We demonstrate ultrafast optical switching of electromagnetic waves through a metallic slot antenna in the terahertz frequency regime. The screening of the terahertz field due to the optical excitation of carriers in GaAs/AlGaAs multiple quantum wells is further enhanced due to the strong field confinement near the antenna, which results in the complete suppression of the far-field transmission of terahertz waves. The ultrafast rising time of the carrier population enables picosecond switching of terahertz transmission. A simulation based on a modified Rayleigh diffraction theory combined with the Drude model confirms that the fast decay of electromagnetic field within GaAs is responsible for the ultrafast optical switching. (C) 2015 Elsevier B.V. All rights reserved.