  A theoretical and numerical investigation of non-ponderomotive absorption at
laser intensities relevant to quantum electrodynamics is presented. It is
predicted that there is a regime change in the dependence of fast electron
energy on incident laser energy that coincides with the onset of pair
production via the Breit-Wheeler process. This prediction is numerically
verified via an extensive campaign of QED-inclusive particle-in-cell
simulations. The dramatic nature of the power law shift leads to the conclusion
that this process is a candidate for an unambiguous signature that future
experiments on multi-petawatt laser facilities have truly entered the QED
regime.
