Far-field super-resolution scanning imaging based on fractal resonator

Gao Qiang , Li Xiao-Qiu , Zhou Zhi-Peng , Sun Lei 为突破传统衍射极限实现远场超分辨率扫描成像, 提出一种基于分形谐振器的微结构阵列用于目标远场超分辨率扫描成像. 该阵列基于局域模谐振原理, 将目标的超分辨率信息包含在频谱中传播到远场, 可在远场通过频谱信息判断目标所在位置, 从而不借助于格林函数实现超分辨率实时成像, 成像过程简单方便. 基于分形谐振器的设计使阵列具有多频点工作的特点, 最终实现了 λ /10的超分辨率成像. 结合分形谐振器一阶谐振和三阶谐振的特点, 提出一种可有效提升成像效率的新型的扫描成像方法. 远场 /  超分辨率扫描成像 /  分形谐振器 /  The resolution of traditional far-field imaging system is generally restricted by half of the wavelength of incident light due to the diffraction limit. The more specific reason is that evanescent waves carrying sub-wavelength information cannot propagate in the far field and make no contribution to the imaging. However, higher imaging resolution is required in practical applications. To realize the far-field super-resolution imaging, the imaging system should be able to collect both propagating waves and evanescent waves. Many designs have been proposed to solve this issue. In 2007, a far-field superlens was proposed by Liu et al. (Liu Z W, Durant S, Lee H, Pikus Y, Fang N, Xiong Y, Sun C, Zhang X 2007 Nano Lett . 7 403) to realize far-field super-resolution in optical range, which consisted of a silver film and a nanoscale grating coupler. The silver film was used to amplify the evanescent waves, which were then converted into propagating waves by the sub-wavelength gratings. However, the special material properties limit the freedom of design. In microwave band, the incident components can be converted into Bloch modes by the resonant metalens, which consists of subwavelength resonators, and then be radiated to far field. Nevertheless, Green function between antenna and target is necessary, which is difficult to obtain due to the complex and even time-dependent imaging environment in practical applications, especially for super-resolution imaging system. It has been demonstrated in recent research that frequency information can be associated with spatial information of imaging target by localization resonant modes. Therefore, super-resolution imaging can be realized based on frequency information, without using Green function. Thus, a novel microstructure array is proposed to realize the far-field super-resolution scanning imaging based on a fractal resonator. The fractal resonator can work at several frequencies because of the self-similarity, which provides higher selectivity according to practical conditions. Several working statuses can be obtained for the resonator by adding photoconductive semiconductor switches, which are controlled by laser. On account of localization mode resonance, the array can realize the conversion between evanescent waves and propagating waves. Then with the help of antennas in the far-field to receive the frequency information, the location of imaging source can be confirmed according to the spectrum. Then by using the magnitude of resonant peak, sub-wavelength image can be reconstructed without using Green function. To verify the super-resolution scanning imaging characteristics of the array, an imaging simulation of “laugh face”-shaped target is performed. The image is reconstructed very well and the resolution determined by the period of the array is 20 mm, corresponding to λ /10. In view of the particularity of proposed fractal resonator, a novel scanning method is proposed. By combining the first and the third resonance, the imaging efficiency can be well improved compared with by the traditional point-by-point scanning method. Keywords: far-field /  super-resolution scanning imaging /  fractal resonator /  imaging efficiency  隋怡晖, 郭星奕, 郁钧瑾, Alexander A. Solovev, 他得安, 许凯亮. 生成对抗网络加速超分辨率超声定位显微成像方法研究 . 物理学报, 2022, 71(22): 224301. doi: 10.7498/aps.71.20220954 张锐, 王建军, 粟敬钦, 刘兰琴, 丁磊, 唐军, 刘华, 景峰, 张小民. 基于波导相位调制器的光谱色散平滑技术实验研究 . 物理学报, 2010, 59(9): 6290-6298. doi: 10.7498/aps.59.6290 丁 莉, 刘代中, 高妍琦, 朱宝强, 朱 俭, 彭增云, 朱健强, 俞立钧. 高功率激光装置光束准直系统新型远场监测技术 . 物理学报, 2008, 57(9): 5713-5717. doi: 10.7498/aps.57.5713 左言磊, 魏晓峰, 朱启华, 刘红婕, 王 逍, 黄 征, 郭 仪, 应纯同. 基于配对误差补偿方法的拼接光栅压缩池理论研究 . 物理学报, 2007, 56(9): 5227-5232. doi: 10.7498/aps.56.5227 左言磊, 魏晓峰, 朱启华, 王 逍, 刘红婕, 黄 征, 郭 仪, 应纯同. 单程拼接光栅压缩池系统中光栅缝隙的衍射效应 . 物理学报, 2007, 56(10): 5784-5789. doi: 10.7498/aps.56.5784