  Fast radio bursts (FRBs) are millisecond transient astrophysical phenomena
and bright at radio frequencies. The emission mechanism, however, remains
unsolved yet. One scenario is a coherent emission associated with the magnetar
flares and resulting relativistic shock waves. Here, we report unprecedentedly
large-scale simulations of relativistic magnetized ion-electron shocks, showing
that strongly linear-polarized electromagnetic waves are excited. The kinetic
energy conversion to the emission is so efficient that the wave amplitude is
responsible for the brightness. We also find a polarization angle swing
reflecting shock front modulation, implicating the polarization property of
some repeating FRBs. The results support the shock scenario as an origin of the
FRBs.
