Giant mid-infrared Goos–Hänchen shifts <i>via</i> resonant optical tunneling in graphene/Weyl-semimetal stacks

We demonstrate a mid-infrared (mid-IR) resonant optical tunneling platform that yields an order-of-magnitude enhancement of the Goos-Hänchen (GH) shift through critical coupling in the transverse magnetic (TM) channel. The device comprises a prism, a monolayer graphene sheet, an ultrathin Weyl-semimetal (WSM, Co₃Sn₃S₃) film and an air gap. The underlying mechanism is a balance between resonant tunneling across the gap and internal dissipation within the graphene/WSM stack, which drives a deep reflectance minimum and a rapid π phase winding and produces giant, sign-controllable lateral displacements. At an incidence wavelength of <i>λ</i> = 10.8 μm, the structure operates very close to critical coupling (<i>R</i>_min < 0.5%) and exhibits a steep TM reflection phase slope, which translates into a millimeter-scale lateral displacement of several hundred wavelengths (10²<i>λ</i>). The GH response is actively reconfigurable, and when the Fermi level of graphene is tuned to <i>u</i>_c = 1.0 eV, the tunneling can induce a sign reversal of the shift and a giant spike with 10³<i>λ</i>. Filling the gap with an analyte further enables refractometric sensing with angular and amplitude sensitivities of <i>S</i>_<i>θ</i> = 25-38 deg/RIU and <i>S</i>_D = (1.5-5) × 10⁴<i>λ</i>/RIU, respectively. These results establish mid-IR resonant tunneling as a compact route to ultra-large, sign-switchable GH shifts with clear utility for precision beam steering, weak measurement readout, and integrated photonic sensing.