A device for modulating terahertz waves includes a metal layer (703) including a continuous metal portion (705) and island metal portions (707). The metal portions (705, 707) are separated by apertures (709). The device further includes a semiconductor layer (715) affixed to a bottom surface of the metal layer (703). The semiconductor layer (715) includes carrier regions (717) located below the apertures (709). The transmission of terahertz waves through the apertures (709) is modulated by changing a voltage applied across the aperture via voltage source (715). By injecting free carriers into carrier regions (717) due to a change of the voltage an extraordinary terahertz transmission effect of the metal layer (703) can be switched off. A small increase in the free-carrier absorption is significantly enhanced by the Fabry-Perot resonance, resulting in a substantial decrease in transmission. The disclosed ring aperture terahertz modulator allows for electrical control of the carrier density only in the area underneath the aperture. This design minimizes the power consumption and maximizes the operation speed.