Theoretical Study On Microwave Modulation Of Nematic Liquid Rystals

With the rapid development of communication,military,navigation,surveillance and other technical fields,microwave equipment has a growing importance in daily life due to its core part of these applications.It is well known that liquid crystals,especially nematic liquid crystals,are widely used in the fields of display,and electronically controlled modulation in the range of visible light and infrared.Fortunately,several studies have shown that nematic liquid crystals still maintain large dielectric anisotropy,high birefringence,low driving voltage and loss in the microwave band.Coupled with its small size,low cost and adjustable electronic control,it has great potential in the field of electronically controlled microwave modulation.The emergence of liquid crystal microwave modulation devices such as liquid crystal microwave phase shifters and filters has promoted the development of liquid crystal microwave modulation.However,these devices also have problems such as slow response time of devices due to the thickness of the liquid crystal layer.In this paper,the modulation of microwave phase shift by ordinary nematic liquid crystal cell is studied to improve the performance of liquid crystal microwave modulation device.The modulation of the microwave by the liquid crystal depends on the distribution of the liquid crystal director under the electric field.Therefore,regardless of the dielectric material characteristics or the device structure,as long as the director distribution can be changed,the modulation characteristics of the liquid crystal to the microwave are affected.Taking a twisted nematic liquid crystal cell as an example,the twist structure,substrate anchoring strength,pre-tilt angle,pre-twist angle,and chiral doping concentration of the filled liquid crystal all affect the distribution of the director under the electric field.In addition,the flexoelectric effect of the nematic liquid crystal or other liquid crystal cell structure changes the distribution of the director,thereby affecting the modulation performance of the nematic liquid crystal to the microwave.This article combines theory,experimentation and simulation.Firstly,the material parameters of liquid crystal E7 at low frequency are accurately measured by the capacitance method.Based on this,the material properties of the nematic liquid crystal and the structure of the liquid crystal cell are studied.The microwave phase shift per unit length of the nematic liquid crystal under the electric field is calculated.The phase shift is used to analyze the effect of the flexoelectric effect in parallel alignment nematic liquid crystal cell and the structure of in-plane switching liquid crystal cell on microwave modulation,and a novel microwave modulation device structure based on twisted nematic liquid crystal cell is proposed.The results show that the existence of flexoelectric effect has an 11% improvement on the phase shift of microwave.The change of azimuth angle in the in-plane switching liquid crystal cell can increase the phase shift of microwave by 28%.The structure of the novel liquid crystal microwave modulation device effectively reduces the working voltage,device weight and fabrication cost,and improves the microwave modulation performance.It is hoped that it can provide a reference for the design and application of future liquid crystal microwave modulation devices.