Material processing of dielectrics with temporally asymmetric shaped femtosecond laser pulses on the nanometer scale

Laser material processing of dielectrics with temporally asymmetric femtosecond laser pulses of identical fluence, spectrum, and statistical pulse duration is investigated experimentally. To that end single shot structures at the surface of fused silica as a function of fluence and pulse shape are analyzed with the help of scanning electron microscopy. Structures for the bandwidth limited pulses show the known expansion in structure size with increasing laser fluence approaching the diffraction limit, which is 1.4 μm for the 0.5NA microscope objective used. In contrast, structures from the asymmetric pulses are remarkably stable with respect to variations in laser fluence and stay below 300 nm despite doubling the fluence. Different thresholds for surface material modification with respect to an asymmetric pulse and its time reversed counterpart are attributed to control of different ionization processes.     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

Excimer laser surface treatment of aluminum alloy in nitrogen

The excimer laser nitriding process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1-bar nitrogen gas. A vapor plasma expands from the surface and a shock wave dissociates and ionizes nitrogen. It is assumed that nitrogen from plasma in contact with the surface penetrates to some depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen-concentration profiles are determined from Rutherford backscattering spectroscopy and scanning electron microscopy coupled to energy-dispersive X-ray analysis. Crystalline quality is evidenced by an X-ray diffraction technique. Transmission electron microscopy gives the in-depth microstructure. Fretting coefficient measurements exhibit a lowering for some experimental conditions. The polycrystalline nitride layer obtained is several micrometers thick and composed of a pure AlN (columnar microstructure) top layer (200–500 nm thick) standing on an AlN (grains) in alloy diffusion layer. From the heat conduction equation calculation it is shown that a 308-nm laser wavelength would be better to increase the nitride thickness, as it corresponds to a weaker reflectance R value for aluminum. Received: 17 October 2000 / Accepted: 19 October 2000 / Published online: 23 May 2001     

Femtosecond study of surface structure and composition and time-resolved spectroscopy in metals

Surface structuring and compositioning in aluminum alloy 2024-T3 were demonstrated using a femtosecond pulse laser. Surface nanostructuring was developed as a function of laser parameters and the surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. Surface compositioning was performed by selectively removing the elements on the surface of the sample. Femtosecond studies of highly excited electrons were performed by a pump–probe technique, and the thermalization time was found to be in a range of 1.5–3 ps, increasing with incident fluence. The time-resolved measurement is well matched to the numerical calculation. Received: 6 September 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-405/744-6811, E-mail: dou@okstate.edu     

Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification

A superhydrophobic ZnO nanorod array film on cotton substrate was fabricated via a wet chemical route and subsequent modification with a layer of n-dodecyltrimethoxysilane (DTMS). The as-obtained cotton sample was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning probe microscope (SPM) and X-ray photoelectron spectroscopy (XPS), respectively. The wettability of the cotton fabric sample was also studied by contact angle measurements. The modified cotton fabrics exhibited superhydrophobicity with a contact angle of 161° for 8 μL water droplet and a roll-off angle of 9° for 40 μL water droplet. It was shown that the proper surface roughness and the lower surface energy both played important roles in creating the superhydrophobic surface, in which the Cassie state dominated.     

Theoretical studies of ultrafast ablation of metal targets dominated by phase explosion

Ultrashort pulse laser ablation of metallic targets is investigated theoretically through establishing a modified two-temperature model that takes into account both the temperature dependent electron–lattice coupling and the electron–electron-collision dominated electron diffusion processes for higher electron temperature regime. The electron–lattice energy coupling rate is found to reduce only slowly with increasing pulse duration, but grow rapidly with laser fluence, implying that the melting time of metallic materials decreases as the laser intensity increases. By taking phase explosion as the primary ablation mechanism, the predicted dependences of ablation rates on laser energy fluences for different laser pulse widths match very well with the experimental data. It is also found that during phase explosion the ablation rate is almost independent of the pulse width, whereas the ablation threshold fluence increases with the pulse duration even for femtosecond pulses. These theoretical results should be useful in having proper understanding of the ablation physics of ultrafast micromachining of metal targets. PACS 52.50.Jm; 61.80.Az; 72.15.Cz; 79.20.Ap; 79.20.Ds     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Dynamics evolution of shock waves in laser–material interaction

In this work, the dynamics and internal structure of shock waves in picosecond laser–material interaction are explored at the atomistic level. The pressure of the shock wave, its propagation, and interaction zone thickness between the plume and ambience are evaluated to study the effect of the laser absorption depth, ambient pressure, and laser fluence. Sound agreement is observed between the MD simulation and theoretical prediction of shock wave propagation and mass velocity. Due to the strong constraint from the compressed ambient gas, it is observed that the ablated plume could stop moving forward and mix with the ambient gas, or move backward to the target surface, leading to surface redeposition. Under smaller laser absorption depth, lower ambient pressure, or higher laser fluence, the shock wave will propagate faster and have a thicker interaction zone between the target and ambient gas.     

Femtosecond laser-induced surface wettability modification of polystyrene surface

In this paper, we demonstrated a simple method to create either a hydrophilic or hydrophobic surface. With femtosecond laser irradiation at different laser parameters, the water contact angle (WCA) on polystyrene’s surface can be modified to either 12.7° or 156.2° from its original WCA of 88.2°. With properly spaced micro-pits created, the surface became hydrophilic probably due to the spread of the water droplets into the micro-pits. While with properly spaced micro-grooves created, the surface became rough and more hydrophobic. We investigated the effect of laser parameters on WCAs and analyzed the laser-treated surface roughness, profiles and chemical bonds by surface profilometer, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). For the laser-treated surface with low roughness, the polar (such as C—O, C=O, and O—C=O bonds) and non-polar (such as C—C or C—H bonds) groups were found to be responsible for the wettability changes. While for a rough surface, the surface roughness or the surface topography structure played a more significant role in the changes of the surface WCA. The mechanisms involved in the laser surface wettability modification process were discussed.     

Electrode patterning on PEDOT:PSS thin films by pulsed ultraviolet laser for touch panel screens

Laser dry etching by a laser driven direct writing apparatus has been extensively used for the micro- and nano-patterning on the solid surface. The purpose of this study is to pattern the PEDOT:PSS thin film coated on the soda-lime glass substrates by a nano-second pulsed ultraviolet laser processing system. The patterned PEDOT:PSS film structure provides the electrical isolation and prevents the electrical contact from each region for capacitive touch screens. The surface morphology, geometric dimension, and edge quality of ablated area after the variety of laser patternings were measured by a 3D confocal laser scanning microscope. After the single pulse laser irradiation, the ablation threshold of the PEDOT:PSS film conducted by the nano-second pulsed UV laser was determined to be 0.135±0.003 J/cm². The single pulse laser interacted region and the ablated line depth increased with increasing the laser fluence. Moreover, the inner line width of ablated PEDOT:PSS films along the patterned line path increased with increasing the laser fluence but the shoulder width increased with decreasing fluence, respectively. The clean, smooth, and straight ablated edges were accomplished after the electrode patterning with the laser fluence of 1.7 J/cm² and 90 % overlapping rate.     

Removal of graffiti from the mortar by using Q-switched Nd:YAG laser

This paper presents part of the larger study on microstructural features of mortars and it's effects on laser cleaning process. It focuses on the influence of surface roughness, porosity and moisture content of mortars on the removal of graffiti by Nd:YAG laser. The properties of this laser are as follows: wavelength (λ) 1.06 μm, energy: 500 mJ per pulse, pulse duration: 10 ns. The investigation shows that the variation of laser fluence with the number of pulses required for the laser cleaning can be divided into two zones, namely effective zone and ineffective zone. There is a linear relationship observed between number of pulses required for laser cleaning and the laser fluence in the effective zone, while the number of pulses required for the laser cleaning is almost constant even though the laser fluence increases in the ineffective zone. Moreover, surface roughness, porosity and moisture content of mortar samples have influence on the laser cleaning process. The effect of these parameters become however negligible at the high level of laser fluence. The number of pulses required for the laser cleaning is low for smooth surface or less porous mortar. Furthermore, the wetness of the samples facilitates the cleaning process.     

Mechanisms of ablation-rate decrease in multiple-pulse laser ablation

We present two sets of experimental results on the ablation-rate decrease with increase of the number of consecutive laser pulses hitting the same spot on the target surface. We have studied laser ablation of a carbon target with nanosecond pulses in two different interaction regimes: one with a XeCl laser (λ=308 nm) and the other with a Nd:YAG laser (λ=1064 nm), in both cases at the intensity ∼5×10⁸ W/cm² Two different mechanisms were found to be responsible for the ablation-rate decrease; they are directly related to the two different laser–matter interaction regimes. The UV-laser interaction is in the regime of transparent vapour (surface absorption). The increase of the neutral vapour density in the crater produced by the preceding laser pulses is the main reason for the decrease of ablation rate. With the IR laser each single laser pulse interacts with a partially ionised plume. With increase of the number of pulses hitting the same spot on the target surface, the laser–matter interaction regime gradually changes from the near-surface absorption to the volume absorption, resulting in the decrease in absorption in the target and thus in the decrease in the ablation rate. The change in the evaporation rate was considered for both vacuum and reactive-gas environments. Received: 21 February 2001 / Accepted: 26 February 2001 / Published online: 23 May 2001     

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8?J?cm^–2 for pulse duration of ～25?fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100?fs, for a constant fluence of 11.8?J?cm^–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4?mJ?cm^–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370?nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25?fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.     

Laser cleaning of copper roofing sheets subjected to long-lasting environmental corrosion

The study was concerned with the effect of the laser fluence and pulse duration on the microstructure of copper roofing of the Wilanów Palace in Warsaw (Poland) subjected to environmental degradation since the turn 19th century. The techniques used for the microstructure examinations included: SEM+EDS, X-ray diffraction analysis, surface profiling and colour analysis. The measurements of colour of the laser-cleaned surfaces showed that series of 100-μs pulses offer the most effective cleaning. It was also found that by controlling the number of laser pulses it is possible to control the roughness of the cleaned surface.     

Control of material removal of fused silica with single pulses of few optical cycles to sub-picosecond duration

Surface ablation of a dielectric material (fused silica) by single femtosecond pulses is studied as a function of pulse duration (7–450 fs) and applied fluence (F _th<F<10F _th). We show that varying the pulse duration gives access to high selectivity (with resolution ∼10 nm) for axial removal of matter but does not influence the transverse ablation selectivity, which only depends on the normalized applied fluence F/F _th. The ablation efficiency is shown to be inversely dependent on the pulse duration and saturates with respect to the applied fluence earlier at ultra-short pulse durations (≤30 fs). The deduced optimal fluence F _opt corresponding to the highest ablation efficiency for each pulse width defines two regimes of laser application. Below F _opt, the removed material depth can be accurately adjusted in a large range (∼40–200 nm) as a function of the applied fluence and the morphology of the ablated pattern almost reproduces the Gaussian beam distribution. Above F _opt, the material removal depth tends to saturate and the morphology of the ablated pattern evolves to a top-hat distribution. The coupled evolution of depth and morphology is related to the dynamics of formation of dense plasma at the surface of the material, acting as an ultra-fast optical shutter.     

Modeling of UV pulsed-laser ablation of metallic targets

A model to describe the laser ablation of metallic targets is presented. It accounts for the main physical processes involved in the laser–solid–plasma interaction by considering the photon absorption and the ionization mechanisms that are active in the plasma, as well as the laser-produced plasma kinetics. The model is used to simulate the laser ablation of aluminum targets irradiated with a 6-ns UV laser pulse at 0.35 μm, and the results are compared with experimental findings. Calculations show that all the investigated plasma parameters strongly depend on the laser intensity until a roll-off is reached at irradiance ≥1.5 GW cm^-2. The satisfactorily good agreement between model predictions and experimental findings confirms that laser–plasma interaction processes and plasma kinetics play a relevant role during nanosecond laser ablation of metals in the laser intensity range of concern in this study. Received: 12 February 1999 / Accepted: 12 April 1999 / Published online: 7 July 1999     

Effect of pulsed laser parameters on the corrosion limitation for electric connector coatings

Materials used in electrical contact applications are usually constituted of multilayered compounds (e.g.: copper alloy electroplated with a nickel layer and finally by a gold layer). After the electro-deposition, micro-channels and pores within the gold layer allow undesirable corrosion of the underlying protection. In order to modify the gold-coating microstructure, a laser surface treatment was applied. The laser treatment suppressing porosity and smoothing the surface sealed the original open structure as a low roughness allows a good electrical contact. Corrosion tests were carried out in humid synthetic air containing three polluting gases. SEM characterization of cross-sections was performed to estimate the gold melting depth and to observe the modifications of gold structure obtained after laser treatment. The effects of the laser treatment were studied according to different surface parameters (roughness of the substrate and thickness of the gold layer) and different laser parameters (laser wavelength, laser fluence, pulse duration and number of pulses). A thermokinetic model was used to understand the heating and melting mechanism of the multilayered coating to optimize the process in terms of laser wavelength, energy and time of interaction.     

Angular laser cleaning for effective removal of particles. from a solid surface

Laser removal of small particles from a metal surface is carried out by changing the incident angle of the laser beam. It has been found that a dramatic improvement of cleaning efficiency in terms of area and energy is observed when using the laser at glancing angle of incidence as compared to perpendicular. Furthermore substrate damage is greatly reduced and probably eliminated at glancing angles. The process mechanism is discussed by considering the adhesion and the laser-induced cleaning forces for different incident angles. It is shown that there are different laser–matter interactions operating. Received: 25 April 2000 / Accepted: 9 May 2000 / Published online: 5 October 2000     

Large area laser surface micro/nanopatterning by contact microsphere lens arrays

Laser surface micro/nanopatterning by particle lens arrays is a well-known technique. Enhanced optical fields can be achieved on a substrate when a laser beam passes through a self-assembled monolayer of silica microspheres placed on the substrate. This enhanced optical field is responsible for ablative material removal from the substrate resulting in a patterned surface. Because of the laser ablation, the microspheres are often ejected from the substrate during laser irradiation. This is a major issue impeding this technique to be used for large area texturing. We explored the possibility to retain the spheres on the substrate surface during laser irradiation. A picosecond laser system (wavelength of 515 nm, pulse duration 6.7 ps, repetition rate 400 kHz) was employed to write patterns through the lens array on a silicon substrate. In this experimental study, the pulse energy was found to be a key factor to realize surface patterning and retain the spheres during the process. When the laser pulse energy is set within the process window, the microspheres stay on the substrate during and after laser irradiation. Periodic patterns of nanoholes can be textured on the substrate surface. The spacing between the nanoholes is determined by the diameter of the microspheres. The depth of the nanoholes varies, depending on the number of laser pulses applied and pulse energy. Large area texturing can be made using overlapping pulses obtained through laser beam scanning.