Low Laser Power Density Research Articles

자유표면변형을 고려한 저에너지밀도 및 고에너지밀도 레이저 용접공정 통합 해석

In this study, a unified numerical investigation has been performed on the evolution of weld pool, key-hole geometry during low-power, high-power density laser welding. Unsteady phase-change heat transfer, fluid flow with the surface tension are examined. The one-dimensional vaporization model is introduced to model the overheated surface temperature, recoil pressure during high-power density laser welding. It is shown that Marangoni convection in the weld pool is dominant at low-power density laser welding,, the keyhole with thin liquid layer, the hump are visible at high-power density laser welding. It is also shown that the transition from conduction welding to penetration welding for iron plate exists when the laser power density is about 10^6W/㎠ .

Investigation of open-air laser-induced chemical vapor deposition of carbon on moving optical fibers

Using a CO2 laser as a heat source, carbon coatings have been successfully deposited on moving optical fibers in an open-air laser-induced chemical vapor deposition (LCVD) reactor. Applications include fusion splice recoat and in-line coating of optical fibers. The relationship between operating parameters and the carbon deposition temperature and rate was investigated. Results indicate that they are strongly dependent on the laser power density and the optical fiber's traverse velocity. In order to provide a deeper understanding of the fundamental principles that govern laser heating and the carbon LCVD processes, a heat transport model was developed to predict the fiber surface temperature during deposition. The surface temperatures obtained from experiments compared well with the predicted temperature. Based on the temperature calculations, various kinetic parameters, including the activation energy EA and pre-exponential factor k0 for the carbon deposition process, are determined. The optical fiber signal loss at 1550 nm induced by the LCVD process is observed to be less than 10–3 dB at low laser power density.

A study of low-power density laser welding process with evolution of free surface

In this study, numerical investigation has been performed on the evolution of weld pool geometry with moving free surface during low-energy density laser welding process. The free surface elevates near the weld pool edge and descends at the center if d σ/d T is dominantly negative. It is shown that the predicted width and depth of the weld pool with moving free surface are a little greater than those with flat weld pool surface. It is also believed that the oscillation of the weld pool surface during the melting process augments the rate of convective heat transfer in the weld pool. The present analysis with moving free surface should be considered when Peclet number is greater and Weber number is much smaller than one since the deformation of the weld pool surface is noticeable as Pe number increases and We number decreases, especially when Pe / We is greater than the order of 10.

Raman Spectra in Vanadate Nanotubes Revisited

In this letter we report the Raman spectra of vanadate nanotubes (VONTs). The spectra present a clear signature that can be used for probing the tubular structure. The temperature effects on the structure of dodecylamine- and Cu-intercalated VONTs were studied by changing the laser power density during the Raman measurements. We have found that low laser power densities promote the decomposition of VONTs, leading to the collapse of the tubular structure and converting the nanotubes into V2O5 oxide. The decomposition occurs through an intermediate compound that is isostructural to V2O5 xerogel. The Raman experiments in VONT-based systems should be performed at extremely low laser power densities.

Preparation and Microstructure Control of Protein Thin Films Deposited by Pulsed Laser Deposition and Via Colloid Chemical Routes

ABSTRACTProtein thin film (mainly silk fibroin) was prepared by pulsed laser deposition (PLD) with 1064nm IR-beam and via colloid chemical routes. Thickness, surface roughness, and microstructures of the deposited film were examined by quartz crystal microbalance sensor, field emission scanning electron microscope (FE-SEM), and atomic force microscope (AFM). The laser power density was varied systematically for PLD to control the microstructures of the film and the secondary structure (β-sheet, α-helix, or random coil) of the protein. Secondary structure of the target and film was examined by FT-IR. Films prepared by PLD comprise by agglomerated particles with their primary particle size around 30nm. The size of the primary particles was uniform, especially for the film prepared at low laser power density. At low laser power density, proportion of β-sheet increased and that of random coil decreased. Proportion of random coil was also increased by the wet colloidal process. PLD with low power density is most suitable to preserve the secondary structure in the protein thin film.

Effect of small crystal size and surface temperature on the Raman spectra of amorphous and nanostructured Si thin films deposited by radiofrequency plasmas

A detailed analysis by Raman spectroscopy of amorphous and nanostructured silicon (Si) thin films deposited by PECVD is presented. The nanostructured Si films, also referred as polymorphous Si, were obtained under plasma conditions where Si nanoparticles and radicals contributed to the film growth. Such films consist of nanometric ordered domains embedded in an amorphous matrix, as demonstrated by TEM [1]. However, these films seem to be non-structured by Raman spectroscopy analysis, as revealed from the spectra performed at very low laser power density (∼1 kW/cm 2), where structural modifications are sure to be not induced. Higher laser powers are found to produce the film crystallization. From these Raman spectra, the surface temperature reached by the sample during laser irradiation, Si-crystallite size and crystalline volume fraction can be determined. Nevertheless, care is needed to well quantify the Raman spectra, because both crystal size and surface temperature affect the crystalline Raman peak in different extent. In this study, a method to quantify the Raman spectra is proposed. The most conventional models for calculations are critically reviewed.

Explosive Vaporization of Metallic Sodium Microparticles by CW Resonant Laser Radiation

Explosive vaporization of metallic Na microparticles stimulated by resonant cw laser radiation has been observed in a glass cell. Vaporization occurs at low laser-power density. The effect consists in the generation of optically thick and sharply localized Na vapor clouds propagating in the cell against the laser beam. The effect is explained by laser excitation of Na atoms, which collide onto the surface of the microparticles and transfer their internal energy. This causes other atoms to be vaporized and to continue the avalanche process.

Crystal size and temperature measurements in nanostructured silicon using Raman spectroscopy

In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The validity and application ranges of the different models that can be used to calculate these parameters are critically discussed.

Investigations on nanosecond laser produced plasma in air from the multi-component material YBa 2Cu 3O 7

The laser produced plasma from the multi-component target YBa 2CU 3O 7 was analyzed using Michelson interferometry and time resolved emission spectroscopy. The interaction of 10 ns pulses of 1.06 μm radiation from a Q-switched Nd:YAG laser at laser power densities ranging from 0.55 GW cm −2 to 1.5 GW cm −2 has been studied. Time resolved spectral measurements of the plasma evolution show distinct features at different points in its temporal history. For a time duration of less than 55 ns after the laser pulse (for a typical laser power density of 0.8 GW cm −2, the emission spectrum is dominated by black-body radiation. During cooling after 55 ns the spectral emission consists mainly of neutral and ionic species. Line averaged electron densities were deduced from interferometric line intensity measurements at various laser power densities. Plasma electron densities are of the order of 10 17 cm −3 and the plasma temperature at the core region is about 1 eV. The measurement of plasma emission line intensities of various ions inside the plasma gave evidence of multiphoton ionization of the elements constituting the target at low laser power densities. At higher laser power densities the ionization mechanism is collision dominated. For elements such as nitrogen present outside the target, ionization is due to collisions only. © 1997 Elsevier Science B.V.

Photochemistry of phosgene in the solid phase: Dissociation, ejection, and thermal desorption

Understanding photochemistry and energy transfer mechanisms in molecular solid films is of interest to many scientific issues, ranging from matrix-assisted laser desorption ionization mass spectrometry to photochemical processes on polar stratospheric cloud particles. We present a study of a model system: the photochemistry (hν=1.2–6.4 eV) of a molecular Cl2CO solid film at low laser power density, 10 μJ–1 mJ/cm2 for ∼10 ns pulses. At hν≥3.5 eV, photon absorption by Cl2CO leads to a major photodissociation channel resulting in CO (g) and Cl (g) and a minor molecular Cl2CO ejection channel. Both photodissociation and molecular ejection are observed at the lowest laser power density and their yields depend linearly on pulse energy. This result establishes a single photon photoexcitation mechanism. The electronically excited Cl2CO in the surface region of the solid film can either dissociate or convert its electronic energy to translational motion in Cl2CO. The translational energy distribution of CO (g) from the photodissociation channel is bimodal: the flux-weighted mean translational energy of the fast channel is photon energy dependent (〈Etrans〉=210, 135, and ∼90 meV at hν=6.4, 5.0, and 3.5 eV, respectively), while the slow channel is independent of photon energy and corresponds to completely thermalized CO molecules (〈Etrans/2k〉=84±3 K). The mean translational energy of photoejected Cl2CO is 〈Etrans〉=220±20 meV. In addition to photoejection, there is also a distinctively different thermal desorption channel due to transient laser heating.

Laser heating of YBa2Cu3O7 films in Raman experiments.

Magnetron-sputtered and laser-deposited ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films on MgO(100) substrates are investigated in a Raman setup. We prepare 25-\ensuremath{\mu}m-wide microbridges using standard photolithography and argon-ion-beam etching. The change of critical currents of illuminated microbridges is used to determine the spot temperature with high accuracy. The thermal boundary resistance of magnetron-sputtered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films on MgO(100) is determined to be ${\mathit{R}}_{\mathrm{bd}}$=(76\ifmmode\pm\else\textpm\fi{}9)/${\mathit{T}}^{3}$ ${\mathrm{K}}^{4}$ ${\mathrm{cm}}^{2}$ ${\mathrm{W}}^{\mathrm{\ensuremath{-}}1}$ and ${\mathit{R}}_{\mathit{b}\mathit{d}}$=(1.7\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ K ${\mathrm{cm}}^{2}$ ${\mathrm{W}}^{\mathrm{\ensuremath{-}}1}$ for temperatures below and above 35 K, respectively. In order to obtain the spot temperature also Stokes and anti-Stokes Raman spectra are taken. The accuracy of this method, however, is only \ifmmode\pm\else\textpm\fi{}10 K and \ifmmode\pm\else\textpm\fi{}30 K at low and high laser power densities, respectively. At room temperature Raman measurements with increasing laser power densities are performed until oxygen rearrangements and finally oxygen loss in the Cu(1)-O(1) chains are observed. By analyzing the spot temperatures in these measurements and comparing with recent oxygen diffusion experiments we show that both the rearrangement and the oxygen loss are thermally activated processes.

The preparation and characterization of AlPO 4 thin films via laser ablation of AlPO 4-H 4

An excimer laser (KrF, 248 nm) was used to deposit well-dispersed thin films of aluminum phosphate onto a titanium nitride substrate from an AlPO 4-H 4 target. It was found that laser ablation of AlPO 4-H 4 results in AlPO 4 cristobalite or aluminum orthophosphate as well as AlPO 4 tridymite and amorphous materials depending upon the conditions. The effects of laser power, substrate distance, atmosphere, and temperature on thin-film formation were studied. Low substrate temperatures and pulse energies between 50 mJ and 100 mJ resulted in growth of AlPO 4 cristobalite or aluminum orthophosphate. Low laser-power density and high substrate temperatures produced AlPO 4 tridymite on the substrate TiN surface. These first examples of laser-ablated AlPO 4 films were characterized by X-ray diffraction, X-ray fluorescence, infrared spectroscopy (FTIR) and scanning electron microscopy.

A new multipurpose CO2 laser therapy instrument.

A new multipurpose CO2 laser therapy instrument has been developed. It is a highly efficient medical instrument. By use of high laser power density to coagulate, evaporate, and cut body tissue on the nidus, the operation can be controlled and has obvious curative effects. Unlike other kinds of CO2 laser therapy instruments, this device has an advanced switching power supply (SPS) and red guiding light system. With an overcurrent protective device, an overvoltage protective device, and a high-voltage shield device, it provides efficiency, stability, reliability, and low loss. The plastic casing does not leak electricity and the film switches are designed for clinical practice convenience. Additionally, the laser power is numerically displayed and can be set prior to the procedure. The distinct visible guiding light of the laser output makes the operation more convenient and accurate. Because of this unique design and properties, it is a leading model in China. The instrument can be widely used for surgery, gynecology, dermatology, and otolaryngology. The radiation therapy of low laser power density has the effect of being antiinflamatory, analgesic, and antipruritic, and promotes cure of the epithelium. Moreover, it is effective to treat all sorts of sprains, scapulohumeral periarthritis, arthritis, sciatica, and surface ulcers.

From green algae to calcium inside buckyballs

AbstractCoorongite and carbonaceous residues from coorongite pyrolysis at 450 and 500°C were studied by laser ablation Fourier transform mass spectrometry. Raw coorongite gave positive‐ion spectra having mainly protonated species of m/z 80–300 when laser ablated with a high laser power density. Endohedral fullerene positive ions of calcium were observed during the laser ablation of coorongite pyrolysis residues. Pyrolysis of the raw coorongite at 450 and 500°C produced residues which on laser ablation using the fundamental frequency of an Nd: YAG laser (1064 nm) gave a series of calcium fullerides. These ions were observed using low laser power densities (100–600 kW cm–2) Mixing the coorongite pyrolysis residue with barium sulphate gave M@Cn+ ionsThe symbol ‘@’, as in Ca@60, is used to represent an endohedral complex, i.e. the adduct is located inside the carbon cage. where M = calcium or barium. Mixing the coorongite pyrolysis residue with strontium oxalate also gave M@Cn+ ions where M = calcium or strontium. No ions containing two or more metals were detected.

The Effect of pH on the Localized Etching Process Induced by Laser Irradiation

The pH of copper sulfate solution affects the formation of the passive layer, mainly , on the copper surface under laser irradiation. The electrode process can be changed from electroless deposition to etching due to the formation of this passive material coupled with the dissolution of copper. The etching effect is obtained even under a low laser power density when the pH of the copper sulfate solution is controlled between 4.3 and 2.0.

Trivalent Ce2O3 and CeO2?x intermediate oxides induced by laser irradiation of CeO2 powders

A strong non-stoichiometry of pure fcc CeO2 was induced by laser irradiation. The increase of laser power and/or energy density had a saturable effect on particle size growth. The possibility of CeO2 reduction to A-Ce2O3 by laser irradiation was demonstrated. Particles of stable Ce7O12 phase were observed in all specimens irradiated at low laser-power densities. An epitaxial relationship between triclinic Ce11O20 and cubic Ce12O22 phases was found. The controversial C-Ce2O3 phase was detected at the limits of a bcc particle. An unknown bcc phase of acicular morphology, strongly related to C-Ce2O3, was also registered. The dose dependence of CeO2 structural modifications obtained by laser irradiation as a function of laser energy density variation could be explained by a simple defect aggregation model implying lattice defects (oxygen vacancies and Ce3+ ions).

In situ trace element determination of carbonates by laserProbe inductively coupled plasma mass spectrometry using nonmatrix matched standardization

Abstract The LaserProbe Inductively Coupled Plasma Mass Spectrometry (LP-ICP-MS) has shown great potential to precisely and quickly determine trace element compositions of minerals in situ at a scale of ≤ 60 μm. However, standardization is complicated, due to the lack of matrix-matched, homogeneous mineral standards. In this study, a nonmatrix matched calibration standard, the NIST 612 silicate glass, was employed to determine Sr, Y, Ba and REE concentrations in calcite and dolomite grain mounts. Using 43Ca as an internal standard, the results demonstrate that it is not essential to have a matrix-matched mineral standard by LP-ICP-MS. Routine limits of detection are ~10 ppb for REEs and ~200 ppb for Sr, Y and Ba, depending on laser operating conditions. Evaluation of precision and accuracy is made complex by heterogeneity of trace elements within single mineral grains. However, the best within-grain precision (determined from several LP sampling pits within an individual mineral grain) is considered to represent the maximum analytical error and is generally 10 grains) were analyzed. Nonmatrix matched standardization for quantitative analysis can successfully be used, because the small size laser probe ( ~ 10 μm) used in this study can generate much higher power density than the conventional size laser beam. At high power density, the sample ablation mainly results from plasma plume expansion induced by the laser, not from absorption of the laser beam and related thermal vaporization, as for conventional, low power density laser analysis. The plasma plume expansion as a major ablation process permits the ablation of even those materials usually transparent to laser (e.g., calcite). This technique can be applied to the analysis of trace elements of carbonates in thin sections, grain mounts and rock or mineral chips, thus avoiding extensive sample preparation with significant implications for studies of near-surface and environmental processes.

Laser-induced chemical vapour deposition of TiSi 2: Aspects of deposition process, microstructure and resistivity

Lines of TiSi 2 (phase C-54) in the width interval 6–30 μm were written on an Si(100) substrate by scanning a continuous wave and focused argon ion laser beam across the substrate surface. Because of the slight volume change upon the reaction between the silicon substrate and the H 2TiCl 4 vapour, the TiSi 2 lines were practically embedded in the substrate. The linewidth was strongly dependent on the laser power density and the scanning velocity. Vapour composition and total pressure had only a slight influence on the linewidth. The surface profile perpendicular to the laser-written lines was also investigated. The gaussian temperature profile, obtained across the laser beam on the substrate surface, caused conditions of both deposition and etching. The substrate etching, observed near the edges of the deposited lines, was induced merely by the chlorine or by the chlorine in combination with the hydrogen in the reaction gas mixture. By a proper selection of the laser power density, scanning velocity, vapour composition and total pressure, the surface profile could be controlled. In order to calculate the laser-induced temperature, the reflectivity as a function of temperature for TiSi 2 was determined experimentally. The TiSi 2 lines were smooth and had a (001) texture. The grain size varied from 0.2–0.5 μm in the centre of the line to about half that size at the line periphery. Defects such as dislocations, voids or pores were never observed. However, a few stacking faults and twin bands were detected. The lines were free from cracks provided that a low laser power density and a high scanning velocity were employed. The lowest resistivity of as-deposited lines, 85 μωcm, was obtained at a low laser power density, 6.4 × 10 5 W cm −2.

Saturation and fragmentation in non resonant laser postionization of sputtered atoms and molecules

Using non resonant two photon ionization at λ=248 nm, the emission of neutral atoms and dimers was investigated during ion beam sputtering of polycrystalline silver and copper with 5 keV Ar+. Saturation effects were examined by measuring the ion intensities as a function of the photon flux density. It is found that with the maximum available laser power density (1·109 W/cm2) the ionization of Ag atoms can be easily saturated, whereas no complete saturation was achieved for Cu atoms. At the same time the saturation curve measured for Ag differs significantly both from that measured for Cu and from the theoretically expected behaviour for non resonant two photon ionization. For Ag2 and Cu2 the ionization is shown to be strongly resonance enhanced already being saturated in the region of 107 W/cm2. From the data taken at low laser power densities (<106 W/cm2), the role of fragmentation processes either by dissociative ionization or by neutral dissociation is discussed. As an example, the results using laser postionization are employed to quantitatively determine the abundance of Ag2 molecules in the flux of neutral particles sputtered from the silver sample.

Laser ablation studies of palladium electrolytically loaded with hydrogen and deuterium.

The kinetic-energy distributions (KED's) of ${\mathrm{Pd}}^{+}$, ${\mathrm{H}}_{\mathit{n}}^{+}$, ${\mathrm{D}}_{\mathit{n}}^{+}$ (n=1 to 3), and electrons created in a picosecond laser-produced plasma from palladium metal were studied using a spherical-sector electrostatic energy analyzer followed by a time-of-flight mass spectrometer. High-resolution, Fourier-transform mass spectroscopy showed no ${\mathrm{PdH}}_{\mathit{n}}^{+}$ or ${\mathrm{PdD}}_{\mathit{n}}^{+}$ ions. The total yield of ${\mathrm{Pd}}^{+}$ ions grew linearly with laser power, while the yields of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ increased exponentially. The yields of ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{3}^{+}$ increased with laser power density slowly, comparable to the yields of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ at low laser power density only. The average ion kinetic energy was a linear function of laser power for all species, but with a much higher average kinetic energy for ${\mathrm{Pd}}^{+}$ than for other species at a given laser power. The KED's for ${\mathrm{Pd}}^{+}$ were dominated by a plasma component with a Maxwellian shape, accompanied by a thermionic component with a kinetic energy in the range of 0--1 eV. The ${\mathrm{H}}^{+}$, ${\mathrm{H}}_{2}^{+}$, ${\mathrm{H}}_{3}^{+}$, and ${\mathrm{D}}^{+}$ KED's were basically Maxwellian, but the origins were offset to positive kinetic energies, with the offset energy increasing linearly with mass. There was no thermal component for the ${\mathrm{H}}_{\mathit{n}}^{+}$ or ${\mathrm{D}}^{+}$ ions.