Laser Pulse Fluence Research Articles

Nonlinear femtosecond laser processing of alkylsiloxane monolayers on surface-oxidized silicon substrates

Femtosecond laser patterning of octadecylsiloxane monolayers on surface-oxidized silicon substrates via single-pulse processing at λ=800nm, τ<30fs, and ambient conditions has been investigated. Depending on the laser pulse fluence, local irradiation results in circular spots of distinct size and morphology. At high fluences, a particular rich complexity of distinct surface morphologies is observed including hole, rim, and ripple formation, and a faint boundary area where monolayer decomposition sets in. At low fluences, subwavelength patterning of the organic monolayer is feasible. In particular, at a 1∕e laser spot diameter of 1.3μm, surface spots with a width down to 300nm are fabricated. Selective processing of the organic monolayer, though, is restricted to a very narrow range of fluences between 1.1 and 1.2J∕cm2. A significantly larger parameter range for selective processing is anticipated in the case of functional monolayers that incorporate aromatic groups. Promising perspectives in femtosecond laser processing of organic monolayers are discussed.

Modeling Carrier-Phonon Nonequilibrium Due to Pulsed Laser Interaction With Nanoscale Silicon Films

Ultrashort-pulsed laser irradiation on semiconductors creates a thermal nonequilibrium between carriers and phonons. Previous computational studies used the “two-temperature” model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the carriers or phonons or when the carriers’ or phonons’ mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this article, the nonequilibrium between carriers and phonons in silicon films is modeled via numerical solution of the Boltzmann transport model (BTM), which is applicable over a wide range of length and time scales. The BTM is solved using the discontinuous Galerkin finite element method for spatial discretization and the three-stage Runge–Kutta temporal discretization. The BTM results are compared with previous computational studies on laser heating of macroscale silicon films. The model is then used to study laser heating of nanometer size silicon films, by varying parameters such as the laser fluence and pulse duration. From the laser pulse duration study, it is observed that the peak carrier number density, and maximum carrier and phonon temperatures are the highest for the shortest pulse duration of 0.05 ps and decreases with increasing pulse duration. From the laser fluence study, it is observed that for fluences equal to or higher than 1000 J/m2, due to the Auger recombination, a second peak in carrier temperature is observed. The use of carrier-acoustic phonon coupling leads to equilibrium phonon temperatures, which are approximately 400 K higher than that of carrier-optical phonon-acoustic phonon coupling. Both the laser pulse duration and fluence are found to strongly affect the equilibrium time and temperature in Si films.

UV laser-ablated surface textures as potential regulator of cellular response

Textured surfaces obtained by UV laser ablation of poly(ethylene terephthalate) films were used to study the effect of shape and spacing of surface features on cellular response. Two distinct patterns, cones and ripples with spacing from 2 to 25 μm, were produced. Surface features with different shapes and spacings were produced by varying pulse repetition rate, laser fluence, and exposure time. The effects of the surface texture parameters, i.e., shape and spacing, on cell attachment, proliferation, and morphology of neonatal human dermal fibroblasts and mouse fibroblasts were studied. Cell attachment was the highest in the regions with cones at ∼4 μm spacing. As feature spacing increased, cell spreading decreased, and the fibroblasts became more circular, indicating a stress-mediated cell shrinkage. This study shows that UV laser ablation is a useful alternative to lithographic techniques to produce surface patterns for controlling cell attachment and growth on biomaterial surfaces.

Mathematical modeling of dynamics of fast phase transitions and overheated metastable states during nano- and femtosecond laser treatment of metal targets

For a mathematical description of pulsed laser heating, melting, and evaporation of an aluminium target in an ambient atmosphere, a one dimensional, multifront hydrodynamic Stephan problem was used, written for both phases (liquid and solid). On the boundary of solid and gaseous forms, the Stephan problem is combined with radiation gas-dynamic equations, with thermal conductivity, and describes processes in the evaporated material and surrounding gas. For the numerical solution, finite difference method of dynamic adaptation, which gives an opportunity of explicitly tracking interphase boundaries and shock waves, was applied. As a result, in the process of the solution, the problem had 6 computational regions and 7 boundaries, 6 of them were moving, including 2 shock waves and one free boundary in the atmosphere. We used this model to calculate the pulsed laser interaction with an aluminium target with the following parameters: λ = 0.8 μm, τ = 10−8–10−15 s, and G0 = 10−9–10−16 W/cm2. Modeling revealed that in the case of long ∼1 ns pulses, most of the energy is spent on melting and heating the liquid. The depth of the molten pool depth constitutes about 1.2 μm. In the case of femtosecond pulses, most of the energy is spent on heating the solid body and the formation of shock waves in it. The depth of the molten pool does not exceed 0.03 μm. Even though the evaporated layers were of almost the same thickness. For nanosecond laser pulses with fluence J less than 30 J/cm2, there is no plasma formation in the evaporated material. For the same fluence of femtosecond laser pulses, plasma is formed after the pulses and is thermal by nature.

Plasmonic Nanobubbles as Transient Vapor Nanobubbles Generated around Plasmonic Nanoparticles

We have used short laser pulses to generate transient vapor nanobubbles around plasmonic nanoparticles. The photothermal, mechanical, and optical properties of such bubbles were found to be different from those of plasmonic nanoparticle and vapor bubbles, as well. This phenomenon was considered as a new complex nanosystem-plasmonic nanobubble (PNB). Mechanical and optical scattering properties of PNB depended upon the nanoparticle surface and heat capacity, clusterization state, and the optical pulse length. The generation of the PNB required much higher laser pulse fluence thresholds than the explosive boiling level and was characterized by the relatively high lower threshold of the minimal size (lifetime) of PNB. Optical scattering by PNB and its diameter (measured as the lifetime) has been varied with the fluence of laser pulse, and this has demonstrated the tunable nature of PNB.

Molecular dynamics simulations of MALDI: laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization

Molecular dynamics simulations of matrix-assisted laser desorption/ionization were carried out to investigate laser pulse width and fluence effects on primary and secondary ionization process. At the same fluence, short (35 or 350 ps) pulses lead to much higher initial pressures and ion concentrations than longer ones (3 ns), but these differences do not persist because the system relaxes toward local thermal equilibrium on a nanosecond timescale. Higher fluences accentuate the initial disparities, but downstream differences are not substantial. Axial velocities of ions and neutrals are found to span a wide range, and be fluence dependent. Total ion yield is only weakly dependent on pulse width, and consistent with experimental estimates. Secondary reactions of matrix cations with analyte neutrals are efficient even though analyte ions are ablated in clusters of matrix.

Optically guided controlled release from liposomes with tunable plasmonic nanobubbles

A new method of optically guided controlled release was experimentally evaluated with liposomes containing a molecular load and gold nanoparticles (NPs). NPs were exposed to short laser pulses to induce transient vapor bubbles around the NPs, plasmonic nanobubbles, in order to disrupt the liposome and eject its molecular contents. The release efficacy was tuned by varying the lifetime and size of the nanobubble with the fluence of the laser pulse. Optical scattering by nanobubbles correlated to the molecular release and was used to guide the release. The release of two fluorescent proteins from individual liposomes has been directly monitored by fluorescence microscopy, while the generation of the plasmonic nanobubbles was imaged and measured with optical scattering techniques. Plasmonic nanobubble-induced release was found to be a mechanical, nonthermal process that requires a single laser pulse and ejects the liposome contents within a millisecond timescale without damage to the molecular cargo and that can be controlled through the fluence of laser pulse.

Control of Surface Shape in Nanostructure Formed with Femtosecond Laser Pulses

This paper reports the experimental results demonstrating that the nanostructured surface of diamondlike carbon film can be shaped so as to have a sawlike pattern with obliquely incident ppolarized femtosecond laser pulses. The nanoscale surface shape was observed as functions of incident angle, superimposed number and fluence of laser pulses and characterized with height and slope angle of the inclined surface. This results show that the inclined shape is formed with the non-uniform spatial distribution of local field enhanced on the nanostructured surface.

Tuning cell adhesion by controlling the roughness and wettability of 3D micro/nano silicon structures

The aim of this study is to investigate fibroblast cell adhesion and viability on highly rough three-dimensional (3D) silicon (Si) surfaces with gradient roughness ratios and wettabilities. Culture surfaces were produced by femtosecond (fs) laser structuring of Si wafers and comprised forests of conical spikes exhibiting controlled dual-scale roughness at both the micro- and the nano-scale. Variable roughness could be achieved by changing the laser pulse fluence and control over wettability and therefore surface energy could be obtained by covering the structures with various conformal coatings, which altered the surface chemistry without, however, affecting morphology. The results showed that optimal cell adhesion was obtained for small roughness ratios, independently of the surface wettability and chemistry, indicating a non-monotonic dependence of fibroblast adhesion on surface energy. Additionally, it was shown that, for the same degree of roughness, a proper change in surface energy could switch the behaviour from cell-phobic to cell-philic and vice versa, transition that was always correlated to surface wettability. These experimental findings are discussed on the basis of previous theoretical models describing the relation of cell response to surface energy. The potential use of the patterned Si substrates as model scaffolds for the systematic exploration of the role of 3D micro/nano morphology and/or surface energy on cell adhesion and growth is envisaged.

Physical properties of La0.7Ba0.3MnO3−δ complex oxide thin films grown by pulsed laser deposition technique

We report on transport properties of oxide manganite La0.7Ba0.3MnO3−δ (LBMO) thin films deposited by pulsed laser deposition (PLD) technique. Detailed analysis of heavy-ion stoichiometric composition has been carried out as a function of laser-pulse fluence and ambient oxygen pressure. Depositions using high-fluence (6 J/cm2) and low oxygen pressure (10−2 mbar) provide the optimal heavy-ion stoichiometric ratio in the LBMO samples. Deviations from the optimal LBMO stoichiometry are observed when decreasing the laser fluence or increasing the background oxygen pressure. This behavior is interpreted by considering the influence of the experimental deposition conditions on the plume dynamics. All these findings provide clear insights on the PLD-growth of manganites and, more in general, of complex oxide materials.

Shaping of nanostructured surface in femtosecond laser ablation of thin films

We report that the nanostructured surface of diamondlike carbon films can be shaped so as to have a sawlike pattern with obliquely incident p-polarized femtosecond laser pulses. The nanoscale surface shape was observed as functions of incident angle, superimposed number and fluence of laser pulses and characterized with height and slope angle of the inclined surface. It is shown that the inclined shape is formed with the non-uniform spatial distribution of the local field enhanced on the nanostructured surface.

Influence of the pulse number and fluence of a nanosecond laser on the ablation rate of metals, semiconductors and dielectrics

We investigated the pulsed laser ablation of metallic (Al), semiconductor (Si), and wide bandgap dielectric (LiNbO 3 ) targets in air at normal atmospheric conditions by using 4.5 ns pulses at 532 nm wavelength. We determined the dependence of the ablation rate on the pulse number and laser fluence. The number of consecutive laser pulses hitting the target on the same area was between 5 and 40, and the laser fluence was varied in the range of 10-250 J/cm 2 by changing the irradiated area at the target surface. We find that the ablation rate of the three targets is approximately constant when the pulse number is smaller than 15. Further increase of the pulse number leads to a decrease of the ablation rate, the fastest decrease of the ablation rate with pulse number being observed for the dielectric target. The dependence of the ablation rate on the laser fluence indicates two different regimes. In the first regime, which is for values of the fluence smaller than the threshold value (~70 J/cm 2 for Al, ~90 J/cm 2 for Si, and ~180 J/cm 2 for LiNbO 3 ), the ablation rate increases approximately logarithmically with the fluence. In the second regime, characterized by values of the fluence greater than the threshold value, there is a steep increase of the ablation rate. This sudden jump of the ablation rate at the threshold fluence is due to the transition from normal vaporisation to phase explosion, and to the changes in the dimensionality of the plasma-plume hydrodynamics from one-dimensional to three-dimensional.

Low Power Visible-to-UV Upconversion

Low power visible-to-UV photon upconversion is demonstrated for the first time, achieved from two simple organic chromophores dissolved in benzene. Selective 442 nm excitation of the triplet sensitizer 2,3-butanedione (biacetyl) in the presence of the laser dye 2,5-diphenyloxazole (PPO) results in the observation of singlet fluorescence from the latter in the UV centered at 360 nm, anti-Stokes shifted by a record 0.64 eV with respect to the excitation. All of the experimental data are consistent with the upconverted singlet PPO fluorescence being produced as a result of biacetyl-sensitized triplet-triplet annihilation (TTA) of triplet excited PPO chromophores. Nanosecond laser flash photolysis performed under pseudo-first-order conditions revealed the bimolecular rate constant of triplet-triplet energy transfer between the biacetyl sensitizer and PPO acceptor, k(q) = 9.0 x 10(8) M(-1)s(-1). The TTA process was confirmed by the quadratic dependence of the upconverted integrated PPO emission intensity measured with respect to incident 442 nm light power density. The maximum quantum yield of the upconverted emission (0.0058 +/- 0.0002) was determined relative to 1,8-diphenyl-1,3,5,7-octatetraene, both measured with 0.389 W/cm(2) incident power density. The PPO triplet-triplet annihilation rate constant (k(TT)) was determined from transient absorption decays monitored at the peak of its characteristic triplet-to-triplet excited-state absorption (500 nm) as a function of incident pulsed laser fluence; this process attains the diffusion limit in benzene at room temperature, k(TT) = 1.1 +/- 0.1 x 10(10) M(-1) s(-1).

A Comparative Study of Two-Temperature and Boltzmann Transport Models for Electron-Phonon Nonequilibrium

Ultrashort-pulsed laser irradiation on metals creates a thermal nonequilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the energy carriers or when the carriers mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this article, the nonequilibrium between energy carriers caused by a laser pulse interaction with a metal film is modeled via a numerical solution of the Boltzmann transport model (BTM) for electrons and phonons. A comparative assessment of the two-temperature model and its variants is carried out relative to the BTM. The higher order Runge-Kutta discontinuous Galerkin (RKDG) method is used for numerical discretization of the models. In this study, the gold film thickness is varied between 2–2000 nm, and the laser pulse duration and fluence are varied between 5 fs to 10 ps and 10–2000 J/m2, respectively. It is found that BTM shows the best agreement with the experimental data compared to the two-temperature models for the electron and phonon temperature profiles and the melting threshold fluence.

Tailoring the wetting response of silicon surfaces via fs laser structuring

Control over the wettability of solids and manufacturing of functional surfaces with special hydrophobic and self-cleaning properties has aroused great interest because of its significance for a vast range of applications in daily life, industry and agriculture. We report here a simple method for preparing stable superhydrophobic surfaces by irradiating silicon (Si) wafers with femtosecond (fs) laser pulses and subsequently coating them with chloroalkylsilane monolayers. It is possible, by varying the laser pulse fluence on the surface, to achieve control of the wetting properties through a systematic and reproducible variation of roughness at micro- and nano-scale which mimics both the topology of the “model” superhydrophobic surface—the natural lotus leaf—, as well as its wetting response. Water droplets can move along these irradiated superhydrophobic surfaces, under the action of small gravitational forces, and experience subsequent immobilization, induced by surface tension gradients. These results demonstrate the potential of manipulating liquid motion through selective laser patterning.

Photothermal Bubbles as Optical Scattering Probes for Imaging Living Cells

We propose and have experimentally studied a new method with improved sensitivity and specificity of imaging of living cells. Intracellular photothermal bubbles generated around gold nanoparticles (NPs) and their clusters were proposed as optical scattering probes for the amplification of scattered light. Microbubbles generated around gold spheres and shells with 10-ns 532-nm laser pulses in individual living cells (leukemia cells, lung and squamous carcinoma cancer cells) have amplified optical side scattering up to 1800-times relative to that of intracellular gold NPs, and without detectable damage to host cells. We explain the discovered optical amplification by the endocytosis-mediated clustering of NPs in cells, and by the selective generation of microbubbles (that do not disrupt the host cell) around these clusters at minimal levels of laser pulse fluence. Photothermal bubbles generated around laser-activated gold NPs may significantly improve the sensitivity and specificity of cell imaging, and can be considered as a new type of optical cellular probes.

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm2 and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm2 (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm2). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.

Surface plasmon scattering on polymer–bimetal layer covered fused silica gratings generated by laser induced backside wet etching

Large amplitude fused silica gratings are prepared by combining the UV laser induced backside wet etching technique (LIBWE) and the two-beam interference method. The periodic patterning of fused silica surfaces is realized by s-polarized fourth harmonic beams of a Nd:YAG laser, applying saturated solution of naphthalene in methyl-methacrylate as liquid absorber. Atomic force microscopy is utilized to analyze how the modulation amplitude of the grating can be controlled by the fluence and number of laser pulses. Three types of plasmonic structures are prepared by a bottom-up method, post-evaporating the fused silica gratings by gold–silver bimetal layers, spin-coating the metal structures by thin polycarbonate films, and irradiating the multilayers by UV laser. The effect of the bimetal and polymer-coated bimetal gratings on the surface plasmon resonance is investigated in a modified Kretschmann arrangement allowing polar and azimuthal angle scans. It is demonstrated experimentally that scattering on rotated gratings results in additional minima on the resonance curves of plasmons excited by second harmonic beam of a continuous Nd:YAG laser. The azimuthal angle dependence proves that these additional minima originate from back-scattering. The analogous reflectivity minima were obtained by scattering matrix method calculations realized taking modulation depths measured on bimetal gratings into account.

Laser pulse induced gold nanoparticle gratings

We report the results of our experimental investigation of laser induced gold nanoparticle gratings and their optical diffraction properties. A single shot of a pair Nd-YAG laser pulses with the same polarization is directed toward a 6 nm thick gold film on a substrate of polymethyl methacrylate. As a result of the laser illumination, the thin gold film is fragmented into an array of nanoparticles. Through the observation of scanning electron and dark-field optical microscopes, we discovered that the morphology of the gold nanoparticle grating is dependent on the fluence of laser pulse. The spectrum of first order diffraction shows the dependence on the absorbance property due to the presence of the nanoparticles. The ablation of nanothickness thin films via the use of laser pulses may provide a simple and efficient method for the fabrication of nanoscale structures, including two dimensional arrays of nanoparticles.

Bio-inspired water repellent surfaces produced by ultrafast laser structuring of silicon

We report here an efficient method for preparing stable superhydrophobic and highly water repellent surfaces by irradiating silicon wafers with femtosecond laser pulses and subsequently coating them with chloroalkylsilane monolayers. By varying the laser pulse fluence on the surface one can successfully control its wetting properties via a systematic and reproducible variation of roughness at micro- and nano-scale, which mimics the topology of natural superhydrophobic surfaces. The self-cleaning and water repellent properties of these artificial surfaces are investigated. It is found that the processed surfaces are among the most water repellent surfaces ever reported. These results may pave the way for the implementation of laser surface microstructuring techniques for the fabrication of superhydrophobic and self-cleaning surfaces in different kinds of materials as well.