Generation Of Large-scale Terahertz Flat-top Beam And Its Application In Computed Tomography

Terahertz(THz)wave is a kind of electromagnetic radiation between microwave and infrared band.As a strong polar liquid,water is very absorbing to terahertz waves,and metal has strong reflectivity to terahertz waves due to its high electrical conductivity,but terahertz wave can penetrate non-polar and non-metallic materials well.These characteristics make it widely used in the field of transmission imaging.Due to the unevenness of the Gaussian beam intensity,its application in illumination imaging for large-scale targets is limited.In view of the above problems,the flat-top beam is a potential solution.This paper proposed a general framework for designing large-scale terahertz flat-top beams based on the phase recovery algorithm.Using the scalar diffraction theory as the calculation theory and the phase recovery algorithm as the core design algorithm,the circular terahertz flat-top beam system was successfully designed,and the system was numerically simulated by the finite difference time domain algorithm to verify the correctness of the designed system.The mature 3D printing technology,with high production accuracy and fast forming speed,had also been used to produce the phase plate structural elements of the system for experiments.The experimental system built successfully produced a large circular flat-top beam in the order of hundreds of millimeters in the 0.1 THz frequency band.Facing a larger-size lighting scene,a circular flat-top beam generation system with larger devices is required,which has greater limitations in practical applications.Considering the application scenarios of large-scale illumination,this paper designed a square that can be spliced and has higher practicality.The terahertz flat-top beam generation system also produced square flat-top beams on the order of hundreds of millimeters in experiments.Used in computed tomography(CT)imaging,a 0.1 THz square flat-top beam CT imaging system was built.In the experiment,the tomograms of different samples were successfully obtained and three-dimensional reconstructions were carried out.For simple samples,the system was able to recover its complete structure information,while for complex samples,the smaller details could not be completely recovered due to imaging quality problems,and only overall structure information could be obtained.The lighting mode of the application experiment system built was all square terahertz flat-top beam homogenized illumination,which greatly improves the imaging efficiency of terahertz CT.This paper also analyzed the factors that affect the quality of the flat-top beam,compared the characteristic parameters in the theoretical,simulation and experimental results,and analyzed the quality of the CT imaging results.The results showed that the flat-top beam can be used for terahertz CT imaging applications.Subsequent research can also make improvements on this basis to further improve the performance of the design system.If a better quality terahertz flat-top beam can be produced,the application of terahertz waves in the imaging field will be broader.