Relativistic-induced opacity of electron-positron plasmas

The interaction of intense electromagnetic waves with electron-positron (e-e+) plasmas is studied by particle-in-cell simulations and theoretical analysis. It is found that an initial underdense e+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 e-e+ 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 e-e+ plasma becomes opaque. This relativistic-induced opacity effect of e-e+ 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 e-e+ plasmas. This relativistic-induced opacity effect is a peculiar phenomenon in the e-e+plasmas, which may be encountered in the high-energy astrophysical phenomena or in the interactions of intense lasers with matters in the laboratories.