  Stimulated Brillouin backscattering of light is shown to be drastically
enhanced in electron-positron plasmas, in contrast to the suppression of
stimulated Raman scattering. A generalized theory of three-wave coupling
between electromagnetic and plasma waves in two-species plasmas with arbitrary
mass ratios, confirmed with a comprehensive set of particle-in-cell
simulations, reveals violations of commonly-held assumptions about the behavior
of electron-positron plasmas. Specifically, in the electron-positron limit
three-wave parametric interaction between light and the plasma acoustic wave
can occur, and the acoustic wave phase velocity differs from its usually
assumed value.
