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High orientation consistency and adjustable convex width of the low-spatial-frequency laser-induced periodic surface structures (LSFLs), crucial to the functional surface characteristics, have remained elusive. This paper proposes a new method to fabricate LSFLs with high orientation consistency on the rough surface of titanium by combining laser polishing and laser induction with LSFLs with a tunable convex width via laser melting as the post-treatment. Picosecond pulses trains with a 50-ns interval are applied to regulate the thermal incubation effect and achieve laser polishing and laser nanoscale melting. The melting time of titanium for laser polishing and laser nanoscale melting is determined to be on a microsecond time scale and around 100 ns, respectively. Experimental studies show that the surface texture of titanium lowers the orientation consistency of LSFLs and its divergence angle is 30°. Picosecond pulses with a sub-pulse number of three are applied to achieve surface polishing and the formation of the rudiment of the LSFLs, followed by the picosecond laser induction. As a result, the divergence angle of LSFLs decreases from 30° to 12°. On this basis, aiming at the problem of the narrow adjustability of the convexity ratio of LSFLs, a nanoscale melting processing method based on picosecond pulse trains with a sub-pulse number of four is proposed, and LSFLs with the tunable convexity ratios from 0.3 to 0.87 are obtained.

While metal oxides and metal sulfides have been extensively studied for gas sensing applications, there are no extensive reports on gas sensing properties of metal diborides. Here, for the first time, we have investigated the conductometric gas sensing behavior of HfB2 nanoparticles. The HfB2 nanoparticles is synthesized via a sol-gel method and characterized using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The gas sensor is fabricated by drop casting the HfB2 nanoparticles on interdigitated Ag/Pd electrodes. The sensor exhibited a promising NO2 sensing performance at the operating temperature of 200 °C. Interestingly, it is found that resistance of the sensor decreases upon exposure to both oxidizing and reducing gases, which does not follow the gas sensing behaviors of the widely investigated metal oxides. In addition, the sensor response is minimally influenced by oxygen background concentration. These results highlight that HfB2 nanoparticles exhibit very unique sensing characteristics.

This paper proposes a new kind of composite micro/nano structure composed of porous nanomaterials including titanium (Ti) gradient material and titania (TiO2) gradient material. The composite structure is fabricated via in-situ deposition of gradiently distributed and oxidized porous titanium nanoparticles on the surface of picosecond laser-induced micro/nano structures. The porous composite layer made up of Ti gradient material and TiO2 gradient material shows the best anti-reflectivity. The ability of TiO2 for absorbing –CH3 hydrophobic group and the air cushion effect of nanostructure endow the TiO2 gradient porous material with hydrophobic characteristics. Then, the period and height of the desired microstructure of the composite micro/nano structure with the best super-hydrophobicity and anti-reflectivity are confirmed to be about 10 μm and more than 10 μm, respectively. The designed composite micro/nano structures are fabricated using picosecond laser induction processing strategy with the etched depth of the materials being only about 12 μm. The reflectance of the designed composite micro/nano structure is around 2 % from the visible band to the near-infrared band and below 20 % from the ultraviolet to middle-infrared band. Moreover, its slip angle is just 4° and contact angle 167.

Cariogenic Streptococcus mutans is known as a predominant etiological agent of dental caries due to its exceptional capacity to form biofilms. From strains of S. mutans isolated from dental plaque, we discovered, in the present study, a polyketide/nonribosomal peptide biosynthetic gene cluster, muf, which directly correlates with a strong biofilm-forming capability. We then identified the muf-associated bioactive product, mutanofactin-697, which contains a new molecular scaffold, along with its biosynthetic logic. Further mode-of-action studies revealed that mutanofactin-697 binds to S. mutans cells and also extracellular DNA, increases bacterial hydrophobicity, and promotes bacterial adhesion and subsequent biofilm formation. Our findings provided an example of a microbial secondary metabolite promoting biofilm formation via a physicochemical approach, highlighting the importance of secondary metabolism in mediating critical processes related to the development of dental caries. The mutanofactin family of lipopeptide natural products, produced by strains of cariogenic Streptococcus mutans, promotes biofilm formation via increased cell-surface hydrophobicity and binding to extracellular DNA.

Chemiresistive CO2 sensing is attractive due to low cost and ease of chip-level integration. Our previous studies (Yong Xia, 2021) showed the well-developed ZnO material fabricated by in-situ annealing exhibited good CO2 sensing performance. Here, we have expanded on those studies, including CO2 cyclic tests under both dry air and N2 background whereby a much higher response to CO2 in N2 background was observed. Detailed density functional theory calculations were conducted to understand the behavior. The results indicated nitrogen doping is mainly responsible for the observed response. In the presence of pre-adsorbed O2, N-doped ZnO can no longer interact with CO2, which agrees well with the observation of higher response in N2 background. Furthermore, density of states analysis showed N sp2 hybridized orbital and N 2p orbital of the N dopant mixed with sp2 hybridized orbital of C atom and 2p orbitals of C/O atoms in CO2 to form σ and π bonds, respectively. However, they mixed with O 2s/2p orbitals of O atom in O2 when pre-adsorbed O2 was present, hindering CO2 interaction with N-doped ZnO, and resulting in limited response in air. The illustrated mechanism does not only further the understanding of metal oxide-based CO2 sensing, but also guide the design of new functional materials for CO2 sensing or capture.

Real-world implementation of artificial compound eye (ACE) has been limited by its poor transparency and high requirement for the stable Cassie state. In general, the improvement of surface dewetting performance sacrifices the transparency of ACE. Herein, ACE was obtained by an integrated manufacturing technology that combined photolithography, microprinting, and chemical growth. Through skillful manipulation of the fabrication process, dewetting hairs were fabricated on the top of micropillars and around the microlens. The combination of nanohairs and micropillars resulted in outstanding superhydrophobicity (∼170°), pristine lotus effect with low sliding angle (∼1°), and contact angle hysteresis (∼2°). Moreover, the surface showed almost no adhesion under a preload of 4 mN, exhibiting excellent stable Cassie state and antiadhesion performance. Furthermore, dynamic impact showed that the impacting droplet was quickly detached from the surface (contact time ∼14.1 ms) without sticking for

We studied the impact dynamics of a droplet on two types of surfaces, i.e., nanostructured/hierarchical (NS/HS) surfaces, with different extents of hydrophobicity. It was found that the contact time is related to wetting hysteresis. It can be concluded that wetting hysteresis plays a significant role in the contact process of bouncing drops based on the work done against resistance produced by contact angle hysteresis (CAH). For similar surface roughness, the work done by CAH dominates, and a lower CAH creates a smaller contact time. Compared with NS surfaces, the energy stored during the Cassie–Baxter/Wenzel state transition because of the more pronounced air pocket formation provides the upward kinetic energy, resulting in rapid detachment of a droplet from HS surfaces. Thus, HS-3 has a smaller contact/elongation time (∼8/2 ms) because of the enhanced air pocket formation and more favorable wettability (larger contact angle (CA) and smaller contact angle hysteresis (CAH)) than other surfaces. In addition, the results show that surface morphology affects the contact time of bouncing drops mainly by influencing the elongation stage. For different Weber numbers (We), the upward energy storage dominates and results in different varying trends of contact time with We for NS-3 and HS-3. For further study, the morphology evolution of bouncing drops with We was also investigated in detail. The results show that a satellite droplet is launched in a certain We range because of high adhesion resulting from the Cassie–Baxter/Wenzel state transition. These findings provide guidelines for the preparation of surfaces for both self-cleaning and anti-icing purposes.

Functional surfaces with broad-band ultralow optical reflection have many potential applications in areas like national defense and energy conversion. For efficient, high-quality manufacturing of material surfaces with antireflection features, a novel machining method for multiscale micro-nano structures is proposed. This method can enable the collaborative manufacturing of both microstructures via laser ablation and micro-nano structures with high porosity via in situ deposition, and it can simplify the fabrication process of multiscale micro-nano structures. As a result, substantially improved antireflection properties of the treated material surface can be realized by optimizing light trapping of the microstructures and enhancing the effective medium effect for the micro-nano structures with high porosity. In ultraviolet-visible-near-infrared regions, average reflectances, as low as 2.21 and 3.33%, are achieved for Si and Cu surfaces, respectively. Furthermore, the antireflection effect of the treated surface can also be extended to the mid-infrared wavelength range, where the average reflectances for the Si and Cu surfaces decrease to 5.28 and 5.18%, respectively. This novel collaborative manufacturing method is both simple and adaptable for different materials, which opens new doors for the preparation of broad-band ultra-low-reflectivity materials.

Many arthropods have compound eyes, which are made up of numerous separate visual units (microlenses) or ommatidia. These natural compound eyes have exceptional optical properties such as wide field of view (FOV), low aberration, and fast motion tracking capability. In this paper, a large-scale artificial compound eye (ACE) is fabricated efficiently using a combination of inkjet printing and air-assisted deformation processes. Both size and geometry of the microlens are controlled via superposed drops on the substrate. The simulation results show that the light intensity of the ACE follows a systematic distribution for tilted incident light, which represents a significant improvement, compared to planar distributed microlenses. We then manufacture ACEs with different heights and diameters, and their FOVs are compared with the theoretically predicted results. The measured FOV was 50°-140°. The acceptance angles for the different ACEs are determined, and their relationship with the ratio of height to radius (H/r) of the microlens is investigated in more detail. Furthermore, the imaging properties of the microlenses with different angles of incidences are studied, which suggest a FOV up to 140° and an acceptance angle of about 50°. The microlens captures images even at an angle of incidence of about 60°. The corresponding distortion in both the

This paper presents a study of the effects of ablation direction, beam profile and outer environment on the width and morphology of micro-grooves ablated by a 10-ps, 532-nm laser with a changing ra...

This paper reports on the fabrication of marking units of controllable size ranging from 300 to 500 μm equipped with anti-reflective micro-structures using defocused femtosecond laser on titanium alloy TC4. The results show that the range of diameters of marking units (laser ablation-based craters) goes through three stages with the increase of the pulse number. The craters are too shallow for the first stage and too deep for the third stage to meet the criterion of depth, so the second stage turns out to be the optimal stage of parameter selection, where the diameter and depth of marking units are within a desired range with modification of laser energy and defocusing amount. Besides meeting the marking requirements of the size and morphology, the anti-reflective micro-structures with great color difference are formed on the surface of marking units, which contributes to the high recognition rate. Compared with the conventional marking methods, this method has a great recognition rate without recast layer and micro-cracks. Therefore, the femtosecond laser-based processing would provide a new marking technology with high efficiency and quality.

This study presents the manufacturing process of bioinspired compound (BIC) eyes. The hierarchical eyes were accomplished by a combination of (i) modified laser swelling, (ii) air-assisted deformation, and (iii) controlled crystal growth. The results show that the addition of nanostructures on the surface effectively improved the water repellent performance with a contact angle (CA) of ∼160° and generally decreased the reflection by ∼25% in the wavelength range of 400-800 nm than the planar surface. Apart from these properties, the BIC eyes showed good optical performance. The convex structure has a circular shape and aspherical profile; this provides optical uniformity and constant resolution (full width at half-maximum = 1.9 μm) in all the directions. Furthermore, the BIC eyes reduced the imaging distortion by 1.5/3.4 and 2.3/3.1 times along the x and y axes, respectively, under 10° and 20° incident lights than a single lens. In the light acceptance range, the image displays almost no distortion.

Carbon nanowires have many interesting properties in electronic, opto-electronic and nanoelectromechanical devices due to their high aspect ratios. Carbon nanotubes are ideal templates for the synthesis of one-dimensional carbon nanowires. T-carbon nanowires have been recently produced by the pseudo-topotactic conversion of multi-walled carbon nanotubes (MWCNTs) under picosecond laser irradiation. There is no data about the relationship between the carbon product morphology and laser pulse duration reported so far. Pulsed laser induced reaction is a fast and far from equilibrium process. Carbon nanostructures with different morphology have been produced from carbon nanotube suspension in methanol under laser irradiation with different pulse duration. Carbon nanoparticles, porous carbon nanorods, and even diamond nanobundles have been obtained under nanosecond laser irradiation. The one-dimensional template growth from MWCNTs to carbon nanowires is not successful under nanosecond laser irradiation. However, different carbon nanowires with diameters similar to shortened carbon nanotubes have been easily produced from carbon nanotube suspension under the irradiation of picosecond and femtosecond pulsed laser. The morphology-directing transformations of carbon nanotubes have also explained using reported mechanisms.

Cariogenic Streptococcus mutans is known as a predominant etiological agent of dental caries due to its exceptional capacity in forming biofilms. From strains of S. mutans isolated from dental plaque, we here discover a polyketide/non-ribosomal peptide biosynthetic gene cluster, muf, which directly correlates with a strong biofilm-forming capability. We then identify the muf-associated bioactive product, mutanofactin-697 that contains a novel molecular scaffold, along with its biosynthetic logic. Further mode-of-action studies reveal mutanofactin-697 binds to S. mutans cells nonspecifically, increases bacterial hydrophobicity, and promotes bacterial adhesion and subsequent biofilm formation. Our findings provide the first example of a microbial secondary metabolite promoting biofilm formation via a physicochemical approach, highlighting the significance of secondary metabolism in mediating critical processes related to the development of dental caries.

This study reported on the high capacity color code obtained by femtosecond laser-induced surface oxidation on titanium alloy. The color code with quaternary numbers was designed. Various colors were obtained by changing the pulse energy and scanning speed. Moreover, the XPS survey spectra showed the surface chemical composition of different colored areas was TiO2. Experimental reflectance data and simulations showed that the increase of oxide layer thickness led to peak shift and color change. Taking into account the effect of color code unit size on coloring, the processing areas with an area of 1 × 1 mm2 and 6 × 6 mm2 were prepared. Since the color difference showed that the colors obtained under different unit sizes were different, the effect of size should be considered in the actual processing. By changing the scanning speed, different color areas with obvious contrast could be obtained to meet the conditions of color code realization. Therefore, this research further realized the encryption of information on the basis of increasing the information capacity of the marking.

The bonding strength between environmental barrier coatings (EBCs) and silicon carbide fiber reinforced silicon carbide (SiC/SiC) was improved by preparing microstructure on SiC/SiC surface with femtosecond laser. The processing morphologies with different parameters were compared. There was oxidation phenomenon at the edge of laser processing, and the degree of graphitization was increased. Without damage the internal fiber structure of SiC/SiC, reasonable processing parameters were selected, and microgroove structure was processed on the SiC/SiC surface using the femtosecond laser. The tension test results showed that, after the femtosecond laser machining, the bonding strength between EBCs and SiC/SiC could be improved by about 5.5%, and the time spent to reach the critical load was extended by 11.2%. The EBCs regularly adheres to the microgrooves formed by the femtosecond laser processing, while EBCs in the area without the laser processing was completely detached.

This work presents the fabrication of convex structures by laser irradiation on dual-layer polymethylmethacrylate (PMMA). The surface PMMA layer can prevent gaseous product from escaping and the high absorption of underlying black PMMA layer can ensure enough gas products produced. It is shown that convex structures can only be formed in a particular focus range. The focus position range for appearance of convex structures is determined in our experiments to be 180–400 μm. And then the dependences of height and diameter of convex structures on pulse energy, pulse number and film thickness of surface layer have been investigated. The result demonstrates that the size (both diameter and height) of convex structures could be tuned by pulse energy; compared with the diameter, the height of convex structures is more sensitive to pulse number and film thickness of surface transparent layer. The formation of convex structures is attributed to the sensitively balanced combination effect between the softening of surface material and expansion of underlying material. Finally, large-area-arrays of convex structures with high consistency and variable tunable sizes were generated. The diameter and height of convex structures were measured to be 149 μm and 43 μm, respectively.

Ultrafast laser-induced surface modification of materials has attracted significant attention in recent years. The mechanism of formation of ultrafast laser-induced periodic surface structures (LIPSS), which were evenly distributed over the large area of indium tin oxide, was studied both theoretically and experimentally. Films were deposited using 10-ps 1064-nm laser. Resistance and transmission characteristics of LIPSSs on ITO film were analyzed. The resistance of LIPSSs increases with the increasing of transmittance in infrared band within a certain range. The paper reports the optimal processing parameters for the realization of LIPSSs on ITO films established based on the resistance and transmission properties of the film. The film with the optimized structure can significantly improve the infrared transmittance function of ITO while ensuring the low resistance, which can greatly improve the power generation efficiency of the thin film solar cells.

This study adopted nanosecond laser to conduct drilling research on nickel-based alloy with thermal barrier coatings (TBCs). The experiments of different processing environments on quality and efficiency of laser drilling were conducted. Through combination of high processing efficiency under air environment and good processing quality under water environment, temporal control of air/water environment on laser drilling is proposed. Since drilling efficiency was not significantly reduced, cold machining effect of water was used to improve processing quality. Research results proved that laser processing method with temporal control of air/water environment has advantages of traditional laser drilling under air and water. Laser drilling of temporal control of air/water environment was improved in quality in comparison with single environment.

This study reported the fabrication of a large area of micro/nano structures with different morphologies and sizes by the deposition of ablated material and melting of material on silicon through a line-shaped femtosecond laser beam irradiation. The evolution of micro/nano structures on the silicon surface was demonstrated with the laser fluence of 0.64 J/cm2. It was found that the melting of material was responsible for the formation of the micro-protrusions from laser-induced periodic surface structures (LIPSSs). Additionally, the deposition fell on the surface of the micro-protrusions in oblique incidence way, causing LIPSSs obscure and even invisible. As a consequence, those micro-protrusions gradually evolved into the micro-spikes with the ladder-like surface. Then, various laser fluences were applied to regulate the deposition and melting behaviors of silicon, to obtain the micro/nano structures with different morphologies and sizes. The formation mechanism of these micro/nano structures was analyzed. On this basis, the optical properties test showed that best anti-reflectivity was referred to the sample full of micro-spikes with the ladder-like surface, and the average reflectance has decreased from ∼38.17% of the planar silicon to∼4.75% in the waveband between 300 and 1000 nm.

Low-power microheater platforms are promising in lowering power consumption during gas sensing processes. However, the small amount of activated-material and poor electrical contact greatly affect the sensitivity. Here, via in-situ annealing of a porous metal organic framework (MOF), ZIF-8, using a miniature heater electrode with a fast ramp rate (ca. 60 °C/s), we demonstrate the formation of a well-connected nanoparticle network with high porosity. Nanoparticle networks prepared in-situ exhibit significantly enhanced response to ethanol, defined as the ratio of sensor’s resistance before and after gas exposure, compared to ex-situ annealed counterparts (>10 times larger response) and to commercially available nanoparticles (∼4 times larger response) at a sensing temperature of 250 °C. The mechanism of the enhanced performance is studied using AC impedance spectroscopy. The results indicate that the large number of highly accessible and effective adsorption sites on the in-situ annealed material are responsible for the enhancement.

Carbon dioxide (CO2) has been shown to contribute to human health consequences indoors, such as shortness of breath, nasal and optic irritation, dizziness, and nausea. In this work, we explore the potential of metal–organic frameworks (MOFs) as highly-porous, crystalline sorbents for sensitive colorimetric CO2 detection. In particular, the zeolitic imidazolate framework (ZIF-8) is chosen as the sorptive material due to its chemical stability and tunable CO2 affinity. The colorimetric gas sensor is developed in methanol with three components: (i) MOF ZIF-8 as a high surface area adsorbent; (ii) ethylenediamine (ED) as the CO2-affinitive basic function; and (iii) phenolsulfonpthalein (PSP) as the pH indicator. Colorimetric assays and ratiometric analysis confirm a colorimetric response to variable CO2 concentrations of relevance to indoor air quality. The color response is attributed to a zwitterion mechanism whereby ED reacts with CO2 to form a zwitterionic intermediate. This intermediate is then deprotonated by the pH indicator, shifting the pH and inducing a color change. Given its simple fabrication, rapid and obvious response, and stability in ambient environment, the ZIF-8-based colorimetric sensor provides a promising route for an improved indoor air quality monitoring.

In this study, we report on the synthesis of the NiO/ZnO heterostructured nanowires by a facile two-step liquid phase route and their gas sensing characteristics employing Au interdigitated electrodes integrated on a miniature ceramic heater. Microstructural characterizations indicate that flocculent NiO particles are uniformly assembled on the outer surfaces of the single-crystalline ZnO nanowires, with diameters around 50 nm and lengths ranging from 500 nm to several μm. The gas sensing investigation indicates that the sensors based on NiO/ZnO heterostructured nanowires exhibit high sensitivity towards ethanol, good reversibility, reproducibility, stability, robustness towards humidity, and fast response/recovery rates at the determined optimum operating temperature of 300 °C. Interestingly, the sensor shows higher ethanol response but longer recovery time in N2 compared with those in air. An ethanol sensing mechanism is proposed to explain the experimental results.

Pseudo-topotactic conversion of carbon nanotubes into one-dimensional carbon nanowires is a challenging but feasible path to obtain desired diameters and morphologies. Here, a previously predicted but experimentally unobserved carbon allotrope, T-carbon, has been produced from pseudo-topotactic conversion of a multi-walled carbon nanotube suspension in methanol by picosecond pulsed-laser irradiation. The as-grown T-carbon nanowires have the same diameter distribution as pristine carbon nanotubes, and have been characterized by high-resolution transmission electron microscopy, fast Fourier transform, electron energy loss, ultraviolet–visible, and photoluminescence spectroscopies to possess a diamond-like lattice, where each carbon is replaced by a carbon tetrahedron, and a lattice constant of 7.80 Å. The change in entropy from carbon nanotubes to T-carbon reveals the phase transformation to be first order in nature. The computed electronic band structures and projected density of states are in good agreement with the optical absorption and photoluminescence spectra of the T-carbon nanowires., T-carbon is a previously predicted but so far unobserved allotrope of carbon, with a crystal structure similar to diamond, but with each atomic lattice position replaced by a carbon tetrahedron. Here, the authors produce T-carbon nanowires via laser-irradiating a suspension of carbon nanotubes in methanol.

We report on the formation of rutile TiO2 flocculent laser-induced periodic surface structures (LIPSSs) with high antireflectivity and superhydrophobicity on the surface of titanium under 10 ns 1064 nm laser irradiation without focusing. The center part of the Gaussian laser beam is used to deposit flocculent structure and the edge part used to produce LIPSSs. The melt and modification thresholds of titanium were determined first, and then, the melt and modification spot-overlap numbers, several responsible for the formation of flocculent structure and LIPSSs, were introduced. It is found that both the melt and modification spot-overlap numbers increase with an increase in laser fluence and spot-overlap number, contributing to the production of flocculent LIPSSs. LIPSSs are obtained with the modification spot-overlap number above 300, and the amount of flocculent structures increases with an increase in the peak laser fluence and spot-overlap number. Then, considering that the fine adjustment of the melt ...

A theoretical study, based on the split as well as experiments, was conducted to investigate the formation of high-spatial-frequency laser-induced periodic surface structures (HSFLs) on rough indium-tin-oxide (ITO) films under 10-ps 532-nm-wavelength laser irradiation. At a peak laser fluence of 0.472 J/cm2, the theoretical periods of HSFLs (130–190 nm) matched the experimental values (128–200 nm). Both the theoretical and experimental results demonstrated that the transformation mechanism of laser-induced periodic surface structures (LIPSSs) from low-spatial-frequency LIPSSs (LSFLs) to HSFLs was attributed to split and the irregular period difference of HSFLs and LSFLs was attributed to the surface roughness. Deeper ablation occurred for LIPSSs with a larger period, and the difference at the ablated depth increased with increasing spot number. Therefore, the LIPSSs with the larger period were clearer demarcated and the initial pits in the convex portion of LIPSSs disappeared due to the laser-induced melting. Consequently, sub-100-nm-perioded HSFLs were invisible in spite of the theoretical minimum period of ~88.5 nm. Then, for pits of different depths, the difference of the ablated depth induced by a subsequent pulse can be narrowed by reducing the laser fluence. On this method, 83-nm-perioded HSFLs were obtained by reducing the peak laser fluence to 0.432 J/cm2. Keywords: Ripples, Split, Indium-tin-oxide film, Picosecond laser, Surface roughness, Drude-Lorentz model

Femtosecond laser processing has been extensively used in micromachining, especially for the precision processing of hard and brittle materials. However, the precision of the materials ablated by femtosecond laser is not easy to control. This paper reports an experimental and theoretical study on the ablation characteristics of fused silica using high repetition rate femtosecond laser. An experimental study of microchannels milling on the fused silica was carried out. The influence of pulse energy, repetition rate, scanning velocity, scanning times on the size and morphology of the microchannels was obtained. Simultaneously, the experimental data on the depth and width of microchannels under different parameter combinations were acquired through the orthogonal experiment. The prediction model of aspect ratio was obtained by BP neural network algorithm. Finally, the verification test was established and showed that the experimental results were consistent with the theoretical results. It would provide a theoretical basis for further study on the microchannels fabrication of femtosecond laser.

This study reported on the formation of sub-5-μm microstructures covered on titanium by cracks growth under 10-ns laser radiation at the wavelength of 532 nm and its induced light modification for production of nanostructures. The electric field intensity and laser power density absorbed by commercial pure titanium were computed to investigate the self-trapping introduced by cracks and the effect of surface morphology on laser propagation characteristics. It is found that nanostructures can form at the surface with the curvature radius below 20 μm. Meanwhile, variable laser fluences were applied to explore the evolution of cracks on commercial pure titanium with or without melt as spot overlap number increased. Experimental study was first performed at the peak laser fluence of 1.063 J/cm 2 to investigate the microstructures induced only by cracks growth. The results demonstrated that angular microstructures with size between 1.68 μm and 4.74 μm was obtained and no nanostructure covered. Then, at the peak laser fluence of 2.126 J/cm 2 , there were some nanostructures covered on the melt-induced curved microstructured surface. However, surface molten material submerged in the most of cracks at the spot overlap number of 744, where the old cracks disappeared. The results indicated that there was too much molten material and melting time at the peak laser fluence of 2.126 J/cm 2 , which was not suitable for obtainment of perfect micro-nano structures. On this basis, peak laser fluence was reduced down to 1.595 J/cm 2 and the sharp sub–5 μm microstructures with nanostructures covered was obtained at spot overlap number of 3720.

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10 ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000 K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800 K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10 ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

This study reported on the fabrication of nanoripples via femtosecond laser-induced periodic surface oxidation on titanium film, which can work both for the structural colors in reflection mode and transmission mode. The periods and heights of nanoripples were obtained under different pulse energies and scanning speeds. To obtain the structural colors in the transmission mode, the height of the nanoripples was above 175 nm. The experimental results showed that the spectral changes in the reflection mode and transmission mode were consistent, which conformed to the principle of grating diffraction. Moreover, it could be seen that the colors were very sensitive to the angles. The ripples orientation affected the intensity of the color, while the viewing angle and the incident angle affected the type of the color. According to the spectral characteristics of structural colors, the display and encryption of information were realized. On this basis, we used the method of pattern segmentation to achieve the encryption and decryption of information on the condition that the overall pattern was not missing. This study provided a new idea to realize the color display, information security, and anti-counterfeiting on both sides of the sample.

The fabrication of uniform and intact nanostructures on semiconductor thin films is an essential process to alleviate the many electronic defects that are usually present. In this study, nanostructures with good photoelectric properties were fabricated on ITO films by direct laser irradiation and secondary sputtering. A Nd:YLF femtosecond laser system, with a wavelength of 800 nm and a repetition rate of 1 kHz based on pulse regenerative amplification, was used. Laser-induced periodic surface structures with different fluences and scan speeds were studied. With a fluence of 1.86 J cm−2, a scan speed of 1.1 mm s−1, a scan spacing of 4.5 μm, and a focused laser spot of 18.5 μm, a large area of uniform nanostructures with little damage are obtained under optimized parameters. The transmittance increases by up to 21%, and the sheet resistance increases more than 10 times. A definite thickness of the ITO thin film was sputtered by secondary sputtering to improve the electrical conductivity, and the heights of nanostructures can be exactly controlled. Compared with the untreated surface of ITO films, the transmittance increases by 19% and the sheet resistance decreases to less than 1.6 times with the optimal nanostructure height of 90 nm. This method to fabricate nanostructures can greatly improve the photoelectric properties without any complex processing and expensive cost, which can be greatly used in a wide area of the industry application.

In this study, a method for dot-matrix marking with femtosecond laser on nickel-based alloy using diaphragm modulation was proposed. The simulation results showed that the edge quality and recognition uniformity of the crater were improved because of the diffraction effect of the diaphragm. Moreover, the confinement of the diaphragm could ensure the position accuracy. First, the reasonable size of diaphragm turned out to be from 8 to 10 mm. With the addition of the diaphragms, the regular variation of the diameters of the craters under different parameters was beneficial to control the size of the marking units. Thanks to the edge diffraction effect, the melt phenomenon on the edge of the craters almost disappeared, so the boundary of the crater was clear. In addition, the roundness of the crater increased by a maximum of 6%. Because the difference of gray value of different areas within the crater decreased by 60%, the recognition uniformity and the recognition rate of the marking were improved. Moreover, it was found that the contaminated area did not affect the identification of the marking. Therefore, with spatial shaping of laser beam with diaphragm, the femtosecond laser marking technology can have the advantage of high position precision, dimensional accuracy, processing quality and recognition rate.

To meet the ultra-broadband perfect absorption of visible-infrared light on silicon surfaces, a green, efficient and economical method for fabricating multi-scale micro-nano composite structures in ambient air is proposed. We experimentally demonstrate laser cleaning assisted femtosecond laser ablation for fabricating anti-reflection structures. Laser cleaning technology not only effectively eliminates oxide deposition on the laser textured surfaces, but also manufactures the small scale fine-microstructures and nanostructures. A focused ellipse laser spot is innovatively applied to realize large area and energy decays continuously multiple laser cleaning of laser-treated surfaces, and solve the problem that new oxide deposition is generated in the cleaning process. The processing efficiency is also increased by 4.8 times. The average reflectance of 2.06% is reached from 300 to 2500 nm. Great enhancement of infrared light absorption of silicon from 2.5 to 16 μm is realized experimentally. The average reflectance is reduced to 4.98% with a broadband reflectance below 6.6%. Especially, a reflectance below 5.0% from 2.5 to 10 μm and an average reflectance of 4.3% is achieved, which is the least reported to date by laser processing techniques as far as we know. This strategy for anti-reflection structures is excellent candidate for future optoelectronic devices.

Microlens arrays were fabricated on polymethyl methacrylate (PMMA) substrate using a microlens array (MLA) with rear-side ps laser swelling in this study. The modified beam energy can ensure the fabricated microlenses are uniform. Using the foci of a microlens array (MLA), approximately 630 microlenses were fabricated with a single series of pulses, which provides greater processing throughput compared to point-to-point direct laser writing. The formation of smooth microlenses is attributed to photolysis of methyl red (MR) and molecular relaxation. The dependence of height and diameter of the microlenses on laser fluence and irradiation time were investigated. The results show that the size of microlenses could be controlled using the appropriate fluence and irradiation time. Finally, an array of mirolenses with high uniformity was fabricated over a large area. The diameter and height of the microlenses were measured to be 55 and 12 µm, respectively.

Micro-structures offer superior functions such as superhydrophobicity, selfcleaning, anti-wear and drag reduction. In this paper, various microstructures were fabricated by rear-side picosecond laser irradiation of two-layer materials. The material of underlying layer was commonly commercial available ink; the material of surface layer was PMMA. The high light absorptivity of underlying material result in significantly reduced absorption depth. The laser source could therefore be regarded as plane heat source, leading to better surface morphologies after the mater-laser interaction. The results showed that convex structures were obtained at a lower laser fluence; with increase of laser fluence, a doughnut-like structures were obtained; with further rise of laser fluence, bowl-like structures would be obtained. Moreover, the size of microstructures could be tuned by adjusting laser processing parameters such as laser power, frequency and laser-mater interaction time. This method provides an insight for fabrication of functional surface.

This study was performed using picosecond pulses to obtain the three-dimensional micro-nano-hierarchical porous structures on the surface of titanium via the combination of the ablation and the deposition of ablated particles. For the repetition rate of 100 kHz and the scanning speed of 10 mm/s, there were secondary nano-tree-like micro-nano structures via the deposition of the ablated material formed on the primary microstructures. However, when the scan speed decreased, the primary microstructures were invisible owing to too much material deposition. When the repetition rate increased to 500 kHz, the ablated particles were irradiated by the posterior pulse before deposition onto the surface of material, and agglomerated into spidernet-like nanostructures. Then, the upper layer of the secondary micro-nano structure was molten and came into micro-nano porous fractal structures. Both the secondary micro-nano porous structures showed the hierarchy in the vertical and horizontal direction of the surface of titanium. Contact angle measurement after 3 months indicated that the nano-tree-like micro-nano structures showed super-hydrophilcity, and the spidernet-like nano and micro-nano porous fractal structures showed super-hydrophobicity.

With greater optical penetration depth and lower ablation threshold fluence, it is difficult to directly fabricate large scales of laser-induced periodic surface structures (LIPSSs) on indium–tin–oxide (ITO) films. This study proposed an approach to obtain optimized LIPSSs by sputtering an Au thin film on the ITO film surface. The concept behind the proposal is that the upper layer of the thin Au film can cause surface energy aggregation, inducing the initial ripple structures. The ripples deepened and become clear with lower energy due to optical trapping. The effective mechanism of Au film was analyzed and verified by a series of experiments. Linear sweep, parallel to the laser polarization direction, was performed using a Nd:VAN laser system with 10-ps Q-switched pulse, at a central wavelength of 532 nm, with a repetition rate of 1 kHz. The complete and clear features of the nanostructures, obtained with the periods of approximately 320 nm, were observed on ITO films with proper laser fluence and scanning speed. The depth of ripples was varying in the range of 15–65 nm with clear and coherent ITO films. The preferred efficiency of fabricating nanostructures and the excellent results were obtained at a scanning speed of 2.5 mm/s and a fluence of 0.189 J/cm2. In this way, the ablation and shedding of ITO films was successfully avoided. Thus, the proposed technique can be considered to be a promising method for the laser machining of special nonmetal films.

We report on the formation of rutile TiO

This paper presents the preparation of different kinds of titanium oxide fractal structures on the surface of titanium by inverse pulsed laser deposition (IPLD) in air and water. In air, two-dimensional fractal structures are obtained with a low pulse energy. However, their branches units are aggregated and nanoscale branches disappear due to the high substrate temperature, causing the low fractal dimension of structure. When a higher laser energy is applied, the preformed deposited material forms a porous film, which reduces heat transfer from substrate. Therefore, three-dimensional and one-dimensional fractal structures with nanoscale branches on the topside of the film can be obtained. Then the desired two-dimensional fractal structures with nano-branches are obtained in water due to the water-induced rapid cooling of substrate temperature and plasma shock wave-induced particle’s expansion along the surface of substrate. Meanwhile, the asymmetry of fractal structure units analyzed by diffusion limited aggregation (DLA) model is caused by the difference of the distance between the initial deposited particles. In addition, when the pulse energy goes up to 111 mJ, the branches of two-dimensional fractal structure units are also aggregated and form isolated particles. The idea about modification of substrate temperature and water can guide the preparation of the desired titanium oxide fractal structures in pulsed laser deposition (PLD), which is also applicable to other materials.

Nanosecond laser ablation of metal is a complicated process, which consists of many strongly coupled physical phenomena, including material heating, melting, evaporation, vapour dynamics, and plasma shield. In this work, the nanosecond laser ablation process of titanium is investigated at 1064 nm wavelength. A multi-physics axisymmetric two-dimensional (2D) model is presented. The evolution and the distribution of titanium target’s temperature were solved using governing equations and the vapour dynamics was determined using the Knudsen relations. The maximum temperature of titanium grown slower with the increase in laser fluence and the maximum flow velocity of liquid materials reached 121 m/s with the laser fluence of 12 J/cm2. In addition, the plasma shield effect was taken into account to correct the energy distribution of the incident laser. As the laser fluence increases, the energy efficiency decreases. At the laser fluence of 12 J/cm only 55.9% of the energy was absorbed at the centre of titanium. Furthermore, the surface morphology profiles were analysed after the laser ablation on different laser fluences lying within the range of 2 - 12 J/cm2. The results showed that the surface morphology after ablation has a crater-like form and the increment of laser fluence leads to a slower non-linear increment in ablation depth and diameter of melt zone. The calculated results are in good agreement with the experimental results. The study provides useful information for nanosecond laser precision fabrication.

Silicon carbide (SiC) ceramics have been widely used for microelectronics, aerospace, and other industrial fields due to their excellent chemical stability and thermal tolerance. However, hard machinability and low machining precision of SiC ceramics are the key limitations for their further applications. To address this issue, a novel method of underwater femtosecond laser machining was introduced in this study to obtain high precision and smooth surface of the microgrooves of SiC ceramics. The removal profiles were characterized in terms of width, depth, and surface morphology, which exhibited high dependence on the femtosecond laser processing parameters. The instability during the underwater processing affected by laser-induced gas bubbles and material deposition, however, limits the high surface accuracy of microgrooves and processing efficiency. The process condition transformation from a bubble-disturbed circumstance to a disturbance-free model was carefully investigated through a high speed camera for the femtosecond laser processing of SiC ceramics in water. The experiment results indicated that degree of disturbed effect was heavily dependent on size, distribution, and motion of laser-induced gas bubble. Furthermore, some typical evolution mechanisms of gas bubble and their influence on the removal profiles of microgrooves were discussed in detail. Bubble evolution has been proven to be mainly responsible for the behavior of laser propagation (focus model, total reflection, etc.), which notably affects microstructural characteristic of the microgrooves.