  Optical Hall conductivity $\sigma_{xy}(\omega)$ is measured from the Faraday
rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz
frequency regime. The Faraday rotation angle ($\sim$ fine structure constant
$\sim$ mrad) is found to significantly deviate from the Drude-like behavior to
exhibit a plateau-like structure around the Landau-level filling $\nu=2$. The
result, which fits with the behavior expected from the carrier localization
effect in the ac regime, indicates that the plateau structure, although not
quantized, still exists in the terahertz regime.
