Probe position correction based on overlapped object wavefront cross-correlation for continuous-wave terahertz ptychography

Rotational position error correction in ptychography

Accurate determination of scan positions is essential for achieving high-quality reconstructions in ptychographic imaging. This study presents and demonstrates a method for determining the rotation angle of the scan pattern relative to the detector pixel array using diffraction data. The method is based on the Fourier-Mellin transform and cross-correlation calculation. It can correct rotation errors up to 60 deg. High-quality reconstructions were obtained for visible light and electron microscopy datasets, and intricate structures of samples can be revealed. We believe that this refinement method for rotary position errors can be valuable for improving the performance of ptychographic four-dimensional scanning transmission electron microscopy.

Resolution enhancement with highly curved illumination in ptychography

By deducing a formula to compute a sample from recorded diffraction intensity directly and analytically, the relationship among the highest reachable resolution of the ptychographic iterative engine (PIE), its illumination angle, and its collection angle was discussed analytically. Curved illumination was then proposed to realize the resolution enhancement for PIE, and a corresponding computing algorithm was proposed to avoid an undersampling effect without increasing the size of the computing matrix, thus realizing speedy high-resolution PIE imaging with a simple experimental setup. While theoretical analysis was carried out, the feasibility of this proposed method was verified both numerically and experimentally.

Full-field high-resolution terahertz imaging based on a high-resistance silicon solid immersion lens

The spatial resolution of the direct imaging system depends on the wavelength and the numerical aperture. In the terahertz (THz) waveband, the wavelength is relatively large, and the higher numerical aperture of the imaging system usually promises the possibility of achieving higher spatial resolution. Solid immersion technique is an effective method to expand the numerical aperture. We design and fabricate a hemisphere lens with high-resistance silicon to achieve the effect of solid immersion, and obtain full-field, high-resolution focal-plane imaging. The characteristics of the direct refraction imaging and the secondary reflection imaging are analyzed by ray-tracing calculations. And the field curvature of the equivalent object plane and the spot diagram on the vertical image plane of the lens are quantifiably evaluated. It is shown that the secondary reflection imaging can effectively reduce the geometric distortion and achieve more ideal imaging quality. The method of blocking different regions before and after the solid immersion lens is proposed to obtain a clear magnified image of a two-dimensional grating with the period of 300 µm. This method provides a powerful tool for THz full-field microscopic imaging.

Ptychography-based high-throughput lensless on-chip microscopy via incremental proximal algorithms

Ptychography-based lensless on-chip microscopy enables high-throughput imaging by retrieving the missing phase information from intensity measurements. Numerous reconstruction algorithms for ptychography have been proposed, yet only a few incremental algorithms can be extended to lensless on-chip microscopy because of large-scale datasets but limited computational efficiency. In this paper, we propose the use of accelerated proximal gradient methods for blind ptychographic phase retrieval in lensless on-chip microscopy. Incremental gradient approaches are adopted in the reconstruction routine. Our algorithms divide the phase retrieval problem into sub-problems involving the evaluation of proximal operator, stochastic gradient descent, and Wirtinger derivatives. We benchmark the performances of accelerated proximal gradient, extended ptychographic iterative engine, and alternating direction method of multipliers, and discuss their convergence and accuracy in both noisy and noiseless cases. We also validate our algorithms using experimental datasets, where full field of view measurements are captured to recover the high-resolution complex samples. Among these algorithms, accelerated proximal gradient presents the overall best performance regarding accuracy and convergence rate. The proposed methods may find applications in ptychographic reconstruction, especially for cases where a wide field of view and high resolution are desired at the same time.

Continuous-Wave THz Imaging for Biomedical Samples

In the past few decades, the applications of terahertz (THz) spectroscopy and imaging technology have seen significant developments in the fields of biology, medical diagnosis, food safety, and nondestructive testing. Label-free diagnosis of malignant tumours has been obtained and also achieved significant development in THz biomedical imaging. This review mainly presents the research status and prospects of several common continuous-wave (CW) THz medical imaging systems and applications of THz medical imaging in biological tissues. Here, we first introduce the properties of THz waves and how these properties play a role in biomedical imaging. Then, we analyse both the advantages and disadvantages of the CW THz imaging methods and the progress of these methods in THz biomedical imaging in recent ten years. Finally, we summarise the obstacles in the way of the application of THz bio-imaging application technology in clinical detection, which need to be investigated and overcome in the future.

Reconstruction method of a ptychographic dataset with unknown positions

Wavefield drift or wobbling occurs quite often in coherent scanning systems such as satellite laser communication, laser pointing of high-power lasers, or microscopy. The uncertainty of wavefront positions might result in blurred images or large measurement errors. Here we propose an iterative approach that can retrieve both the drift positions and complex-valued distribution of the wavefield from a ptychographic diffraction intensity dataset. We demonstrate the feasibility and effectiveness of the method in numerical simulation and an optical experiment. The method requires little a priori knowledge and thus would open up new opportunities in many fields.

Continuous-wave terahertz reflective ptychography by oblique illumination

Massive usage scenarios prompt the prosperity of terahertz (THz) reflective imaging methods. In this Letter, we apply ptychography to continuous-wave THz reflective imaging. Our scheme has a compact lensless layout and uses a full-field oblique-illumination recording mode. Diffraction patterns are corrected through tilted plane correction. This method can be used to retrieve the complex-valued object function and to suppress the negative effect of non-uniform illumination. The feasibility is investigated using two metal samples.

Ptychography imaging by 1-D scanning with a diffuser

It is beneficial to improve the resolution by a diffuser in imaging systems, because higher frequency information could be involved into the captured patterns via scattering effect. In this paper, a lensless imaging method is designed by 1-D scanning. A diffuser is placed upstream of the object, which is translated in a one-dimensional path and corresponding positions are corrected by cross-correlation. Our method requires a diffraction pattern of the object without a diffuser to speed up convergence and improve resolution. In field reconstruction, the amplitude constraint is added into the iterative phase retrieval algorithm. The high-quality complex-valued images can be obtained with ∼15 patterns. As a ptychography, the proposed method only needs a 1-D device, which could simplify the experimental equipment for reducing costs and measurement time.

Multi-layered full-field phase imaging using continuous-wave terahertz ptychography

Due to the unique properties of terahertz (THz) waves, THz phase imaging has been widely investigated to retrieve the absorption and phase modulation of dielectric two-dimensional thin samples, as well as multiple stacked samples. In this Letter, we apply the three-dimensional ptychographic iterative engine algorithm for continuous-wave THz full-field multi-layered phase imaging. The complex-valued transmission function of two-layered polypropylene thin plates and the corresponding probe function are reconstructed, respectively, which are immune to crosstalk of different layers. The phenomenon of the field-of-view enlargement at the second object layer is observed. This lensless compact imaging method can be potentially used for THz three-dimensional imaging.

Continuous-wave terahertz self-referencing digital holography based on Fresnel's mirrors

Continuous-wave terahertz digital holography (TDH) is a booming full-field phase-contrast imaging method validated in both in-line and Mach-Zehnder off-axis geometries. In this Letter, a self-referencing TDH approach is proposed based on the Fresnel's mirrors, by which the object wavefront is partitioned and reflected. Two beams interfere with each other to form an off-axis hologram. The proposed recording configuration is immune from a superposed twin image and has higher temporal stability than Mach-Zehnder interferometers. To evaluate the phase-contrast imaging performance, different types of samples are measured.

General method for complex-wave fields registration with high fidelity

In the field of optical imaging, the image registration method could be applied to realize a large field of view along with high resolution. The traditional image registration methods are mostly conceived for intensity images and might fail for complex-valued images. Especially, those methods do not account for the random phase offset associated with phase. In this paper, we proposed a general method for complex-wave field registration. A similar procedure has been proposed for the reconstruction of the ptychographic dataset, but here is modified for the registration of general wave fields. The method can efficiently separate the illumination and object function, refine the positions of each wavefront, and thus provide a stitched wide-field object wave with high fidelity. Simulation and experimental results applied to register the wave fields obtained from digital holographic microscopy are given to verify the feasibility of the method. This method would have potential applications in large-field high-resolution microscopy, adaptive imaging, remote sensing and the measurement of structured optical fields.

Phase retrieval using axial diffraction patterns and a ptychographic iterative engine

We propose a phase retrieval method using axial diffraction patterns under planar and spherical wave illuminations. The proposed method uses a ptychographic iterative engine (PIE) for the phase retrieval algorithm. The proposed approach uses multiple diffraction patterns. Thus, adjusting the alignment of each diffraction pattern is mandatory, and we propose a method to adjust the alignment. In addition, a random selection of the measured diffraction patterns is used to further accelerate the convergence of the PIE-based optimization. To confirm the effectiveness of the proposed method, we compare the conventional and proposed methods using a simulation and optical experiments.

THz coherent lensless imaging

Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.