When a femtosecond laser pulse is tightly focused inside a transparent material, the intensity in the focal volume is high enough to cause absorption through nonlinear processes. The absorption of the laser energy creates a micrometer-sized, highly excited plasma inside the material. The energy, initially contained in the plasma, is subsequently transferred to the lattice in the form of heat and shock waves. In solids, this energy transfer permanently alters the material. Because the absorption is strongly intensity-dependent, it is possible to create permanent damage inside the material, leaving the surface unaffected.
Our initial work focused on determining damage thresholds in various materials and studying the effects of self-focusing. We measured the damage threshold using a dark-field scattering technique as a function of the numerical aperture of the focusing objective. Under tight-focusing conditions, for numerical apertures (NA) of 0.65 or higher, damage occurs for peak powers below the threshold for self-focusing. From the dependence of the energy threshold on NA we obtained an intensity threshold. This intensity threshold increases somewhat as the band gap of the material increases, but much less than one would expect on the basis of multiphoton excitation alone. Our results therefore suggest that avalanche ionization plays an important role in femtosecond micromachining.
We recently demonstrated the microstructuring of bulk transparent samples using pulses with less than 10 nJ energy. This processing technique has the capability of inducing morphological and optical changes. The changes can be as large as tens of micrometers and evolve over microseconds. Applications include data storage and the writing of waveguides and waveguide splitters in bulk glass, fabrication of micromechanical devices in polymers, subcellular photodisruption inside single cells, and nanosurgery in nematodes.