The interaction of intense electromagnetic waves with electron–positron () plasmas is studied by particle-in-cell simulations and theoretical analysis. It is found that an initial underdense plasma can become opaque under the irradiation of a relativistically intense laser pulse. The strong ponderomotive force of the relativistic laser pulse and the small mass density of the plasma can combine to induce the efficient pile-up of the electrons and positrons at the front of the laser pulse. Therefore, the local plasma density at the laser pulse front increases dramatically and finally the initial underdense plasma becomes opaque. This relativistic-induced opacity effect of plasmas is opposite to the well-known relativistic-induced transparency effect, in which an initial overdense electron-ion plasma can become transparent to a relativistically intense laser pulse. Further, the significant red shift of reflected lights as well as the efficient generation of energetic positrons are investigated in the relativistic-induced opacity of plasmas. This relativistic-induced opacity effect is a peculiar phenomenon in the plasmas, which may be encountered in the high-energy astrophysical phenomena or in the interactions of intense lasers with matters in the laboratories.
Plasma based charged-particle accelerators