  We demonstrate arbitrary helicity control of circularly polarized light (CPL)
emitted at room temperature from the cleaved side-facet of a lateral-type
spin-polarized light-emitting diode (spin-LED) with two ferromagnetic
electrodes in an anti-parallel magnetization configuration. Driving alternate
currents through the two electrodes results in polarization switching of CPL
with frequencies up to 100 kHz. Furthermore, tuning the current density ratio
in the two electrodes enables manipulation of the degree of circular
polarization. These results demonstrate arbitrary electrical control of
polarization with high speed, which is required for the practical use of
lateral-type spin-LEDs as monolithic CPL light sources.
