Accelerated electrons for in situ peak intensity monitoring of tightly focused femtosecond laser radiation at high intensities

Abstract

A simple technique for diagnosing the peak intensity of a tightly focused high-power femtosecond laser beam is proposed. The method is based on tracking the angular distribution of electrons directly accelerated in the waist of linearly polarized tightly focused radiation. Field ionization of a low density (<1016 cm−3) gas (e.g. helium) creates electrons, which are accelerated by the electromagnetic field and leave the focal area with relativistic residual energy. Three dimensional simulations utilizing the particle-in-cell code and test particle method revealed that the maximum of angular distribution of high energy electrons shifts to the beam axis with increase in the peak laser intensity. Hence, the peak laser intensity may be well evaluated by detecting a direction of the flux of large momentum particles. The experimentally measured angular characteristics of electrons accelerated in the caustic region of tightly focused ∼1 TW laser radiation with an estimated vacuum peak intensity above 1019 W cm−2 have shown reasonable agreement with the numerical simulations. Shot-to-shot variation of the pulse energy leads to deviations in the particles angular distribution, opening the opportunity to control the focusing quality in situ, in each laser shot.