Broadband Achromatic Spatial Differential Imaging with Optical Spin-Orbit Coupling

Background Introduction

In image processing, traditional spatial differentiation is typically accomplished through digital electronic computation. However, many big data applications require real-time and high-throughput image processing, which poses a tremendous challenge for digital computation. Optical analog spatial differentiation holds the potential to overcome this challenge, as it can perform large-scale parallel processing of entire images with low energy consumption. Furthermore, optical spatial differentiation can image pure phase objects (such as transparent biological cells), which digital electronic computation cannot achieve. Therefore, optical differentiation has recently garnered widespread attention and has extensive applications in label-free cell imaging, image processing, and computer vision.

Paper Source

This paper, titled “spin–orbit optical broadband achromatic spatial differentiation imaging,” is authored by Hongwei Yang, Weichao Xie, Huifeng Chen, Mingyuan Xie, Jieyuan Tang, Huadan Zheng, Yongchun Zhong, Jianhui Yu, Zhe Chen, and Wenguo Zhu. The authors are primarily from the Institute of Optoelectronic Information and Technology at Jinan University in Guangzhou. The paper was published in the 11th volume, 7th issue of “Optica” in July 2024.

Research Process

Research Objective

This paper proposes a compact broadband achromatic optical spatial differential imaging method based on natural thin crystals with intrinsic spin-orbit coupling. By inserting a uniaxial crystal before a traditional microscope camera, optical vortices are embedded into the image field to perform second-order topological spatial differentiation of the field, capturing isotropic differential images.

Experimental Setup and Methods

1. Design and Use of Uniaxial Crystal: The optical axis of the crystal is perpendicular to the crystal interface, with dielectric constant ε=diag[n_o², n_o², n_e²], where n_o and n_e are the refractive indices of the o-wave and e-wave, respectively. By adjusting the incidence angle and polarizer of the uniaxial crystal, the geometric Berry phase and angular gradient of the crystal can be selected to achieve spatial differentiation.

2. Calculation of Optical Field Characteristics: The incident polarized light field, after passing through the uniaxial crystal, contains spin-unconverted and spin-converted components. The spin conversion process imposes amplitude and phase modulation on the angular spectrum of the incident light field. The birefringent differential imaging under spin-orbit coupling was specifically calculated.

3. Construction of Experimental Imaging System: The experimental system comprises a 4x plan achromatic objective lens and a tube lens with a focal length of 200 mm. To verify achromaticity, spatial differential imaging of the object edges was performed under different color light sources.

Experimental Results and Analysis

1. Measurement of Transfer Function: By measuring the spectrum under different color light sources, the isotropy of the transfer function and vortex phase characteristics were verified. The experimental results were highly consistent with theoretical predictions.

2. Verification of Imaging Quality: Under irradiation with red, green, blue, and white light, the imaging clarity and gradient at the object edges were significant. Even with minimal phase jumps (such as 0.08π), differential imaging could extract the edges of the objects.

3. Imaging of Biological Samples: A homemade phase contrast microscope was built to image biological samples such as onion epidermal cells and mouse skin melanoma cells. The results showed that under edge enhancement mode, the edges and internal details of the cells were highlighted, facilitating the tracking and observation of cell morphology.

Conclusion and Significance

The broadband achromatic differential imaging method proposed in this paper, based on spin-orbit coupling, features compactness, isotropy, and phase detection capabilities, achieving high-contrast full-color object differential imaging. Its main contributions include:

1. Scientific Value: The proposed method enables high-sensitivity detection of pure phase objects, overcoming the limitation of previous methods that could only detect large phase jumps, providing a new pathway for optical analog computation.

2. Application Value: This method can be widely applied in label-free biological imaging, real-time high-throughput image processing, and also in computer vision for high-contrast imaging of high-quality transparent biological samples.

Research Highlights

1. Innovative Imaging Method: The optical differential imaging method based on spin-orbit coupling utilizes uniaxial crystals and reflection multiplication technology to achieve second-order topological differentiation.

2. High-Sensitivity Phase Detection: Successfully achieved detection of small phase jumps in pure phase objects (as low as 0.08π), significantly enhancing detection sensitivity.

3. Comprehensive Biological Imaging Demonstration: In actual biological sample imaging, it demonstrated ultra-high edge detection performance and color aberration resistance, providing a new tool for biomedical research.

This paper presents a novel method for broadband achromatic spatial differential imaging based on optical spin-orbit coupling, showcasing its broad application prospects in biological imaging and providing critical technical support for phase contrast microscopy and optical analog computation.