Spatial-domain interferometer for measuring plasma mirror expansion

We present a practical spatial-domain interferometer for characterizing the electronic density gradient of laserinduced plasma mirrors with sub-30-femtosecond temporal resolution. Time-resolved spatial imaging of an intensityshaped pulse reflecting off an expanding plasma mirror induced by a time-delayed pre-pulse allows us to measure characteristic plasma gradients of 10-100 nm with an expansion velocity of 10.8 nm/ps. Spatial-domain interferometry (SDI) can be generalized to the ultrafast imaging of nm to m size laser-induced phenomena at surfaces.