The highly nonlinear interaction of femtosecond laser pulses with transparent materials (glasses) allows the deposition of optical energy into the bulk of dielectrics.
This provides the possibility to induce micrometer-sized structural changes, which are associated with local refractive index changes d(elta)n. For dn>0, the writing of waveguides with fs-laser pulses has been demonstrated successfully in many different glasses. However, neither the dynamics nor the processes leading to the local refractive-index changes are fully understood, which prevents an optimum control of the laser induced modification process.

With our experiments we address the problems relevant for application. We demonstrate that we can guide the material-dependent interaction channels and the energy flow dynamics in laser irradiated solids by impulsive and large bandwidth excitation with THz repetition-rate pulse trains.
For BK7 we are able to reverse the material response and obtain an increase of refractive index (in the single shot regime) Furthermore, we compensate for the influence of spherical aberrations by properly temporally shaped laser pulses in the fs- to ps-time domain.

In addition our results indicate a significant thermal contribution to the refractive index changes along with strong absorption or scattering of the probe-beam radiation in the focal region. Both last for more than 10 ns.