Ihtesham H. Chowdhury

HEAT TRANSFER IN FEMTOSECOND LASER PROCESSING OF METAL

The short time scales and high intensities obtained during femtosecond laser irradiation of metals require that heat transfer calculations take into account the nonequilibrium that exists between electrons and the lattice during the initial laser heating period. Thus, two temperature fields are necessary to describe the process--the electron temperature and the lattice temperature. In this work, a simplified one-dimensional, parabolic, two-step model is solved numerically to predict heating, melting, and evaporation of metal under femtosecond laser irradiation. Kinetic relations at the phase-change interfaces are included in the model. The numerical results show close agreement with experimental melting threshold fluence data. Further, it is predicted that the solid phase has a large amount of superheating and that a distinct melt phase develops with duration of the order of nanoseconds.

Ultrafast double-pulse ablation of fused silica

Ultrafast pump-probe experiments were used to study high-intensity ultrafast pulse-ablation dynamics in fused silica. Two laser pulses with varied time delay and pulse energy were used to irradiate fused silica samples and observe the transient reflectivity and transmissivity of the probe pulse. It was seen that the probe reflectivity initially increased due to the formation of free-electron plasma and then dropped to a low value within a period of about 10ps caused by a rapid structural change at the surface. The time-resolved measurements of reflectivity and transmissivity were also related to atomic force microscopy measurements of the depth of the laser-ablated hole. It was seen that the depth peaked at zero delay between the pulses and decreased within a period of about 1ps as the temporal separation between the pulses was increased caused by the screening by the plasma produced by the first pulse. When the temporal separation is about 100ps or longer, evidence for melting and resolidification during...

Molecular Dynamics Study of Phase Change Mechanisms During Femtosecond Laser Ablation

In this work, Molecular Dynamics (MD) simulation is employed to investigate femtosecond laser ablation of copper, with an emphasis on the understanding of the mechanism of phase change during laser ablation. Laser induced heat transfer, melting, surface evaporation, and material ablation are studied. Theoretically, it has been suggested that under intense femtosecond laser irradiation, the material undergoes a volumetric phase change process; its maximum temperature can be close to or even above the thermodynamic critical point. The MD simulations allow us to determine the transient temperature history of the irradiated material and to reveal the exact phase change process, which explains the mechanisms of femtosecond laser ablation. A finite difference calculation is also performed, which is used to compare results of heating and melting prior to a significant amount of material being ablated.

Plasma formation in fused silica induced by loosely focused femtosecond laser pulse

The focusing position inside fused silica irradiated by a loosely focused high power femtosecond laser pulse is studied both experimentally and numerically. The experimental measurement of plasma radiation shows that the laser pulse is focused behind the focal plane, which is also found in the numerical calculation and is attributed to a complex interplay between self-focusing due to the Kerr effect and defocusing because of the free electron plasma. Also, when more than one pulse is incident at the same spot in the sample, plasma radiation is observed at more than one spot along the laser propagation direction.

Ultrafast pulse train micromachining

A new micromachining technique using user-defined trains of amplified femtosecond laser pulses is described. In this method, a 2-fold Michelson interferometer is used to split each output pulse of an amplified femtosecond laser system operating at 1 kHz into four different pulses at desired seperations ranging from 1 ps to 1 ns. These quadruple pulses are then focused on metal, semiconductor and dielectric samples and the material removal characteristics are noted. The experimental results show that there is a distinct effect of the pulse separation on the machining characteristics. It is observed that, in some cases, use of the quadruple pulses separated by 1 ns provides better material removal than the original pulses separated by 1 ms. The femtosecond laser-material interaction is also modeled for the case of metal samples using the two-temperature model. Numerical simulations that were carried out show that irradiation with quadruple pulses lead to a reduction in the predicted melting threshold fluence, which agrees with the experimental observation.

Numerical Simulation of Femtosecond Laser Ablation of Copper: Comparison Between Molecular Dynamics and Finite Difference Calculations

Molecular Dynamics (MD) and Finite Difference (FD) methods are applied to investigate femtosecond laser ablation of copper. Laser induced heat transfer, melting, vaporization, and material ablation are studied. Phase change relevant parameters, such as the velocity of solid-liquid and liquid-vapor interfaces are reported. It is shown that results of MD and FD are in good agreement before strong material removal occurs. However, only MD is capable of capturing the ablation phenomena accurately.© 2003 ASME