Resonance restoration of terahertz metamaterials in thinner film

Abstract

High demands on metamaterials with desired resonance frequency and high field enhancement led to an extensive investigation on the sub-wavelength gaps, especially in terms of structural parameters. The role of most parameters including gap width, length, period and dielectric environment are widely studied and well understood, while the effect of metal thickness on the gap performance still remains elusive. Here, we theoretically and experimentally investigate resonance frequencies and field enhancements of sub-wavelength gaps with different heights. Scope of this study encompasses thicknesses as small as 10 nm, accessing by far the thinnest terahertz nano-resonators reported in the literature. The thickness decrease leads to huge field enhancement and resonance blueshift in transmission spectra of the sub-10nm gaps, restoring the transmission from the initially gap-plasmon dominated, red-shifted spectra. Such behavior is interpreted as a result of decreased effective refractive index of the gap, which is also supported by a modal-expansion analysis.