Relativistic transparency and opacity in femtosecond laser-plasma interactions

High-intensity ultrashort-pulsed lasers can produce electric fields sufficient to accelerate electrons to relativistic energies within femtoseconds, substantially changing the optical properties of plasmas. Here, we examine the reflectivity and transmissivity of relativistic laser pulses interacting with near-critical-density plasmas using particle-in-cell (PIC) simulations. We describe the dynamics of both relativistic transparency and relativistically enhanced reflectivity across several distinct regimes depending on the relative importance of the relativistic mass increase and ponderomotively driven plasma density modification. We also show that observed frequency shifts are well-matched to an analytic model based on the Doppler effect. The optics of relativistic plasmas play a fundamental role in applications of high-intensity lasers and strongly determine the efficiency of coupling intense light into plasma systems.