Cheng Liu

Ptychographic microscopy via wavelength scanning

A wavelength scanning Ptychographic Iterative Engine (ws-PIE) is proposed to reconstruct high-quality complex images of specimens. Compared with common ptychography, which required the user to transversely scan the sample during data acquisition, the ws-PIE fundamentally reduces the data acquisition time and can avoid the heavy dependence on the accuracy of the scanning mechanism. This method can be easily implemented in the field of material and biological science as the wavelength-swept laser source is currently commercially available. The feasibility of the ws-PIE is demonstrated numerically and experimentally.

Single-shot Fourier ptychography based on diffractive beam splitting

An optical setup and corresponding reconstruction algorithm are proposed to realize single-shot Fourier ptychography (FP). Multiple angle-varied object waves are generated by placing a Dammann grating at a certain distance behind the object, and the generated image array of low resolution corresponding to different diffraction orders formed on the detector plane is recorded simultaneously in a single exposure. The amplitude, as well as the phase information of the object, can be properly reconstructed with a common FP algorithm from the recorded image array. This method eliminates the requirement for the angular scanning of common FP, and the total acquisition time is dramatically reduced. The feasibility of this proposed method was demonstrated both numerically and experimentally. The proposed method has the advantages of fast data acquisition and corresponding high temporal resolution, making it very suitable for applications in which high imaging speed is required.

Ptycholographic iterative engine with self-positioned scanning illumination

A special optical alignment is adopted and corresponding reconstruction algorithm is developed to reduce the reconstruction error induced by the hysteresis or backlash error of the translation stage in Ptychographical Iterative Engine (PIE) imaging with weak scattering specimen. In this suggested method, the positions of the scanning probe are determined directly from the recorded diffraction patterns rather than from the readout of the stage meter. This method not only remarkably improves the reconstruction quality, but also completely lowers the dependency of PIE on the device accuracy and accordingly enhances its feasibility for many applications with weak scattering specimen.

Compensation for the setup instability in ptychographic imaging

The high-frequency vibration of the imaging system degrades the quality of the reconstruction of ptychography by acting as a low-pass filter on ideal diffraction patterns. In this study, we demonstrate that by subtracting the deliberately blurred diffraction patterns from the recorded patterns and adding the properly amplified subtraction to the original data, the high-frequency components lost by the vibration of the setup can be recovered, and thus the image quality can be distinctly improved. Because no prior knowledge regarding the vibrating properties of the imaging system is needed, the proposed method is general and simple and has applications in several research fields.

Ptychographic phase microscope based on high-speed modulation on the illumination beam

Abstract. A type of ptychography-based phase microscope was developed by integrating a spatial light modulator (SLM) into a commercial wide-field light microscope. By displaying a moving pattern on the SLM to change the sample illumination and record the diffraction intensities formed, both the modulus and phase of the transmission function of the sample could be accurately reconstructed with formulas similar to those of common ptychography. Compared with other kinds of phase microscopes, the developed microscope has several advantages, including its simple structure, high immunity to coherent noise, and low requirement for quality optics. In addition, defects in the illumination beam are also removed from the reconstructed image. Further, this microscope’s fast data acquisition ability makes it highly suitable for many applications where highly accurate quantitative phase imaging is important, such as in living cells or other fragile biological samples that cannot sustain continuous imaging over a long period of time.

Single-shot phase retrieval based on beam splitting

A kind of beam-splitting-based single-shot phase retrieval is proposed, where the transmitted field of a sample to be observed is diffracted into many replicas by a Dammann grating and then incident on a weakly scattering phase plate with a known structure, and the exiting beams propagate roughly along their original directions and form a diffraction pattern array on the detector. All sub-diffraction patterns isolated with each other were recorded with a single measurement, and the complex amplitude of the radiation incident on the grating and accordingly transmitted from the sample can be iteratively reconstructed from the recorded diffraction pattern array. Since the weakly scattering plate is accurately measured in advance, fast convergence and high accuracy can be achieved with a small number of sub-diffraction patterns. The feasibility of this proposed method is verified with experiments using visible light.

Measurement of large optical elements used for inertial confinement fusion with ptychography

Abstract High-power laser drivers are located in huge laser facilities built for inertial confinement fusion, and have achieved important progresses in the past decade; however, many unconventional optical elements implemented still cannot be accurately measured. To solve this problem, the ptychographic iterative engine (PIE), which is a recently developed technique that can detect both the phase and modulus of the light field simultaneously, is adopted to measure the transmission function of these optical elements and then to accurately characterize their key parameters. The distinctive advantage of PIE over other traditional metrology techniques in measuring large optical elements is demonstrated in this paper by detecting the focal length of a lens array and the surface profile of a continuous phase plate.