The most promising application is for use in laser-driven plasma accelerator science, such as laser wakefield acceleration of electrons and a laser driving foil to accelerate protons. Laser-plasma interaction can also be an efficient source of high-order harmonic generation (HHG) , x rays ,and even gamma rays.One of the key issues in the above mechanisms is how to make use of the laser ponderomotive force efficiently to pump a strong charge separation field in plasma, which is the origin of particle acceleration. Hence, it is the force (the accelerating force, the confining force, etc.) that people care about most in relativistic laser plasma physics. The effect of another important dynamical quantity, the torque, although as important as force, has not been revealed for a relativistic laser pulse. How to observe the orbital angular momentum (OAM) in laser-plasma interaction and how the appearance of OAM would essentially affect the process are of special interest.

Although several methods are available for obtaining twisted laser pulses, a relativistic twisted light pulse has not been generated yet. Since intense LG pulses are also useful elsewhere, there have been several attempts to produce them with the direct approach. However, because the amplified spontaneous emission in the middle of the laser pulse is inevitably very large during amplification, the amplified laser is no longer a LG pulse. The OAM effect of relativistic laser-plasma interaction remains unknown.

The research group headed by Prof. Shen BaiFei at Shanghai Institute of Optics and Fines Mechanics, Chinese Academy of Sciences, have reported a simple and effective method for generating a relativistic twisted laser pulse and showing the OAM effect.It is called “light fan”: a relativistic laser pulse (with a very high photon density) impinges on a spiral foil (the fan);hence, both the fan and the reflected pulse achieve a net large OAM. This study demonstrates that a reflect fan structure can be applicable in the relativistic regime. More importantly, the dynamic process of such a structure presents new and unique features in the relativistic intensity regime.[Phys. Rev. Lett. 112, 235001; http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.112.235001]