GTMs. (a) Electro-optical GTM. Gating voltage is applied between the top electrode and back electrode (also used to reflect the terahertz beam) to modulate the terahertz absorption of graphene. The line plot shows the electric field in the substrate. The optical thickness provides a critical influence on the electric field in the substrate and graphene. Reproduced from ref. . (b) All-optical GTM. Incident infrared beam modulates the terahertz absorption of graphene. The inset shows the power flow in the substrate. The conducting silicon layer also contributes to attenuate the terahertz beam. Reproduced from ref. . (c) GTM with split-ring resonators (SRRs). Right: the structure of SRR GTM. A SiO2 was deposited to separate the graphene sheet and metal SRRs defined by e-beam lithography. Left: the reflected power of terahertz from the top and back sample surfaces at different bias VTG of the split-ring arrays with respect to graphene can be measured. Reproduced from ref. .