Critical thickness for near-field resonance in ultrathin terahertz metamaterials

Abstract

Terahertz metamaterials provide an effective platform for manipulating terahertz waves and have attracted significant interest for sensing applications. However, realizing compact devices that simultaneously achieve high sensitivity and high spatial resolution remains challenging. To address this issue, we investigate the thickness-dependent resonance behavior of nanoscale terahertz metamaterial sensors excited by a near-field point terahertz source. This source is locally generated via optical rectification in a nonlinear optical crystal under femtosecond laser irradiation at the subwavelength scale. Owing to the highly localized excitation, strong resonance is achieved even in an ultrathin, 20-nm-thick 3 × 3 metamaterial array with a subdiffraction-limited footprint of 0.54 × 0.54 λTHz2. We specifically examine the skin-depth (δ) effect by varying metal thickness below the skin depth and analyzing the resulting electric-field distributions and surface current patterns, which govern the coupling modes among the meta-atoms, the fundamental building blocks of the metamaterial. Numerical simulations show that transmission resonance emerges at a metal thickness of approximately 5 nm (0.045 δ), predicting the existence of an ultrathin critical thickness, while a thickness of 20 nm (0.18 δ) represents a practical boundary for achieving sufficiently strong resonance both in the simulation and experiment. Using a 20-nm-thick 3 × 3 meta-atom array, we further demonstrate trace-level biosensing by discriminating between single-stranded and double-stranded DNA, producing resonance frequency shifts of 30 and 40 GHz, respectively, in transmission measurements. These results identify metal thickness as a key physical parameter governing near-field-induced resonance and highlight the potential of ultrathin, ultracompact terahertz metamaterials for highly sensitive sensing applications.