Laser short-pulse heating of silicon film with the presence of metallic substrate

Laser interaction of silicon film located at he top of metallic substrate is examined and energy transport in electron and lattice sub-systems are formulated using the electron kinetic theory approach. The simulations are repeated for different substrate materials, namely gold, silver, and copper. It is found that electron temperature in the silicon film rises in the vicinity of the silicon–metallic substrate interface, despite the fact that energy absorption from the irradiated filed is significantly low in the silicon film. Lattice site temperature rises rapidly in the early heating period at the interface. In addition, lattice site temperature increase is higher in the silicon film than that corresponding to the metallic substrate.     

Laser short-pulse heating of silicon-aluminum thin films

Energy transport in silicon-aluminum thin films is examined during the laser short-pulse irradiation subjected to the silicon film. The silicon film is considered to be at the top of the aluminum film. Thermal boundary resistance at the interface of the films is incorporated in the analysis. The absorption of laser radiation in the silicon and aluminum films is modeled using the transfer matrix method. Since the silicon film is dielectric, the phonon radiative transport basing the Boltzmann transport equation is incorporated to determine equivalent equilibrium temperature in the film while modified two-equation model is used to account for the non-equilibrium energy transport due to thermal separation of electron and phonon sub-systems in the aluminum film during the laser short-pulse heating process.     

Laser repetitive pulse heating of tool surface

Laser heating of a cemented carbide tool is considered and the temperature field as well as phase changes in the heated region is modeled. Temperature rise, liquid layer thickness, and mushy size are predicted numerically. A control volume approach is introduced to solve the governing equations of heat transfer and phase change. Consecutive pulses with the duty cycle of 60% are accommodated in the simulations in line with the experimental conditions. An experiment is carried out to treat the cemented carbide tool surfaces using the CO₂ laser delivering consecutive pulses. The treated surfaces and their cross-sections are examined using the scanning electron microscope (SEM). It is found that the temperature gradient is high along the laser beam axis resulting in cracks at the irradiated surface. The rapid solidification of the surface causes compact structures with very fine grains in the surface region of the laser irradiated spot.     

Laser pulse heating of steel surface and flexural wave analysis

Laser gas-assisted material processing finds wide application in industry. The modelling of heating, elastic response of the substrate material, and the wave analysis gives insight into the laser workpiece interaction. In the present study, laser gas-assisted heating of steel is considered. The normal component of the thermal stress is taken as the source of load for the flexural wave generation in the material. The flexural wave generated is simulated and the wave characteristics are analyzed at four locations at the workpiece surface. The numerical scheme employing a control volume approach is introduced when solving the governing equations of flow and heat transfer while finite element and spectran element methods are used when solving the stress and wave equations. It is found that the normal component of the stress is tensile. The dispersion effect of the workpiece material, interference of the reflected beam, and partial overlapping of second mode of the travelling wave enable to identify a unique pattern in the travelling wave in the substrate.     

泽贝克/佩尔捷效应演示实验

应用半导体温差发电模块制作了半导体温差发电演示装置,该装置可直观演示泽贝克效应及佩尔捷效应,同时还具有测量泽贝克系数等功能.     

短脉冲激光对硅基探测器的热作用模型选择

从傅里叶模型和非傅里叶模型的基本方程出发,通过有限差分方法对方程进行数值求解。分别分析了10,1．0,0．1,0．01 ns这4种脉宽的脉冲激光作用于硅材料时两种传热模型温度曲线的相对变化;讨论了热弛豫时间对非傅里叶模型数值结果的影响。结果表明:脉宽小于或等于100 ps的激光作用于硅材料时,表层温度上升缓慢,会发生载流子效应,非傅里叶模型可以合理地反映这种现象;对于一般材料,载流子效应发生的条件是脉宽小于或等于材料热弛豫时间,此时应当用非傅里叶模型描述加热过程。     

Time-dependent 3-D modelling of laser surface heating for the hardening of metallic materials

A numerical code for the time-dependent three-dimensional modelling of the laser surface heating for the hardening of metallic materials has been developed by the authors. The temperature-dependence of the thermal properties of the material (stainless steel) is taken into account in the frame of a heating process that doesnt lead to material melting or evaporation. Calculations have been carried out for various dimensions of the parallelepiped-shaped and of the square-shaped spot of the laser beam, as well as for different scanning velocity and for different levels of the laser source power. Various patterns of the laser spot path have also been studied, including a single-pass hardening pattern, a double-pass hardening pattern with and without overlapping, multiple discontinuous and continuous hardening patterns and spiral hardening patterns. The presented results show how the proposed model can be usefully employed in the prediction of the time-evolution of temperature distribution which arises in the workpiece as a consequence of the laser-workpiece interaction under operating conditions typically encountered in industrial applications of the laser hardening process.Received: 29 July 2003, Published online: 30 September 2003PACS: 42.62.Cf Industrial applications - 44.05. + e Analytical and numerical techniques     

Laser studies of metallic artworks

Museum curators and archaeologists use analytical science to provide important information on artworks and objects. For example, scientific techniques provide information on artwork elemental composition, origin and authenticity, and corrosion products, while also finding use in the day-to-day conservation of many historical objects in museums and archaeological sites around the world. In this work two special cases are being discussed. In the first part of our work, physicochemical studies of an icon on a metal substrate were carried out using non-destructive, qualitative analysis of pigments and organic-based binding media, employing various microscopic and analytical techniques, such as Optical Fluorescence Microscopy, XRF, and Gas Chromatography. In the second part of our work, laser cleaning of late Roman coins has been performed using a Q-switched Nd:YAG laser (1064 nm, 6 ns) and a GaAlAs diode laser (780 nm, 90 ps). The corrosion products have been removed, while we observe increased concentrations in Ag, which is the main material of the silver plating found in late Roman coins.     

Analytical solution of hyperbolic heat conduction equation in relation to laser short-pulse heating

In the present study, the hyperbolic heat conduction equation is derived from the Boltzmann transport equation and the analytical solution of the resulting equation appropriate to the laser short-pulse heating of a solid surface is presented. The time exponentially decaying pulse is incorporated as a volumetric heat source in the hyperbolic equation to account for the absorption of the incident laser energy. The Fourier transformation is used to simplify the hyperbolic equation and the analytical solution of the simplified equation is obtained using the Laplace transformation method. Temperature distribution in space and time are computed in steel for two laser pulse parameters. It is found that internal energy gain from the irradiated field, due to the presence of the volumetric heat source in the hyperbolic equation, results in rapid rise of temperature in the surface region during the early heating period. In addition, temperature decay is gradual in the surface region and as the depth below the surface increases beyond the absorption depth, temperature decay becomes sharp.     

Convergence of electron kinetic,two-temperature,and one-temperature models for laser short-pulse heating

The laser short-pulse heating of metallic workpieces initiates the non-equilibrium heating in the surface vicinity of the substrate. The material response to the non-equilibrium heating cannot be predicted accurately by the one-temperature model. Consequently, new models pertinent to laser short-pulse heating are needed. In the present study, laser short-pulse heating of gold, copper, and lead is considered. The material responses to the laser short-pulse due to the electron kinetic theory and the two-temperature and the one-temperature models are examined in detail. The differences between the collisional and diffusional heating mechanisms are presented. The conditions for the convergence of conduction mechanisms are discussed. The electron kinetic theory, the two-temperature, and the one-temperature predictions are compared for three substrates. It is found that the electron kinetic theory predictions differ from the predictions of the one-temperature model in the surface vicinity of the substrate during the early heating duration. As the heating progresses, both models predict similar temperature profiles. The electron kinetic theory and the two-temperature model predictions are in good agreement. PACS 44.10.+i; 42.62.-b     

Dielectric layer-dependent surface plasmon effect of metallic nanoparticles on silicon substrate

The electromagnetic interaction between Ag nanoparticles on the top of the Si substrate and the incident light has been studied by numerical simulations. It is found that the presence of dielectric layers with different thicknesses leads to the varied resonance wavelength and scattering cross section and consequently the shifted photocurrent response for all wavelengths. These different behaviours are determined by whether the dielectric layer is beyond the domain where the elcetric field of metallic plasmons takes effect, combined with the effect of geometrical optics. It is revealed that for particles of a certain size, an appropriate dielectric thickness is desirable to achieve the best absorption. For a certain thickness of spacer, an appropriate granular size is also desirable. These observations have substantial applications for the optimization of surface plasmon enhanced silicon solar cells.     

Laser interference metallurgy: A new method for periodic surface microstructure design on multilayered metallic thin films

Methods for micro- and nanostructuring are essential for functionalization of materials surfaces. In particular, photon-assisted methods for synthesis of functional surfaces have been intensively investigated in the last years. In this study, a new method for surface modification and production of long-range order periodical structures called “laser interference metallurgy” is explored. A metallic thin film sample consisting of three layers composed of Fe, Cu and Al (from top to bottom) on a glass substrate was irradiated with an interference pattern using a Nd:YAG laser (wavelength of 355 nm, 10 ns of pulse duration). For the interference pattern, a configuration producing a line-type energy distribution was chosen. The laser fluence was high enough to melt the aluminium and copper layers at the interference maxima but the iron layer remained in the solid state. Thus, diffusive and convective exchange occurred between aluminium and copper at the energy maxima positions leading to periodical alloy formation with a long-range order. Because it remained in solid state, the iron layer at the top acted as a protective layer effectively preventing removal of the molten layers. The interaction of the different layers was characterized using FIB, TEM and EDX in STEM mode.     

Charging of heated colloidal particles using the electrolyte Seebeck effect

We propose a novel actuation mechanism for colloids, which is based on the Seebeck effect of the electrolyte solution: Laser heating of a nonionic particle accumulates in its vicinity a net charge Q, which is proportional to the excess temperature at the particle surface. The corresponding long-range thermoelectric field E is proportional to 1/r(2) provides a tool for controlled interactions with nearby beads or with additional molecular solutes. An external field E(ext) drags the thermocharged particle at a velocity that depends on its size and absorption properties; the latter point could be particularly relevant for separating carbon nanotubes according to their electronic band structure.     

Laser heating versus phonon confinement effect in the Raman spectra of diamond nanoparticles

In this study, PbSe nanocubes were obtained by high-energy milling, and their optical properties were investigated by measuring the UV?CVIS?CIR spectra in the range of 200?C2,000?nm. The optical absorption of all samples showed a strong UV emission band at 1.45?eV. Previously, to obtain only PbSe nanocubes, an intermediate phase was identified, PbSeO₃. Although both PbSeO₃ and PbSe were traced through this study, a major effort is devoted to characterize the latter. To trace how chemical transitions evolve from precursors to PbSe, X-ray diffraction and Rietveld refinement were carried out. Therefore, the following parameters were evaluated as a function of milling time: phase percentages, area-to-volume ratio, average crystallite dimensions, specific surface area, and morphology changes. To corroborate previous findings, nitrogen adsorption and transmission electron microscopy techniques were used. All the set experimental results unambiguously confirm that crystallites show a cubic morphology, with its average crystallite size distribution being around 24?nm.     

Laser heating of optically thin samples

Summary A mathematical model able to describe the temperature profiles generated in a thin film by the steady-state illumination by a Gaussian laser beam is presented. The film is supposed to be made by a weakly absorbing liquid sample bounded by two parallel transparent plates, the cell walls, whose thermal exchange to the surrounding ambient may be assumed to be linear with the temperature difference. An analytical solution of the problem is presented in form of Hankel transform and a matrix numerical approach to the computation of the temperature profiles is reported. It is particularly well suited to a computer implementation and allows one to get very accurate results in very short computing times. The influence of the heat exchange coefficient to the ambient is shown in an example. A check of the method accuracy is performed by comparison with literature results in the particular case of infinite heat exchange coefficient. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Signatures of the Unruh effect via high-power, short-pulse lasers

The ultra-high fields of high-power short-pulse lasers are expected to contribute to understanding fundamental properties of the quantum vacuum and quantum theory in very strong fields. For example, the neutral QED vacuum breaks down at the Schwinger field strength of 1.3×10¹⁸ V/m, where a virtual e⁺e^- pair gains its rest mass energy over a Compton wavelength and materializes as a real pair. At such an ultra-high field strength, an electron experiences an acceleration of a_S=2×10²⁸g and hence fundamental phenomena such as the long predicted Unruh effect start to play a role. The Unruh effect implies that the accelerated electron experiences the vacuum as a thermal bath with the Unruh temperature. In its accelerated frame the electron scatters photons off the thermal bath, corresponding to the emission of an entangled pair of photons in the laboratory frame. While it remains an experimental challenge to reach the critical Schwinger field strength within the immediate future even in view of the enormous thrust in high-power laser developments in recent years, the near-future laser technology may allow to probe the signatures of the Unruh effect mentioned above. Using a laser-accelerated electron beam (γ～300) and a counter-propagating laser beam acting as optical undulator should allow to create entangled Unruh photon pairs (i.e., signatures of the Unruh effect) with energies of the order of several hundred keV. An even substantially improved experimental scenario can be realized by using a brilliant 20 keV photon beam as X-ray undulator together with a low-energy (γ≈2) electron beam. In this case the separation of the Unruh photon pairs from background originating from linearly accelerated electrons (classical Larmor radiation) is significantly improved. Detection of the Unruh photons may be envisaged by Compton polarimetry in a 2D-segmented position-sensitive germanium detector.     

Laser heating: an analytical approach to the theory

The theory of pulsed and scanned laser beams is developed and the results are shown to agree with similar analyses by other authors. A new expression, however, for the temperature rise caused by a very small laser spot agrees well with recent experiments.     

Laser short pulse heating of metal nano-wires

Non-equilibrium energy transfer takes place in a solid substrate during a short-pulse laser irradiation and temperature field can be obtained analytically in the irradiated region. In the present study, laser short-pulse heating of metal nano-wire is considered and the analytical solution for two-dimensional axisymmetric nano-wire is presented. Since the absorption of the incident beam takes place in the skin of the irradiated surface, a volumetric heat source resembling the absorption process is incorporated in the analysis. Three different nano-wire materials are introduced in the analysis for the comparison reason. These include silver, chromium, and copper. It is found that temperature decay is gradual on the surface vicinity and temporal variation of the surface temperature follows almost the laser pulse intensity profile at the irradiated center.