Impact Of Laser Pulse Research Articles

Drop Shaping by Laser-Pulse Impact

The impact of a laser pulse on a microdrop of opaque liquid induces a spectacular response: The drop is propelled forward rapidly, and is strongly deformed until it breaks up or even explodes. A detailed understanding of this process is crucial for generating extreme-ultraviolet (EUV) light in nanolithography. The authors reveal that droplet propulsion is driven by localized boiling on the illuminated side and subsequent expansion, in proportion to the energy of the laser pulse.

Nanosecond laser treatment of graphene

Laser processing of graphene is of great interest for cutting, patterning and structural engineering purposes. Tunable nanosecond lasers have the advantage of being relatively widespread (compared to e.g. femtosecond or high-power continuous wave lasers). Hereby we have conducted an investigation of the impact of nanosecond laser pulses on CVD graphene. The damage produced by sufficiently strong single shots (pulse width 5ns, wavelength 532 or 266nm) from tunable optical parametric oscillator was investigated by the methods of scanning electron microscopy and optical microspectroscopy (Raman and fluorescence). Threshold of energy density for producing visible damage was found to be ~200mJ/cm2. For UV irradiation the threshold could be notably less depending on the origin of sample. Surprisingly strong fluorescence signal was recorded from damaged areas and is attributed to the residues of oxidized graphene.

Complex processing of interferograms of light-erosion gas-plasma streams in vacuum

A technique for automated processing of the results of interferometry of light-erosion gas-plasma flows using the local thermodynamic equilibrium approximation and data on the mass flow rate of the target substance was developed. The application of this technique allowed evaluation of the spatiotemporal distributions of the optical (refraction and absorption coefficients), thermophysical (temperature, density, and electron concentration), gas-dynamic (velocity distribution of particles, average-mass velocity, and pressure), and optomechanical (momentum coupling coefficient) characteristics of light-erosion gas-plasma flows. The features of application of frequency filters at various stages of automatic processing of measurement results are considered. Examples of processing the experimental results that were obtained under the impact of ultrashort laser pulses on condensed media are presented.

Simulation of the Impact of Refractive Surgery Ablative Laser Pulses with a Flying-Spot Laser Beam on Intrasurgery Corneal Temperature

To evaluate ablation algorithms and temperature changes in laser refractive surgery. The model (virtual laser system [VLS]) simulates different physical effects of an entire surgical process, simulating the shot-by-shot ablation process based on a modeled beam profile. The model is comprehensive and directly considers applied correction; corneal geometry, including astigmatism; laser beam characteristics; and ablative spot properties. Pulse lists collected from actual treatments were used to simulate the temperature increase during the ablation process. Ablation efficiency reduction in the periphery resulted in a lower peripheral temperature increase. Steep corneas had lesser temperature increases than flat ones. The maximum rise in temperature depends on the spatial density of the ablation pulses. For the same number of ablative pulses, myopic corrections showed the highest temperature increase, followed by myopic astigmatism, mixed astigmatism, phototherapeutic keratectomy (PTK), hyperopic astigmatism, and hyperopic treatments. The proposed model can be used, at relatively low cost, for calibration, verification, and validation of the laser systems used for ablation processes and would directly improve the quality of the results.

Diagnostics of plasma produced by femtosecond laser pulse impact upon a target with an internal nanostructure

X-ray diagnostics of the interaction of femtosecond laser pulses with intensities of 1016–1018 W/cm2 with CO2 clusters and frozen nanosize water particles is carried out. The stage of cluster expansion and the formation of a plasma channel, which governs the parameters of the formed X-ray radiation source and accelerated ion flows, is studied. The measurements are based on recording spatially resolved X-ray spectra of H- and He-like oxygen ions. Utilization of Rydberg transitions for spectra diagnostics makes it possible to determine plasma parameters on a time scale of t ∼ 10 ps after the beginning of a femtosecond pulse. The role of the rear edge of the laser pulse in sustaining the plasma temperature at a level of ∼100 eV in the stage of a nonadiabatic cluster expansion is shown. The analysis of the profiles and relative intensities of spectral lines allows one to determine the temperature and density of plasma electrons and distinguish the populations of “thermal” ions and ions that are accelerated up to energies of a few tens of kiloelectronvolts. It is shown that the use of solid clusters made of frozen nanoscale water droplets as targets leads to a substantial increase in the number of fast He-like ions. In this case, however, the efficiency of acceleration of H-like ions does not increase, because the time of their ionization in plasma exceeds the time of cluster expansion.

Light-Induced Precession of Magnetization in Ferromagnetic Semiconductor (Ga,Mn)As

Precession of magnetization induced by laser pulses in (Ga,Mn)As was studied by the pump-and-probe technique. We concentrated on various experimental aspects that can be used to identify a part of the measured magneto-optical signals that provides information about the dynamics of magnetization. We also revealed that that it takes about 80 ps before the quasi-equilibrium precession of magnetization is initiated by the impact of laser pulse.

Self‐organized regular surface patterning by pulsed laser ablation

AbstractThe impact of intense ultra short laser pulses on solid surface ‐ as in laser ablation ‐ results in a transient perturbation of the material to a state far from equilibrium. Due to ultrafast relaxation of the transient disorder in a few picoseconds, self‐organized surface patterns occur, with a typical feature size at the order of 100 nm or less, similar as observed in ion sputtering and explained by non‐linear dynamics models. The feature size of these structures is determined by the deposited energy dose, their shape and orientation crucially depends on the state of polarization of the incident light. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Colossal Magnetoresistive Manganite Based Fast Bolometric X-ray Sensors for Total Energy Measurements of Free Electron Lasers

Bolometric detectors based on epitaxial thin films of rare earth perovskite manganites have been proposed as total energy monitors for X-ray pulses at the Linac Coherent Light Source free electron laser. We demonstrate such a detector scheme based on epitaxial thin films of the perovskite manganese oxide material Nd{sub 0.67}Sr{sub x0.33}MnO{sub 3}, grown by pulsed laser deposition on buffered silicon substrates. The substrate and sensor materials are chosen to meet the conflicting requirements of radiation hardness, sensitivity, speed and linearity over a dynamic range of three orders of magnitude. The key challenge in the material development is the integration of the sensor material with Si. Si is required to withstand the free electron laser pulse impact and to achieve a readout speed three orders of magnitude faster than conventional cryoradiometers for compatibility with the Linac Coherent Light Source pulse rate. We discuss sensor material development and the photoresponse of prototype devices. This Linac Coherent Light Source total energy monitor represents the first practical application of manganite materials as bolometric sensors.

Second order dispersive effect on zero-group velocity Lamb modes

After a local and transient loading of a plate, the acoustic energy decay in the source area can be explained by three mechanisms: the energy transport phenomenon at the Lamb wave group velocity, the material damping and the second-order dispersive effect. Generally, the first mechanism dominates the other two. However, some dispersion curves of Lamb modes present a minimum frequency for a nonzero wave number. Since at these points the group velocity vanishes, no energy transport occurs, and the slower other two phenomena can be observed. Using laser-based ultrasonic techniques, we show that the mechanical response of various thin metallic plates to a laser pulse impact is dominated by the resonance of the zero group velocity S1-Lamb mode. In the first microseconds, we observed a t-1/2 decay of the local vibration amplitude, which cannot be explained by the material damping. We demonstrated that this effect is ascribed to the second-order term in the dispersion relation. After this power law decay, the mechanical displacement undergoes an exponential decay corresponding to the wave damping. Then, we show that the local attenuation of the plate material can be measured without any mechanical contact.

Electron-positron pair creation by powerful laser-ion impact

In view of recently achieved extreme laser powers for which the ponderomotive energy ${U}_{p}\ensuremath{\gg}2m{c}^{2}$, it has become of interest to reconsider fundamental processes of quantum electrodynamics in such powerful laser fields. In the present work, we evaluate the rates of producing electron-positron pairs by the impact of laser pulses of extreme power on highly charged ions at rest. Contrary to previous investigations, we shall consider a linearly polarized laser pulse, since at extreme laser powers the contribution of the ${A}^{2}$ part of the electromagnetic interaction becomes significant. As long as the duration of the laser pulses considered is in the femtosecond domain, the description of the laser beam by a powerful, monochromatic electromagnetic plane wave will still be justified, which simplifies considerably the calculations required.

Laser pulse impact characterization using ultrasonic transducers

Experiments were conducted with an excimer laser (wavelength 248 nm) to cause damage to gelatinous materials, which have optical properties similar to skin tissue. The laser pulses, when they impact the specimen, generate an elastic wave that propagates through the material. A 150-MHz broadband ultrasonic transducer was placed underneath the specimen to observe the material response and the pressure generated by the laser pulse. Different incident laser energies and laser beam spot sizes were utilized to obtain a range of input parameters, and the corresponding transducer responses were recorded. Fourier analysis of the signals was performed to identify the frequency response from the laser pulse. Additional tests were carried out to observe the effects due to a train of laser pulses. Earlier tests were performed with 200-kHz hydrophone-calibrated transducer but due to the shortness of the laser pulses (∼20 ns), a higher frequency transducer was used to accurately characterize the laser impact. The initial results indicate that there is a dominant frequency response around the 25–30-MHz regime.

Dynamics of laser-induced desorption from dielectric surfaces on a sub-picosecond timescale

Abstract The impact of ultra-short laser pulses induces intensity-dependent non-equilibrium processes in the surface region of dielectric targets, resulting in desorption of surface constituents. We report time-resolved studies on particle ejection from CaF 2 and BaF 2 targets. Pump–probe time-of-flight mass spectrometry (ToF MS) was used to measure the particle yields as a function of the delay time between pairs of sub-damage threshold laser pulses, thus obtaining the temporal dynamics of the laser-excited charged particle emission. In a correlative manner, the positive ion, electron and negative ion desorption yields dependence on the pump–probe delay time reveal a coherence peak around zero delay (similar to a two-pulse autocorrelation), accounting for the increase of the ion yield with laser intensity. Additionally, the measurements reveal a delayed peaks at ∼900 fs (BaF 2 ) and ∼300 fs (CaF 2 ). For comparison, we present time-resolved studies on electron emission from aluminum targets with pump and probe pulses below the damage threshold. The measurements show, again, the autocorrelation peak in the coherence region and an additional increase in the electron yield when the pulses are several picoseconds apart. The autocorrelation could also stand for the dephasing time of the electronic coherence, while the delayed peaks may reveal for the time needed for the collisional energy to be transferred to the lattice. The pump pulse induces a new unstable phase, which is further destabilized by the probe pulse. A corresponding qualitative picture for temporal dynamics of femtosecond (fs) laser-induced particle emission is proposed.

An optimal process of femtosecond laser cutting of NiTi shape memory alloy for fabrication of miniature devices

The mechanical properties of NiTi shape memory alloy (SMA) components are sensitive to thermal influence during laser machining. To make the femtosecond laser cutting of NiTi material meet the strict fabrication requirements for miniature SMA devices with high precision, complex patterns and minimal heat affected zone (HAZ) along with high throughput, we report an optimal process of sideways-movement path planning in this article. Femtosecond laser processing of NiTi SMA using the fundamental wavelength of 775 nm from a Ti:sapphire laser along with its second and third harmonic irradiations were systematically investigated. We observed that the main impact of ultrashort laser pulse induced air breakdown on materials processing was beam widening. The laser beam at fundamental wavelength suffered less widening than its harmonic wavelengths. Femtosecond laser machining of metals is still basically a thermal mechanism. High ablation rates at higher laser fluences causes significant recast formation, while lower fluences resulted in better cutting quality at the expense of efficiency. The optimal process involving the method of sideways-movement path planning enables recast-free high-precision features at higher laser fluences with better throughput.

Computer Modeling of Physical Processes Occurring in Laser-Plasma Spraying of Diamond-Type Films

Physical and mathematical models of the dynamics of heating and evaporation of a carbon target have been developed. The dynamics of the erosion torch and its interaction with the substrate under the impact of nanosecond laser pulses on the graphite target have been modeled. The impact of thermodynamic and optical characteristics of the graphite target on the dynamics of heating and evaporation has been investigated.

Laser pulse impact on rat mesenteric blood vessels in relation to laser treatment of port wine stain

Background and Objective To study the impact of laser pulses on animal microvasculature as a model for laser treatment of port wine stains. Study Design/Materials and Methods Rat mesenteric blood vessels were irradiated with a laser pulse (585 nm, 0.2–0.6 ms pulse duration, 0.5–30 J/cm2 radiant exposure). Video microscopy was used to assess vessel dilation, formation of intravascular thrombi, bubble formation, and vessel rupture. Changes in reflection during a laser pulse were measured by simultaneously recording the temporal behavior of the incident and reflected signals. Results A threshold radiant exposure of approximately 3 J/cm2 was found for changes in optical properties of blood in vivo, confirming previous in vitro results. Often, laser exposure induced a significant increase in vessel diameter, up to three times the initial diameter for venules and four times for arterioles, within 200 ms after laser exposure. Arterioles were more likely to dilate than venules. Sometimes, immediately after the pulse, round structures, interpreted as being gas bubbles, were seen within the vessel lumen. Conclusions A variety of phenomena can occur when blood vessels of sizes comparable to those in port wine stains are irradiated with laser pulses as used in port wine stain treatment. Thrombus formation and vessel rupture have been described before from histological sections of laser-irradiated port wine stains. However, vessel dilation and formation of non-transient gas bubbles as found in this study have not been described before. Lasers Surg. Med. 39:461–468, 2001. © 2001 Wiley-Liss, Inc.

Laser pulse impact on rat mesenteric blood vessels in relation to laser treatment of port wine stain

To study the impact of laser pulses on animal microvasculature as a model for laser treatment of port wine stains. Rat mesenteric blood vessels were irradiated with a laser pulse (585 nm, 0.2-0.6 ms pulse duration, 0.5-30 J/cm(2) radiant exposure). Video microscopy was used to assess vessel dilation, formation of intravascular thrombi, bubble formation, and vessel rupture. Changes in reflection during a laser pulse were measured by simultaneously recording the temporal behavior of the incident and reflected signals. A threshold radiant exposure of approximately 3 J/cm(2) was found for changes in optical properties of blood in vivo, confirming previous in vitro results. Often, laser exposure induced a significant increase in vessel diameter, up to three times the initial diameter for venules and four times for arterioles, within 200 ms after laser exposure. Arterioles were more likely to dilate than venules. Sometimes, immediately after the pulse, round structures, interpreted as being gas bubbles, were seen within the vessel lumen. A variety of phenomena can occur when blood vessels of sizes comparable to those in port wine stains are irradiated with laser pulses as used in port wine stain treatment. Thrombus formation and vessel rupture have been described before from histological sections of laser-irradiated port wine stains. However, vessel dilation and formation of non-transient gas bubbles as found in this study have not been described before.

Contribution of vaporization and boiling to thermal-spike sputtering by ions or laser pulses.

Here we consider what, in our terminology, we designate as normal vaporization, normal boiling, and phase explosion. In the case of vaporization, one is dealing with the emission of particles (atoms or molecules) from the extreme outer surface of either a solid or liquid for any temperature exceeding 0 K. In the case of boiling, one is (at least ideally) dealing with heterogeneously nucleated bubbles which diffuse to the outer surface of a liquid or solid and then escape, the latter being possible for temperatures equal to or exceeding the boiling temperature (T(b)). In the case of phase explosion one is dealing with the consequences of what happens when a liquid approaches the thermodynamic critical temperature (T(tc) or T(c)), and massive homogeneous nucleation takes place. Although these three mechanisms have been reviewed in reasonable detail in recent work, we will here present evidence, apparently not previously considered, that boiling, whether the distance scale is atomically small (5-15 nm, as for laser-pulse impact on a metal in the absence of thermal diffusion) or much larger, has a prohibitive kinetic obstacle because it requires bubble diffusion if the bubbles are formed other than at the outer surface. That is to say, boiling will never be a significant process whether with ion or laser-pulse impact. This leaves vaporization and phase explosion as the only possible thermal-spike processes capable of expelling material from an ion- or laser-pulse bombarded surface in a significant quantity. But even with vaporization it can be shown that a kinetic obstacle, although not as severe as for boiling, will enter. The final result is that only phase explosion will normally be relevant for sufficiently short time scales.

High enrichment of carbon and volatile elements in the surface layers of Luna 16 soil sample 1635: Result of comet or meteorite impact?

Surface analysis by X-ray photoelectron spectroscopy (XPS) in combination with Ar etching was performed on a lunar regolith sample from the Luna 16 drill-core. The sample contains only ∼10% of glass spherules; these are of highly variable composition, which distinguishes them from the pristine (volcanic) lunar glasses with their restricted compositional range. The XPS analyses revealed that practically all grains in this sample are coated by layers of different chemical composition. The outer 1200 Å have a normal Fe,Mg-silicate composition. The middle zone between 1200 and 4400 Å is strongly enriched in carbon (up to 60 at%) and in Zn (4 at%). From the electron binding energy of the carbon atoms, it can be concluded that most of the carbon is present in the form of nanodiamonds and graphite. Other volatile elements (Cd, Pb, Ga, P, and Cl) were detected at the 0.1–0.2 at% level in this zone. The inner zone below 4400 Å has again a silicate composition, but is more refractory than the outer zone, i.e. higher in Ca and Al. We propose that this layering was formed by condensation from a carbon-rich cloud, originating from the impact of a comet or a carbonaceous chondrite-like body. XPS analysis of the surface layers showed variations in the chemical state of the main elements (Mg, Si, Al, Ca, Fe) in these layers, corresponding to different molecular structures. Similar variations were found in condensate films produced in laser-pulse impact simulation experiments.

Ultrashort pulse laser ablation of silicon: an MD simulation study

Molecular Dynamics (MD) simulation has been employed in order to study laser ablation of silicon surfaces. The impact of laser pulses with lengths ranging from 5 ps down to 10 fs on a target of approximately 100A x 100A x 50A was investigated. The ablation shows a strong dependence on pulse length, on pulse energy and on the number of laser shots. With decreasing pulse length the amount of removed particles increases, and with decreasing pulse energy the holes become narrower. Especially in multishot ablations, holes with a diameter of just a fraction of the focus could be observed. This can be attributed mainly to ablation of atoms from lower areas and their redeposition close to the surface, leading to amorphous areas around the ablation hole. For pulses of picosecond duration, and even for femtosecond pulses, the main material removal occurs on a timescale of a few ps. Interestingly, the simulations show two thresholds: the onset of damage at the surface. which depends on the pulse energy but only insignificantly on the pulse length; and the onset of the removal of particles, which shows a strong dependence on the pulse length of the laser.

Laser Cleaning - A New Surface Cleaning Method Without Pollutions

AbstractSurface contaminations are removed by laser irradiation with pulse output and short wavelength from various substrate such as magnetic head slide, glass and metals. Laser cleaning is a new dry process to remove surface organic contaminations without using ultrasonic cleaning in organic solvents. This provides a new dry process to clean different substrate surfaces and can take the place of conventional wet cleaning processes such as ultrasonic cleaning with CFC and other organic solvents. The mechanisms of laser cleaning may include laser photodecomposition, laser ablation and surface vibration due to the impact of laser pulse. It is found that short wavelength and short pulse duration are necessary for effective cleaning. It is also found that an appropriate energy density is critical to achieve effective cleaning without causing surface oxidation and secondary contamination.