonon dissipation of the nanowires, assisted by light trapping in the micro structures, ultralow total reflectance of 0.6% is achieved at the infrared wavelength around 17 μm and keeps steadily below 3% over a broad band of 14-18 μm. The precursor structures and the nanowires can be flexibly tuned by controlling the laser processing procedure to achieve desired antireflection performance. The presented approach possesses the advantages of material simplicity, structure reconfigurability, and cost-effectiveness for mass production. It opens a new path to realize unique functions by integrating semiconductor nanowires onto metal surface structures.

Superhydrophobic surfaces with tunable water adhesion have attracted much interest in fundamental research and practical applications. In this paper, we used a simple method to fabricate superhydrophobic surfaces with tunable water adhesion. Periodic microstructures with different topographies were fabricated on copper surface via femtosecond (fs) laser irradiation. The topography of these microstructures can be controlled by simply changing the scanning speed of the laser beam. After surface chemical modification, these as-prepared surfaces showed superhydrophobicity combined with different adhesion to water. Surfaces with deep microstructures showed self-cleaning properties with extremely low water adhesion, and the water adhesion increased when the surface microstructures became flat. The changes in surface water adhesion are attributed to the transition from Cassie state to Wenzel state. We also demonstrated that these superhydrophobic surfaces with different adhesion can be used for transferring small water droplets without any loss. We demonstrate that our approach provides a novel but simple way to tune the surface adhesion of superhydrophobic metallic surfaces for good potential applications in related areas.

Studies regarding the wettability transition of micro- and nano-structured metal surfaces over time are frequently reported, but there seems to be no generally accepted theory that explains this phenomenon. In this paper, we aim to clarify the mechanism underlying the transition of picosecond laser microstructured aluminum surfaces from a superhydrophilic nature to a superhydrophobic one under ambient conditions. The aluminum surface studied exhibited superhydrophilicity immediately after being irradiated by a picosecond laser. However, the contact angles on the surface increased over time, eventually becoming large enough to classify the surface as superhydrophobic. The storage conditions significantly affected this process. When the samples were stored in CO2, O2 and N2 atmospheres, the wettability transition was restrained. However, the transition was accelerated in atmosphere that was rich with organic compounds. Moreover, the superhydrophobic surface could recover their original superhydrophilicity by low temperature annealing. A detailed XPS analysis indicated that this wettability transition process was mainly caused by the adsorption of organic compounds from the surrounding atmosphere onto the oxide surface.

To render metal surfaces to various desirable colors without changing their chemical composition is necessary and important in many applications including decoration, marking, data storage, and anti counterfeiting. In this paper, we report a surface micro/nano texturing technique by utilizing a high power (with an average power up to 100 W) and high repetition rate (with a maximum of 2 MHz) picosecond laser beam to colorize metal surfaces. Sequential colors varying from golden to deep plum were realized on steel surfaces by simply changing the ps laser scanning speeds. Systematic analyses on the spectral response and micro/nano structure characterization were reported. Spectrum shifting effect corresponding to color change was explained through surface plasmon resonance mechanism. The current research demonstrates that a high power and high repetition rate ps laser is capable of forming sequential colors on steel surfaces with a processing speed up to several meters per second, making this technology an efficient and affordable candidate for practical industrial applications. The research can pave the way for ps laser surface texturing technique to be applied to colorize conventional engineering materials.To render metal surfaces to various desirable colors without changing their chemical composition is necessary and important in many applications including decoration, marking, data storage, and anti counterfeiting. In this paper, we report a surface micro/nano texturing technique by utilizing a high power (with an average power up to 100 W) and high repetition rate (with a maximum of 2 MHz) picosecond laser beam to colorize metal surfaces. Sequential colors varying from golden to deep plum were realized on steel surfaces by simply changing the ps laser scanning speeds. Systematic analyses on the spectral response and micro/nano structure characterization were reported. Spectrum shifting effect corresponding to color change was explained through surface plasmon resonance mechanism. The current research demonstrates that a high power and high repetition rate ps laser is capable of forming sequential colors on steel surfaces with a processing speed up to several meters per second, making this technology an e...

Former studies have found that water droplets on the rice leaves exhibit anisotropic sliding behavior, which is mainly caused by the anisotropic groove-like surface microstructures. Similar groove-like surface microstructures and anisotropic sliding behavior of water droplets can also be found on some other plants, such as the reed leaves. In this paper, biomimetic groove-like surface microstructures are fabricated on copper surfaces by femtosecond laser micromachining. Thanks to the flexibility of this method, the period and height of the periodic micro-grooves can be precisely controlled, and the nanoscale surface roughness can also be regulated. The influences of these surface morphologies on the wetting state and anisotropic sliding behavior of water droplets are studied systematically, and theoretic analyses are also conducted. Our results prove that the anisotropic sliding behavior is mainly decided by the wetting state of external water droplets. Besides, plenty of surface nanostructures have no benefits to the anisotropic sliding behavior of surface water droplets.

There is growing demand for improving the seizure resistance of cast iron cylinder liners. We developed a novel approach involving ns pulsed fiber laser texturing to form regularly patterned micro-scale pits on the pre-modulated liner surface. As a result, periodically distributed pits with a dimension of about 100 μm in width, 10 μm in depth and with an area ratio of about 12.5% were formed on the liner surface. Systematic SRV tribological tests were performed to approve that the seizure resistance was much improved by our approach. Considering that ns pulsed fiber laser processing is cost effective, good potential applications of this approach can be forecasted to improve the performance of the conventional automobile engines.

The wettability of metal oxides is vital to many applications including water erosion, filtration, and bioimplantation. In this work, the authors studied the wettability conversion behavior of picosecond laser structured copper surfaces in different atmospheres. The copper surfaces showed hydrophilicity initially after being irradiated by a picosecond laser. However, when they were stored in ambient air, their contact angles increased over time and became highly hydrophobic finally. The storage atmosphere influenced this process greatly, the atmosphere rich in CO2 or O2 would restrain the wettability transition, but the organic-rich and vacuum atmosphere would accelerate it. Detailed surface chemical analysis revealed that the adsorption of organic matters from the air played an important role in this wettability conversion process.

The simultaneous generation of nanostructures and redeposited nanoparticles on copper surfaces through their direct ablation in air by a high power and high repetition rate ps laser was demonstrated. Basic and detailed analysis on the formation and the size distribution of nanoparticles spreading over the nanostructured copper surfaces was performed. Lower scanning speed causes more laser pulse input onto the target surface, resulting in a more dense distribution of the nanoparticles with a nearly constant mean radius. The changes in the particle distribution render the copper surfaces to unique reflection spectra responses and surface colors, which are independent of the viewing angles. The present research can pave the way for the practical applications of ps laser in generating nanoparticles on metal surfaces and tailoring their optical properties.

A surface micro/nano structuring technique was demonstrated for colorizing metal surfaces with a picosecond laser. Sequential color change from black to pink was realized on copper surfaces by simply changing the ps laser scanning speeds. Systematic analyses on the spectral response and microstructure characteristics were reported. The spectrum shifting effect corresponding to color change was explained through the surface plasmon resonance mechanism. The current research shows that a high power and high repetition rate ps laser is capable of structuring metal surfaces with a speed up to several meters per second, presenting an efficient and affordable candidate for practical industrial applications.

A surface micro/nano structuring technique was demonstrated by utilizing a picosecond laser beam to rapidly modify the optical property of copper surfaces with a scanning speed up to tens of millimeters per second. Three kinds of surface micro/nanostructures corresponding to three levels of reflectance were produced which are obviously different from those induced by a femtosecond or nanosecond laser. Specifically, a porous coral-like structure results in over 97% absorptivity in the visible spectral region and over 90% absorptivity in average in the UV, visible, and NIR regions (250 - 2500 nm). Potential applications may include solar energy absorbers, thermal radiation sources, and radiative heat transfer devices.