The role of focusing in the interaction of femtosecond laser pulses with transparent materials Jonathan B. Ashcom Ph.D. Thesis, Harvard University, 2003, 146 pages  export citation  alerts       email to friend Femtosecond lasers can generate extremely intense optical pulses, where the electric field is comparable to or greater than the field that binds electrons to their parent ions. In this regime, the interaction of light with transparent materials can become strongly nonlinear. Nonlinear absorption of the pulse can create an energetic plasma leading to damage in the material. The material, too, can modify the propagation of the pulse, leading to self- focusing and white light continuum generation, a dramatic broadening of the pulse spectrum. This dissertation identifies the numerical aperture (NA), or the strength of the external focusing as a critical parameter in controlling the interaction between short pulses and transparent materials. Using fused silica as a model optical material, we show that at high NA, single shot, catastrophic damage occurs and continuum generation is not observed. At low NA, continuum generation is produced, but bulk material modification accumulates over time at energies above the continuum generation threshold. The continuum spectrum decreases with increasing NA. Bulk micromachining using femtosecond lasers is practical for numerical apertures of 0.25 NA and above, where self-focusing effects are minimal. The bulk modification of natural diamond and surface machining of transparent polymers for microprinting and microfluidic channel fabrication are presented as applications of these results at high NA.

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