Study of the temperature distribution in Si nanowires under microscopic laser beam excitation

The use of laser beams as excitation sources for the characterization of semiconductor nanowires (NWs) is largely extended. Raman spectroscopy and photoluminescence (PL) are currently applied to the study of NWs. However, NWs are systems with poor thermal conductivity and poor heat dissipation, which result in unintentional heating under the excitation with a focused laser beam with microscopic size, as those usually used in microRaman and microPL experiments. On the other hand, the NWs have subwavelength diameter, which changes the optical absorption with respect to the absorption in bulk materials. Furthermore, the NW diameter is smaller than the laser beam spot, which means that the optical power absorbed by the NW depends on its position inside the laser beam spot. A detailed analysis of the interaction between a microscopic focused laser beam and semiconductor NWs is necessary for the understanding of the experiments involving laser beam excitation of NWs. We present in this work a numerical analysis of the thermal transport in Si NWs, where the heat source is the laser energy locally absorbed by the NW. This analysis takes account of the optical absorption, the thermal conductivity, the dimensions, diameter and length of the NWs, and the immersion medium. Both free standing and heat-sunk NWs are considered. Also, the temperature distribution in ensembles of NWs is discussed. This analysis intends to constitute a tool for the understanding of the thermal phenomena induced by laser beams in semiconductor NWs.

Laser ablation modelling of aluminium, silver and crystalline silicon for applications in photovoltaic technologies

Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (108–109 W cm−2), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed.

Interaction between a laser beam and semiconductor nanowires: application to the raman spectrum of Si nanowires

One presents in this work the study of the interaction between a focused laser beam and Si nanowires (NWs). The NWs heating induced by the laser beam is studied by solving the heat transfer equation by finite element methods (fem). This analysis permits to establish the temperature distribution inside the NW when it is excited by the laser beam. The overheating is dependent on the dimensions of the NW, both the diameter and the length. When performing optical characterization of the NWs using focused laser beams, one has to consider the temperature increase introduced by the laser beam. An important issue concerns the fact that the NWs diameter has subwavelength dimensions, and is also smaller than the focused laser beam. The analysis of the thermal behaviour of the NWs under the excitation with the laser beam permits the interpretation of the Raman spectra of Si NWs, where it is demonstrated that temperature induced by the laser beam play a major role in shaping the Raman spectrum of Si NWs

Self-similar motion of laser fusion plasmas. Absorption in an unbounded plasma

The one-dimensional motion generated in a cold, infinite, uniform plasma of density na by the absorption, in a certain plane, of a linear pulse of energy per unit time and area = 4>0t/r, 0< t< r, is considered, the analysis allows for thermal conduction and viscosity of ions and electrons, their energy exchange, and an electron heat flux limiter The resulting motion is self-similar and governed by a single nondimensional parameter a«(n0 2T/0)2/3 Detailed asymptotic results are obtained for both a < l and a > l , the general behavior of the solution for arbitrary a is discussed The analysis can be extended to the case of a plasma initially occupying a half-space, and throws light on how to optimize the hydrodynamics of laser fusion plasmas Known approximate results corresponding to motion of a plasma submitted to constant irradiation (<()) are recovered in the present work under appropriate limiting processes

Non-uniform target illumination in deflagration regime : thermal smoothing

Thermal smoothing in the plasma ablated from a laser target under weakly nonuniform irradiation is analyzed, assuming absorption at nc and a deflagration regime (conduction restricted to a thin quasisteady layer next to the target). Magnetic generation effects are included and found to be weak. Differences from results available in the literature are explained; the importance of the character of the underdense flow at uniform irradiation is emphasized.

Coronal fluid-dynamics in laser fusion

The fluid-dynamics of the corona ejected by laser-fusion targets in the direct-drive approach (thermal radiation and atomic physics unimportant) is discussed. A two-fluid model involves inverse bremsstrahlung absorption, refraction, different ion and electron temperatures with energy exchange, different ion and electron velocities and magnetic field generation, and their effect on ion-electron friction and heat flux. Four dimensionless parameters determine coronal regimes for one-dimensional flows under uniform irradiation. One additional parameter is involved in two-dimensional problems,including the stability of one-dimensional flows, and the smoothing of nonuniform driving.

New laser-based approaches to improve the passivation and rear contact quality in high effiency crystalline silicon solar cells

Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.

Análisis de la aplicación de Soldadura de acero de calidad "A" mediante láser de Neodimio-YAG en construcción naval

El proceso de soldadura por láser desarrollado en los últimos años ha puesto de manifiesto las posibilidades de aplicación de esta tecnología en diferentes sectores productivos, principalmente en la industria automovilística, en la cual se han demostrado sus ventajas en términos de productividad, eficiencia y calidad. El uso de la tecnología láser, ya sea híbrida o pura, reduce el input térmico al limitar la zona afectada por el calor, sin crear deformaciones y, por tanto, disminuye los re-trabajos post-soldadura necesarios para eliminarlas. Asimismo, se aumenta la velocidad de soldadura, incrementando la productividad y calidad de las uniones. En la última década, el uso de láseres híbridos, (láser + arco) de gran potencia de Neodimio YAG, (Nd: YAG) ha sido cada vez más importante. La instalación de este tipo de fuentes de láser sólido de gran potencia ha sido posible en construcción naval debido a sus ventajas con respecto a las instalaciones de láser de C02 existentes en los astilleros que actualmente utilizan esta tecnología. Los láseres de C02 están caracterizados por su gran potencia y la transmisión del haz a través de espejos. En el caso de las fuentes de Nd:YAG, debido a la longitud de onda a la cual se genera el haz láser, su transmisión pueden ser realizada a través de fibra óptica , haciendo posible la utilización del cabezal láser a gran distancia de la fuente, aparte de la alternativa de integrar el cabezal en unidades robotizadas. El proceso láser distribuye el calor aportado de manera uniforme. Las características mecánicas de dichas uniones ponen de manifiesto la adecuación de la soldadura por láser para su uso en construcción naval, cumpliendo los requerimientos exigidos por las Sociedades de Clasificación. La eficiencia energética de los láseres de C02, con porcentajes superiores al 20%, aparte de las ya estudiadas técnicas de su instalación constituyen las razones por las cuales este tipo de láser es el más usado en el ámbito industrial. El láser de gran potencia de Nd: YAG está presente en el mercado desde hace poco tiempo, y por tanto, su precio es relativamente mayor que el de C02, siendo sus costes de mantenimiento, tanto de lámparas como de diodos necesarios para el bombeo del sólido, igualmente mayores que en el caso del C02. En cambio, el efecto de absorción de parte de la energía en el plasma generado durante el proceso no se produce en el caso del láser de Nd: YAG, utilizando parte de esa energía en estabilizar el arco, siendo necesaria menos potencia de la fuente, reduciendo el coste de la inversión. En función de la aplicación industrial, se deberá realizar el análisis de viabilidad económica correspondiente. Dependiendo de la potencia de la fuente y del tipo de láser utilizado, y por tanto de la longitud de onda a la que se propaga la radiación electromagnética, pueden existen riesgos para la salud. El láser de neodimio se propaga en una longitud de onda, relativamente cercana al rango visible, en la cual se pueden producir daños en los ojos de los operadores. Se deberán establecer las medidas preventivas para evitar los riesgos a los que están expuestos dichos operadores en la utilización de este tipo de energía. La utilización del láser de neodimio: YAG ofrece posibilidades de utilización en construcción naval económicamente rentables, debido su productividad y las buenas características mecánicas de las uniones. Abstract The laser welding process development of the last years shows broad application possibilities in many sectors of industry, mostly in automobile production. The advantages of the laser beam process produce higher productivity, increasing the quality and thermal efficiency. Laser technology, arc-hybrid or pure laser welding, reduces thermal input and thus a smaller heat-affected zone at the work piece. This means less weldment distortion which reduces the amount of subsequent post-weld straightening work that needs to be done. A higher welding speed is achieved by use of the arc and the laser beam, increasing productivity and quality of the joining process. In the last decade use of hybrid technology (laser-GMA hybrid method) with high power sources Nd:YAG lasers, gained in importance. The installation of this type of higher power solid state laser is possible in shipbuilding industrial applications due to its advantages compare with the C02 laser sources installed in the shipyards which use this technology. C02 lasers are characterised by high power output and its beam guidance is via inelastic system of mirrors. In the case of Nd:YAG laser, due to its wavelength, the laser beam can be led by means of a flexible optical fibre even across large distances, which allows three dimensional welding jobs by using of robots. Laser beam welding is a process during which the heat is transferred to the welded material uniformly and the features of the process fulfilled the requirements by Classification Societies. So that, its application to the shipbuilding industry should be possible. The high quantum efficiency of C02 laser, which enabled efficiency factors up to 20%, and relative simple technical possibilities of implementation are the reasons for the fact that it is the most important laser in industrial material machining. High power Nd: YAG laser is established on the market since short time, so that its price is relatively high compared with the C02 laser source and its maintenance cost, lamp or diode pumped solid state laser, is also higher than in the case of C02 lasers. Nevertheless effect of plasma shielding does not exist with Nd:YAG lasers, so that for the gas-shielding welding process the optimal gases can be used regarding arc stability, thus power source are saved and the costs can be optimised. Each industrial application carried out needs its cost efficiency analysis. Depending on the power output and laser type, the dangerousness of reflected irradiation, which even in some meters distance, affects for the healthy operators. For the YAG laser process safety arrangements must be set up in order to avoid the laser radiation being absorbed by the human eye. Due to its wavelength of radiation, being relatively close to the visible range, severe damage to the retina of the eye is possible if sufficient precautions are not taken. Safety aspects are of vital importance to be able to shield the operator as well as other personal. The use of Nd:YAG lasers offers interesting and economically attractive applications in shipbuilding industry. Higher joining rates are possible, and very good mechanical/technological parameters can be achieved.

Non-Maxwellian electron distributions in time-dependent simulations of low-Z materials illuminated by a high-intensity X-ray laser

The interaction of high intensity X-ray lasers with matter is modeled. A collisional-radiative timedependent module is implemented to study radiation transport in matter from ultrashort and ultraintense X-ray bursts. Inverse bremsstrahlung absorption by free electrons, electron conduction or hydrodynamic effects are not considered. The collisional-radiative system is coupled with the electron distribution evolution treated with a Fokker-Planck approach with additional inelastic terms. The model includes spontaneous emission, resonant photoabsorption, collisional excitation and de-excitation, radiative recombination, photoionization, collisional ionization, three-body recombination, autoionization and dielectronic capture. It is found that for high densities, but still below solid, collisions play an important role and thermalization times are not short enough to ensure a thermal electron distribution. At these densities Maxwellian and non-Maxwellian electron distribution models yield substantial differences in collisional rates, modifying the atomic population dynamics.

Laser Shock Micro-Forming as an Emerging Technique for Microsystems Shaping and Adjustment

Continuous and long-pulse lasers have been extensively used for the forming of metal sheets for macroscopic mechanical applications. However, for the manufacturing of Micro-Mechanical Systems (MMS), the applicability of such type of lasers is limited by the long relaxation time of the thermal fields responsible for the forming phenomena. As a consequence, the final sheet deformation state is attained only after a certain time, what makes the generated internal residual stress fields more dependent on ambient conditions and might difficult the subsequent assembly process. The use of short pulse (ns) lasers provides a suitable parameter matching for the laser forming of an important range of sheet components used in MEMS. The short interaction time scale required for the predominantly mechanic (shock) induction of deformation residual stresses allows the successful processing of components in a medium range of miniaturization (particularly important according to its frequent use in such systems). In the present paper, Laser Shock Micro-Forming (LSμF) is presented as an emerging technique for Microsystems parts shaping and adjustment along with a discussion on its physical foundations and practical implementation possibilities developed by the authors.

Ion acceleration in underdense plasmas by ultra-short laser pulses

We report on the ion acceleration mechanisms that occur during the interaction of an intense and ultrashort laser pulse ( λ > μ I 2 1018 W cm−2 m2) with an underdense helium plasma produced from an ionized gas jet target. In this unexplored regime, where the laser pulse duration is comparable to the inverse of the electron plasma frequency ωpe, reproducible non-thermal ion bunches have been measured in the radial direction. The two He ion charge states present energy distributions with cutoff energies between 150 and 200 keV, and a striking energy gap around 50 keV appearing consistently for all the shots in a given density range. Fully electromagnetic particle-in-cell simulations explain the experimental behaviors. The acceleration results from a combination of target normal sheath acceleration and Coulomb explosion of a filament formed around the laser pulse propagation axis

Interaction between a laser beam and semiconductor nanowires: application to the raman spectrum of Si nanowires

One presents in this work the study of the interaction between a focused laser beam and Si nanowires (NWs). The NWs heating induced by the laser beam is studied by solving the heat transfer equation by finite element methods (FEM). This analysis permits to establish the temperature distribution inside the NW when it is excited by the laser beam. The overheating is dependent on the dimensions of the NW, both the diameter and the length. When performing optical characterisation of NWs using focused laser beams, one has to consider the temperature increase introduced by the laser beam. An important issue concerns the fact that the NW's diameter has subwavelength dimensions, and is also smaller than the focused laser beam. The analysis of the thermal behaviour of the NWs under the excitation with the laser beam permits the interpretation of the Raman spectrum of Si NWs. It is demonstrated that the temperature increase induced by the laser beam plays a major role in shaping the Raman spectrum of Si NWs.

Effect of Thermal Relaxation on LSP Induced Residual Stresses and Fatigue Life Enhancement of AISI 316L stainless steel

Effect of Thermal Relaxation on LSP Induced Residual Stresses and Fatigue Life Enhancement of AISI 316L stainless steel

High-Power High-Efficiency Laser Power Transmission at 100m Using Optimized Multi-Cell GaAs Converter

A high-power high-efficiency laser power transmission system at 100m based on an optimized multi-cell GaAs converter capable of supplying 9.7W of electricity is demonstrated. An I-V testing system integrated with a data acquisition circuit and an analysis software is designed to measure the efficiency and the I-V characteristics of the laser power converter (LPC). The dependencies of the converter’s efficiency with respect to wavelength, laser intensity and temperature are analyzed. A diode laser with 793nm of wavelength and 24W of power is used to test the LPC and the software. The maximum efficiency of the LPC is 48.4% at an input laser power of 8W at room temperature. When the input laser power is 24W (laser intensity of 60000W/m2), the efficiency is 40.4% and the output voltage is 4 V. The overall efficiency from electricity to electricity is 11.6%.

Experimental implementation of a biometric laser synaptic sensor

We fabricate a biometric laser fiber synaptic sensor to transmit information from one neuron cell to the other by an optical way. The optical synapse is constructed on the base of an erbium-doped fiber laser, whose pumped diode current is driven by a pre-synaptic FitzHugh–Nagumo electronic neuron, and the laser output controls a post-synaptic FitzHugh–Nagumo electronic neuron. The implemented laser synapse displays very rich dynamics, including fixed points, periodic orbits with different frequency-locking ratios and chaos. These regimes can be beneficial for efficient biorobotics, where behavioral flexibility subserved by synaptic connectivity is a challenge.