Low-Loss Graphene Waveguide Modulator for Mid-Infrared Waves

Graphene waveguide plays an important role in modulating optical signal. But it is hard to make a tradeoff between low propagating loss and high field confinement. Here, we propose a bilayer graphene waveguide in a thin topas film and a high refractive index material-germanium cladded with thin metallic film. The influences of structural parameter and chemical potential of graphene are studied to optimize the dimension and working mode. Simulation reveals that our structure can make a balance between high figure of merit (FOM) and low propagation loss, and it reaches a high modulation depth of 2.5 dB/μm. The design can work with FOM over 200, propagation loss lower than 0.2 dB/μm, and propagation length beyond 30 μm in a wide band from 6 THz to 18 THz. Compared with previous graphene waveguides, our structure operates from terahertz band to mid-infrared band, and it has longer propagation length due to the existence of bilayer graphene. Besides, benefiting from the thin metallic film, our structure can be integrated on chip.