Near-Field Characterization of Conductive Micro-resonators for Terahertz Sensing

Abstract

Near-field (NF) terahertz (THz) time-domain spectroscopy (TDS) is an excellent tool for direct studies of THz electromagnetic resonances occurring on a micrometre scale. Micro-resonators are at the heart of numerous promising THz sensing and detecting solutions, including the metamaterial approach available through functional materials constructed from pre-designed resonant micrometresized 'meta-atoms'. Experimental studies of individual micrometre-scale THz resonances are essential, yet extremely challenging for the common far-field spectroscopic methods due to extreme sensitivity requirements. NF THz spectroscopy and microscopy are non-contact techniques for spectroscopic studies of individual micro-resonators and mapping the field patterns of THz resonant modes excited in individual conductive or insulating micro-objects. They give access to essential parameters of micro-resonators, including their resonance frequency, local field enhancement and quality factors. It allows for material and structural characterisation of micro-objects. Using the example of carbon micro-fibres, we show the advantages of NF THz TDS for non-contact THz conductivity probing and direct observation of the fundamental and the third-order surface-plasmon resonance modes in conductive THz micro-resonators.