Graphene-gated plasmonic directional-coupler logic gate with electrically tunable extinction ratio for C-band optical communications

Abstract A graphene-gated plasmonic directional coupler for C-band optical communications is analyzed with full-wave finite-element mode analysis and local-mode propagation. Two coplanar silver strips (20 nm × 200 nm) sit in a crystal cladding ( n = 1.926); an Al 2 O 3 layer fills the gap and forms a 5 nm gate dielectric under monolayer graphene. The coupler operates through bonding and antibonding supermodes of the edge-localized strip mode, whose large splitting rules out weak-coupling coupled-mode theory. Because the bonding supermode concentrates its tangential field on the graphene, gating changes its loss up to 12 times more than the antibonding one; a first-order perturbation model evaluated without fitting reproduces the finite-element results. The coupling section alone (12 nm gap) gives a contrast ratio (CR) of 32.1 dB with 0.49 dB insertion loss, above 28 dB across the C band. The access S-bends supply most of the coupling phase and convert power between supermodes of equal symmetry unless their gap profile is shaped. A complete 3.1 µm crossbar with shaped bends reaches CR = 26.1 dB with 2.0 dB loss, tunable by gating between 21.4 and 26.1 dB; a larger minimum gap raises CR to 46 dB at 4.4 dB loss. With a quarter-wave phase bias, a 4.8 µm device provides XOR and OR simultaneously with 27.3 dB XOR contrast.