Time- and Frequency-Domain THz Imaging for the Characterization of Subwavelength Tags in Anticounterfeiting Devices

Hidden tags are robust and efficient security systems for protecting assets but their identification requires nondestructive techniques. Here, a contactless, efficient, terahertz (THz) investigation protocol based on the electromagnetic interaction between THz radiation and materials with subwavelength features is presented. On top of an opaque, lossy dielectric substrate a metal "open tag" is patterned as reference. This contains features that are visible to the naked eye, but subwavelength in the THz range. An identical dielectric substrate is then used to cover the metallic pattern and fabricate a "hidden tag." Both tags are then scanned using a THz time-domain spectroscope (TDS). The collected spectra are used to evaluate different metrics, which provide valuable pixel-specific information for sample imaging. While the most common imaging techniques are demonstrated to be unsuitable for decoding hidden tags, a thorough study of the THz response allows to determine the best conditions for creating high-resolution images in both time- and frequency-domain. Effective metrics are obtained by isolating the peaks of the signal reflected from the tag or the substrates, or by suitably considering the interaction between the two signals. The proposed method provides a rigorous protocol for the THz imaging of hidden logos in multilayer systems, which can serve as anticounterfeiting tags.