Date of Award

December 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Member

Xin Zhao

Committee Member

Gang Li

Committee Member

Laine Mears

Committee Member

Hongseok Choi

Abstract

Ultrafast lasers have great capability and flexibility in micromachining of various materials. Due to the involved complicated multi-physical processes, mechanisms during laser-material interaction have not been fully understood. To improve and explore ultrafast laser processing and treatment of dielectric materials, numerical and experimental investigations have been devoted to better understanding the underlying fundamental physics during laser-material interaction and material micromachining. A combined continuum-atomistic model has been developed to investigate thermal and non-thermal (photomechanical) responses of materials to ultrafast laser pulse irradiation. Coexistence of phase explosion and spallation can be observed for a considerably wide range of laser fluences. Phase explosion becomes the primary ablation mechanism with the increase of laser fluence, and spallation can be restrained due to the weakened tensile stress by the generation of recoil pressure from ejection of hot material plume. For dielectric materials, due to the much lower temperature gradient by non-linear absorption, the generated thermal-elastic stress is much weaker than that in non-transparent materials, making spallation less important. Plasma dynamics is studied with respect to ejection directions and velocities based on fluorescence and shadowgraph measurements. The most probable direction (angle) is found insensitive to laser fluence/energy. The plasma expansion velocity is closely related to electron thermal velocity, indicating the significance of thermal ablation in dielectric material decomposition by laser irradiation. A numerical study of ultrafast laser-induced ablation of dielectric materials is presented based on a one-dimensional plasma-temperature model. Plasma dynamics including photoionization, impact ionization and relaxation are considered through a single rate equation. Material decomposition is captured by a temperature-based ablation criterion. Dynamic description of ablation process has been achieved through an improved two-temperature model. Laser-induced ablation threshold, transient optical properties and ablation depth have been investigated with respect to incident fluences and pulse durations. Good agreements are shown between numerical predictions and experimental observations. Fast increase of ablation depth, followed by saturation, can be observed with the increase of laser fluence. Reduction of ablation depth at fluences over 20 J/cm2 is resulted from plasma defocusing effect by air ionization. Thermal accumulation effect can be negligible with repetition rate lower than 1 kHz for fused silica and helps to enhance the ablation depth at 10 kHz (100 pulses) to almost double of that with single pulse. The ablation efficiency decreases with fluence after reaching the peak value at the fluence twice of the ablation threshold. The divergence of tightly focused Gaussian beam in transparent materials has been revealed to significantly affect the ablation process, particularly at high laser fluence. A comprehensive study of ultrafast laser direct drilling in fused silica is performed with a wide range of drilling speeds (20-500 μm/s) and pulse energy (60-480 μJ). Taper-free and uniform channels are drilled with the maximum length over 2000 μm, aspect ratio as high as ~40:1 and excellent sidewall quality (roughness ~0.65 μm) at 270 μJ. The impacts of pulse energy and drilling speeds on channel aspect ratio and quality are studied. Optimal drilling speeds are determined at different pulse energy. The dominating mechanisms of channel early-termination are beam shielding by material modification at excessive laser irradiation for low speed drilling and insufficient laser energy deposition for high speed drilling, respectively. An analytical model is developed to validate these mechanisms. The feasibility of direct drilling high-aspect-ratio and high-quality channels by ultrafast laser in transparent materials is demonstrated.

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