Ultrafast Electron Dynamics in Femtosecond Optical Breakdown of Dielectrics

We measured the optical breakdown threshold (OBT) in dielectrics with different band gaps for single and double 25-fs 800-nm transform-limited laser pulses. Our pump-probe double pulse measurements indicate that the plasma energy in dielectrics experiences ultrafast decay which lasts only $\sim 100\mathrm{fs}$ and does not follow an exponential decay curve. Therefore, a decay term must be included in the electron density rate equation. Our double pulse measurements also demonstrate that the OBT is temperature dependent. The OBT in dielectrics was determined using a novel technique, which eliminates the ambiguity in its definition and also allows real-time data acquisition.