Femtosecond Laser-Induced Damage for Micromachining of Transparent Materials”.Abstract. 2002. “
Femtosecond laser pulses have extremely short temporal profiles. When tightly focused with a microscope objective, one micron spatial profiles can be achieved. Even with a very modest per-pulse energy, ranging from ten nanojoules to several microjoules, one can easily reach intensities over ten terawatts per square centimeter. Under such conditions, highly intensity-dependent nonlinear absorption processes such as multiphoton absorption and avalanche ionization take place, leading to permanent structural and chemical changes in transparent materials. Since transparent materials have electronic band gaps too large to bridge with a single photon, linear absorption of laser light does not occur. This allows beams to be focused into the bulk of a transparent material without damaging the surface. The nature of such nonlinear interactions, the physics of focusing in the bulk, the chemical changes induced, as well as a few practical applications are explored in this thesis. I first derive an equation relating the threshold intensity for damage in the bulk of a transparent material to physical quantities like the threshold energy, numerical aperture, and the critical power for self-focusing. Then I investigate the potential of bulk micromachining in transparent polymers to have applications in micromechanical devices and microcircuitry, and I study surface patterning of polydimethylsiloxane and its applications in microcontact printing. Finally, taking advantage of the derived equation, I explore the basic physics and chemistry of making damage in the bulk of diamond, a remarkably hard material with an extremely high refractive index.