Threshold Laser Fluence Research Articles

Submicrosecond dynamics of water explosive boiling and lift-off from laser-heated silicon surfaces

Explosive boiling and lift-off of a thin layer of micron-sized transparent water droplets from an absorbing Si substrate heated by a nanosecond KrF laser were studied using a contact photoacoustic technique. The compressive photoacoustic response increases steeply to an asymptotic value on the order of the water critical pressure starting at a threshold laser fluence of 0.20J∕cm2, where lift-off of the water layer also occurs. Above this threshold, several reproducible discrete multimegahertz components are revealed in Fourier spectra of the acoustic transients, corresponding to nanosecond oscillations of steam bubbles inside the water droplets on the microsecond time scale of the lift-off process. The acoustic pressure buildup, bubble dynamics, and the subsequent lift-off of the thin water layer are interpreted as relaxation stages after near-spinodal explosive boiling of the superheated interfacial water.

Emission and electron/ion analysis of the ablated plume from LiYF 4 crystals doped with Tm 3+ or Nd 3+ ions

We performed temporal as well as frequency-resolved emission spectroscopy of the plasma plume generated in vacuum by 355 nm pulsed laser ablation of a LiYF 4:Tm 3+ crystal (1% Tm 3+ concentration) and a LiYF 4:Nd 3+ crystal (1.5% Nd 3+ concentration). The plume emission spectrum is very rich in features and shows the elemental lines belonging to all the components of the host lattice, either neutral or ionized. The time-resolved analysis of some emission features is discussed and some stream velocities are derived. A possible model for the time evolution of the plume components is reported. The electronic detection technique was used to detect the onset of the plasma plume and to measure the ablation laser fluence threshold.

Nanoparticle Formation of Pentacene by Laser Irradiation in Ethanol Solution

Laser irradiation using a Nd3+:YAG laser (532 nm, 8 ns, 10 Hz) on an ethanol suspension of micrometer-sized pentacene crystals induced absorption spectral changes in the solution, as a result of the formation of pentacene nanoparticles. The nanoparticles were platelike crystals. The nanoparticles formed by irradiation with a fluence of 50 mJ/cm2 for 30 min were 4–13 nm in height and 10–70 nm in width. The threshold laser fluence for the formation was determined to be ca. 20 mJ/cm2, and irradiation with fluences above 80 mJ/cm2 induced the decomposition of the pentacene nanoparticles. The nanoparticle size decreased with an increase in the laser fluence, which was reflected in the longest-wavelength peak position of the absorption bands of the pentacene nanoparticles. On the basis of these results, the mechanism of pentacene nanoparticle formation by laser irradiation is discussed in connection with those of vanadyl phthalocyanine and quinacridone systems reported by Masuhara and coworkers [J. Phys. Chem. A 106 (2002) 2135; Jpn. J. Appl. Phys. 42 (2003) 2725; Jpn. J. Appl. Phys. 45 (2006) 384].

Adjustable Fragmentation in Laser Desorption/Ionization from Laser-Induced Silicon Microcolumn Arrays

Laser-induced silicon microcolumn arrays (LISMA) were developed as matrix-free substrates for soft laser desorption/ionization mass spectrometry (SLDI-MS). When low-resistivity silicon wafers were irradiated in air, sulfur hexafluoride, or water environment with multiple pulses from a 3 x omega mode-locked Nd:YAG laser, columnar structures were formed on the surface. The radii of curvature of the column tips varied with the processing environment, ranging from approximately 120 nm in water, to <1 mum in SF6, and to approximately 2 mum in air. In turn, these microcolumn arrays were used as matrix-free soft laser desorption substrates. In SLDI-MS experiments with a nitrogen laser, the microcolumn arrays obtained in water environment readily produced molecular ions for peptides and synthetic polymers at low laser fluence. These surfaces demonstrated the best ion yield among the three arrays. The threshold laser fluence and ion yield were comparable to those observed in matrix-assisted laser desorption/ionization. Low-femtomole sensitivity and approximately 6000 Da mass range were achieved. At elevated laser fluence, efficient in-source decay was observed and extensive peptide sequence information was extracted from the resulting mass spectra. The versatility of LISMA was attributed to confinement effects due to the submicrometer morphology and to the surface, thermal, and optical properties of processed silicon.

Selective laser nano‐thermolysis of human leukemia cells with microbubbles generated around clusters of gold nanoparticles

Previously reported studies on laser nano-thermolysis of cancerous cells demonstrated insufficient efficacy and specificity of malignant cell damage. Safety, that is, absence of damage to normal cells in the course of the laser thermolysis was also low due to less than optimal protocol of cancer cell targeting with nanoparticles (NP). The objective of this study was two-fold: to optimize NP targeting to real tumor (human) cells and to better understand physical mechanisms of cell damage for improved control of the laser ablation. We have suggested (1) two-stage targeting method to form clusters of light-absorbing gold NPs selectively in target cells, and (2) the cell damage mechanism through laser-induced generation of vapor bubbles around NP clusters. Experimental investigation of laser nano-thermolysis of leukemia cells was performed using 30 nm spherical gold nanoparticles as a light absorbing agent, and photothermal and fluorescent microscopies as well as flow cytometry as methods to monitor microbubble formation and resulting damage of leukemia cells in human bone marrow specimens. NP clusters were formed and visualized using fluorescence microscopy at cell membranes and in cytoplasm of B-lymphoblasts. Laser irradiation of cells (532 nm, 10 nanoseconds, 0.6 J/cm2) induced microbubbles selectively in leukemia cells with large clusters, but not in cells with single NPs or small clusters. Quantitative analysis demonstrated that only 0.1%-1.5% of tumor cells and 77%-84% of normal bone marrow cells survived laser pulse. Two-stage cell targeting method permits formation of NP clusters selectively in diagnosis-specific tumor cells. The clusters serve as effective sources of photothermally-induced microbubbles, which kill individual target cells after a single laser pulse. The laser fluence threshold for generation of microbubbles is inversely proportional to the volume of NP clusters.

Study on metal microparticle content of the material transferred with Absorbing Film Assisted Laser Induced Forward Transfer when using silver absorbing layer

Abstract Absorbing Film Assisted Laser Induced Forward Transfer (AFA-LIFT) is a modified LIFT method where a high absorption coefficient thin film coating of a transparent substrate is used to transform the laser energy into kinetic in order to transfer the “target” material spread on it. This method can be used for the transfer of biomaterials and living cells, which could be damaged by direct irradiation of the laser beam. In previous experiments, ∼50–100 nm thick metal films have been used as absorbing layer. The transferred material can also contain metal microparticles originating from the absorbing thin film and acting as non-desired impurities in some cases. The aim of our work was to study how the properties (number, size and covered area) of metal particles transferred during the AFA-LIFT process depend on film thickness and the applied fluence. Silver thin films with different thickness (50–400 nm) were used as absorbing layers and real experimental conditions were modeled by a 100 μm thick water layer. The particles transferred without the use of water layer were also studied. The threshold laser fluence for the complete removal of the absorber from the irradiated area was found to strongly increase with increasing film thickness. The deposited micrometer and submicrometer particles were observed with optical microscope and atomic force microscope. Their size ranged from 100 nm to 20 μm and depended on the laser fluence. The increase in fluence resulted in an increasing number of particles of smaller average size.

Femtosecond laser ablation of carbon reinforced polymers

Interaction of intense ultrashort laser pulses (120 fs at 795 nm) with polymer based composites has been investigated. We have found that carbon filled polymers exhibit different ultrafast ablation behaviour depending on whether the filling material is carbon black or carbon fiber and on the polymer matrix itself. The shape and dimensions of the filling material are responsible for some geometrical bad quality effects in the entrance and inner surfaces of drilled microholes. We give an explanation for these non-quality effects in terms of fundamentals of ultrafast ablation process, specifically threshold laser fluences and material removal paths. Since carbon fiber reinforced polymers seemed particularly concerned, this could prevent the use of ultrafast ablation for microprocessing purposes of some of these materials.

A general continuum approach to describe fast electronic transport in pulsed laser irradiated materials: The problem of Coulomb explosion

We present a continuum model, based on a drift-diffusion approach, aimed at describing the dynamics of electronic excitation, heating, and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond and nanosecond pulsed laser irradiation. The laser-induced charging of the targets is investigated at laser intensities above the material removal threshold. It is demonstrated that, for near-infrared femtosecond irradiation, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. On the other hand, application of the model to UV nanosecond pulsed laser interaction with bulk silicon has pointed out the possibility of Coulomb explosion in semiconductors. For such regimes a simple analytical theory for the threshold laser fluence of CE has been developed, showing results in agreement with the experimental observations. Various related aspects concerning the possibility of CE depending on different irradiation parameters (fluence, wavelength and pulse duration) and material properties are discussed. This includes the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the reflection and absorption characteristics.

Scaling laws of femtosecond laser pulse induced breakdown in oxide films

The scaling of the single-pulse laser threshold fluence for dielectric breakdown with respect to pulse duration and material band gap energy was investigated in the subpicosecond pulse regime using oxide films (${\mathrm{TiO}}_{2}$, ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$, ${\mathrm{HfO}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{SiO}}_{2}$). A phenomenological model attributes the pulse duration dependence to the interplay of multiphoton ionization, impact ionization, and subpicosecond electron decay out of the conduction band. The observed linear scaling of the breakdown fluence with band gap energy can be explained within the framework of this model by invoking the band gap dependence of the multiphoton absorption coefficient from Keldysh photoionization theory. The power exponent $\ensuremath{\kappa}$ of the observed dependence of the breakdown threshold fluence ${F}_{\mathit{th}}$ on pulse duration ${\ensuremath{\tau}}_{p}$, ${F}_{\mathit{th}}\ensuremath{\propto}{\ensuremath{\tau}}_{p}^{\ensuremath{\kappa}}$, is independent of the material and is attributed to photoionization seeded avalanche ionization.

Structuring of metallic bi- and tri-nano-layer films by laser interference irradiation: control of the structure depth

A periodical structuring on bi-metallic thin layer films was performed by irradiation in air with an Nd:YAG laser operating in pulsed mode. The samples consist on Fe–Al, Cu–Al, Ni–Al and Fe–Al–Fe over a glass substrate. The influence of the laser fluence, the layer thickness, and the substrate on the structure depth (SD) was studied. A thermal simulation was carried out to estimate the temperature distribution and the quantities of the molten material in each layer. It was found that the maximal SD in the lower energy regime is reached at the threshold laser fluence value at which the topography type changes from a low to a high structured profile type. In addition, it was found that as the thickness of the second layer and the difference between the melting points of the metallic layers increase, the structure depth also increases. In the case of bi-layer films, the structuring of the samples is conducted by the expansion of the Al and the glass–substrate at the interference peaks. Nevertheless, if a third metallic layer is placed over the substrate, the structure depth decreases as the thickness of this layer increases. This occurs because the third layer helps to evacuate the heat from the first two layers much faster than the glass substrate and therefore decreasing the structure depth.

Combined atomistic-continuum model for simulation of laser interaction with metals: application in the calculation of melting thresholds in Ni targets of varying thickness

The threshold laser fluence for the onset of surface melting is calculated for Ni films of different thicknesses and for a bulk Ni target using a combined atomistic-continuum computational model. The model combines the classical molecular dynamics (MD) method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons based on the two-temperature model (TTM). In the hybrid TTM-MD method, MD substitutes the TTM equation for the lattice temperature, and the diffusion equation for the electron temperature is solved simultaneously with MD integration of the equations of motion of atoms. The dependence of the threshold fluence on the film thickness predicted in TTM-MD simulations qualitatively agrees with TTM calculations, while the values of the thresholds for thick films and bulk targets are ∼10% higher in TTM-MD. The quantitative differences between the predictions of TTM and TTM-MD demonstrate that the kinetics of laser melting as well as the energy partitioning between the thermal energy of atomic vibrations and energy of the collective atomic motion driven by the relaxation of the laser-induced pressure should be taken into account in interpretation of experimental results on surface melting.

Visualization of microparticle explosion and flow field in nanoparticle synthesis by pulsed laser ablation

Laser ablation of microparticles is one of the promising methods for efficient nanoparticle synthesis but the physical mechanisms of nanoparticle formation are not yet clearly understood, particularly in the case of metallic or ceramic particles. In this work, we report, for the first time, the results of in situ visualization of the metal microparticle explosion. Ablation of a Cu microparticle in CuO nanoparticle synthesis by a Q-switched Nd:YAG laser is visualized using laser-flash shadowgraphy. The dynamics of the ablation plume and shock wave is also analyzed by monitoring the photoacoustic probe-beam deflection. Strong shock-wave emission from a single particle has been observed in the velocity range 1000–4000 m/s even at near-threshold fluences. The threshold laser fluence for particle ablation has been found to be substantially lower than that required for bulk metal ablation. Consequently, the results confirm that the photomechanical mechanism associated with the shock-wave generation plays a significant role in the ablation process.

Fabrication of microstructures in photoetchable glass ceramics using excimer and femtosecond lasers

We discuss laser fabrication of microstructures in photoetchable glass ceramics called Foturan (Schott Company, Elmsford, NY). A KrF excimer laser (= 248 nm, = 25 ns) is used for surface micromachining, and a femtosecond laser (= 800 nm, = 80 fs) is used for fabricating 3-D structures. Important aspects of the machining, such as depth of machining resulting from different laser processing parameters and threshold laser fluences, are presented. A detailed analysis of the absorption process of both lasers in photoetchable glass ceramics is provided.

Photoacoustic study of explosive boiling of a 2-propanol layer of variable thickness on a KrF excimer laser-heated Si substrate

The dynamics of explosive boiling of a 2-propanol layer of variable thickness on a Si substrate heated by a nanosecond KrF excimer laser was studied using a contact photoacoustic technique. The transition from acoustic generation at a free Si boundary to that at a rigid alcohol/Si boundary accompanied by a sharp increase of acoustic generation efficiency was found above a laser fluence threshold of 0.17 J/cm2 and a liquid layer thickness greater than 0.25 μm due to subnanosecond near-critical explosive boiling of the superheated liquid layer near the hot absorbing Si substrate. The gradual increase of the photoacoustic response of the superheated alcohol with increasing thickness of the liquid film at fluences above the explosive boiling threshold was attributed to a diffraction effect due to the fluence- and time-dependent increase of the area undergoing explosive boiling. A model describing photoacoustic generation and subsequent lift-off of the entire liquid layer in this experimental “thin transparent liquid layer/solid absorbing substrate” geometry under near-critical explosive boiling conditions has been proposed.

Limit analysis for laser removal of micron contaminant colloidal silicon dioxide particles from the super-smooth optical glass substrate by pulse Nd:YAG laser

Multimode Nd:YAG pulse laser was applied to remove micron and submicron particles by vaporizing a thin paint film pre-coated on super-smooth optical substrate surface. By analyzing the poor absorption of the optical glass substrate to the irradiative Nd:YAG pulse laser, the removal mechanism of contaminated colloidal particles from the super-smooth surface through vaporization of a volatile solid film is described. A limit analysis was proposed to determine the lower and the upper threshold of laser fluence for cleaning the SiO 2 contaminants from super-smooth K8 optical substrate. Relevant experiments on laser cleaning of micron-polishing particles from super-smooth K8 optical substrate confirmed the usefulness of this method in assisting the selection of effective cleaning fluence for accomplishing high cleanliness, which was in a range of 80–90% of the predicted upper threshold.

Ultraviolet laser surface treatment for biomedical applications of ? titanium alloys: morphological and structural characterization

To improve the surface properties of β titanium alloys developed for biomedical applications we have recently suggested a methodology involving laser-assisted nanostructuration. This strategy would benefit from superficial laser heat treatment since laser annealing displays many advantages as compared to the conventional methods: high resolution, high operating speed, low cost and retaining the initial bulk properties. Therefore, this paper reports results concerning the laser treatment of β titanium alloys under vacuum. Our interest has been focused on the excimer laser single-pulse irradiation (λ=248 nm) of a model β titanium alloy (Ti6.8Mo4.5Fe1.5Al). The threshold laser fluences corresponding to β transus, melting and ablation temperatures as well as the resulting modification depth were first approached theoretically. KrF laser annealings were then carried out in vacuum, varying the fluence conditions from submelting heating to ablation regimes. Subsequent atomic force microscopy and scanning electron microscopy observations were performed to follow the structural and topographical modifications of as-treated specimens and were then discussed as regards the above-mentioned theoretical parameters .

Influence of the processing parameters on the formation and deposition of particles in UV pulsed laser ablation of Al 2O 3–TiC ceramics

In this work, the influence of the ambient gas (He, Ne, N 2, air, Ar and Kr) on the mechanisms of particle formation and deposition in laser micromachining of Al 2O 3–TiC ceramics using pulsed 193 and 248 nm radiation was investigated. Two kinds of particles redeposited around the processed area could be identified: ultrafine particles, of a few tens of nanometers (debris), which aggregate upon redeposition to form a continuous film; and larger particles, with a diameter of 0.5–8 μm (particulates), which remained individualised. The behaviour of debris follows blast wave theory predictions, indicating that this type of particles results from collisions behind the ablation plume front. The particulates are formed above a threshold laser fluence, which decreases with increasing ambient gas atomic/molecular weight and pressure. This threshold is lower for 193 than for 248 nm radiation, but shows the same dependence on the atmosphere for both wavelengths. Experimental evidence and theoretical analysis suggest that the particulates result from ejection of droplets from growing protuberances on the irradiated surface, due to the recoil pressure produced by the plume.

A time-resolved quadrupole mass spectrometric study on 355 nm laser ablated species ejected from LiMn2O4

Mass, velocity and angle distributions of the ablated species generated from 355 nm pulsed laser ablation of a LiMn2O4 target were investigated with an angle- and time-resolved mass spectrometric technique. Both neutral and ionic species of Li, O, LiO, LiO2, Mn, Li2, Li4, Li6, LiMn, MnO and MnO2 were observed at the laser fluence of 0.8 J · cm-2. The yield and variety of the ablated species increase with increasing the laser fluence. The time-of-flight spectra of ablated species can be fitted by a Maxwell-Boltzmann distribution with a center-of-mass velocity. There exist laser fluence thresholds for the ablated LiMn, Li2O and LiO2 species, and the fluence threshold of ionic species is higher than that of neutral species. The angular distributions of the ionic and neutral ablated species can be simulated by a cosηθ or a bicosine function α cosθ + (1-α) cosηθ. In addition, the ablation mechanism of LiMn2O4 by a 355 nm pulsed laser is discussed.

Size and Shape Transformation of TiO2 Nanoparticles by Irradiation of 308-nm Laser Beam

TiO2 particles in a colloidal solution changed their size and shape upon irradiation of a 308 nm laser beam which corresponded to the inter-band absorption of the particles. The particles with a diameter size distribution between 170 to 1050 nm were transformed to smaller particles of approximately 10 nm diameter with a narrow size distribution. The particles of 2 nm diameter were also converted to the particles of 10 nm diameter. The irradiated particles were spherical, indicating that particles were melted upon laser irradiation. The threshold of laser fluence was observed for the size and shape transformation of the particles, which was several times larger than the energy required to melt the particles. Agglomeration of the particles, which was controlled by pH of the solution, enlarged the average size and broadened the range of size distribution of the irradiated particles. The mechanism determining the size of the irradiated particles has been discussed.

A thermal-controlling mechanism for laser ablation of a colossal magnetoresistant oxide target

Both angle- and time-resolved quadrupole mass spectrometric and time-resolved optical emission spectrometric techniques were employed to investigate the laser ablation mechanism of a colossal magnetoresistant target. It is suggested that the ablation process is generally thermal in nature when the laser fluence is not very high (<2.5 J/ cm 2) . This viewpoint is supported by many experimental observations, such as the independence of the kinetic energy of the ablated species on their mass, the ambient gas pressure effect on the kinetic energy of ablated species, the existence of a laser fluence threshold as well as the plume expansion in an ambient gas.