We study synchrotron maser emission and associated particle acceleration
at electron-ion relativistic shocks by PIC simulations. Relativistic shocks emit intense
electromagnetic waves from the shock front via the synchrotron maser instability.
This synchrotron maser emission induces electrostatic plasma waves (i.e., wakefield) and
transverse filamentary structures in the upstream region via the stimulated/induced Raman
scattering and filamentation instability, respectively. Some lucky particles in the phase of
the deceleration by the wakefield enter the filamentary structures. Then they are reflected by
the wakefield and decoupled from the upstream bulk flow. These decoupled particles can feel
the motional electric field and efficiently accelerated.