Frequency Tunable Metamaterial Absorber Using Hygroscopicity of Nature Cork

Abstract

In this letter, a novel frequency tunable metamaterial absorber is proposed for wireless humidity sensor applications made with natural cork. Natural cork has excellent hygroscopicity and its electrical properties can be changed depending on the moisture content. In this work, the absorption ratio of the proposed absorber are simulated and measured in different moisture content ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm M} = 0\% $</tex> </formula> , 1.44%, 5.88%, 13.32%, 18.8%) of the cork substrate. The resonant frequency changed from 4.92 GHz in the dry ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm M} = 0\% $</tex> </formula> ) state to 3.49 GHz in the wet ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm M} = 18.8\% $</tex> </formula> ) state; the measured absorptivity is higher than 90% in all moisture content values. To the best of our knowledge, it is the first metamaterial absorber using natural cork.