Graphene films are characterized by high electrical conductivity and high optical transmittance. In this paper, we mixed graphene solution and polyvinyl alcohol (PVA) solution to make different concentrations of graphene solution. The electrical conductivity, optical transmittance, and phase shift of multilayer graphene films were measured in the THz range by coating graphene solution on the substrate. According to experimental results, the following conclusions are drawn: the film with a larger proportion of graphene has a smaller square resistance, that is, a better conductivity, a smaller transmittance, and a phase shift in the terahertz range under the same thickness. Subsequently, the solution made of 1:1.75 PVA and graphene solution was coated on the outside of the glass sheets containing E7 and HTD liquid crystals, respectively. The distance between the two glass sheets was 800um, which was the thickness of the liquid crystals. Then, a magnetic field is added along one side of the glass sheet (defined as the X direction). Through experiments, it is found that when the voltage is added to 100V (130V), the orientation of E7 (or HTD) liquid crystal can completely change the direction of the electric field. And in the case of the only magnetic field, E7 (or HTD) liquid crystal produces 0.5π phase difference corresponding frequency is 0.8THz (0.6THz), π phase difference corresponding frequency is 1.4THz (1.0THz).
Strong terahertz chiral manipulation plays a vital role in polarization imaging, chiral spectroscopy, and multichannel communications. Here, a composite structure based on the double-layer metasurface sandwiched with the ultrathin liquid crystal layer is proposed to realize a strong terahertz chiral manipulation. On account of breaking mirror symmetry in the propagation direction and the local field enhancement effect in the cavity, the composite structure possesses a strong terahertz chiral response. Moreover, this terahertz chiral response can achieve a large tuning range by changing the optical axis of the liquid crystal: the experimental co-polarization circular dichroism can achieve a large controllable range of −31dB to 30dB at 0.772 THz and a chirality control range from 11dB to −22dB at 0.759 THz. This composite device demonstrates rich characteristics for the feasible manipulation of THz chiral response, which has greatly promoted the development and practical application of THz polarization and chiral control devices.
Terahertz waves have a wide range of applications in the field of sensing because of their good coherence, high signal-noise ratio, low radiation energy, and non-ionization. In addition, THz sensing also has the advantages of real-time, non-contact, and label-free, so it has important application in biological sensing, especially in the sensing of active biological substances. But it also has some defects, such as low sensitivity, strong water absorption, limited detection information, and poor applicability. Herein, we propose a THz time-domain polarization spectroscopy (TDPS) sensing system. The metal metasurface structures are used as the sensor, and the transmission or reflection sensing method is used to detect the PVA thin film, tumor cell and amino acid solution respectively. The experimental results show that: compared with the traditional resonant sensing method, the quality (Q) factor and figure of merit (FoM) of polarization sensing method are improved by at least 4~5 times, so the sensing sensitivity is significantly improved. For tumor cell sensing, its minimum detection accuracy has reached 103 cells/ml. For amino acid solution sensing, the minimum detection accuracy is 10-5 g/ml magnitude. In addition, using the chiral metasurface structure as sensor, this method can also identify the difference between the chiral enantiomer solutions of amino acids.
Phase and polarization are the basic parameters of electromagnetic wave, which can not only carry useful electromagnetic information, but also manipulate the propagation and states of light. With the development of terahertz (THz) technology and its application system, high-performance THz phase control devices are urgently needed. Our recent research work on THz phase control devices based on liquid crystals integrating with artificial microstructures or nanomaterials was reviewed in this paper. The proposed dielectric metasurface can realize a polarization-dependent electromagnetically induced transparency effect with a large artificial birefringence and a negative-dispersion phase difference with large artificial anisotropy. The double-layer graphene gratings can achieve a switchable function to switch between linear-to-linear and linear-to-circular polarization states. On this basis, we presented to combine the artificial microstructures with liquid crystals for controllable THz phase shifters and broadband wave plates. Moreover, by using the strong interaction between liquid crystals and the electrically or magnetically sensitive nanomaterials, we proposed a series of high-efficiency control THz phase devices. This work has greatly promoted the development and practical application of THz phase and polarization control devices.
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