Interactions of Femtosecond Laser Pulses with Transparent Materials

Usually when light goes through a piece of glass, nothing happens to either the light nor the glass, i.e. the glass is transparent. With a powerfull femtosecond laser pulse, however, both the laser light and the glass can be changed. We study the interaction of intense, femtosecond laser pulses with bulk transparent materials. The intensity of a tightly-focused, femtosecond laser pulse can be high enough to cause nonlinear absorption of laser energy by the transparent material. When enough energy is deposited, permanent material change results. The absorption and therefore the material alteration are confined to the extremely small focal volume. Using this technique have produced clyindrical-shaped bulk optical damage as small as 200-nm diameter and 1-um long. By scanning the focus throughout the material, a three-dimensional structure can be micromachined. Our work concentrates on by fundamental and applied aspects of the interactions described above. In our talk, we will try to convey some of this diversity by presenting several kinds of results. First, by measuring the damage threshold for the material, one learns something about the nature of the nonlinear ionization mechanism. We will show measurements of damage threshold for different laser parameter and material combinations and disucss how ionization mechanisms change with these parameters. Second, the morphology of the damage structure reveals how energy deposited by the laser redistributs in the material. We will show multiple different characterizations of bulk optical damage, including high-reslolution optical and electron microscopy, and discuss the importance of these measurements for real-world micromachining applications. Finally, we will show several micromachining applictions. Three-dimensional binary data storage and the direct writing of waveguides and diffractive optical elements have been demonstrated. In addition, the use of these techniques in biological samples for sub-cellular photodisruption will be discussed.