Preparation And Properties Of Large Birefringence Liquid Crystals In Terahertz Bands

Terahertz waves have unique properties and are widely used in terahertz spectroscopy,terahertz imaging and terahertz communications.The liquid crystal material has good photoelectric anisotropy and mature industrial technology,making the terahertz liquid crystal device rapidly developed.At the same time,there are still some problems.The general liquid crystal material in the terahertz band birefringence than the visible range of small,high absorption loss.In order to improve the work efficiency of the device and meet the design requirements of the device,it is necessary to study the liquid crystal with the large birefringence and the low absorption loss in the terahertz band.In order to enhance the birefringence of the liquid crystal and reduce its absorption loss,this article provides a solution.In this paper,the molecular formula of the target compound was designed by increasing the degree of conjugation of liquid crystal molecules.The target compounds with trixylenyl skeleton as the core and different polar groups as the substituent were selected.Sonogashira reaction with halogen-containing benzene ring gave the desired product.The physical properties such as TG,DSC,POM rotational viscosity,dielectric anisotropy and elastic constants were characterized and compared.The relationship between the molecular structure and the properties of the compounds was preliminary analyzed.The metamaterial absorber can test the absorption peak of the liquid crystal in the low-frequency terahertz area.The commercial electromagnetic analysis software(CST)can accurately predict the S-parameter of the MM absorber and calculate the birefringence and the tangent loss of the liquid crystal.Terahertz time-domain spectroscopy can test the birefringence and absorption coefficients of liquid crystals in higher frequency bands.Among them,the highest birefringence index was 0.581 for the compound 1-propylphenyl-3-fluoro-4-isothiocyanatophenyl-pnynylbenzene(D-3-NCS);The least absorption is the compound 1,4-dipropylphenyl-alkynylbenzene(A-3),below 12 cm-1.Monomer liquid crystal because of its high melting point can not be directly applied,so the use of different methods of blending liquid crystal mixed crystal to enhance the performance of liquid crystal.The mixed crystal S200-2 has higher birefringence and lower absorption loss in the terahertz band,and the 360GHz reflective phase shifter made by it reaches a phase shift of 363.4° at 355GHz.The simulation software and the measurement result are good agreement.