Orientation-dependent multiphoton ionization in wide band gap crystals.

Using different crystalline dielectrics and intense femtosecond laser pulses, we show that nonlinear absorption depends on sample orientation. This arises primarily because of the direction dependence of the effective mass of the electron. The multiphoton nature of the interaction creates a local probe that can be used anywhere in the material. We show that the structure of crystal quartz is not changed by repeated illumination of the focal region with 45 fs pulses.