Increasing Laser Fluence Research Articles

High-vacuum time-resolved laser-induced incandescence of flame-generated soot

We have measured time-resolved laser-induced incandescence of flame-generated soot under high-vacuum conditions (4.1×10−6 mbar) at an excitation wavelength of 532 nm with laser fluences spanning 0.06–0.5 J/cm2. We generated soot in an ethylene/air diffusion flame, introduced it into the vacuum system with an aerodynamic lens, heated it using a pulsed laser with a spatially homogeneous and temporally smooth laser profile, and recorded LII temporal profiles at 685 nm. At low laser fluences LII signal decay rates are slow, and LII signals persist beyond the residence time of the soot particles in the detection region. At these fluences, the temporal maximum of the LII signal increases nearly linearly with increasing laser fluence until reaching a plateau at ∼0.18 J/cm2. At higher fluences, the LII signal maximum is independent of laser fluence within experimental uncertainty. At these fluences, the LII signal decays rapidly during the laser pulse. The fluence dependence of the vacuum LII signal is qualitatively similar to that observed under similar laser conditions in an atmospheric flame but requires higher fluences (by ∼0.03 J/cm2) for initiation. These data demonstrate the feasibility of recording vacuum LII temporal profiles of flame-generated soot under well-characterized conditions for model validation.

Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m λ ). At low fluence, small permanent increases of E(m λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.

Compelling Evidence for Lucky Survivor and Gas Phase Protonation: The Unified MALDI Analyte Protonation Mechanism

This work experimentally verifies and proves the two long since postulated matrix-assisted laser desorption/ionization (MALDI) analyte protonation pathways known as the Lucky Survivor and the gas phase protonation model. Experimental differentiation between the predicted mechanisms becomes possible by the use of deuterated matrix esters as MALDI matrices, which are stable under typical sample preparation conditions and generate deuteronated reagent ions, including the deuterated and deuteronated free matrix acid, only upon laser irradiation in the MALDI process. While the generation of deuteronated analyte ions proves the gas phase protonation model, the detection of protonated analytes by application of deuterated matrix compounds without acidic hydrogens proves the survival of analytes precharged from solution in accordance with the predictions from the Lucky Survivor model. The observed ratio of the two analyte ionization processes depends on the applied experimental parameters as well as the nature of analyte and matrix. Increasing laser fluences and lower matrix proton affinities favor gas phase protonation, whereas more quantitative analyte protonation in solution and intramolecular ion stabilization leads to more Lucky Survivors. The presented results allow for a deeper understanding of the fundamental processes causing analyte ionization in MALDI and may alleviate future efforts for increasing the analyte ion yield.

Room Temperature Fabricated ZnO:Al with Elevated and Unique Light-Trapping Performance

ABSTRACTWe present a novel ZnO:Al fabrication process consisting of room-temperature vacuum sputtering followed by an excimer laser annealing (ELA). The ELA treatment improves the optical transmission of the films, and the film resistivities (&lt;1 mΩ·cm) remain stable or improve with increasing laser fluence up to 0.6 J/cm2, as the carrier density increases but the carrier mobility is degraded. This process is followed by a standard dilute HCl chemical texturing step, and produces substrates with suitable texture, conductivity, and transparency properties for thinfilm photovoltaic applications. Substrates resulting from this process display elevated haze levels (80% at 600 nm and 50% at 800 nm) after the wet-chemical etching step. Such substrates have been used to make single junction hydrogenated nanocrystalline silicon solar cells, and an increase in the short-circuit current of up to 2.2 mA/cm2 is observed compared to a substrate deposited by a standard room-temperature sputtering + wet-etch process. This gain is primarily due to increased photo-response in the red due to improved light-scattering, as at wavelengths greater than 600 nm, a gain in photocurrent of up to 1.7 mA/cm2 is observed.

The morphological and optical characteristics of femtosecond laser-induced large-area micro/nanostructures on GaAs, Si, and brass

We systematically study the morphological and optical characteristics of the large-area micro/nanostructures produced by femtosecond laser irradiation on GaAs, Si, and brass. The experimental results demonstrate that along with the increase of laser fluence, significant changes in the surface morphology can be observed, and the most prominent phenomenon is the enlarging of the feature size of formed structures. Interestingly, by the fourier analysis of the treated areas, a peculiar phenomenon can be revealed: as laser fluence increases, the spatial frequencies of the structures change following a specific law--the allowed main frequencies are discrete, and appear to be a sequence of 2f, f, f/2, f/4, and f/8 (f is the fundamental frequency corresponding to the near-subwavelength ripples). In our opinion, the new frequency components of f/2, f/4, and f/8 originate in the 2-order, 4-order, and 8-order grating coupling. The law can offer us new insights for the evolving mechanisms of a variety of laser-induced micro/nanostructures in different scales. Furthermore, the optical characteristics of the treated surface are strongly dependent on the morphological characteristics that are mainly determined by laser fluence, such as the feature size of the micro/nanostructures, the topology of the surface morphology, the surface roughness, and the irregular degree of the formed structures. In general, as laser fluence increases in a moderate range, the specular reflectance of the structured surface would be significantly reduced. However, if laser fluence is excessive, the anti-specular-reflection effect would be much weakened. In ideal laser fluence, the micro/nanostructures produced by the near-infrared laser can achieve an ultra-low specular reflectance in the visible and near-infrared spectral region, which exhibits an attracting application prospect in the field of utilizing solar energy.

Bioactivities of a Ti surface ablated with a femtosecond laser through SBF

Micro-patterns with deposited calcium/phosphorus (Ca/P) elements on a pure titanium (Ti) surface were synthesized in 2.5× simulated body fluid (SBF) by femtosecond laser ablation, and their bioactivities were evaluated by co-culture with osteoblasts. Scanning electron microscope observation showed that a three-order roughness micro-pattern, which was composed of grooves, islands, holes and sub-micro-particles, was obtained, and the size of holes and islands was increased from 0.5 µm and 2 µm to 5 µm and 6 µm, respectively, gradually with the increase of laser fluence from 3.3 J cm−2 to 12.5 J cm−2. Irradiation of femtosecond lasers promoted the deposition of Ca/P on the surface of a Ti plate. The osteoblasts grew faster and extended more slender filopodia on the femtosecond laser-ablated surface than on a sand blast followed by an acid surface, which demonstrated that the micro-patterns with deposited Ca/P on the surface were favorable for osteoblasts’ growth.

Atomic force microscopy, Raman spectroscopy and nonlinear absorption properties of femtosecond laser irradiated CR-39

Investigations have been performed to explore ultrashort laser irradiation effects on the surface topography as well as structural and nonlinear absorption properties of a polymer CR-39. For this purpose, a CR-39 target was exposed in air to 25 fs, 800 nm Ti:sapphire laser radiation at fluences ranging from 0.25 J cm−2 to 3.6 J cm−2. The surface features, structural changes and nonlinear absorption were explored by AFM, Raman Spectroscopy and a Z-scan technique, respectively. Several topographical structures like bumps, explosions and nano cavities have been observed on the irradiated surface. Raman spectroscopy reveals changes in the fundamental structure of the polymer after the irradiation. Nonlinear absorption data contained by the Z-scan technique predict the dominance of three-photon absorption in case of pristine CR-39. Furthermore, nonlinear absorption (three or two photon) increases with increasing laser fluences and is well correlated with surface and structural changes revealed by AFM and Raman spectroscopy.

Effects of laser fluence on the structural properties of pulsed laser deposited ruthenium thin films

Ruthenium (Ru) has received great interest in recent years for applications in microelectronics. Pulsed laser deposition (PLD) enables the growth of Ru thin films at low temperatures. In this paper, we report for the first time the characterization of pulsed laser deposited Ru thin films. The deposition processes were carried out at room temperature in vacuum environment for different durations with a pulsed Nd:YAG laser of 355-nm laser wavelength, employing various laser fluences ranging from 2 J/cm2 to 8 J/cm2. The effect of the laser fluence on the structural properties of the deposited Ru films was investigated using surface profilometry, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Ru droplets, some spherical in shape and some flattened into round discs were found on the deposited Ru. The droplets were correlated to ripple formations on the target during the laser-induced ejection from the target. In addition, crystalline Ru with orientations of (100), (101), and (002) was observed in the XRD spectra and their intensities were found to increase with increasing laser fluence and film thickness. Grain sizes ranging from 20 nm to 35 nm were deduced using the Scherrer formula. Optical emission spectroscopy (OES) and energy-dispersive X-ray spectroscopy (EDS) show that the composition of the plume and the deposited Ru film was of high purity.

Plasma Membrane Integrity and Survival of Melanoma Cells After Nanosecond Laser Pulses

Circulating tumor cells (CTCs) photoacoustic detection systems can aid clinical decision-making in the treatment of cancer. Interaction of melanin within melanoma cells with nanosecond laser pulses generates photoacoustic waves that make its detection possible. This study aims at: (1) determining melanoma cell survival after laser pulses of 6 ns at λ = 355 and 532 nm; (2) comparing the potential enhancement in the photoacoustic signal using λ = 355 nm in contrast with λ = 532 nm; (3) determining the critical laser fluence at which melanin begins to leak out from melanoma cells; and (4) developing a time-resolved imaging (TRI) system to study the intracellular interactions and their effect on the plasma membrane integrity. Monolayers of melanoma cells were grown on tissue culture-treated clusters and irradiated with up to 1.0 J/cm2. Surviving cells were stained with trypan blue and counted using a hemacytometer. The phosphate buffered saline absorbance was measured with a nanodrop spectrophotometer to detect melanin leakage from the melanoma cells post-laser irradiation. Photoacoustic signal magnitude was studied at both wavelengths using piezoelectric sensors. TRI with 6 ns resolution was used to image plasma membrane damage. Cell survival decreased proportionally with increasing laser fluence for both wavelengths, although the decrease is more pronounced for 355 nm radiation than for 532 nm. It was found that melanin leaks from cells equally for both wavelengths. No significant difference in photoacoustic signal was found between wavelengths. TRI showed clear damage to plasma membrane due to laser-induced bubble formation.

Nonlinear absorption properties correlated with the surface and structural changes of ultra short pulse laser irradiated CR-39

We have investigated femtosecond laser irradiation effects on the surface topography, structural changes and nonlinear absorption properties of CR-39. For this purpose, a CR-39 target was exposed in air to 25 fs, 800 nm Ti: sapphire laser radiation at fluences ranging from 0.25 J cm−2 to 3.6 J cm−2. The surface of irradiated CR-39 probed by an Atomic Force Microscope (AFM) exhibits the formation of several topographical structures, like bumps, explosions and nano cavities. Raman spectroscopy is performed to explore chemical and structural modification of the irradiated target. The spectroscopy reveals changes such as cross linking, bond breaking, formation of new bonds etc. in the fundamental structure of the polymer after irradiation. In order to establish a correlation between morphological and structural changes with the changes in the nonlinear absorption of the irradiated CR-39, a Z-scan technique was employed. A comparison of experimentally obtained data from Z-scan measurements with our calculations predicts the dominance of three-photon absorption in the case of pristine CR-39, whereas for irradiated targets concurrence of three- and two-photon absorption is probable. Nonlinear absorption increases with increasing laser fluences and is well correlated by surface and structural changes revealed by AFM and Raman spectroscopy.

Micromachining of silicon with excimer laser in air and water medium

Laser micromachining of silicon was performed by KrF excimer laser, utilising the beam energy density (fluence) in the range of 1.33-5 J/cm 2 . Experiments were conducted on silicon wafer surface both in air and water medium. A qualitative study of the surface topography was carried out from the micrographs of the different machined spots. A mathematical model was developed to represent the process. The ablation/etch depth is found to increase rapidly with increase in laser fluence after a threshold value. The variation is nonlinear, irrespective of the surface topography and it gets a saturation value at higher fluence.

Laser Induced Modification in Glasses by CW Laser Backside Irradiation (CW-LBI)

We studied on the modification inside glass formed in the trajectory of a platinum particle implanted by laser illumination. A 1-μm-thick platinum film was deposited on the surface of Pyrex glass. The platinum film was irradiated by a focused Ar ion laser beam with a laser power of 4.2 W from the other side through the glass. As a result, platinum film was melted and a platinum particle with the diameter of 5 μm was implanted into the glass. The glass around the platinum particle heated by the laser illumination was softened, which enables the platinum particle migration in the glass. The threshold fluence for the platinum particle implantation was 0.1 MW/cm2. The speed of the platinum particle migration increased in the range from 0.1 mm/s to 10 mm/s with increasing laser fluence. Platinum was not detected and no clear composition change was detected in the trajectory of the platinum particle migration by energy dispersive X-ray spectroscopy. Etching of the modified zone formed cone shaped structures. The modified zone was bended and curved by changing the direction of the laser beam.

The mulDamage characteristics of HfO2/SiO2 high reflector at 45° incidence in 1-on-1 and N-on-1 tests

P-polarization high reflectors are deposited by e-beam from hafnia and silica. 1-on-1 and N-on-1 tests at 1064-nm wavelength with P-polarization at 45° incidence are carried out on these samples. Microscope and scanning electron microscope are applied to investigate the damage morphologies in both 1-on-1 and N-on-1 tests. It is found that the laser damage threshold is higher in N-on-1 tests and nodular defect is the main inducement that leads to the damage because nodular ejection with plasma scalding is the typical damage morphology. Similar damage morphology observed in the two tests indicates that the higher laser damage threshold in N-on-1 test is attributed to the mechanical stabilization process of nodular defects, owing to the gradually increased laser fluence radiation. Based on the typical morphology study, some process optimizations are given.

Time-domain investigation of laser-induced diffusion of CO on a vicinal Pt(111) surface

We report on a time-domain study of diffusion of CO on a vicinal Pt(111) surface at low substrate temperature induced electronically via substrate electrons, which have been optically excited by femtosecond-laser pulses. Hopping rates from step sites to terrace sites have been determined using optical second-harmonic generation as a sensitive probe of the step coverage. The dynamics of the electronic energy transfer between the excited substrate electrons and the adsorbate degrees of freedom have been investigated by the application of a two-pulse correlation scheme. We observe a nonlinear dependence of the hopping rate on the laser fluence $(\ensuremath{\propto}{F}^{6})$ in a fluence range of $F=1.9--4.3\text{ }\text{mJ}/{\text{cm}}^{2}$ and a narrow two-pulse correlation with a width of below 0.5 ps that increases with increasing laser fluence. It will be shown that the broadening of the two-pulse correlation with laser fluence is a general phenomenon for substrate mediated laser-induced surface reactions.

Internal energy deposition with silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry

The use of silicon nanoparticles for laser desorption/ionization (LDI) is a new appealing matrix-less approach for the selective and sensitive mass spectrometry of small molecules in MALDI instruments. Chemically modified silicon nanoparticles (30 nm) were previously found to require very low laser fluence in order to induce efficient LDI, which raised the question of internal energy deposition processes in that system. Here we report a comparative study of internal energy deposition from silicon nanoparticles to previously explored benzylpyridinium (BP) model compounds during LDI experiments. The internal energy deposition in silicon nanoparticle-assisted laser desorption/ionization (SPALDI) with different fluorinated linear chain modifiers (decyl, hexyl and propyl) was compared to LDI from untreated silicon nanoparticles and from the organic matrix, α-cyano-4-hydroxycinnamic acid (CHCA). The energy deposition to internal vibrational modes was evaluated by molecular ion survival curves and indicated that the ions produced by SPALDI have an internal energy threshold of 2.8–3.7 eV. This is slightly lower than the internal energy induced using the organic CHCA matrix, with similar molecular survival curves as previously reported for LDI off silicon nanowires. However, the internal energy associated with desorption/ionization from the silicon nanoparticles is significantly lower than that reported for desorption/ionization on silicon (DIOS). The measured survival yields in SPALDI gradually decrease with increasing laser fluence, contrary to reported results for silicon nanowires. The effect of modification of the silicon particle surface with semifluorinated linear chain silanes, including fluorinated decyl (C10), fluorinated hexyl (C6) and fluorinated propyl (C3) was explored too. The internal energy deposited increased with a decrease in the length of the modifier alkyl chain. Unmodified silicon particles exhibited the highest analyte internal energy deposition. These findings may suggest a role of the modifier as a moderator in the energy dissipation and relaxation process. The relatively low internal energy content of SPALDI-produced ions indicates that this is a “soft” desorption technique, with potential advantages in the analysis of labile compounds.

Thermal and gasdynamic analysis of ablation of poly(methyl methacrylate) by pulsed IR laser irradiation under conditions of nanoparticle formation

We present comprehensive experimental and theoretical studies of poly(methyl methacrylate) (PMMA) ablation by single pulses of 9.17 µm laser radiation in a wide range of fluences. Ablation was carried out in a quartz cell through which nitrogen under atmospheric pressure was pumped. The composition of the ablation products was analysed with the use of an automatic diffusion battery. Both the irradiated surface and the substrate with the deposited ablation products were examined by transmission electron microscopy. Different aspects and stages of ablation have been investigated: laser heating and vaporization of the PMMA surface, mechanisms of ablation, dynamics of the laser-induced plume, nanoparticle formation in the plume and/or ejection from the irradiated surface. It has been found that the size distribution of nanoparticles formed during ablation changes its form from a single peak to a bimodal shape with increasing laser fluence. The transformation of the size distribution is analysed with the help of thermal and gasdynamic modelling which gives a basis for insight into the mechanisms and dynamics of ablation.

The Role of Thermal and Electronic Pressure in the Picosecond Acoustic Response of Femtosecond Laser-excited Solids

AbstractUltrafast time-resolved X-ray diffraction has been used to study the dynamics of coherent acoustic phonons in fs laser-excited Ge and Au, with the particular goal to clarify the interplay of the electronic and thermal pressure contributions. For semiconductors it is usually assumed that the electronic pressure is the dominant driving force. Our measurements reveal that in Ge the relative strength of the electronic pressure decreases with increasing laser fluence. Only for low fluences the electronic pressure dominates, while at high fluences the thermal pressure exceeds the electronic pressure. For the case of Au the data are well described within the established theoretical framework using the known values for those material parameters which determine the laser-induced pressure, namely the energy relaxation time and the electronic and lattice Grüneisen parameters.

Effects of femtosecond laser ablation on Vitrovac 6025X

In the development of extremely sensitive magnetic sensors, the processing of the soft magnetic materials is extremely critical as this affects the performance of the materials in the sensing element. Laser cutting is a fast, direct and simple approach to process soft magnetic materials into magnetic sensors. Traditionally, nanosecond laser introduces thermal stresses which greatly deteriorate magnetic properties. Femtosecond laser, which has a lower ablation threshold and smaller heat affected zones, stands to be a potential candidate to solve such thermal and processing issues of processed magnetic materials. In this paper, Ti: sapphire femtosecond laser was used to pattern and cut commercially available magnetic ribbon (Vitrovac 6025X) into a square of 5 mm × 5 mm with a 2 mm long channel at the centre of the specimens. Laser fluence was varied to investigate the effects of femtosecond laser on (i) quality of laser ablated channel; (ii) material composition; and (iii) magnetic properties of the magnetic ribbon. Channels with well-defined edges were observed at lower laser fluences. With increasing laser fluence, molten walls were formed. Chemical composition of the magnetic ribbon remains similar at the channel walls, regions just beside the walls and no laser-processed areas. Hysteresis loops obtained for varying laser fluences and varying the number of ablated channels in the magnetic foil suggest that femtosecond laser processing does not significantly affect the coercivities and magnetic anisotropies of the workpiece material.

Structural and optical properties of nano-structured tungsten-doped ZnO thin films grown by pulsed laser deposition

Novel highly c-oriented tungsten-doped zinc oxide (WZO) thin films with 1 wt% were grown by pulsed laser deposition (PLD) technique on corning 1737F glass substrate. The effects of laser energy on the structural, morphological as well as optical transmission properties of the films were studied. The films were highly transparent with average transmittance exceeding 87% in the wavelength region lying between 400 and 2500 nm. X-ray diffraction analysis (XRD) results indicated that the WZO films had c-axis preferred orientation with wurtzite structure. Film thickness and the full width at half maximum (FWHM) of the (0 0 2) peaks of the films were found to be dependent on laser fluence. The composition determined through Rutherford backscattering spectroscopy (RBS) appeared to be independent of the laser fluence. By assuming a direct band gap transition, the band gap values of 3.36, 3.34 and 3.31 eV were obtained for corresponding laser fluence of 1, 1.7 and 2.7 J cm −2, respectively. Compared with the reported undoped ZnO band gap value of 3.37 eV, it is conjectured that the observed low band gap values obtained in this study may be attributable to tungsten incorporation in the films as well as the increase in laser fluence. The high transparency makes the films useful as optical windows while the high band gap values support the idea that the films could be good candidates for optoelectronic applications.

Ultrafast melting and resolidification of gold particle irradiated by pico- to femtosecond lasers

Ultrafast melting and resolidification of a submicron gold particle subject to pico- to femtosecond laser pulse are studied in this paper. The nonequilibrium heat transfer in the electrons and lattice is described using a two-temperature model, and the locations of the solid-liquid interface are determined using an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. The results showed that the maximum melting depth, peak interfacial temperature, and velocity increase with the decreasing particle size and pulse width or with the increasing laser fluence.