Effects of structural parameters on terahertz field enhancement in metal nano slit

Abstract

Electric field enhancement inside metal-insulator-metal gaps, especially slit structure, is investigated in Terahertz range. Dependencies of field enhancement on slit width and thickness are calculated by modal expansion method for perfect electric conductor approximation and numerical finite difference time domain method for real metal. Field enhancement increases as slit width and thickness decrease. Slit thickness as well as width has a large influence on field enhancement when the width becomes small. To explain the tendencies we propose a model based on charge and capacitor concept. In practice, fabricated metal slit which has a few nanometer sized width usually shows asymmetric and tapered shape due to complexity of fabrication processes. Dependence of field enhancement on tapered angle in symmetric and asymmetric slit is also investigated. Numerical results show that thickness of smallest gap region is the most important for enhancement factor if tapered angle is less than 70 degree. This confined field can be utilized to enhance light-matter interaction. By placing metal nano gap structure on vanadium dioxide film which has transition property between Mott insulator and metal phase at the specific temperature, we can observe enhanced interaction between vanadium dioxide film and terahertz field.