Active THz Optoelectronics Using Multilayer Graphene: tunable filters, phase modulators, and resonators.

We present the active manipulation of THz waves by means of optoelectronic devices based on multilayer graphene (MLG) structures (~100 layers thick). The devices are based on the reversible intercalation of ions, which changes the carrier density of the MLG by as much as 2 orders of magnitude, thus, tuning the complex dielectric function ε(ω) in a controllable and reversible way. We tested our devices using terahertz time domain spectroscopy (THz TDS), and we quantified our ability to modulate terahertz radiation/light. Furthermore, finite difference time domain (FDTD) calculations were carried out to model and interpret the voltage-induced changes in ε(ω), extract phase information, and predict the behavior of more complex structures and devices. We observed under various applied voltages a clear shift in the reflected peak position to earlier times (for positive applied voltages) and later times (for negative applied voltages). The apparent peak delay and peak advance can be understood on the basis of phase shifts obtained by a modulation in the complex dielectric function (i.e., ε1 and ε2). Assuming a frequency of 1 THz, this modulation here corresponds to phase shifts of Δφ = ±120⁰. These results stand in contrast to those reported previously in which negligible phase shifts are observed.