Metamaterial near-field sensor for deep-subwavelength thickness   measurements and sensitive refractometry in the terahertz frequency range
  We present a metamaterial-based terahertz (THz) sensor for thickness measurements of subwavelength-thin materials and refractometry of liquids and liquid mixtures. The sensor operates in reflection geometry and exploits the frequency shift of a sharp Fano resonance minimum in the presence of dielectric materials. We obtained a minimum thickness resolution of 12.5 nm (1/16000 times the wavelength of the THz radiation) and a refractive index sensitivity of 0.43 THz per refractive index unit. We support the experimental results by an analytical model that describes the dependence of the resonance frequency on the sample material thickness and the refractive index. 
Benjamin Reinhard, Klemens M. Schmitt, Viktoria Wollrab, Jens Neu,   Ren\'e Beigang, Marco Rahm
physics.optics