  Fabrication and optical characteristics of a spin light-emitting-diode
(spin-LED) having dual spin-injection electrodes with anti-parallel
magnetization configuration are reported. Alternating a current between the two
electrodes using a PC-driven current source has led us to the observation of
helicity switching of circular polarization at the frequency of 1 kHz. Neither
external magnetic fields nor optical delay modulators were used. Sending
dc-currents to both electrodes with appropriate ratio has resulted in
continuous variation of circular polarization between the two opposite
helicity, including the null polarization. These results suggest that the
tested spin-LED has the feasibility of a monolithic light source whose circular
polarization can be switched or continuously tuned all electrically.
