Sensitive distinction between herbs by terahertz spectroscopy and a metamaterial resonator

Terahertz spectroscopy, combined with a metamaterial resonator, was used to distinguish varieties of herbs that look alike to the naked eye. The metamaterial resonator, based on an ohm-ring, was experimentally demonstrated to exhibit two significant resonant peaks in the range from 0.2 to 1.4 THz. The resonant peaks were shifted to lower frequency with changes of the surface permittivity, and this feature was applied to identify varieties of herbs. In the experiment, different borneols were effectively identified according to the different degree of redshift. This work demonstrates that metamaterial resonator is a sensitive tool for terahertz spectral sensing, and shows a new detection method for different herbs.