Material Removal and Ejection Dynamics in Femtosecond Laser Machining of Microchannels in Transparent Materials

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High aspect ratio and high quality microchannels in transparent materials are critical in many important areas, such as micro-optics, microelectronics, micromechanics, and biomedicine. However, it is difficult to fabricate them using traditional machining techniques, due to the brittle nature and low thermal conductivity often found in transparent materials. Femtosecond pulsed lasers offer the potential to overcome these difficulties. However, the aspect ratio and quality of microchannels produced by femtosecond pulsed lasers are limited. This award supports fundamental research to enable significant improvement in the quality and aspect ratio of microchannels produced by femtosecond pulsed lasers.The research objectives are to establish the relationships between (1) ablation mechanisms (spallation, phase explosion, fragmentation, etc.) and machining conditions (laser intensity, pulse duration, etc.); (2) ablation mechanisms and ejected particle size/velocity distributions; and (3) the size/velocity of an ejected particle and its capability of escaping a long channel. To achieve the first two objectives, a physics-based atomistic model, consisting of a molecular dynamics method, a Monte Carlo method, and a particle-in-cell method, will be developed, with laser parameters and material properties as the inputs. By predicting the distributions of temperature, pressure, and electric field within the materials, dominating ablation mechanisms will be revealed under different machining conditions. This model will also predict the sizes and velocities of the ejected particles by simulating the atom evolution during the laser-matter interaction. To verify the simulation outputs, the sizes/velocities of the ejected particles under the same conditions will be experimentally measured by the time-resolved pump-probe imaging technique. To achieve the third objective, outputs of the atomistic model, such as the temperature, pressure, and the sizes/velocities of the ejected particles after the initial laser-matter interaction, will be used as inputs into a subsequently developed smooth particle hydrodynamics model, to simulate the ejected particle evolution within the channel in a large time scale. For particles with given sizes and initial velocities, the model will predict their escape or redeposition onto the channel side walls, based on the temperature, pressure, and ambient environment inside the channel. Ejected particle moving dynamics, such as their transient locations and velocities, will also be observed in-situ using the time-resolved pump-probe imaging technique, and compared with model simulation results.