Period measurement of a periodic structure by using a heterodyne grating interferometer.

This paper proposes an alternative method for grating period measurement based on heterodyne grating interferometry. The optical configurations for measuring the period of reflection/transmission gratings were demonstrated, and four commercially available gratings were used to evaluate the effectiveness of the proposed method. Based on the phase-lock technique, the grating period could be obtained immediately through the phase wrapped/unwrapped process. Under precise measurement conditions, the grating period measurement error of the proposed method was better than 1 nm, and the grating period difference between product specifications was less than 1%. In addition, the measurement results of the proposed method also exhibited high similarity with optical microscopy measurements.

Common-path digital holographic microscopy based on a volume holographic grating for quantitative phase imaging.

Digital holographic microscopy (DHM) is a powerful quantitative phase imaging (QPI) technique that is capable of recording sample's phase information to enhance image contrast. In off-axis DHM, high-quality QPI images can be generated within a single recorded hologram, and the system stability can be enhanced by common-path configuration. Diffraction gratings are widely used components in common-path DHM systems; however, the presence of multiple diffraction beams leads to system power loss. Here, we propose and demonstrate implementation of a volume holographic grating (VHG) in common-path DHM, which provides single diffraction order. VHG in common-path DHM (i.e., VHG-DHM) helps in improving signal-to-noise ratio as compared to the conventional DHM. In addition, VHG, with inherently high angular selectivity, reduces image noise caused by stray light. With a simple fabrication process, it is convenient to utilize VHG to control the beam separation angle of DHM. Further, by using Bragg-matched wavelength degeneracy to avoid potential cell damaging effect in blue light, the VHG is designed for recording at a maximum sensitive wavelength of ∼488 nm, while our VHG-DHM is operated at the longer wavelength of red 632.8 nm for cell observation. Experimental results, measured by the VHG-DHM, show the measurement of target thickness ranging from 100 nm to 350 nm. In addition, stability of the system is quantitatively measured. High-contrast QPI images of human lung cancer cells are demonstrated.

Intelligent Phase Contrast Meta-Microscope System.

Phase contrast imaging techniques enable the visualization of disparities in the refractive index among various materials. However, these techniques usually come with a cost: the need for bulky, inflexible, and complicated configurations. Here, we propose and experimentally demonstrate an ultracompact meta-microscope, a novel imaging platform designed to accomplish both optical and digital phase contrast imaging. The optical phase contrast imaging system is composed of a pair of metalenses and an intermediate spiral phase metasurface located at the Fourier plane. The performance of the system in generating edge-enhanced images is validated by imaging a variety of human cells, including lung cell lines BEAS-2B, CLY1, and H1299 and other types. Additionally, we integrate the ResNet deep learning model into the meta-microscope to transform bright-field images into edge-enhanced images with high contrast accuracy. This technology promises to aid in the development of innovative miniature optical systems for biomedical and clinical applications.

Optical scanning holography with a polarization directed flat lens.

Recently, an optical scanning holographic system with a polarization directed flat lens was proposed to realize coaxial scanning holography (CSH). The advantage of CSH is its small form factor and the stability. However, the diffraction efficiency of the polarization directed flat lens cannot be 100%, and thus there is always zeroth order light in the scanning beam. The imperfect diffraction property of the polarization directed flat lens results in an incomplete scanning Fresnel zone plate. Consequently, the reconstructed image is blurred and noisy. In this paper, we compared different methods, including the back propagation, the phase correlation, and inverse filtering, for the hologram reconstruction. It is demonstrated that inverse filtering is the only method that can retrieve the high-frequency component of the hologram. However, additional noise also arises with the use of inverse filtering. Therefore, the imaging performance of CSH by using a polarization directed flat lens is inherently worse than that of conventional OSH.