A point source analytical model of inverse pulsed laser deposition

A simple analytical model for inverse pulsed laser deposition is proposed. In the model the motion of the evaporated material is assumed to emerge as from a point source located above the surface of evaporation at some distance. The obtained thickness profiles of inverse deposited films agree well with those calculated by the test particle Monte Carlo method. The proposed approach has been applied for analysis of experimental data on inverse pulsed laser deposition of graphite in nitrogen atmosphere with nanosecond pulses of laser fluences between 1 and 7 J/cm². The model describes well the thickness profiles and pressure dependence of film growth rate for inverse deposition.     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Study of laser fragmentation process of silver nanoparticles in aqueous media

Laser fragmentation of Ag nanoparticles in Ag hydrosol was studied by simultaneous measurements of the transmitted fluence of the incident laser beam and the time evolution of the surface plasmon extinction (SPE) spectra. The experiments showed that the laser fragmentation in a small volume of hydrosol proceeds during first 20 pulses and then reaches saturation. The value of the transmitted fluence corresponding to saturation increases with incident pulse fluence, but the impact of the first pulse applied to the hydrosols shows an optical limitation. Fluences above 303 mJ/cm² cause the formation of less stable, aggregating nanoparticles, while fluences below 90 mJ/cm² do not provide sufficient energy for efficient fragmentation. The interval of fluences between 90–303 mJ/cm² is optimal for fragmentation, since stable hydrosols constituted by small, non-aggregated nanoparticles are formed.     

Cumulative and continuous laser vaporization synthesis of single wall carbon nanotubes and nanohorns

The conditions for the scaled synthesis of single wall carbon nanotubes (SWNTs) and single wall carbon nanohorns (SWNHs) by laser vaporization at high temperatures are investigated and compared using in situ diagnostics. An industrial Nd:YAG laser (600 W, 1–500 Hz repetition rate) with tunable pulse widths (0.5–50 ms) is utilized to explore conditions for high-yield production. High-speed videography (50000 frames/s) of the laser plume and pyrometry of the target surface are correlated with ex situ high resolution transmission electron microscopy analysis of the products for pure carbon targets and carbon/catalyst targets to understand the effects of the processing conditions on the resulting nanostructures. Carbon is shown to self-assemble into single-wall nanohorn structures at rates of ∼1 nm/ms, which is comparable to the catalyst-assisted SWNT growth rates. Two regimes of laser ablation, cumulative ablation by multiple pulses and continuous ablation by individual pulses, were explored. Cumulative ablation with spatially overlapping 0.5-ms pulses is favorable for the high yield and production rate of SWNTs at ∼6 g/h while continuous ablation by individual long laser pulses (∼20 ms) at high temperatures results in the highest yield of SWNHs at ∼10 g/h. Adjustment of the laser pulse width is shown to control SWNH morphology.     

Growth of ZnO nanoparticles and nanorods with ultrafast pulsed laser deposition

In this work, we study the application of ultrafast pulsed laser deposition (PLD) in ZnO nanomaterial synthesis, including nanoparticles and nanorods. PLD using long pulse (nanosecond) lasers has been widely used as a method for growing prototype materials. The recently-emerged ultrafast PLD is expected to be able to overcome the problem of large liquid droplet formation. Using near infrared and femtosecond laser pulses in ablation, we first characterize the ablation plume using a Langmuir probe and plasma optical emission spectroscopy. We then examine the structural properties of the nanoparticles generated during low-fluence ablation. Finally, we demonstrate that using nanoparticle aggregates as templates, assisted by plume-excited nitrogen radicals at a high fluence, high quality ZnO nanorods can be grown free of metal catalysts.     

Single-pulse drilling study on Au,Al and Ti alloy by using a picosecond laser

Picosecond laser single pulse ablation of Au, Al and Ti alloy (Ti6Al4V) was experimentally investigated with a laser pulse width of 10 ps at a wavelength of 1064 nm for potential industrial micromachining applications. The diameters, depths and morphologies of the drilled craters were studied. Two novel phenomena were found: as hole diameters decreased with fluence, a change of slope of the trend line indicated a change in ablation mechanism for Al and Ti alloy, metallic materials with short electron-phonon coupling times (<10 ps), while Au showed no such transition: an isolated island structure was also observed on Au due to significant melt expulsion. A one-dimensional two-temperature model has been used to discriminate different ablation phenomena. It is shown that metallic materials with different electron–phonon coupling constant have different ablation characteristics in the ps regime. This study could be very helpful for metallic material micromachining with high repetition rate ps lasers pulses which indicates that high throughput may be achieved as well as good machining quality.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

Ion extraction from the surface ablated materials in electric fields using an intense femtosecond laser pulse

In order to develop a femtosecond laser ablation (fsLA) ion source for TOF mass spectrometry, we have analyzed time-resolved images of laser-induced fluorescence from Sm⁺ ions produced by fsLA of a solid samarium in electric fields. The polarity and the strength of electric fields had a remarkable effect on the expansion of Sm⁺ ions. Moreover, accelerating electric fields elongated the duration of the ion emission from the samarium surface in fsLA, which degraded time-focusing of the ions. We have found that suppression the continuous ion emission caused by fsLA in electric fields is most important in TOF measurements.     

Optical extinction for determining the size distribution of gold nanoparticles fabricated by ultra-short pulsed laser ablation

This work presents a method, based on measurements of the optical extinction spectra, to determine the size of spherical gold nanoparticles produced using the femtosecond laser ablation process in deionized water. By using an improved theoretical model that modifies the contribution of the free electrons to the dielectric function introducing a size-dependent term, it is possible to fit the full experimental extinction spectrum considering a certain size distribution. Additionally, in order to obtain complementary measurements of the size distribution, TEM analysis was performed. The results obtained showed that the predominant nanoparticle size distribution ranges from 1 to 11 nm in terms of radii. An optical extinction measurement together with an appropriate theoretical model based on Mie’s theory represents a simple, low-cost, fast and easy method to describe a multimodal size distribution of spherical gold nanoparticles.     

Ultrafast dynamic ellipsometry measurements of early time laser ablation of titanium thin films

The dynamics of laser ablated titanium thin films are investigated using a recently developed technique that measures time-resolved and one-dimensional spatially-resolved ablation dynamics in a single shot. Ultrafast dynamic ellipsometry, a technique based on space-shifted spectral interferometry, uses the time-dependent frequency of a chirped laser pulse to provide time encoding, allowing the picosecond probing of material dynamics in a single shot. With this technique, the sample is probed at two different incident angles with both s- and p-polarized light, which measures the motion of the material and any change in its complex refractive index. Ultrafast dynamic ellipsometry is applied to study the mechanism of initiation by laser-based optical detonators that employ the ablation of titanium thin films. The resulting data indicate that the titanium is ablated as a fragmented flyer and not as an expanding plasma.     

Analysis of the laser ablation processes for thin-film silicon solar cells

A detailed analysis of the monolithical series connection of thin-film silicon modules with ZnO/Ag back contact is presented. In this study, pulsed lasers with wavelengths of 1064 nm and 532 nm were used. The influence of various laser parameters like laser power, pulse overlap, etc., on the different patterning steps is discussed. The focus of this study was on the back contact patterning process. Here (i) the flake formation process during the ablation and (ii) the influence of a NIR-laser source as an alternative approach to the green laser were investigated in detail. The latter would reduce system costs if only one NIR-laser source could be used for all patterning steps.     

Nonlinearity and time-resolved studies of ion emission in ultrafast laser ablation of graphite

We have investigated the laser fluence dependence of the ion emission process in ultrafast laser ablation of graphite using a time-of-flight technique. Two regimes of ion emission have been identified: (1) a highly nonlinear laser absorption process accompanied by generation of a transient electrical field on the surface and collisionless emission of ions due to electrostatic repulsion; (2) a saturation regime for laser power absorption characterised by nearly equal kinetic energy of ejected carbon clusters. We also show the effect of the surface temperature on the emitted clusters’ stability and the influence of nonlinearity on the intensity autocorrelation traces.     

Optodynamic monitoring of laser micro-drilling of glass by using a laser probe

We present a new method for monitoring of the laser micro-drilling process. The method is based on detection of acoustic waves in air above the processed sample using a laser beam deflection probe (LBDP). We present an augmentation of the experimental set-up by means of a digital micrographic system which enables the acquisition of images of the plasma plume and of the hole cross-section during a drilling sequence. Comparing the acquired images with the detected LBDP signals, we examine the drilling process from a new perspective.     

Nonlinear photoionization and laser-induced damage in silicate glasses by infrared ultrashort laser pulses

We show that photoionization of wide band gap silicate glasses by infrared ultrashort laser pulses can occur without laser-induced damage. Two glasses are studied, fused silica and a multi-component silicate photo-thermo-refractive (PTR) glass. Experiments are performed by low numerical aperture focusing of ultrashort laser pulses (100 fsec<τ<1.5 psec) at the wavelengths 780 nm, 1430 nm, and 1550 nm. Filaments form inside both glasses and are visibly observable due to intrinsic luminescence. Keldysh’s theory of nonlinear photoionization is used to model the formation of filaments and values of about 10¹³ W cm⁻² for the laser intensity and 10¹⁹ cm⁻³ for the free electron density are estimated for stable filaments to arise. Laser-induced damage is studied by the generation of a third harmonic from an interface created between a damage site and the surrounding glass matrix. It is found that third harmonic generation occurs only after several thousands of laser shots indicating that damage is not a single-shot phenomena. The ability to photoionize PTR glass without damage by ultrashort laser pulses offers a new approach for fabricating diffractive optical elements in photosensitive glass.     

Femtosecond laser modification of living neuronal network

An all-wet femtosecond laser microprocessing technique was utilized for patterning and cutting functional network of living neuronal cells on a multi-electrode dish (MED). The neuronal cells cultured on a source substrate were transferred onto an electrode in a MED probe in solution by utilizing a femtosecond laser-induced impulsive force and a pattern of neuronal cells were formed on the MED probe. The cellular activity of the detached neurons was supported that neurites could be regenerated around the electrodes. As another processing method, the neurons stretching between electrodes were selectively cut by the direct femtosecond laser irradiation and the spontaneous electrical activity of the neuronal network was evaluated. While the spontaneous action potentials of neurons were synchronized before the cutting, the synchronization disappeared after the cutting, indicating that the neuronal network is locally disconnected by the laser cutting. The present method is applicable to artificial reconstruction of living neuronal network.     

Plasma plume induced during laser ablation of graphite

The plasma plume induced during ArF laser ablation of a graphite target is studied. Velocities of the plasma expansion front are determined by the optical time of flight method. Mass center velocities of the emitting atoms and ions are constant and amount to 1.7×10⁴ and 3.8×10⁴ m s⁻¹, respectively. Higher velocities of ions result probably from their acceleration in electrostatic field created by electron emission prior to ion emission. The emission spectroscopy of the plasma plume is used to determine the electron densities and temperatures at various distances from the target. The electron density is determined from the Stark broadening of the Ca II and Ca I lines. It reaches a maximum of ∼9.5×10²³ m⁻³ 30 ns from the beginning of the laser pulse at the distance of 1.2 mm from the target and next decreases to ∼1.2×10²² m⁻³ at the distance of 7.6 mm from the target. The electron temperature is determined from the ratio of intensities of ionic and atomic lines. Close to the target the electron temperature of ∼30 kK is found but it decreases quickly to 11.5 kK 4 mm from the target.     

Laser etching of glass substrates by 1064 nm laser irradiation

TiB₂-based cermets are prepared by combustion synthesis followed by a pressing stage in a granulate medium. Products obtained by combustion synthesis are characterized by a large remaining porosity (typically 50%). To produce dense cermets, a subsequent densification step is performed after the combustion process and when the reacted material is still hot. To design the process, numerical simulations are carried out and compared to experimental results. In addition, physical and electrical properties of the products related to electrical contact applications are evaluated. PACS 81.20.Ka; 81.05.Mh; 72.80.Tm     

Influence of fast electrical discharge on expansion of laser produced plasma

For some applications of laser produced plasma it may be advantageous to implement some degree of control on the ion flux and velocity distribution in the plasma expansion. We have shown how a fast, high current discharge in a laser produced plasma leads to a dramatic increase in the plasma ionization and expansion velocity. These observations are consistent with a magnetohydrodynamic discharge model whereby the self-induced magnetic field causes radial compression of an open-ended plasma column.     

Underwater excimer laser ablation of polymers

In this paper, we study the photoablation kinetic of poly (ethylene terephthalate) (PET), polycarbonate (PC), polyimide (PI) and polystyrene (PS) in both air and water. Compared to the results obtained in air, we highlight the decrease of the ablation threshold (AT) of polyesters in contact with water as a function of polymer chemical structure. In order to check the expected hydrolytic reaction of polyesters near the ablation threshold, the chemical modification of the polymer surfaces, as well the composition of the ablation products, were investigated after irradiation near the fluence of ablation threshold in air (air-F _t ) by X-ray photoelectron spectroscopy (XPS) and confocal Raman microspectroscopy. The morphology of polymers obtained by underwater irradiation and near the air-F _t was also examined using scanning electron microscopy (SEM). To understand the process and its dynamics in contact with water, we consider the model of temperature at the polymer-water interface based on the semi-analytical solution of the transit heat-diffusion equation.