In this paper, we propose a metamaterial-based terahertz (THz) absorber with optically tunable absorbance. The unit cell of the structure consists of a cross-shaped resonator, a dielectric spacing layer for wave impedance matching and a high-resistivity silicon (HRS) substrate. Without illumination, the structure acts as a capacitive metal mesh filter that has minimum transmittance and maximum reflectance at its resonance frequency. When the HRS substrate is optically illuminated, its conductivity increases, effectively blocking transmission through the structure. Therefore, this device will have a low reflectance if
the impedance is matched. The optimized structure shows a high
absorbance of 98% at 0.25 THz in simulations. This concept can
be used for the realization of dynamic control of absorbance and
emissivity for applications in the THz and infrared (IR) range.