Anomalous laser-induced group velocity dispersion in fused silica

We present 20fs(2) accuracy laser-induced group velocity dispersion (LI-GVD) measurements, resulting from propagation of a femtosecond laser pulse in 1mm of fused silica, as a function of peak intensity. For a 5.5 × 10(11) W/cm(2) peak intensity, LI-GVD values are found to vary from -3 to + 15 times the material GVD. Normal induced dispersion can be explained by the Kerr effect, but anomalous LI-GVD, found when the input pulses have negative pre-chirp, cannot. These findings have significant implications regarding self-compression and the design of femtosecond lasers.