Microstructuring of bulk transparent solids using nanojoule, femtosecond laser pulses

We produce sub-micron sized permanent damage in the bulk of dielectric materials using 110-fs laser pulses with only 40 nJ of energy. Tight external focusing (0.65 NA) of the ultrashort laser pulses enables us to achieve a high intensity at the focus with low laser energy. The high intensity leads to nonlinear absorption of the laser pulse by the material, resulting in permanent damage. Achieving high intensity with low energy reduces the effects of self-focusing, and eliminates the need for an amplified laser system. We report thresholds for damage and critical self-focusing in fused silica using 110-fs laser pulses of 400-nm and 800-nm wavelength. We find that, in our focusing geometry, the damage thresholds are more than an order of magnitude smaller than the critical self-focusing thresholds, indicating that self-focusing does not play a dominant role in the damage formation process. Without self-focusing, the laser spot size and intensity can be known, allowing straightforward interpretation of the wavelength and band gap dependence of bulk damage thresholds. Furthermore, the nanojoule level energies needed for permanent damage are readily available from cavity-dumped laser oscillators, enabling precise microstructuring of transparent solids without an amplified laser system.