  We report a terahertz optical modulator consisting of randomly stacked
trilayer graphene (TLG) deposited on an oxidized silicon substrate by means of
THz- Time Domain Spectroscopy (THz-TDS). Here, the gate tuning of the Fermi
level of the TLG provides the fundamental basis for the modulation of THz
transmission. We measured a 15% change in the THz transmission of this device
over a broad frequency range (0.6-1.6THz). We also observed a strong absorption
>80% in the time-domain signals and a frequency independence of the
conductivity. Furthermore, unlike previous studies, we find that the underlying
silicon substrate, which serves as a gate electrode for the graphene, also
exhibits substantial modulation of the transmitted THz radiation under applied
voltage biases.
