  Fast radio bursts (FRBs) are cosmological radio transients with unclear
generation mechanism. Known characteristics such as their luminosity, duration,
spectrum and repetition rate, etc. suggest that FRBs are powerful coherent
radio signals at GHz frequencies, but the status of FRBs near source remain
unknown. As an extreme astronomical event, FRBs should be accompanied by energy
-- comparable or even more powerful x/{\gamma}-ray counterparts. Here,
particle-in-cell simulations of ultra-strong GHz radio pulse interaction with
GeV photons show that at 3*10^12V/cm field-strengths, quantum cascade can
generate dense pair plasmas, which greatly dampen the radio pulse. Thus, in the
presence of GeV photons in the source region, GHz radio pulses stronger than
3*10^12V/cm cannot escape. This result indicates an upper field-strength limit
of FRB at the source.
