Showing papers by "Hai-Lung Tsai published in 2012"

Formation mechanisms of sub-wavelength ripples during femtosecond laser pulse train processing of dielectrics

Abstract: A new model is proposed to investigate femtosecond laser pulse train processing of dielectrics by including the laser wave properties in the photon particle properties based plasma model. In the case studies, the pulse duration is 50?fs and the pulse delays are 0, 25, 50 and 75?fs. Both the laser wave?particle duality and transient localized changes of material properties are considered in the proposed model, and the formation mechanism of sub-wavelength ripples is revealed. This study shows that the interference between surface plasmons and laser field plays a key role in the formation of sub-wavelength ripples for which the excitation of surface plasmons is necessary. The predicted period of sub-wavelength ripples is in agreement with the experiments.

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation.

Abstract: A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron–photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon–electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.

A method to generate pressure gradients for molecular simulation of pressure-driven flows in nanochannels

Abstract: One of the difficulties in molecular simulation of pressure-driven fluid flow in nanochannels is to find an appropriate pressure control method. When periodic boundary conditions (PBCs) are applied, a gravity-like field has been widely used to replace actual pressure gradients. The gravity-fed method is not only artificial, but not adequate for studying properties of fluid systems which are essentially inhomogeneous in the flow direction. In this paper, a method is proposed which can generate any desired pressure difference to drive the fluid flow by attaching a “pump” to the nanofluidic system, while the model is still compatible with PBCs. The molecular dynamics model based on the proposed method is applied to incompressible flows in smooth nanochannels, and the predicted velocity profiles are identical to those by the gravity-fed method, as expected. For compressible flows, the proposed model successfully predicts the changes of fluid density and velocity profile in the flow direction, while the gravity-fed method can only predict constant fluid properties. For fluid flows in nanochannels with a variable cross-sectional area, the proposed model predicts higher mass flow rates as compared to the gravity-fed method and possible reasons for the difference are discussed.

Hybrid laser-arc welding of aerospace and other materials

Abstract: This chapter first describes the origin and major characteristics of the hybrid laser-arc welding technique. Second, fundamentals of this welding technique, such as laser-plasma interaction, keyhole formation and collapse, weld pool dynamics, metal melting and solidification, etc., are elaborated. Finally, applications, current research and development, and future challenges and development of hybrid laser-arc welding of aeronautical materials, such as magnesium, aluminium and magnesium alloys, are discussed.

Effect of Interlayer Between Semiconductors on Interfacial Thermal Transport

Abstract: Due to the high surface–to–volume ratio in nanostructured components and devices, thermal transport across the solid–solid interface strongly affects the overall thermal behavior. Materials such as Si, Ge, SiO2 and GaAs are widely used in advanced semiconductor devices. These materials may have differences in both crystal structure and Debye temperature. We have shown that the thermal transport across such interfaces can be improved by inserting an interlayer between the two confining solids.If the two confining solids are similar in crystal structure and lattice constant but different in Debye temperature, it is predicted from the molecular dynamics modeling that an over 50% reduction of the thermal boundary resistance can be achieved by inserting a 1– to 2–nm–thick interlayer which has similar crystal structure and lattice constant as the two solids. In this case, the Debye temperature of the optimized interlayer is approximately the square root of the product of the Debye temperatures of the two solids. However, if the interlayer has large lattice mismatches with the two confining solids, a thin disordered layer is formed in the solid and in the interlayer adjacent to their interface. Such a disordered layer can distort the phonon density of states at the interface and strongly affects the interfacial phonon transport. In this case, it is found that a 70% reduction of the thermal boundary resistance can be achieved if the lattice constant of the interlayer is smaller than that of the two solids and the Debye temperature of the interlayer is approximately the average of the Debye temperatures of the two solids.On the other hand, if the two solids have a large difference in both lattice constant and Debye temperature, the optimized interlayer should have a lattice constant near the average of the lattice constants of the two solids. For this case, an over 60% reduction of the thermal boundary resistance can be achieved if the Debye temperature of the interlayer is equal to or slightly higher than the square root of the product of the Debye temperatures of the two solids. The calculated phonon density of states shows that the distorted phonon spectra induced by large lattice mismatches are generally broader than the phonon spectra of the corresponding undistorted case. The broader interfacial phonon spectra increase the overlap between the phonon spectra of the two solids at the interface which leads to improved thermal boundary transport.Copyright © 2012 by ASME