Interaction of Strong Electromagnetic Waves with Unmagnetized Pair Plasmas

We investigate analytically and numerically the interaction of strong electromagnetic waves with unmagnetized pair plasmas. We show that the interaction is governed by a single nonlinearity parameter, $\varepsilon_{\rm p}$, defined as the ratio of the wave strength parameter to the wave frequency in units of the plasma frequency (with both frequencies measured in the plasma rest frame prior to the interaction). When $\varepsilon_{\rm p}<1$, the number of wavelengths that propagate through the plasma without attenuation from induced Compton scattering is approximately $\varepsilon_{\rm p}^{-2/3}$. This attenuation can imprint sub-structures as narrow as a few wavelengths on the pulse profile. When $\varepsilon_{\rm p}>1$, the electromagnetic pulse acts as a relativistic piston and drives a shock into the plasma. Our results establish a framework for the interaction of strong electromagnetic waves with pair plasmas, a process relevant for intense radio pulses from neutron stars and for next-generation pair plasma experiments at multi-petawatt laser facilities.