Experimental verification of laser intensity is extremely important for fundamental physics research at ultrahigh laser intensities. However, reliable diagnosis of the focal spot and peak intensity face huge challenges. Our work demonstrates for the first time that the farfield pattern of coherent radiation produced from the laser-thin foil interaction can be utilized as an in-situ, real-time and convenient diagnostic of the laser focus. The spatially variant laser field accelerates curved electron sheets that leave the foil at nearly the speed of light, producing coherent radiation with a doughnut-shaped farfield distribution. The farfield pattern depends on the shape of the focal spot and peak intensity. Assisted by simulations, we can infer the peak intensity and the focal spot size. In addition, the farfield pattern can provide immediate feedback on the focal spot shape and guide the optimization of ion acceleration experiments.

laser plasma interaction,laser focus diagnosis,high power laser