Methodological Exploration Of High Time And Spatially Resolved Ptychography

As an advanced lens-free imaging technology,coherent diffractive imaging(CDI)has been widely used in the fields of biology and materials science,such as two-dimensional microstructure imaging,sample quantitative analysis,chemical element distribution imaging,nano-crystal strain field imaging and so on.Among the many coherent diffractive imaging methods,the scanning coherent diffractive imaging method(ptychography)has great expectation because of its advantages of accurate imaging,fast convergence speed and imaging of large samples.The scanning coherent diffractive imaging method has been fast developed in the past ten years,but neither the basic theory nor the experimental method can be called perfect.Although the method of scanning imaging makes it possible to image samples with large transverse size in the plane perpendicular to the optical axis,the scanning coherent diffractive imaging has no advantage over the traditional CDI while imaging thick samples due to the defocusing effect caused by the Ewald sphere effect,the researches on the theory and method of this field are still relatively lacking;For biological samples such as protein,the scattering ability is generally weak and the diffraction signal quality is poor.So,new methods are urgently required to reconstruct the diffraction data set of weak scatter samples;During the scanning process,if the consistency of probes at different neighborhood scanning positions is poor,the reconstruction quality will be seriously affected,and the relevant algorithm is needed to correct the inconsistency of the scanning probes.The high harmonic light source has great advantages in high time resolution imaging because of its ultra-short pulse length,but its low coherence is a major factor limiting its application in the field of real space imaging.The existing multi-mode ptychographic iterative engines(multi-mode PIE)can somehow solve the low coherence question,but it is difficult to achieve a balance between the fineness of wavelength sampling and crosstalk between neighboring modes.To address the above problems,this paper carries out methodological exploration of high time and spatially resolved ptychography,including:(1)Proposed a new kind of ptychography method,called cascade scanning coherent diffractive imaging method(or cascaded ptychography).Taking the dual cascaded ptychography as an example,the number of samples changed from one of the traditional ptychography to two,and the distance between the two samples is limited within the applicable range of projection approximation,and the two samples are scanned in a cascaded way.Diffraction patterns corresponding to different scanning positions are collected by detector.Comparing with traditional ptychography the redundancy of data set generated by cascaded scanning increased exponentially.According to the principle of multiplication approximation,the process of probe illuminating sample 1,followed by irradiation of Sample 2,can be expressed mathematically by continuous multiplication of real spatial information of probe and two samples.According to the convolution theorem,the point multiplication in real space is equivalent to convolution in Fourier space,and the diffraction patterns collected by CCD is the convolution result of the three frequency-domain information.In order to separate and reconstruct the probe and two samples simultaneously,phase recovery and frequency domain deconvolution(Wigner distribution deconvolution)are conducted at the same time.Based on this train of thoughts,a reconstruction algorithm for this kind of cascaded scanning data set is developed on the basis of ePIE.The new algorithm is also based on the idea of iteration,via iteratively conduct real space constrain and frequency domain constrain,the true phase of diffraction data set can be finally recovered,meanwhile,Wigner distribution deconvolution were used to achieve the simultaneous reconstruction of the probe and two samples.This method can be regarded as a three-dimensional scanning coherent diffractive imaging method without rotating samples,compared with the previous multilayer ptychography method called 3PIE algorithm,our method also takes advantage of the high data redundancy of ptychography,and developed this advantage to a much higher level,based on which,the interlayer space of the samples in this method can be infinitely small,and the interlayer crosstalk is completely eliminated.The front and rear two samples can be regarded as a single thick sample,which is of great significance for studying the scattering mechanism of thick samples and the characteristics of coherent diffractive imaging.In addition,when the front sample with high scattering characteristic,the method can play the effect of diffraction enhancement imaging of the posterior layer sample.The verification experiment using visible light laser as light source has been conducted,and obtained ideal experimental results.(2)Proposed a method that introduce the diffraction information of probe into the iterative process to enhance the phase retrieval capability in ptychography.Demonstrated the rationality of using the diffraction pattern of pin-hole or zone plate as the frequency information of probe in practical experiments.The enhancement effect on the imaging quality of the introduced frequency information of probe and the realization mechanism are also discussed.In order to better image the weak scattering samples such as biological samples,researchers proposed many methods to further improve the imaging ability of ptychography,among them,adding strong scattering reference is a feasible idea.In the most widely used ptychography devices,the scanning probe generating device such as pin-hole or zone plate itself has strong scattering characteristics.This means that in the diffraction pattern,signal from the probe generating device is the dominant part and with better quality.The scanning process is equivalent to using the sample diffraction signal to modulate the diffraction signal of the scanning probe in the form of convolution.Using visible laser and synchrotron radiation X-ray as light source,the feasibility of the new method is verified in two kinds of most common ptychography devices(using pin-hole or zone plate as the probe generating device)respectively,and good experimental results are obtained.Both the reconstruction quality and the reconstruction stability have significantly improved comparing with the common ePIE algorithm and the common multi-mode ePIE algorithm.(3)Proposed a new ptychography reconstruction algorithm that can correct the inconsistency of scanning probes at various scanning positions caused by the concave and convex of sample surface.The traditional ptychography reconstruction algorithms are all based a basic assumption that the probe morphology at each scanning local area is exactly the same.Ideally,the sample plane is an ideal two-dimensional plane perpendicular to the optical axis、and the reconstructed sample matrix correspond to the two-dimensional projection of the sample on that plane,at which point the probes at each scanning position is generated by the Fresnel transmission of the probe formed by the probe generating device,so,the morphology of probe at each local scanning area is consistent.However,the real sample in the actual ptychography experiment is three-dimensional,usually a thin layer with micron scale concave and convex on the surface,so the sample projection is not an ideal two-dimensional plane located in the sample plane,but an approximate plane with micron scale fluctuation.Because in ptychography a pin-hole or zone plate is generally used to generate the scanning probe,the morphology of probe is constantly changing in the transmission process along the optics axis,the axial drift of the sample projection plane caused by the concave and convex of the sample surface will inevitably make consistency of probe morphology destroyed.The ptychography device using synchrotron radiation X-ray as the light source and zone plate as the probe generating device is the most widely used,and the micron level fluctuation of the sample projection plane along the optics axis can cause the probe morphology changed dramatically.The consistency of probe morphology over all of the scanning positions is the prerequisite to ensure the imaging quality in ptychography,and the specificity of the probe morphology caused by the axial draft the sample projection plane well definitely affect the imaging quality.To address this problem,a new algorithm is developed to correct the inconsistency of scanning probe morphology caused by this problem.After each certain times of iterations,at each local scanning position,the algorithm search for the actual local sample projection plane along the optics axis based on the average sample plane position.With the concave and convex information of sample found out before,conduct compensatory transmission to the scanning probe at each local scanning position to overcome the imaging blur problem.Firstly,the simulation experiment of the method is carried out,and then the method is verified by the experimental data set with synchrotron radiation soft X-ray as the light source,and the imaging quality is obviously improved.(4)In order to realize the ultra-high time resolution scanning coherent diffractive imaging with an ultra-short pulse as the light source,a multi-mode reconstruction algorithm with cross-wavelength probe consistency limitation is proposed,which can effectively solve the low coherence problem in broad band ultra-short pulse.Combined with ghost mode method absorbing the photon energy not accurately corresponding to the sampled wavelength reconstruction modes,the new method can greatly improves the imaging quality,and accurately recover the spectrum information of the incident light.In order to solve the wavelength widening problem of incident light,the existing ptychography reconstruction methods needs to find a balance between the fineness of wavelength sampling and crosstalk between neighboring modes.If the sampling density is too low to fully approximate the continuous incident spectrum,and the non-sampled photon energy will exist in the form of noise to reduce the imaging quality.On the other hand,a too high fineness of wavelength sampling will make the reconstruction parameters of adjacent modes too similar,and the crosstalk between the adjacent modes will also seriously reduce the imaging quality.The cross-wavelength probe consistency limitation added in the new method can effectively anchor each wavelength mode accurately at the sampling wavelength value,as a result,can avoid the crosstalk between adjacent modes under the condition that the incident spectrum is fully sampled.Because the discrete sampled wavelengths can never perfectly approximate the continuous actual incident spectrum,some photon energy will certainly fall in the gap between the sampled wavelengths.If not treated,those photon energy will interfere the reconstruction process in the form of noise,so,ghost modes are added in the reconstruction algorithm to absorb this part of photon energy.Because all the existing ultra-short pulse light source such as HHG are not capable enough to be applied in the actual imaging experiment,on the basis of numerical simulation experiment,we verified the new algorithm using the white LED as the light source,and obtained good results.