The research team led by Professor Kun Huang and Professor Heping Zeng from the State Key Laboratory of Precision Spectroscopy at East China Normal University (ECNU) has made a significant breakthrough in high-resolution mid-infrared (mid-IR) imaging. They proposed a novel mid-infrared nonlinear Fourier ptychographic imaging method. By combining pump field spatial intensity modulation with a Fourier-domain aperture synthesis algorithm, they overcame the long-standing bottleneck in traditional upconversion imaging systems, where performance is limited by the cross-sectional size of the nonlinear medium. This breakthrough enables large-field-of-view, high-resolution, ultra-sensitive room-temperature mid-IR single-photon imaging, providing a powerful tool to meet the urgent need for high-throughput infrared detection in fields such as materials science, chemical analysis, and biomedicine. The related findings, titled Mid-infrared Fourier ptychographic upconversion imaging, were recently published online in the prestigious academic journal Optica (Fig. 1). ECNU is the first affiliation of the paper. Doctoral students Tingting Zheng and Zhuohang Wei are the co-first authors, and Researcher Kun Huang and Professor Heping Zeng are the co-corresponding authors.
Fig. 1 The latest research findings of the team published online in Optica.
Background and Challenges
Mid-IR imaging can acquire unique information about an object's radiation temperature and chemical composition, finding broad applications in biomedicine, materials science, and environmental monitoring. For a long time, developing mid-IR imaging technologies with large fields of view, high resolution, and high sensitivity has been a key pursuit in the field of infrared measurement and control. Such technologies are crucial for supporting applications in extreme scenarios demanding high throughput and low illumination, such as long-range infrared remote sensing, deep-penetration imaging, low-phototoxicity biological tissue observation, and non-destructive photosensitive material detection. Currently, mid-IR detection and imaging devices are constrained by the narrow-bandgap semiconductor materials they use, typically requiring cryogenic cooling to suppress significant dark current and background noise. Achieving highly sensitive mid-IR imaging at room temperature remains challenging. In this context, nonlinear upconversion detection technology emerged. It utilizes an optical parametric frequency conversion process to faithfully convert infrared signals to the visible or near-infrared (NIR) band. This leverages the mature and high-performance photon detection and optical field manipulation devices available in these bands, providing an effective means to achieve room-temperature mid-IR single-photon measurement and control.
To date, upconversion detection technology has been successfully applied in many high-performance IR imaging scenarios, demonstrating significant advantages in detection sensitivity and imaging frame rates. However, because upconversion imaging systems rely on nonlinear conversion, they inevitably suffer from spectral aperture limitations, significantly impacting imaging spatial resolution. Consequently, achieving imaging performance that combines both a large field of view and high resolution has been difficult for a long time. Notably, to enhance system conversion efficiency, the upconversion process often employs quasi-phase-matched optical parametric wavelength conversion. This avoids spatial walk-off between optical field wavevectors and allows for long interaction lengths. However, this technique requires specially designed nonlinear crystals, where a regular domain inversion is induced, typically by applying a strong electric field (tens of kV/mm) within the crystal, to introduce periodic jumps in the nonlinear susceptibility. Limited by current fabrication processes, the thickness of periodically poled crystals, while ensuring precision and consistency of the poling structure, is typically on the millimeter scale. The resulting crystal aperture severely limits the working bandwidth of the spatial spectrum in 4f imaging systems. Therefore, to achieve higher spatial resolution, there is an urgent need to develop novel nonlinear imaging architectures that can overcome the physical limitations imposed by the cross-sectional size of existing nonlinear media, laying the foundation for advancing the broader application of IR upconversion imaging technology.
Innovation and Results
To address this, the research team proposed a novel mid-infrared nonlinear Fourier ptychographic imaging method. This method utilizes optical field intensity modulation technology to elliptically shape and control the size of the pump beam, thereby fully utilizing the transverse dimensions of the nonlinear crystal. By rotating the elliptical aperture in the Fourier domain, spatial high-frequency components of the object in various directions are captured. Combined with the Fourier ptychographic algorithm for aperture synthesis, the range of spatial frequencies that the imaging system can convert is expanded. This ultimately achieves high-resolution mid-IR upconversion imaging, effectively breaking through the aperture limitation imposed by the nonlinear crystal in traditional schemes. Figure 2 illustrates the imaging performance under different pump conditions. Under Gaussian pumping, limited by the crystal thickness, the beam diameter is small, and the aperture effect is particularly significant, restricting the spatial resolution of the imaging system. In contrast, the width of the crystal cross-section is limited only by the wafer size, which can reach several centimeters, far exceeding the thickness. Therefore, elliptical pumping can fully utilize the crystal's width dimension, achieving a significant resolution improvement along the long axis direction. Similarly, rotating the elliptical aperture in the Fourier plane enables resolution improvement in specific directions. By fusing image information from two orthogonal directions, the captured spatial frequencies space can be effectively expanded, resulting in an overall improvement in resolution along both horizontal and vertical directions.
Fig. 2 Schematic of mid-infrared Fourier ptychographic upconversion imaging.
Figure 3 shows the schematic of the mid-infrared Fourier ptychographic upconversion imaging setup. To acquire spatial frequency information of the target object from different directions, researchers precisely rotated the sample using a rotation stage while capturing the corresponding upconverted image at each angle. This method is simpler and more convenient than rotating the crystal (i.e., the aperture stop) and enables automatic optical alignment and data acquisition using electronically controlled translation and rotation stages. Although the captured images contain only intensity information, the Fourier ptychographic algorithm can stably recover the spectral phase information within a finite number of iterations. This allows for the fusion of multiple sets of spectral information to reconstruct a high-definition image of the sample. It is worth mentioning that the experiment employed a chirped poling nonlinear crystal. This crystal has a linearly chirped poling period along the direction of light field propagation, forming a gradient reciprocal lattice vector to satisfy the phase matching for wavevectors from different incident directions. This enables large-field-of-view, high-resolution upconversion imaging.
Fig. 3 Setup for mid-infrared Fourier ptychographic upconversion imaging.
To better characterize the imaging performance of this technique, the research team used a star-shaped resolution target to study the optical resolution in different directions. As shown in Figure 4, researchers rotated the sample every 30 degrees, collecting upconverted images at 12 angles in total, equivalent to rotating the elliptical pump aperture by one full circle. The image captured at a single angle has higher resolution in a specific direction, corresponding to the orientation of the elliptical aperture's long axis. In the orthogonal direction, the image resolution is lower, corresponding to the short axis. At different rotation angles, the direction with high-resolution imaging capability changes linearly. Using the Fourier ptychography-based optical aperture synthesis algorithm, the researchers iteratively fused the multiple captured intensity images to recover a high-resolution image isotropic in all directions. Within an imaging field of view of 25 mm diameter, a spatial resolution of 39 micrometers was achieved, corresponding to a space-bandwidth product (number of resolvable pixels) as high as 3.2×105. This represents an improvement of at least one order of magnitude compared to previously reported records. Furthermore, benefiting from the low-noise conversion process and a highly sensitive silicon-based camera, the researchers also demonstrated ultra-sensitive imaging performance at the single-photon level, with incident infrared light intensity as low as 1 photon/pulse/pixel. This achieves mid-IR single-photon imaging performance combining a large field of view, high resolution, and ultra-high sensitivity.
Fig. 4 Experimental results of mid-infrared Fourier ptychographic upconversion imaging.
Summary and Outlook
The developed nonlinear Fourier ptychographic imaging technique achieves an effective aperture size difficult to attain with existing nonlinear media, overcoming the long-standing dilemma where mid-IR upconversion imaging systems struggled to simultaneously achieve a large field of view and high resolution. In the future, this technology can be extended to other wavelengths lacking efficient imaging methods. By selecting suitable nonlinear media, it holds promise for achieving highly sensitive detection and high-resolution imaging in the long-wave infrared or terahertz spectral bands, supporting non-invasive, non-destructive high-precision material analysis and characterization. Furthermore, by integrating advanced spectral imaging techniques and computational imaging algorithms, it is anticipated that multi-dimensional mid-IR imaging architectures with high spatial-spectral-temporal resolution can be developed, providing new means to acquire high-throughput infrared information for biology, materials, chemistry, and other fields.
Recently, the research group has achieved a series of breakthroughs in mid-IR nonlinear measurement and control: constructing a low-threshold mid-IR fiber optical parametric oscillator [Photon. Res. 12, 2123 (2024)], realizing ultra-sensitive mid-IR single-photon detection [Adv. Photon. Nexus 3, 046002 (2024), Photonics Res. 12, 1294 (2024)], developing broadband mid-IR single-pixel spectroscopy technology [Laser Photon. Rev. 18, 2301272 (2024), Laser Photon. Rev. 18, 2401099 (2024)], and demonstrating high-frame-rate mid-IR hyperspectral imaging [Nat. Comm. 15, 1811 (2024]. The related work received substantial support from the Ministry of Science and Technology, the National Natural Science Foundation of China (NSFC), Shanghai Municipality, Chongqing Municipality, and ECNU.
Paper Link：Mid-infrared Fourier ptychographic upconversion imaging
