Tuning the optical modulation of wideband terahertz waves by the gate voltage of graphene field effect transistors

In this paper, a broadband terahertz wave was optically modulated by laser signal using a graphene field effect transistor. The modulation speed and depth were tuned by varying the gate voltage. The modulating laser and gate voltage together can trigger both inter- and intra-band transitions in graphene. The laser light penetrates through graphene into the p-Si substrate forming a thin loss layer at the graphene/p-Si interface, while the gate voltage tuned the intra-band transitions in graphene by changing the carrier density of states and the Fermi level. The change in carrier concentration, density of states, and mobility in the loss layer directly affected the optical conductivity and refractive index, and thus the modulation depth. Modulation depth of the whole device is between 28.5% and 39.3% at modulation frequencies from 100 kHz to 1.0 MHz.