A technique to measure the intensity profile of a focused laser pulse at full power is a long-standing desire. High power lasers allow experiments at relativistic intensities (1018 W/cm2 and beyond). At those photon densities all atoms are ionized and therefore it is very difficult to measure directly the peak intensity or the 3D-profile of the focal spot intensity. We would like to present a way to measure it, based on residual atoms in the experimental chamber.
A low-density gas will imply a number of atoms at the laser focal volume. Those atoms will be instantaneously ionized, and the released electrons will move at relativistic speeds driven by the laser field. Plasma effects, at low density, can be neglected, and electrons move independently driven only by the laser field. Nearly 50 years ago, an approach was suggested that is based on relativistic Thomson scattering, which consists of a rich spectrum of Doppler shifted radiation of the laser light, and its harmonics . This reference provides very simple expressions for the scattered Doppler shift vs. intensity. Therefore, such scattered photons give very valuable information about the intensity profile. We propose to measure the Doppler shift of the low order harmonics as an in-situ direct measure of the intensity.
In particular, we will present the first preliminary experimental observation of such a shift of the second harmonic as a non-destructive way to measure the intensity profile of the Salamanca VEGA-2 laser focal profile. The spectrum is consistent with a peak intensity beyond 1018 W/cm2, which correlates well with the expected intensity. This promising result is the theme of this presentation. Details of the experiment, numerical simulations, related experiments and prospects for exploiting relativistic Thomson scattering to develop an in situ intensity profiler will be discussed.
 E. S. Sarachik and G. T. Schappert, Phys. Rev. D 1, 2738 (1970).