Efficient terahertz modulator based on photoexcited graphene

An efficient terahertz (THz) modulator consisting of a single-layer graphene on a silicon substrate has been investigated using an external 450 nm continuous-wave laser with a low photoexcitation power. Upon photoexcitation, both transmission and reflection are measured using a THz time-domain spectroscopy. The experimental results show that the modulation depth of the transmission can reach 74% for the proposed modulator under external photoexcitation. The difference between the transmission through the sample and bare silicon substrate can reach a maximal value of 49.3% with a pump power of 420 mW, which indicates that the modulation effect of graphene is dominant with minimal contribution from the silicon substrate. The analyses of the strong attenuation of the transmitted THz waves reveals that an enhanced absorption of the THz wave takes place, which is shown to be rooted in the increase of the doping level in graphene, the accumulation of the photo-induced carrier at the interface of the graphene and silicon substrate, as well as the scattering between carriers, phonons and defects. The results of this study imply promising applications for the development of high-performance THz modulators and absorbers.