Application Of Phase-retrieval Algorithms Research Articles

A minimalist approach to 3D photoemission orbital tomography: algorithms and data requirements

Photoemission orbital tomography provides direct access from laboratory measurements to the real-space molecular orbitals of well-ordered organic semiconductor layers. Specifically, the application of phase retrieval algorithms to photon-energy- and angle-resolved photoemission data enables the direct reconstruction of full 3D molecular orbitals without the need for simulations using density functional theory or the like. However, until now this procedure has remained challenging due to the need for densely-sampled, well-calibrated 3D photoemission patterns. Here, we present an iterative projection algorithm that completely eliminates this challenge: for the benchmark case of the pentacene frontier orbitals, we demonstrate the reconstruction of the full orbital based on a dataset containing only four simulated photoemission momentum measurements. We discuss the algorithm performance, sampling requirements with respect to the photon energy, optimal measurement strategies, and the accuracy of orbital images that can be achieved.

Effective Pattern Intensity Artifacts Treatment for Electron Diffractive Imaging

We present a method to treat spurious intensities in electron diffraction experiments. Coherent electron diffraction imaging requires proper data reduction before the application of phase retrieval algorithms. The presence of spurious intensities in the electron diffraction patterns makes the data reduction complicated and time consuming and jeopardizes the application of mathematical constraints to maximize the information that can be extracted from the experimental data. Here we show how the experimental diffraction patterns can be treated to remove the unwanted artifacts without corrupting the genuine intensities scattered by the specimen. The resulting diffraction patterns are suitable for the application of further processes and constraints aimed at deriving fundamental structural information by applying phase retrieval algorithms or other approaches capable of deriving quantitative atomic resolution information about the specimen structure.

Noise propagation in x-ray phase-contrast imaging and computed tomography

Three phase-retrieval algorithms, based on the transport-of-intensity equation and on the contrast transfer function for propagation-based imaging, and on the linearized geometrical optics approximation for analyser-based imaging, are investigated. The algorithms are compared in terms of their effect on propagation of noise from projection images to the corresponding phase-retrieved images and further to the computed tomography (CT) images/slices of a monomorphous object reconstructed using filtered backprojection algorithm. The comparison is carried out in terms of an integral noise characteristic, the variance, as well as in terms of a simple figure-of-merit, i.e. signal-to-noise ratio per unit dose. A gain factor is introduced that quantitatively characterizes the effect of phase retrieval on the variance of noise in the reconstructed projection images and in the axial slices of the object. Simple analytical expressions are derived for the gain factor and the signal-to-noise ratio, which indicate that the application of phase-retrieval algorithms can increase these parameters by up to two orders of magnitude compared to raw projection images and conventional CT, thus allowing significant improvement in the image quality and/or reduction of the x-ray dose delivered to the patient.

Longitudinal electron bunch profile reconstruction by performing phase retrieval on coherent transition radiation spectra

The application of phase-retrieval algorithms to the reconstruction of the longitudinal bunch profile of an electron bunch from the spectrum of the coherent transition radiation (CTR) it produces is considered. The development of a new algorithm for this application, the Bubblewrap algorithm, is described. Tests with synthetic data show successful reconstruction of the longitudinal profile of single and double electron bunches, provided that the CTR spectrum is known over a sufficiently wide range. The application of the Bubblewrap algorithm to the reconstruction of laser-accelerated electron bunch profiles from experimental data is demonstrated. The results are shown to be consistent with estimates of the bunch length obtained by other methods.

相位恢复算法在反射层析激光雷达成像中的应用

相位恢复算法在反射层析激光雷达成像中的应用

Application of phase retrieval algorithm in reflective tomography laser radar imaging

Application of phase retrieval algorithm in reflective tomography laser radar imaging

相位恢复算法在反射层析激光雷达成像中的应用

We apply phase retrieval method to align projection data for tomographic reconstruction in reflective tomography laser radar imaging. In our experiment, the target is placed on a spin table with an unknown, but fixed, axis. The oscillatory motion of the target in the incident direction of the laser pulse is added at each view to simulate the real satellites random motion. The experimental simulation results demonstrate the effectiveness of this method to improve image reconstruction quality. Future research also includes the development of projection registration based on phase retrieval for targets with more complicated structure.

Phase retrieval from the power spectrum of a periodic object

The phase-retrieval problem in crystallography is analyzed for the purpose of studying the application of phase-retrieval algorithms to crystallographic data of power spectra to obtain directly the electronic density distribution in the crystal. The theoretical importance of considering that the periodic object is formed by a finite or by an infinite number of unit cells is shown. A phase-retrieval method that is frequently used in optics, the iterative Fourier transform (IFT) algorithm, has been adapted for this application. To test this method I have performed computer simulations of power spectra that correspond to infinite periodic objects and have obtained satisfactory reconstructions in some interesting cases. From these results I have established some approximate conclusions about the probability of having a unique solution or multiple solutions. The importance of the existence of noncrystallographic symmetry in the unit cell for the resolution of this phase-retrieval problem has been proved, as has the ability of the IFT to incorporate and use this additional information successfully.