The preplasma origin of the plasma-mirror reflectivity response

Plasma mirrors (PM) enhance the temporal contrast of high-power lasers, but declining reflectivity at subrelativistic intensities limits their energy throughput. Here we show that prepulse-generated preplasma drives this decline in p polarization primarily through absorption. Particle-in-cell simulations using a measured prepulse profile reproduce the reflectivity decrease, with near- and far-field energy accounting showing a weak contribution from scattering. Optical-cycle-resolved electron tracking reveals successive electron bunches extracted from the plasma surface, supporting a vacuum-heating interpretation. Under the simulated conditions, s-polarized absorption remains weak and no comparable decline occurs. Applying the model to two representative double-plasma-mirror (DPM) systems, BELLA and CoReLS, it captures both the p-polarized and s-polarized reflectivity responses across the operating range. Yet, the model still misses the high-fluence s-polarized reflectivity rollover, and further investigation of the scattering channel might be needed. These findings connect prepulse history to polarization-dependent energy loss, informing the choice of plasma-mirror operating conditions.