Laser Pulse Energy Density Research Articles

Cooling-down of thermal thick probes after flash excitation – A measure for the real energy density?

Though flash lamps are one of the most applied heat sources in the field of Thermographic Testing (TT) using active thermography, only little is known about the actually achieved energy input into test objects. In this paper, an easy to realize sensor concept is proposed and experimentally evaluated. The concept is based on the measurement of the surface temperature of a thermal thick probe after flash excitation. After considering the sensor concept with FEM simulations the experimental investigation of four materials (two polymer and two building materials) is described. It will be shown that a suited coating is essential for the realization of the sensor concept. The experimental results prove the suitability of black rigid PVC as the most promising material. Using a coated PVC sample the energy density of short laser pulses, similar to flashes of flash lamps, could be determined exactly with an estimated relative uncertainty of only a few percent.

Photoluminescence Spectra of CdTe Single Crystals Subjected to Nanosecond Laser Irradiation

Low temperature photoluminescence (PL) of high-resistivity detector-grade Cl-compensated CdTe semiconductor crystals subjected to irradiation with nanosecond (τ = 7 ns) laser pulses of the second harmonic (λ = 532 nm) of a YAG:Nd laser is studied. Irradiation of CdTe crystals within the certain range of laser pulse energy densities results in a relative decrease in the emission intensity in both the deep energy level and edge regions and an increase in the exciton band intensity in the PL spectra. The evolution of the PL spectra depending on laser energy density, excitation level and temperature under excitation are analyzed. Laser-stimulated transformation of the point defect structure of the CdTe surface region and mechanisms of laser-induced defect formation are discussed. The optimal regimes of laser processing have been obtained which result in the minimum ratio of the defect and exciton bands that is an evidence of an increase in the structural perfection of the irradiated crystals.

Phosphorus out-diffusion in laser molten silicon

Laser doping via liquid phase diffusion enables the formation of defect free pn junctions and a tailoring of diffusion profiles by varying the laser pulse energy density and the overlap of laser pulses. We irradiate phosphorus diffused 100 oriented p-type float zone silicon wafers with a 5 μm wide line focused 6.5 ns pulsed frequency doubled Nd:YVO4 laser beam, using a pulse to pulse overlap of 40%. By varying the number of laser scans Ns = 1, 2, 5, 10, 20, 40 at constant pulse energy density H = 1.3 J/cm2 and H = 0.79 J/cm2 we examine the out-diffusion of phosphorus atoms performing secondary ion mass spectroscopy concentration measurements. Phosphorus doping profiles are calculated by using a numerical simulation tool. The tool models laser induced melting and re-solidification of silicon as well as the out-diffusion of phosphorus atoms in liquid silicon during laser irradiation. We investigate the observed out-diffusion process by comparing simulations with experimental concentration measurements. The result is a pulse energy density independent phosphorus out-diffusion velocity vout = 9 ± 1 cm/s in liquid silicon, a partition coefficient of phosphorus 1 < kp < 1.1 and a diffusion coefficient D = 1.4(±0.2)cm2/s × 10−3 × exp[−183 meV/(kBT)].

Micro-Raman study of nanocomposite porous films with silver nanoparticles prepared using pulsed laser deposition

Nanocomposite porous films with silver nanoparticle (Ag NP) arrays were prepared by the pulsed laser deposition from the back flux of erosion torch particles in argon atmosphere on the substrate placed at the target plane. Preparation conditions of the films were set by argon pressure, energy density of laser pulses, their amount and substrate position relatively to the torch axis. The films were prepared with gradient thickness, variable Ag NP sizes and distance between them along the length of substrate as well as corresponding maxima in the spectra of local surface plasmon absorption. Plasmon effects of the Raman scattering enhance in the matter of the Ag NP shell gave the opportunity to register the spectral bands caused by an extremely small quantity of silver compounds with oxygen and carbon. The possible nature of individual bands in the Raman spectrum was analyzed. The obtained results are important for interpretation of the Raman spectra of analytes based on the prepared SERS substrates.

Optical diagnostics of the laser-induced phase transformations in thin germanium films on silicon, sapphire, and fused silica

The in-situ procedure is used to study the modification of thin (200–600 nm) germanium films induced by nanosecond pulses of a ruby laser. The films are produced using the ion-beam or magnetron sputtering on single-crystalline silicon (Si), sapphire (Al2O3), and fused silica (α-SiO2) substrates. The results on the dynamics of the laser-induced processes are obtained using the optical probing of the irradiated region at wavelengths of λ = 0.53 and 1.06 μm. The results of probing make it possible to determine the threshold laser energy densities that correspond to the Ge and Si melting and the generation of the Ge ablation plasma versus the amount of deposited Ge and thermophysical parameters of the substrate. The reflection oscillograms are used to obtain the dependences of the melt lifetime on the laser-pulse energy density.

Acoustic emissions during the sublimation of a gallium nitride micron layer induced by KrF laser radiation

The efficiency of gallium nitride layer ablation when exposed to pulses of a KrF laser through a layer of sapphire is investigated. The resulting dependences of the evolution of acoustic emission upon varying the laser pulse energy density are of a threshold nature, allowing us to control the efficiency of ablation and the quality of gallium nitride crystals separated from a sapphire substrate.

SENSING DYNAMICS OF THE ZONE TYPE MULTI-LEVEL SYSTEM IN A LASER FIELD: STOCHASTIZATION OF VIBRATIONAL MOTION FOR MOLECULES IN THE MULTI-PHOTON PHOTOEXCITATION REGIME

We present new theoretical scheme to sensing dynamics of the zone type multi-level system in a laser field, based on the quantum stochastic kinetic approach. The multi-photon dissociation yield, selectivity and absorbed energy upon the laser pulse energy density for BCl3 molecules in the oxygen O2 buffer gas are calculated. It has been studied a stochastization of vibrational motion for molecules in the multi-photon photo-excitation regime on example of CF3 I, SF6 and BCl3 molecules within the stochastic Focker-Plank model.

(Invited) UV Excimer Laser Assisted Heteroepitaxy of (Si)GeSn on Si(100)

The perspective to achieve Photonic Integrated Circuits (PICs) using CMOS compatible direct bandgap Group-IV heterostructures is highly attractive from scientific, technological, and economic point of view. Research activities on modeling, growing and processing adequate Group-IV materials and heterostructures for developing new emitters, waveguides and detectors have therefore been intensified over the last decade.Several approaches directed to material and strain engineering of Silicon and Germanium micro- or nanostructures were studied and already led to significant progress in this field. However, to ensure further prospects, more versatile material systems would be of interest and theoretical as well as experimental studies in this direction identified the Silicon-Germanium-Tin (SiGeSn) ternary as an attractive but also challenging system. These studies converged finally to the intensive exploration on how to produce and process Germanium-Tin (GeSn) binary and Silicon containing Germanium-Tin (Si)GeSn alloys, that are heteroepitaxially grown on Silicon wafers or virtual Germanium (v-Ge) substrates. As a result, the possibility to tune independently band gap and lattice constant over a wide range and to achieve, by adjusting strain and composition, the predicted direct bandgap material within this system, is currently explored. As a consequence, (Si)GeSn is now expected to boost the development of new high speed micro- and nanoelectronic components, as well as of CMOS compatible opto-electronic devices and of new GeSn and SiGeSn strain platforms on conventional Si wafers, as successfully employed in Si(Ge) technology [1-4].Major issues in growing and processing these binary and ternary (Si)GeSn alloys are the large mismatch between the lattices of the pure elements (0.54nm for Si, 57nm for Ge, and 0.69nm for a-Sn), the very low solid phase solubility of Sn in Ge of less than 1% and the tendency of Sn to cluster and to form surface precipitates, when subjected to thermal treatment [5-7].Due to these interconnected issues, increased attention has been devoted to, Si-technology compatible, non-equilibrium growth techniques that allow such an independent adjustment of lattice parameters and band gaps in a precise and cost efficient way. Among several well established techniques, like low temperature Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD), also Pulsed Laser Induced Epitaxy (PLIE) using Excimer lasers has been identified as a promising candidate.This contribution will give an overview on recent achievements obtained through 193 nm ArF-Excimer Laser PLIE of (Si)GeSn alloys on v-Ge substrates and on Si wafers. The technique itself is based on local rapid melting and solidification processes, that are induced by 25 ns short ArF-Excimer laser pulses and are completed within an ultra-short time scale (<500 ns). Applying these heat treatments to Si, Ge and Sn heterostructures that are grown on Si(100), leads to epitaxial alloys with gradients in composition and strain that depend on the intermixing of these elements in the molten phase and ultimately on the thermal gradients produced by adjusting “precursor-heterostructure”, laser energy density and number of laser pulses [8-11].This contribution will give an overview on the following PLIE approaches that have been used to obtain the binary and ternary alloys with different crystallinity, strain and stoichiometry, including Sn content of up to 20% and their respective advantages or drawbacks:i) PLIE treatment of low cost precursor heterostructures, like thermally evaporated Sn on Si(100) or on v-Ge.ii) PLIE of v-Ge wafers coated with Sn films through Molecular Beam Epitaxy (MBE).iii) PLIE of Boron doped or intrinsic a-Si:H or a-Ge:H films, produced by Laser Induced Chemical Vapour Deposition (LCVD) and MBE, on Sn/v-Ge.iv) Scanning of the substrate through the laser beam to achieve large area treatment.v) Production of micro-patterns by Mask Projection Assisted PLIE. Acknowledgment This work was partially financed by Spanish (MAT2008-02350, MAT2011-24077), Galician (2010/83) and local P.P.00VI131H64102 (UVigo) grants, co-financed by FEDER funds.

The ejection threshold of molten aluminum induced by millisecond pulsed laser

The laser pulse energy density threshold of melt ejection induced by millisecond pulsed laser can vary widely due to the variation of focused laser spot area. An axisymmetric model of the flow of liquid Aluminum under the effect of recoil force from evaporation is built up to analyze the threshold of ejection through the temporal evolution of the temperature of melted pool. Temperature of the central point of focused laser spot, drag force induced by surface tension in melted pool, and recoil force of evaporation on molten liquid surface could be obtained through resolving heat conduction equation analytically and numerically with assumed Gaussian temperature distribution function. The relationship of laser pulse energy density threshold of the ejection and area of the focused laser spot is obtained through the theoretical calculation. The shape of the melted pool will be different when the relative magnitude of recoil force and drag force is varied. The validity of the obtained threshold is verified through comparison of melted pool shape with the experimental result reported in literature. The criterion proposed in this paper provides a reliable fast-diagnostic method for laser processing.

Ablation experiment and threshold calculation of titanium alloy irradiated by ultra-fast pulse laser

The interaction between an ultra-fast pulse laser and a material's surface has become a research hotspot in recent years. Micromachining of titanium alloy with an ultra-fast pulse laser is a very important research direction, and it has very important theoretical significance and application value in investigating the ablation threshold of titanium alloy irradiated by ultra-fast pulse lasers. Irradiated by a picosecond pulse laser with wavelengths of 1064 nm and 532 nm, the surface morphology and feature sizes, including ablation crater width (i.e. diameter), ablation depth, ablation area, ablation volume, single pulse ablation rate, and so forth, of the titanium alloy were studied, and their ablation distributions were obtained. The experimental results show that titanium alloy irradiated by a picosecond pulse infrared laser with a 1064 nm wavelength has better ablation morphology than that of the green picosecond pulse laser with a 532 nm wavelength. The feature sizes are approximately linearly dependent on the laser pulse energy density at low energy density and the monotonic increase in laser pulse energy density. With the increase in energy density, the ablation feature sizes are increased. The rate of increase in the feature sizes slows down gradually once the energy density reaches a certain value, and gradually saturated trends occur at a relatively high energy density. Based on the linear relation between the laser pulse energy density and the crater area of the titanium alloy surface, and the Gaussian distribution of the laser intensity on the cross section, the ablation threshold of titanium alloy irradiated by an ultra-fast pulse laser was calculated to be about 0.109 J/cm2.

Decay of photoinduced oscillations of the optical reflection coefficient of bismuth

A model describing the decay of photoinduced oscillations of the optical reflection coefficient R of bismuth is constructed, taking the crystal lattice anharmonicity into account. The decay time of oscillations of R is calculated as a function of the energy density of a laser pulse. The results of calculations explain the experimental data on the anomalously strong decay of oscillations of the optical reflection coefficient of bismuth (the decay time decreases by more than an order of magnitude with an increase in the laser pulse energy density from 0 to 4 mJ/cm2).

Heat-transfer processes upon laser heating of inert-matrix-hosted inclusions

A model to describe the heating of metal inclusions in inert media by a laser radiation pulse with allowance for the heat-transfer and melting processes in the matrix and inclusion materials is proposed. The time regularities of the heating of the matrix and inclusions were examined, and the dependences of the maximum temperature on the particle surface on the laser pulse energy density and on the particle radius were obtained. Approximate formulae for the maximum heating temperature and for the radius of most heated particles are proposed. We show that melting processes result in a reduction of the maximum heating temperature and in an insignificant variation of the radius of most heated particles.

Micron-scale copper wires printed using femtosecond laser-induced forward transfer with automated donor replenishment

We demonstrate the use of laser-induced forward transfer (LIFT) in combination with a novel donor replenishment scheme to print continuous copper wires. Wires of mm length, a few microns wide and submicron in height have been printed using a 800 nm, 1 kHz repetition rate, 150 fs pulsed laser. A 120 nm thick copper donor was used along with laser pulse energy densities of 0.16-0.21 J cm−2 to print overlapping few-micron sized pads to form the millimeter long wires. The wires have a measured resistivity of 17 ± 4 times that of bulk copper.

Raman spectroscopic studies of pulsed laser‐induced defect evolution in graphene

Raman spectra were obtained for graphene after irradiating the samples by pulsed laser (λ = 248 nm). Changes in the spectra were observed as the pulse laser energy density (PLED) was varied from 0.1 to 0.25 J/cm2. Changes in bilayer graphene were accompanied by the appearance of the D peak and the broadening of the G peak. Changes in multilayer graphene are more profound as the Raman spectra changes from a multilayer to bilayer and subsequently to monolayer graphene in response to a slow increase in the PLED. The threshold PLED was found to be dependent on the number of graphene layers. We also irradiate graphene with very high PLED (much above the threshold), and the Raman spectra were found to be significantly changed. The G‐band became broader, and red shifted, while the intensity of the 2D‐band was drastically reduced and an intense defect‐related D peak appeared at about 1350 cm−1. The laser ablation of graphene, both with low‐ and high‐energy intensity, is consistent with the reported theoretical predictions. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Effect of laser radiation absorption efficiency on the heating temperature of inclusions in transparent media

The efficiency of light absorption by inclusions of various metals in transparent media is calculated using as an example silver azide, lead, and PETN. It is shown that the absorption efficiency, along with the laser pulse energy density, has a decisive influence on the maximum temperature of heating of the inclusion. Dependences of the maximum heating temperature on the radius of the inclusions are plotted for a pulse duration of 30 ns. Asymptotic expressions are obtained for the dependence of the maximum heating temperature on the pulse duration for an ensemble of inclusions.

Laser-induced desorption of atomic and molecular fragments from a tin dioxide surface modified by a thin organic covering of copper phthalocyanine

The systematic features of laser-induced desorption from an SnO2 surface exposed to 10-ns pulsed neodymium laser radiation are studied at the photon energy 2.34 eV, in the range of pulse energy densities 1 to 50 mJ/cm2. As the threshold pulse energy 28 mJ/cm2 is achieved, molecular oxygen O2 is detected in the desorption mass spectra from the SnO2 surface; as the threshold pulse energy 42 mJ/cm2 is reached, tin Sn, and SnO and (SnO)2 particle desorption is observed. The laser desorption mass spectra from the SnO2 surface coated with an organic copper phthalocyanine (CuPc) film 50 nm thick are measured. It is shown that laser irradiation causes the fragmentation of CuPc molecules and the desorption of molecular fragments in the laser pulse energy density range 6 to 10 mJ/cm2. Along with the desorption of molecular fragments, a weak desorption signal of the substrate components O2, Sn, SnO, and (SnO)2 is observed in the same energy range. Desorption energy thresholds of substrate atomic components from the organic film surface are approximately five times lower than thresholds of their desorption from the atomically clean SnO2 surface, which indicates the diffusion of atomic components of the SnO2 substrate to the bulk of the deposited organic film.

Controlled p-type to n-type conductivity transformation in NiO thin films by ultraviolet-laser irradiation

We report the systematic changes in structural, electrical, and optical properties of NiO thin films on c-sapphire introduced by nanosecond ultraviolet excimer laser pulses. Epitaxial nature of as deposited NiO was determined by x-ray diffraction phi scans and transmission electron microscopy (TEM) and it was established that NiO film growth takes place with twin domains on sapphire where two types of domains have 60° in-plane rotation with respect to each other about the [111] growth direction. We determined that at pulsed laser energy density of 0.275 J/cm2, NiO films exhibited conversion from p-type semiconducting to n-type conductive behavior with three orders of magnitude decrease in resistivity, while maintaining its cubic crystal structure and good epitaxial relationship. Our TEM and electron-energy-loss spectroscopy studies conclusively ruled out the presence of any Ni clustering or precipitation due to the laser treatment. The laser-induced n-type carrier transport and conductivity enhancement were shown to be reversible through subsequent thermal annealing in oxygen. This change in conductivity behavior was correlated with the nonequilibrium concentration of laser induced Ni0-like defect states.

Photoinitiated grafting polymerization of acrylic acid onto the surface of polyethylene under pulsed laser radiation

Photoinduced grafting polymerization on the surface of PE films induced by nanosecond pulsed laser radiation is studied. The grafting is performed from the liquid phase composed of acrylic acid and a photoinitiator (benzophenone) dissolved in it. Pulsed laser radiation with a wavelength of 355 nm, a pulse duration of 11 ns, and a repetition rate of 10 Hz is used. Formation of the surface-graft polymer is followed by IR-ATR spectroscopy and contact-angle measurements. It is found that the time of laser treatment sufficient for the efficient modification of the PE surface with the grafted poly(acrylic acid) is in the range from 0.5 to 1.0 s at a laser-pulse energy density of 200–500 mJ/cm2. At energy densities beyond this range, the efficiency of the reaction decreases rapidly. The results on laser grafting are compared with the results of grafting during UV irradiation with a lamp at a wavelength of 365 nm.

Generation of nanostructures on metals by laser ablation in liquids: new results

Surface nanostructuring of titanium, nickel, molybdenum, and tungsten by ablation with pico- and femtosecond laser pulses in liquids is studied experimentally for the first time. The morphology and properties of obtained nanostructures are investigated using a field emission scanning electron microscope and Raman spectroscopy. The size of nanostructures depends on the laser pulse duration and energy density and on the target material. As a rule, the size distribution of structures is bimodal. Potential applications of such nanostructured substrates are discussed.

FEM for modelling 193 nm excimer laser treatment of SiO2/Si/Si(1‐x)Gex heterostructures on SOI substrates

AbstractResearch on epitaxial crystaline silicon (c‐Si) and silicon‐germanium (Si1‐xGex) alloys growth and annealing for microelectronic purposes, such as Micro‐ or Nano‐Electro‐Mechanical Systems (MEMS or NEMS) and Silicon–On‐Nothing (SON) devices is continuously in progress. Laser assisted annealing techniques using commercial ArF Excimer Laser sources are based on ultra‐rapid heating and cooling cycles induced by the 193 nm pulses of 20 ns, which are absorbed in the near surface region of the heterostructures. During and after the absorption of these laser pulses, complex physical processes appear that strongly depend on sample structure and applied laser pulse energy densities. The control of the experimental parameters is therefore a key task for obtaining high quality alloys. The Finite ElementsMethod (FEM) is a powerful tool for the optimization of such treatments, because it provides the spatial and temporal temperature fields that are produced by the laser pulses. In this work, we have used a FEM commercial software, to predict the temperatures gradients induced by ArF excimer laser over a wide energy densities range, 0.1&lt;Φ &lt;0.4 J/cm2, on different SiO2/Si/Si(1‐x)Ge(x) thin films deposited on SOI substrate. These numerical results allow us to predict the threshold energies needed to reach the melting point (MP) of the Si and SiGe alloy without oxidation of the thin films system. Therefore, it is possible to optimize the conditions to achieve high quality epitaxy films. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)