Anti-counterfeiting technologies of banknotes are essential to avoid financial fraud and maintain the stability of the socio-economic system. In recent years, the technologies used by counterfeiters have become more and more advanced, bringing great challenges to traditional methods of authenticity check. Therefore, the identification technologies of banknotes urgently need to be improved. Terahertz spectroscopy is widely used for analysis and material identification due to its properties (such as biomolecular fingerprinting spectrum, ultra-weak photon emission, non-destructive and time-resolution detection). In this paper, we apply the transmission terahertz imaging to the identification of anti-counterfeit labels of RMB. We designed and built a THz-TDS confocal imaging system, which has higher image resolution than traditional THz-TDS imaging systems. We used this system to perform terahertz imaging on anti-counterfeit labels of banknotes. The results show that our designed and built THz-TDS confocal imaging system not only provides higher image quality, but also can identify more details of the anti-counterfeit labels of banknotes.
In this paper, we reported the design, fabricate, and characterize of an electronically controlled terahertz (THz) amplitude modulator composed of a four-open ring metamaterial structure and a high electron mobility transistor (HEMT) structure, HEMT area is located at the openings of the four-opening ring metamaterial structure. The concentration of 2D electron gas of the HEMT is adjusted by applying an external electrical field and results in the resonance shifts. Both static and dynamic characterizations of the modulator were carried out by using an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system in the transmission type. Experimental results show that the amplitude modulator can modulate the incident THz waves with two orthogonal polarization directions. The modulation depths are 56% at 0.32 THz for vertical polarized beam and 40% at 0.34 for horizontal polarized beam respectively. The modulator has potential application in wireless communication and real-time imaging.
Ultrathin crystalline silicon wafers for photovoltaic applications have attracted intensive attention because of potential benefits in cost-effectiveness. Structural design with high light absorption is important for photovoltaics because planar ultrathin silicon is poor in absorption. We conduct a comparative investigation on designs of light absorption enhancement for 2-μm-thick ultrathin crystalline silicon, where the front texture is a nanopyramidal structure and the rear adopts several designs. Our calculation results show that both of the ultrathin silicon with front nanopyramids and rear silver nanoarrays and the ultrathin silicon with two-sided nanopyramids are promising for photovoltaic applications. For the latter design, the calculated photocurrent achieves the highest value of 35.1 mA/cm2 when a perfect electric conductor layer is applied at the bottom. In contrast, the former design has a lower photocurrent value of 31.2 mA/cm2. But, this design is of practical significance because the majority of experimental reports on ultrathin crystalline silicon solar cells are single-sided front-textured at present and the fabrication techniques of plasmonic Ag nanoarrays are matured. Compared with previous reports, the present work offers a multiple option of structural designs for ultrathin crystalline silicon to enhance the light absorption for photovoltaic applications.
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