  The nonlinear interaction between electromagnetic waves and plasmas attracts
significant attention in astrophysics because it can affect the propagation of
Fast Radio Bursts (FRBs) -- luminous millisecond-duration pulses detected at
radio frequency. The filamentation instability (FI) -- a type of nonlinear
wave-plasma interaction -- is considered to be dominant near FRB sources, and
its nonlinear development may also affect the inferred dispersion measure of
FRBs. In this paper, we carry out fully kinetic particle-in-cell simulations of
the FI in unmagnetized pair plasmas. Our simulations show that the FI generates
transverse density filaments, and that the electromagnetic wave propagates in
near vacuum between them, as in a waveguide. The density filaments keep merging
until force balance between the wave ponderomotive force and the plasma
pressure gradient is established. We estimate the merging timescale and discuss
the implications of filament merging for FRB observations.
