Spatio-temporal X-wave.

We introduce an illumination configuration which is a spatiotemporal analog of a non-diffracting X-wave. By interfering multiple ultrashort converging plane waves, we generate a tight central spot at which a transform limited ultrashort pulse is formed. Outside this tight focus a spatiotemporal speckle field with longer duration and reduced peak power is created. We investigate this spatiotemporal X-wave configuration analytically, numerically, and experimentally demonstrate the effect using two photon excitation fluorescence.