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Credit: by Meiyan Pan, Yifei Fu, Mengjie Zheng, Hao Chen, Yujia Zang, Huigao Duan, Qiang Li, Min Qiu, and Yueqiang Hu
Optical lenses have been playing a critical role in imaging systems for plentiful applications such as picture-capturing, sensing, and endoscopy. The developing wearable and portable devices require state-of-the-art imaging devices with higher integration, smaller size, and better performance. However, Conventional imaging relies on the bending of light at the lenses’ surface and the gradually accumulated phase change during the propagation. Sufficient thickness of the lenses is necessary so the size of conventional lenses is difficult to downscale. Furthermore, conventional imaging systems usually contain cascading lenses due to the limited performance of a single lens. Hence, miniaturized imaging systems are challenging for conventional optics.
As an alternative, the metasurface-based flat lens, so-called metalens, could meet the growing requirement of device miniaturization and system integration. Metalens focusing is achieved by abrupt phase change locally imparted by subwavelength structures, namely, meta-atoms. In particular, all-dielectric metalenses have attracted attention for their high efficiencies. Utilizing the planar structure, a large number of degrees of freedom is introduced to control the light wavefront. The ultra-lightweight and ultra-thin flat optics is then enabled.
To encourage the development of metalens-based compact devices, in a new review article published in Light: Science & Application, researchers from Jihua Laboratory and Hunan University in China, and co-workers have emphasized the progress of dielectric metalenses in compact imaging systems whilst highlighting the challenges obstructing future advancements.
Attributed to the customized engineering capability on the wavefront, dielectric metalenses possess aberration-correction and dispersion-engineering capabilities, so they offer a way to solve monochromatic and achromatic focusing problems via single-piece configurations. Furthermore, multiple functionalities can be implemented within the same shared aperture of a single metalens owing to the high flexibility in controlling light. The recent progress in versatile metalens-based applications such as three-dimensional imaging, real-time polarization imaging, and optical analog computing have shown various opportunities for single-piece metalenses to replace their complicated conventional counterparts.
Despite the tremendous progress achieved so far, it is pointed out that several challenges should be addressed for the practical development of metalens-based imaging systems.
“High focusing efficiency is challenging for high-NA metalenses due to the fundamental phase discretization and diffraction constraints. The breakthrough broadband achromatic feature of metalens suffers from some fundamental constraints, design limitations, and fabrication challenges. Moreover, conflicts in metalens parameters and aberration correction capabilities should be manipulated. Regarding multifunctional metalenses, the overall efficiency is limited by not only the efficient component of power but also the crosstalk among the sub-units. Balancing the performances among different focal spots is also a tough task. In addition, common challenges are hindering further development of dielectric metalens-integrated systems, including limitations of conventional design methods, the scaling up of dielectric metalenses, and the approaches for integrated devices.”
“In turn, these challenges provide some possible future development directions of metalenses: (i) New strategies to break the conflicts among metalens performances. (ii) Multifunctional and reconfigurable metalenses that could replace complicated configurations of conventional optics. (iii) High-efficiency design methods for high-performance metalenses. (iv) High-efficiency fabrication and mass manufacturing methodologies for large-area metalenses.” the researchers added.
“By addressing the present challenges of metalenses and combining the non-local optics, we envision that metalens-based imaging systems would be increasingly compact and widely employed in future applications, ranging from camera modules for consumer photography and autonomous vehicles to wearable displays for AR/VR/MR and machine vision, to bioimaging and endoscopy, to signal processing and optical computation.” the researchers forecast.
Journal
Light Science & Applications