image: Fig.1.Schematic diagram of optical machine structure and its point diffusion function and modulation transfer function.
Credit: Wang, J., Deng, Y., Wang, C. et al.
1. Introduction
Recently, the research team of Jianli Wang of Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, has made progress in the astronomical observation based on metalens. In this study, the initial optical mechanical structure and the modulation phase of the metalens are optimized on the basis of the image-side telecentric optical system. At the same time, the 2D concentric-ring structure is used as the basic configuration of the metalens, and the TE, TM mode compatible structural design is carried out for the 2D meta-atoms in its cross section. Compared with the conventional 3D meta-atoms array structure, this approach greatly reduces computational resources and time consumption. As a result, a combined telescopic system with metalens with a diameter of 5cm as the main lens is developed, achieving high-resolution detection within a 20° field of view, and is the largest full-field high-resolution imaging system with the reported metalens aperture. This achievement further advances the deep application of large-diameter metalens in the field of astronomical observation." Portable Astronomical Observation System Based on Large-Aperture Concentric-Ring Metalens" was published in the international top optical journal Light: Science & Applications. Jianli Wang and Yongding Deng are the first authors of the paper, and Prof. Jan G. Korvink and Yongbo Deng from Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT) are the corresponding authors.
2. Research background
Metalens modulate the light field with subwavelength structure, also known as meta-atoms, offering numerous advantages such as lightweight design, ease of integration, and high controllability. Currently, the applications of metalens are predominantly concentrated in miniature optical imaging systems, including micro-robots, endoscopes, and portable microscopes. However, their application in fields requiring large aperture sizes, on the order of centimeters or even decimeters, such as visible light telescopes, thermal infrared detection, and optical remote sensing, is still somewhat constrained by the associated design and manufacturing costs.
Owing to their outstanding multifunctional modulation capabilities, metalens based on nano-posts arrays play an irreplaceable role in applications such as ultra-high resolution microscopy, broadband achromatic imaging, holography and polarization multiplexing. However, with the increase of the diameter of the building-block-array metalens to the order of 104λ~105λ, the number of nano-posts contained in the metalens reaches 108 to 1011, and the fly in the ointment of long processing cycle and high processing cost caused by the vast amount of layout data gradually appeared. Correspondingly, concentric-ring metalens with fixed height inherit the advantages of building-block-array metalens in polarization-independent focusing scenarios. With significantly fewer nano-structures, the calculation and processing costs of the concentric-ring metalens are substantially reduced, while its mechanical stability is significantly improved .It is expected to become a new choice with more application value.
Based on the above problems, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences developed a portable telescopic system with a concentric-ring metalens with a diameter of 5cm as the main lens, achieving high-resolution detection within a 20° field of view (MTF>0.4@46lp/mrad).
Innovative research
Optical mechanical structure design
Common hyperboloid phase metalens can achieve diffraction-limited focusing of normally incident parallel light, but Such designs are highly sensitive to off-axis aberrations, making it difficult to achieve wide-field high-resolution imaging. Therefore, the image-side telecentric optical system is adopted as the design basis ,introducing a front aperture stop to limit the edge ray aberration, which is conducive to limiting the chief ray angle, thereby reducing the sensitivity of Point Spread Function (PSF). Building upon this, the target phase is expressed as an even-order polynomial, being used as joint optimization parameters together with the basic optical machine dimensions .Under the condition, the imaging-side resolution matches the cut-off frequency of the detector (MTF>0.4@46lp/mrad) across the entire FOV, maximizing the entrance pupil diameter.
Micro-nano structure design of metalens
The micro-nano structure design in this study focuses on the interior section of the concentric-ring metalens. The two dimensional meta-atoms with sub-wavelength width are divided according to different topological structure, and the complex transmittance of both polarizations of TM wave and TE wave were recorded respectively .However, according to the mathematical model of 2D truncated waveguide, it can be judged that the scattering effect of TE and TM modes is not consistent ,and how to achieve the optimal compatibility of dual modes is the key to structural design.To this end, For this purpose, the study employs the optimized modulation phase result obtained from optical machine design as the ideal value. Based on the principle of maximizing the complex transmittance projections for both polarization states, the overall cross-sectional structure of the assembled metalens is determined.The whole 3D structure of the concentric-ring metalens can be finally obtained by rotating the arranged nano-ring cross-section around the optical axis for one complete revolution.
Imaging effect
Finally, the meta-tube, a narrowband filter with a bandwidth of 5nm and the detector were reassembled into a meta-camera for the real scene imaging test. During this process, an image edge enhancement method was utilized to slightly increase the contrast of the image, making the surface details of the moon more abundant and apparent. Fig.3 shows an image result of a distant building target, which is about 300 meters away from the meta-camera. In the zoomed-in portion of the image, the meta-camera can effectively resolve the vertical window frame gap with a width of 5mm, and the corresponding resolution angle is approximately 13µrad, demonstrating the significant resolution capability for single-line targets .Fig.4 shows the dynamic imaging effect with an imaging distance of about 150 meters.
3. Summary and prospect
The research team developed a portable telescopic system that achieve high-resolution detection within a 20° field of view, with a diameter of 5 centimeters. In this work, the 2D concentric-ring configuration is easier to prepare large scale micro-nano structures, and the corresponding 2D meta-atoms assembly structure design method are proposed, and the optical mechanical structure design is realized by combining the the image-side telecentric optical system. Finally, the prototype of metalens imaging principle is developed and the real scene imaging test is carried out. The follow-up research of domain will continue to focus on the development of large-caliber metalens. Further optimize the large-format micro-nano structure preparation process, increase the effective aperture of the image system, and improve compactness of structure, high-resolution field of view, and broadband characteristic in order to be applied more widely in the field of astronomical observation.
Article Title
Portable astronomical observation system based on large-aperture concentric-ring metalens