Nanogap-enhanced terahertz sensing of 1 nm thick (λ/10⁶) dielectric films

We experimentally show that terahertz (THz) waves confined in sub-10 nm metallic gaps can detect refractive index changes caused by only a 1 nm thick (∼λ/10⁶) dielectric overlayer. We use atomic layer lithography to fabricate a wafer-scale array of annular nanogaps. Using THz time-domain spectroscopy in conjunction with atomic layer deposition, we measure spectral shifts of a THz resonance peak with increasing Al₂O₃ film thickness in 1 nm intervals. Because of the enormous mismatch in length scales between THz waves and sub-10 nm gaps, conventional modeling techniques cannot readily be used to analyze our results. We employ an advanced finite-element-modeling (FEM) technique, Hybridizable Discontinuous Galerkin (HDG) scheme, for full three-dimensional modeling of the resonant transmission of THz waves through an annular gap that is 2 nm in width and 32 μm in diameter. Our multiscale 3D FEM technique and atomic layer lithography will enable a series of new investigations in THz nanophotonics that has not been possible before.