The Resolution Improvement Of Continuous-wave Terahertz Ptychography

Terahertz(THz)waves are located between the infrared radiations and microwaves on the electromagnetic spectrum,which have unique propagation and penetration properties for security inspection,nondestructive testing,and biomedical imaging,e.g.,penetrability for non-metallic and non-polar materials,safety for most organisms,and water sensitivity.THz phase imaging not only reflects the material absorption from the intensity images,e.g.,the change of water content,but also obtains the information of the thickness,refractive index,morphology and inner structure of the object by phase measurement.Therefore,THz phase imaging has become an important branch of THz imaging.ptychography is a lensless phase imaging approach,in which redundant diffraction patterns are recorded in the Fresnel or Fraunhofer region at a set of overlapping probe positions of the object.By using the extended ptychographical iterative engine(e PIE),both the complex-valued object transmittance function and the probe function are retrieved.Continuous-wave(CW)THz ptychography realizes large field of view imaging of hidden samples,and avoids the influence of illumination beam quality on imaging quality.The low imaging resolution is caused by the detectors which have characteristics of large pixel size,small number of pixels,low sensitivity.So the signal-to-noise ratio of the recorded diffraction patterns are poor,and the recorded diffraction information are limited by the size of the detector target.The research status of THz phase imaging,ptychography resolution of other bands and the methods for improving resolution are fully surveyed.The recording process and reconstruction algorithm of ptychography are described in detail,and the application process of the extrapolation algorithm in other lensless imaging methods is introduced in detail,too.Introducing the divergent spherical wave illumination into CW THz ptychography.By building the theoretical models,the difference and relationship between the planar wave probe and the divergent spherical wave probe are analyzed,and the ptychography reconstruction results is analyzed.The theoretical resolution of ptychography under the planar probe and the divergent probe are explained theoretically.Then,the influence of various parameters based on the divergent spherical wave probe on the ptychography reconstruction results are verified by simulation.Finally,two setups of CW THz ptychography based on the planar wave probe and the divergent spherical wave probe are built by the pumped far-infrared gas laser(2.52 THz)and the PY-IV pyroelectric array detector.Finally,The reconstruction results of the two systems are compared and analyzed.An extrapolation extended ptychographical iterative engine is proposed for THz ptychography.Firstly,the principle of this algorithm is introduced,and the effectiveness is verified through simulation.The simulation results show that the extrapolation region is related to the characteristics of the sample.The experimental reconstruction results of the proposed algorithm and e PIE based two systems mentioned are compared,and the feasibility of this algorithm is verified which improving the experimental resolution.Finally,the two systems and two algorithms are combined,respectively,and the experimental resolution in four cases is quantitatively analyzed,which verified the resolution improvement of ptychography based on the divergent spherical wave probe and extrapolation algorithm.