Ultra-broadband (1.3-2.5 μm) and highly efficient graphene electro-optic modulator based on a buried silicon waveguide

We numerically demonstrate an ultra-broadband graphene electro-optic modulator based on a buried silicon-on-silica (SoS) waveguide. The device comprises a single-layer graphene sheet on a rectangular silicon core, separated by an Al₂O₃ spacer to form a parallel-plate nanocapacitor. A silicon slab links the core to the base electrode, while a platinum contact connects the graphene to the gate, minimizing parasitic absorption and series resistance. By systematically optimizing the waveguide dimensions (800 × 350 nm² core), and Al₂O₃ spacer thickness (7 nm), the design efficiently enhances transverse magnetic (TM) mode light-matter interaction. Through electrical tuning of the Fermi level of the graphene sheet, the high modulation efficiency of the graphene enables a compact 32-µm² active area. The device achieves a high modulation bandwidth of 6.57 GHz across an ultra-broadband operation spectrum ranging from 1.30 to 2.50 µm, which is scalable to 46.94 GHz via a 50-nm dielectric modification. A key advantage of this mid-infrared operation is the monotonic reduction in switching voltage from 4.0 V at 1.30 µm to an ultra-low 1.5 V at 2.50 µm. This scaling amplifies modulation efficiency up to 2.00 dB/V and drives energy consumption down from 1.90 to 0.44 pJ/bit. While practical constraints at extended interaction lengths (L_3 dB ≈ 86 µm) limit the bandwidth (f_3 dB) to 3.51 GHz, performance remains fully adequate for mid-infrared sensing and communications. This energy-efficient, complementary metal-oxide semiconductor (CMOS)-compatible architecture establishes a promising building block for next-generation photonic integrated circuits unifying data communications and mid-infrared sensing.