Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces

Femtosecond laser-induced periodic surface structures(LIPSS)have been extensively studied over the past few decades.In particular,the period and groove width of high-spatial-frequency LIPSS(HSFL)is much smaller than the diffraction limit,making it a useful method for efficient nanomanufacturing.However,compared with the low-spatial-frequency LIPSS(LSFL),the structure size of the HSFL is smaller,and it is more easily submerged.Therefore,the formation mechanism of HSFL is complex and has always been a research hotspot in this field.In this study,regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference using an 800 nm,50 fs femtosecond laser.The ultrafast dynamics of HSFL formation on the silicon surface of prefabricated LSFL under single femtosecond laser pulse irradiation were observed and analyzed for the first time using collinear pump-probe imaging method.In general,the evolution of the surface structure undergoes five sequential stages:the LSFL begins to split,becomes uniform HSFL,degenerates into an irregular LSFL,undergoes secondary splitting into a weakly uniform HSFL,and evolves into an irregular LSFL or is submerged.The results indicate that the local enhancement of the submerged nanocavity,or the nanoplasma,in the prefabricated LSFL ridge led to the splitting of the LSFL,and the thermodynamic effect drove the homogenization of the splitting LSFL,which evolved into HSFL.

Laser-induced breakdown spectroscopy as a method for millimeter-scale inspection of surface flatness

A non-contact method for millimeter-scale inspection of material surface flatness via Laser-Induced Breakdown Spectroscopy(LIBS)is investigated experimentally.The experiment is performed using a planished surface of an alloy steel sample to simulate its various flatness,ranging from 0 to 4.4 mm,by adjusting the laser focal plane to the surface distance with a step length of 0.2 mm.It is found that LIBS measurements are successful in inspecting the flatness differences among these simulated cases,implying that the method investigated here is feasible.It is also found that,for achieving the inspection of surface flatness within such a wide range,when univariate analysis is applied,a piecewise calibration model must be constructed.This is due to the complex dependence of plasma formation conditions on the surface flatness,which inevitably complicates the inspection procedure.To solve the problem,a multivariate analysis with the help of Back-Propagation Neural Network(BPNN)algorithms is applied to further construct the calibration model.By detailed analysis of the model performance,we demonstrate that a unified calibration model can be well established based on BPNN algorithms for unambiguous millimeter-scale range inspection of surface flatness with a resolution of about 0.2 mm.

Two-dimensional laser-induced periodic surface structures formed on crystalline silicon by GHz burst mode femtosecond laser pulses

Femtosecond laser pulses with GHz burst mode that consist of a series of trains of ultrashort laser pulses with a pulse interval of several hundred picoseconds offer distinct features in material processing that cannot be obtained by the conventional irradiation scheme of femtosecond laser pulses(single-pulse mode).However,most studies using the GHz burst mode femtosecond laser pulses focus on ablation of materials to achieve high-efficiency and high-quality material removal.In this study,we explore the ability of the GHz burst mode femtosecond laser processing to form laser-induced periodic surface structures(LIPSS)on silicon.It is well known that the direction of LIPSS formed by the single-pulse mode with linearly polarized laser pulses is typically perpendicular to the laser polarization direction.In contrast,we find that the GHz burst mode femtosecond laser(wavelength:1030 nm,intra-pulse duration:220 fs,intra-pulse interval time(intra-pulse repetition rate):205 ps(4.88 GHz),burst pulse repetition rate:200 kHz)creates unique two-dimensional(2D)LIPSS.We regard the formation mechanism of 2D LIPSS as the synergetic contribution of the electromagnetic mechanism and the hydrodynamic mechanism.Specifically,generation of hot spots with highly enhanced electric fields by the localized surface plasmon resonance of subsequent pulses in the bursts within the nanogrooves of one-dimensional LIPSS formed by the preceding pulses creates 2D LIPSS.Additionally,hydrodynamic instability including convection flow determines the final structure of 2D LIPSS.

Characteristics of Laser-Induced Surface and Bulk Damage of Large-Aperture Deuterated Potassium Dihydrogen Phosphate at 351 nm

Deuterated potassium dihydrogen phosphate damage performance at 351 nm is studied on a large-aperture laser system. Bulk and rear-surface damage are initiated under the 3ω fluences of 6.T J/cm2 and 33/cm2, and show different growth characteristics under multiple laser irradiations with the fluence of 6 J/cm2. The size and number of bulk damage keep unchanged once initiated. However, surface damage size also does not grow, while surface damage number increases linearly with laser shots. Different damage thresholds and growth behaviors suggest different formations of bulk and surface damage precursors. The cause of surface damage is supposed to be near-surface absorbing particles buried under the sol-gel coating.

Conditions for laser-induced plasma to effectively remove nano-particles on silicon surfaces

Particles can be removed from a silicon surface by means of irradiation and a laser plasma shock wave.The particles and silicon are heated by the irradiation and they will expand differently due to their different expansion coefficients,making the particles easier to be removed.Laser plasma can ionize and even vaporize particles more significantly than an incident laser and,therefore,it can remove the particles more efficiently.The laser plasma shock wave plays a dominant role in removing particles,which is attributed to its strong burst force.The pressure of the laser plasma shock wave is determined by the laser pulse energy and the gap between the focus of laser and substrate surface.In order to obtain the working conditions for particle removal,the removal mechanism,as well as the temporal and spatial characteristics of velocity,propagation distance and pressure of shock wave have been researched.On the basis of our results,the conditions for nano-particle removal are achieved.

Investigation of the factors affecting the limit of detection of laser-induced breakdown spectroscopy for surface inspection

Laser-induced breakdown spectroscopy(LIBS) was examined to detect a trace substance adhered onto Al alloys for the surface inspection of materials to be adhesively bonded. As an example of Si contamination, silicone oil was employed and sprayed onto substrates with a controlled surface concentration. LIBS measurements employing nanosecond UV pulses(λ?=?266 nm) and an off-axis emission collection system with different detecting heights were performed. Because surface contaminants are involved in the plasma formed by laser ablation of the substrates, the relative contribution of the surface contaminants and the substrates to the plasma emission could be changed depending on the conditions for plasma formation. The limit of detection(LOD) was evaluated under several detecting conditions for investigating the factors that affected the LOD. A significant factor was the standard deviation values of signal intensities obtained for the clean substrates. This value varied depending on the measurement conditions.For the Al alloy(A6061), the smallest LOD obtained was 0.529 μg?·?cm^(-2). Furthermore, an improved LOD(0.299 μg?·?cm^(-2)) was obtained for the Al alloy with a lower Si content.

Laser-induced periodic surface structured electrodes with 45% energy saving in electrochemical fuel generation through field localization

Electrochemical oxidation/reduction of radicals is a green and environmentally friendly approach to generating fuels.These reactions,however,suffer from sluggish kinetics due to a low local concentration of radicals around the electrocatalyst.A large applied electrode potential can enhance the fuel generation efficiency via enhancing the radical concentration around the electrocatalyst sites,but this comes at the cost of electricity.Here,we report about a~45%saving in energy to achieve an electrochemical hydrogen generation rate of 3×10^(16) molecules cm^(–2)s^(–1)(current density:10 mA/cm^(2))through localized electric field-induced enhancement in the reagent concentration(LEFIRC)at laser-induced periodic surface structured(LIPSS)electrodes.The finite element model is used to simulate the spatial distribution of the electric field to understand the effects of LIPSS geometric parameters in field localization.When the LIPSS patterned electrodes are used as substrates to support Pt/C and RuO_(2) electrocatalysts,the η_(10) overpotentials for HER and OER are decreased by 40.4 and 25%,respectively.Moreover,the capability of the LIPSS-patterned electrodes to operate at significantly reduced energy is also demonstrated in a range of electrolytes,including alkaline,acidic,neutral,and seawater.Importantly,when two LIPSS patterned electrodes were assembled as the anode and cathode into a cell,it requires 330 mVs of lower electric potential with enhanced stability over a similar cell made of pristine electrodes to drive a current density of 10 mA/cm^(2).This work demonstrates a physical and versatile approach of electrode surface patterning to boost electrocatalytic fuel generation performance and can be applied to any metal and semiconductor catalysts for a range of electrochemical reactions.

The Formation of Colored Film on Stainless Steel and the Study for Surface Structure

This paper discusses the coloration process on the stainless steel and the properties of the film. The compositions, morphology and structure of colored films on stainless steel are studied by using SEM,AES,AFM,STM. The diffusion controlled mechanisms of films and calculation formula of surface electropotential difference are discussed.

Seasonal influence of freshwater discharge on spatio-temporal variations in primary productivity, sea surface temperature, and euphotic zone depth in the northern Bay of Bengal

Ocean productivity is the foundation of marine food web,which continuously removes atmospheric carbon dioxide and supports life at sea and on land.Spatio-temporal variability of net primary productivity(NPP),sea surface temperature(SST),sea surface salinity(SSS),mixed layer depth(MLD),and euphotic zone depth(EZD) in the northern B ay of Bengal(BoB) during three monsoon seasons were examined in this study based on remote sensing data for the period 2005 to 2020.To compare the NPP distribution between the coastal zones and open BoB,the study area was divided into five zones(Z1-Z5).Results suggest that most productive zones Z2 and Zl are located at the head bay area and are directly influenced by freshwater discharge together with riverine sediment and nutrient loads.Across Z1-Z5,the NPP ranges from 5 315.38 mg/(m^(2)·d) to 346.7 mg/(m^(2)·d)(carbon,since then the same).The highest monthly average NPP of 5 315.38 mg/(m^(2)·d) in February and 5 039.36 mg/(m^(2)·d) in June were observed from Z2,while the lowest monthly average of 346.72 mg/(m^(2)·d) was observed in March from Z4,which is an oceanic zone.EZD values vary from 6-154 m for the study area,and it has an inverse correlation with NPP concentration.EZD is deeper during the summer season and shallower during the wintertime,with a corresponding increase in productivity.Throughout the year,monthly SST shows slight fluctuation for the entire study area,and statistical analysis shows a significant correlation among NPP,and EZD,overall positive between NPP and MLD,whereas no significant correlation among SSS,and SST for the northern BoB.Long-term trends in SST and productivity were significantly po sitive in head bay zones but negatively productive in the open ocean.The findings in this study on the distribution of NPP,SST,SSS,MLD,and EZD and their seasonal variability in five different zones of BoB can be used to further improve the management of marine resources and overall environmental condition in response to climate changes in BoB as they are of utmost relevance to the fisheries for the three bordering countries.

Surface photometry and radial color gradients of nearby luminous early-type galaxies in SDSS Stripe 82

We make use of the images from the Sloan Digital Sky Survey Stripe 82（Stripe 82） to present an analysis of r band surface brightness profiles and radial color gradients（g-r,u-r） in our sample of 111 nearby early-type galaxies（ETGs） .Thanks to the Stripe 82 images,each of which is co-added from about 50 single frames,we are able to pay special attention to the low-surface-brightness areas（LSB areas） of the galaxies.The LSB areas affect the Sérsic fittings and concentration indices by making both of the indices less than the typical values for ETGs.In the Sérsic fits to all the surface brightness profiles,we found some Sérsic indices that range from 1.5 to 2.5,much smaller than those of typical de Vaucouleur profiles and relatively close to those of exponential disks,and some others much larger than four but still with accurate fitting.Two galaxies cannot be fitted with a single Sérsic profile,but once we try double Sérsic profiles,the fittings are improved：one with a profile relatively close to the de Vaucouleur law in the inner area and a profile relatively close to an exponential law in the LSB area,the other with a nice fitting in the inner area but still having a failed fitting in the outer area.About 60%of the sample has negative color gradients（red-core） within 1.5Re,much more than the approximately 10%positive ones（blue-core） within the same radius.However,taking into account the LSB areas,we find that the color gradients are not necessarily monotonic：about one third of the red-core（or blue-core） galaxies have positive（or negative） color gradients in the outer areas. So LSB areas not only make ETGs’Sérsic profiles deviate from de Vaucouleur ones and shift to the disk end,but also reveal that quite a number of ETGs have opposite color gradients in inner and outer areas.These outcomes remind us of the necessity of double-Sérsic fitting.These LSB phenomena may be interpreted by mergers and thus have different metallicity in the outer areas.Isophotal parameters are also discussed briefly in this paper with the following conclusion：there are more disky nearby ETGs that are identified than boxy ones.

THE EFFECT OF WOOD SURFACE ROUGHNESS OF DIFFERENT SPECIES OF TREES ON WOOD COLOR IN WORKING OPERATION

The multinomial wood surface roughness parameters and wood color parameters of 5 kinds of species of trees were measured in working operation by the stylus surface roughness meter and colorimeter, and the measurement results were compared. The results indicated: The variations of wood surface roughness had the effect on the wood color of some kinds of species of trees on working operation, with the surface roughness decreasing, the brightness and luminance exponent increased, the hue angle increased lightly, but the variation of color saturation was different to the different wood color hue of species of trees.

Design and fabrication of structural color by local surface plasmonic meta-molecules

In this paper, we propose a new form of nanostructures with Al film deposited on a patterned dielectric material for generating structural color, which is induced by local surface plasmonic resonant(LSPR) absorption in sub-wavelengthindented hole/ring arrays. Unlike other reported results obtained by using focus ion beam(FIB) to create metallic nanostructures, the nano-sized hole/ring arrays in Al film in this work are replicated by high resolution electron beam lithography(EBL) combined with self-aligned metallization. Clear structural color is observed and systematically studied by numerical simulations as well as optical characterizations. The central color is strongly related to the geometric size, which provides us with good opportunities to dye the colorless Al surface by controlling the hole/ring dimensions(both diameter and radius), and to open up broad applications in display, jewelry decoration, green production of packing papers, security code,and counterfeits prevention.

The Effects of Porcelain Polishing Techniques on the Color and Surface Texture of Different Porcelain Systems

The aim of this study was to compare the effects of different porcelain polishing techniques on the color change and surface roughness of feldspathic porcelains and lithium disilicate glass-ceramic system. 80 porcelain specimens were divided into two subgroups, each subgroup was divided into 4 groups of 10 specimens. Surfaces were polished with different ceramic polishers. VITA Easyshade spectrophotometer was used to measure color differences. Surface roughness was evaluated using a profilometer. The color change and surface roughness of porcelains were statistically analyzed by Two-way ANOVA followed by a Tukey HSD test (α = 05). There were relationships between the porcelain systems and polishing techniques (p 0.05). For △E values, there was relationship between the porcelain systems and polishing techniques (p 0.05). Chairside porcelain polishing systems were not able to provide a porcelain surface as smooth as the glazed surface for the IPS empress and Ceramco 3 porcelains.

Large-area straight,regular periodic surface structures produced on fused silica by the interference of two femtosecond laser beams through cylindrical lens

Inhomogeneity and low efficiency are two important factors that limit the application of laser-induced periodic surface structures(LIPSSs),especially on glass surfaces.In this study,two-beam interference(TBI)of femtosecond lasers was used to produce large-area straight LIPSSs on fused silica using cylindrical lenses.Compared with those produced us-ing a single circular or cylindrical lens,the LIPSSs produced by TBI are much straighter and more regular.Depending on the laser fluence and scanning velocity,LIPSSs with grating-like or spaced LIPSSs are produced on the fused silica sur-face.Their structural colors are blue,green,and red,and only green and red,respectively.Grating-like LIPSS patterns oriented in different directions are obtained and exhibit bright and vivid colors,indicating potential applications in surface coloring and anti-counterfeiting logos.

Laser-induced jigsaw-like graphene structure inspired by Oxalis corniculata Linn.leaf

The laser scribing of polyimide(PI, Kapton) film is a new, simple and effective method for graphene preparation. Moreover,the superhydrophobic surface modification can undoubtedly widen the application fields of graphene. Herein, inspired by the hydrophobic and self-cleaning properties of natural Oxalis corniculata Linn. leaves, we propose a novel bionic manufacturing method for superhydrophobic laser-induced graphene(LIG). By tailoring the geometric parameters(size, roughness and height/area ratio) and chemical composition, the three-dimensional(3D) multistage LIG, i.e., with micro-jigsaw-like and porous structure, can deliver a static water contact angle(WCA) of 153.5° ± 0.6°, a water sliding angle(WSA) of 2.5° ±0.5°, and great superhydrophobic stability lasting for 100 days(WCAs ≈ 150°). This outstanding water repellency is achieved by the secondary structure of jigsaw-like LIG, a porous morphology that traps air layers at the solid–liquid interface. The robust self-cleaning and anti-stick functions of 3D bionic and multistage LIG are demonstrated to confirm its great potential in wearable electronics.

Surface modifications of biometallic commercially pure Ti and Ti–13Nb–13Zr alloy by picosecond Nd:YAG laser

The effects of picosecond Nd:YAG laser irradiation on chemical and morphological surface characteristics of the commercially pure titanium and Ti–13Nb–13Zr alloy in air and argon atmospheres were studied under different laser output energy values.During the interaction of laser irradiation with the investigated materials,a part of the energy was absorbed on the target surface,influencing surface modifications.Laser beam interaction with the target surface resulted in various morphological alterations,resulting in crater formation and the presence of microcracks and hydrodynamic structures.Moreover,different chemical changes were induced on the target materials’surfaces,resulting in the titanium oxide formation in the irradiation-affected area and consequently increasing the irradiation energy absorption.Given the high energy absorption at the site of interaction,the dimensions of the surface damaged area increased.Consequently,surface roughness increased.The appearance of surface oxides also led to the increased material hardness in the surface-modified area.Observed chemical and morphological changes were pronounced after laser irradiation of the Ti–13Nb–13Zr alloy surface.

Effect of lens focusing distance on laser-induced silicon plasmas at different sample temperatures

We investigated the dependence of laser-induced breakdown spectral intensity on the focusing position of a lens at different sample temperatures(room temperature to 300 ℃) in atmosphere.A Q-switched Nd:YAG nanosecond pulsed laser with 1064 nm wavelength and 10 ns pulse width was used to ablate silicon to produce plasma. It was confirmed that the increase in the sample's initial temperature could improve spectral line intensity. In addition, when the distance from the target surface to the focal point increased, the intensity firstly rose, and then dropped.The trend of change with distance was more obvious at higher sample temperatures. By observing the distribution of the normalized ratio of Si atomic spectral line intensity and Si ionic spectral line intensity as functions of distance and temperature, the maximum value of normalized ratio appeared at the longer distance as the initial temperature was higher, while the maximum ratio appeared at the shorter distance as the sample temperature was lower.

Damage characteristics of laser plasma shock wave on rear surface of fused silica glass

The damage to the rear surface of fused silica under the action of high power laser is more severe than that incurred by the front surface,which hinders the improvement in the energy of the high power laser device.For optical components,the ionization breakdown by laser is a main factor causing damage,particularly with laser plasma shock waves,which can cause large-scale fracture damage in fused silica.In this study,the damage morphology is experimentally investigated,and the characteristics of the damage point are obtained.In the theoretical study,the coupling and transmission of the shock wave in glass are investigated based on the finite element method.Thus,both the magnitude and the orientation of stress are obtained.The damage mechanism of the glass can be explained based on the fracture characteristics of glass under different stresses and also on the variation of the damage zone’s Raman spectrum.In addition,the influence of the glass thickness on the damage morphology is investigated.The results obtained in this study can be used as a reference in understanding the characteristics and mechanism of damage characteristics induced by laser plasma shock waves.

Femtosecond laser-induced periodic structures:mechanisms, techniques, and applications

Over the past two decades,femtosecond laser-induced periodic structures(femtosecond-LIPSs)have become ubiquitous in a variety of materials,including metals,semiconductors,dielectrics,and polymers.Femtosecond-LIPSs have become a useful laser processing method,with broad prospects in adjusting material properties such as structural color,data storage,light absorption,and luminescence.This review discusses the formation mechanism of LIPSs,specifically the LIPS formation processes based on the pump-probe imaging method.The pulse shaping of a femtosecond laser in terms of the time/frequency,polarization,and spatial distribution is an efficient method for fabricating high-quality LIPSs.Various LIPS applications are also briefly introduced.The last part of this paper discusses the LIPS formation mechanism,as well as the high-efficiency and high-quality processing of LIPSs using shaped ultrafast lasers and their applications.

Generating micro/nanostructures on magnesium alloy surface using ultraprecision diamond surface texturing process

The lightness and high strength-to-weight ratio of the magnesium alloy have attracted more interest in various applications.However,micro/nanostructure generation on their surfaces remains a challenge due to the flammability and ignition.Motivated by this,this study proposed a machining process,named the ultraprecision diamond surface texturing process,to machine the micro/nanostructures on magnesium alloy surfaces.Experimental results showed the various microstructures and sawtooth-shaped nanostructures were successfully generated on the AZ31B magnesium alloy surfaces,demonstrating the effectiveness of this proposed machining process.Furthermore,sawtooth-shaped nanostructures had the function of inducing the optical effect and generating different colors on workpiece surfaces.The colorful letter and colorful flower image were clearly viewed on magnesium alloy surfaces.The corresponding cutting force,chip morphology,and tool wear were systematically investigated to understand the machining mechanism of micro/nanostructures on magnesium alloy surfaces.The proposed machining process can further improve the performances of the magnesium alloy and extend its functions to other fields,such as optics.