Metamaterial radiation from attenuated total reflection at terahertz frequencies

Abstract

The focus of this research was to explore the behavior of two-dimensional planar metamaterials or metafilms and understand the various excitation schemes for application of metafilms to Terahertz-Attenuated Total Reflection spectroscopy (THz-ATR). A standard THz time domain spectroscopy system based on photoconductive switches was modified to implement the ATR technique. Finite metamaterial arrays with varying singly- split ring resonator sizes were excited in the Kretschmann ATR configuration using finite sized terahertz beams. Numerical approaches using commercial software were looked into to explain the experimental observations. Various theoretical models were used to explain the observed phenomena. The ATR measurements showed an unexpected strengthening of the resonance when the metafilm sample was illuminated near the edge. This phenomenon referred to in this study as "the anomalous edge enhancement" was observed strongly in metafilms with closely spaced rings. A re-radiation signal was observed across the total internal reflection barrier where no signal is expected. It consisted of two peaks one at the fundamental metamaterial resonance and the second peak was due to the periodicity of the metafilm array. The anomalous behavior seen in the ATR measurements is attributed to the edge currents at the boundary of the metafilm array giving rise to this re-radiation signal. Results from analytic treatments based on Floquet method and method of moments were able to qualitatively model the measurements. The observed re-radiation signal is a potential loss mechanism that could impact observations from commonly used transmission measurements on metamaterials.