Guanhua Ren, Zhongjie Zhu, Jianbing Zhang, Hongwei Zhao, Yanfeng Li, Jiaguang Han,
Paper and paper products banknotes play an important role in daily life. Two terahertz (THz) time-domain spectrometers (TDS) with combined spectral coverage from 0.5 to 10 THz were used to characterize the optical properties of several types of paper and banknotes. We found that the paper and banknotes show remarkable fingerprints in the terahertz regime, which are highly related to the vibrational modes of the main component of paper and banknotes — celluloses. Meanwhile, THz transmission imaging measurements had been performed and shown the capability of imaging the key anti-counterfeit labels of banknotes such as watermarks, security threads and optically variable inks. The presented study here indicates that the THz spectroscopy together with the imaging method could provide a visible, contactless and nondestructive alternative to delineating paper and paper product banknotes.
