ABSTRACT
In recent years, femtosecond lasers have been used for a multitude of micromachining tasks [1]. Several groups have shown that femtosecond laser pulses cleanly ablate virtually any material with a precision that consistently meets or exceeds that of other laser-based techniques, making the femtosecond laser an extremely versatile surface micromachining tool [2-7]. For large-band-gap materials, where laser machining relies on nonlinear absorption of high-intensity pulses for energy deposition, femtosecond lasers offer even greater benefit. Because the absorption in a transparent material is nonlinear, it can be confined to a very small volume by tight focusing, and the absorbing volume can be located in the bulk of the material, allowing three-dimensional micromachining [8,9]. The extent of the structural change produced by femtosecond laser pulses can be as small as or even smaller than the focal volume. Recent demonstrations of three-dimensional micromachining of glass using femtosecond lasers include threedimensional binary data storage [8,10] and the direct writing of optical waveguides [912] and waveguide splitters [13]. The growing interest in femtosecond laser micromachining of bulk transparent materials makes it more important than ever to uncover the mechanisms responsible for producing permanent structural changes.
