Femtosecond Micromachining of Transparent Materials

We present an overview of femtosecond microstructuring of transparent materials. Bulk structuring of transparent materials can be achieved by focusing high-intensity femtosecond pulses. The morphology of the structures depends on the incident energy per pulse and on the focusing conditions. At high focusing conditions the damage threshold in silicate glasses is just a few nanojoules. This energy range is available from an oscillator. We have demonstrated laser writing of embedded waveguides in silicate glasses with a femtosecond oscillator. Laser machining at high laser repetition rate results in a cumulative thermal mechanism of material modification which leads to structural index of refraction changes beyond the focal volume. The cross-sectional index profile of the fabricated structure depends on the number laser pulses irradiated. We present a parametric study of the role of the laser repetition rate in the size of the machined structures. We identify two distinct regimes of processing in the kHz to MHz range. As the time interval between pulses is reduced, we observe a transition from a repetitive modification process (identical to what is frequently called multiple shot damage) to a cumulative thermal mechanism. In the repetitive regime, each pulse acts independently and the energy deposited diffuses out of the focal volume before the next pulse arrives. In the cumulative thermal regime, the heat deposited is additive and leads to structures of significantly different morphology whose size increases dramatically with the decrease of the time interval between pulses.