Time-resolved measurements on reflectivity of an ultrafast laser-induced plasma mirror

Using a linearly chirped laser pulse to irradiate antireflection coated targets, the time-varying reflectivity of a plasma mirror (PM) has been measured at various laser intensities from 1012 to 1017 W/cm2. The onset of plasma generation as well as the formation process of a PM with the highest reflectivity has been observed. The rise time of the PM's reflectivity reaching up to the maximum varies from 300-500 fs at lower laser intensities but goes up to 900 fs at higher intensity of >1016 W/cm2. This long rise time can be attributed to a slowly rising shoulder of the laser pulse, which will trigger the generation of preplasma well in advance of the laser peak. The detailed measurements on both time-integrated and time-resolved reflectivity of a PM, which is induced by p- and s-polarized laser pulses, respectively, indicate that an s-polarized pulse is favorable to obtain the maximal reflectivity and the best contrast improvement as well. This difference can be attributed to the weaker absorption and a smaller plasma scale length generated for s-polarization.