Continuous-wave Terahertz Multi-plane Ptychography

Terahertz(THz)waves,located between the microwaves and infrared radiations in the electromagnetic spectrum,have unique penetration and non-ionizing interaction properties.Since the phase modulation of the optical field carries on more information about the sample,the THz phase image can supply higher imaging contrast for non-conducting quasi-transparent samples.In Chapter 1,the thesis makes a sort summary of the cutting-of-art of the terahertz imaging strategies based on terahertz time-domain spectroscopy or the continuous-wave terahertz system.With the development of powerful continuous-wave(CW)THz sources and array detectors,THz phase imaging in full-field recording mode becomes available for multi-layered dielectric samples.At the same time,a series of terahertz phase imaging techniques has been summarized in Chapter 1.For instance,CW THz in-line digital holography could reconstruct the complexed wave-front of two small and isolated objects were implemented without occlusion using autofocusing algorithm.Although a complementary phase retrieval technique for imaging behind a moving and scattering barrier broke the limitation of sample size,a numbers of diffraction patterns were collected along the optical axis with moving the detector.Ptychography is a type of coherent diffraction imaging,where a set of diffraction patterns are processed by iterative algorithms to reconstruct a sample's complex-valued transmission function and its corresponding illumination probe function.Ptychography has been achieved in the optical band,X-ray,ultra-violet,scanning electronic microscopy(SEM)and so on.The family of ptychographic iterative engine(PIE)algorithm are always used in the process of the reconstruction.In the Chapter 2,the thesis has introduced the principle of ptychography and detailed the algorithm flow.Extended PIE(ePIE)algorithm could retrieve the complex transmission function of the probe beam and the sample at the object plane with the advantages of using the computer memory efficiently and simple flow compared to other algorithms.However,for some special types of testing sample in THz band,the problem of algorithm convergence delay makes the reconstruction process unable to proceed quickly.In the chapter 3,the thesis applies the dual-plane ePIE(dp-ePIE)algorithm in the experiment of continuous-wave terahertz ptychography to speed up the convergence of the reconstruction process.In order to evaluate the dp-ePIE algorithm convergence rate and the reconstruction quality,the correlation coefficient(CC)index are used as metrics in the MATLAB simulation,while the no-reference structural similarity(NRSS)index are used in the experiment,both of which ranges from 0 to 1.In the family of thePIE algorithm,3PIE(three-dimensional PIE)algorithm is utilized to reconstruct the distribution of the multi-layered samples.Based on the forward propagation model,the amplitude and phase images of different layer could be retrieval clearly using this method.In Chapter 4,the 3D-PIE algorithm was proposed to achieve the THz multi-layered ptychographic phase imaging.The feasibility of the introduced method is experimentally verified using a 2.52 THz CW laser.The complexed-valued transmission functions of two-layer polypropylene samples and the corresponding probe function are reconstructed from a set of far-field diffraction patterns.The phenomenon of field-of-view enlargement at the second object layer is observed and analyzed.This method can be potentially used for THz three-dimensional imaging.Finally,the thesis points out the conclusion,and prospect the continuous-wave terahertz ptychography for further study.Future work could focus on enhancing the resolution and explore the influence of layer interval.When the interval distance of each layer could small enough and with enough layer,this method has the potential to achieve three-dimensional imaging in the terahertz imaging and non-destructive testing.