Fundamentals of Femtosecond Laser Modification of Bulk Dielectrics

Femtosecond laser pulses focused beneath the surface of a dielectric are absorbed through nonlinear photoionization mechanisms, giving rise to a permanent structural modification with dimensions on the order of a micrometer. At low pulse energies, the modification in many glasses is a smooth refractive index change, enabling photonic device fabrication. At higher pulse energies, the laser-induced modification may contain birefringent, periodic nanoplanes which align themselves orthogonally to the laser polarization. These nanogratings are not ideal for most waveguide devices but when the sample is exposed to hydrofluoric acid after writing, preferential chemical etching along the direction of the nanoplanes forms several millimeter-long buried microchannels which are useful for microfluidic applications. At even higher pulse energies, ultrahigh pressures within the focal volume lead to microexplosions causing empty voids which can be used for three-dimensional photonic bandgap devices and memories. In addition to pulse energy, other parameters have been shown to strongly influence the resulting morphology after femtosecond laser exposure including repetition rate, scan speed, focusing condition, polarization, pulse duration, depth, and direction.