Dynamics of femtosecond laser induced ionization in dielectrics

This study has developed a setup for femtosecond time-resolved imaging Mach-Zehnder interferometry. This interferometer is capable of measuring the electron concentration with temporal resolution of about 70 fs and a lateral spatial resolution of about 1 μm. A strong pump-pulse (Ti:sapphire, 800 nm, 40 fs) excites the sample whereas a co-propagating weak time-delayed probe-pulse (400 nm, 60 fs) experiences the changes of the refractive index induced by the pump pulse. Both the probe and the reference pulses pass through a high-resolution microscope objective and interfere in the image plane, where an interferogram is recorded by a CCD-camera. The application of a 2D-Fourier-transform technique allows to reconstruct the changes in the amplitude and phase of the probe pulses due to excitation of the sample with great accuracy. The experimental results are compared with the numerical simulation of ultrashort pulse propagation, which take into account self-focusing, dispersion and known ionization mechanisms.