Synthetic Aperture Metalens
Fig. 1. Outdoor imaging experiment with the synthetic aperture metalens-integrated camera.
Metalens is an ultrathin optical element that can focus light using densely arranged subwavelength structures. Due to its extremely small form factor, it has been considered promising for imaging applications that desire an extreme system size, weight, and power, such as in consumer electronics and remote sensing. However, as a major impediment prohibiting the wide adoption of the metalens technology, the aperture size, and consequently the imaging resolution, of a metalens is often limited by lithography processes that are not scalable.
A synthetic aperture can mix signals from a collection of sub-apertures to render an image having a resolution comparable to an aperture the size of the circumcircle of all sub-apertures. It is a technology that has been widely used in the radiofrequency regime. As a landmark example, the Event Horizon Telescope project captured and retrieved the first image of the black hole M87 through aperture synthesis of a global network of radio telescopes. Over the last decades, the synthetic aperture approach has also been adopted in the optical domain for applications ranging from remote sensing to microscopy.
In this work, a team led by Prof. Yuanmu Yang at Tsinghua University and Prof. Xiangning Chen at Space Engineering University propose to adopt a synthetic aperture approach to alleviate the issue. They experimentally demonstrate that, assisted by computational reconstruction, a synthetic aperture metalens composed of multiple metalenses with relatively small aperture size can achieve an imaging resolution comparable to a conventional lens with an equivalent large aperture.
They validate the concept via an outdoor imaging experiment performed with a synthetic aperture metalens-integrated near-infrared camera using natural sunlight for target illumination. Related results were published in Photonics Research Vol. 9, Issue 12, 2021 (Feng Zhao, Zicheng Shen, Decheng Wang, Bijie Xu, Xiangning Chen, Yuanmu Yang. Synthetic aperture metalens[J]. Photonics Research, 2021, 9(12): 12002388).
In this work, the authors address the aperture size limitation of optical metalens by adopting the synthetic aperture approach. They show that, by combining multiple metalenses with relatively small aperture size, the maximum cut-off frequency (MCF) of the modulation transfer function (MTF) of the synthetic aperture metalens can be improved to a level that is comparable to a lens with an equivalent large aperture.
For a conventional single-aperture metalens, the image can be obtained through the convolution of the target object and the point spread function (PSF) of the metalens. On the other hand, if we consider a synthetic aperture metalens composed of two sub-apertures, the as-obtained image may be blurry due to the additional side lobes in the PSF as well as the reduced intermediate frequency response of the system's MTF.
However, the maximum cut-off frequency (MCF) of the synthetic aperture is improved to a level comparable to an aperture the size of the circumcircle of both sub-apertures, thus the fine feature of the target object is retained in the convoluted signal. Subsequently, a sharper image can be recovered from the as-obtained image using recovery algorithms such as the Wiener filtering and Richardson-Lucy deconvolution.
The authors explore multiple metalens sub-aperture arrangements and experimentally demonstrate that, assisted by computational reconstruction, the synthetic aperture metalens composed of 3 or 4 sub-apertures can achieve an imaging resolution comparable to a conventional lens with an aperture the size doubling the size of all sub-apertures combined. They further build a metalens-integrated near-infrared camera and demonstrate the enhancement of the imaging resolution using synthetic aperture metalens in an outdoor setting using natural sunlight for target illumination (Fig. 1).
Natural sunlight was used to illuminate the target (a Tsinghua University logo) placed about 7 meters away from the camera. To demonstrate resolution enhancement using the synthetic aperture metalens-integrated camera, two images were first taken with a conventional lens with an aperture radius corresponding to the sub-aperture and circumcircle aperture radius of the 3-sub-aperture metalens, respectively. In comparison, the recovered image from the 3-sub-aperture metalens can resolve the fine lines of the target, with a resolving power comparable to the conventional circumcircle-aperture lens, and greatly improved in comparison with the conventional sub-aperture lens.
Using flat optical components, a plethora of novel sub-aperture arrangements that are difficult to realize by bulk lenses due to manufacturing constraints can be envisaged. By combining even more metalenses with a millimeter or centimeter size, it may provide a pathway towards constructing synthetic aperture metalens with an equivalent aperture size on a much larger scale, thus opening up new avenues for numerous exciting applications ahead.
合成孔径超透镜
图1、 合成孔径超透镜相机户外成像实验。
超透镜是一种超薄光学元件，可以通过密集排列的亚波长结构调控光的相位、振幅和偏振等属性。由于其轻薄、多功能等特点，超透镜在消费电子和遥感等对光学系统尺寸、重量和功率有极高要求的应用领域具有很大的应用前景。然而超透镜元件受到现有器件加工工艺的限制，孔径难以做大。现有超透镜孔径最大只能做到厘米量级。这一问题当前是阻碍超透镜技术广泛应用的主要障碍之一。
合成孔径技术通过混合一组精心排列的子孔径镜头阵列所获取的信号，可以生成与更大孔径（相当于所有子孔径外接圆大小）镜头具有相似分辨率的图像。这是一种此前被广泛应用于射频成像领域的技术。
作为一个标志性的案例，2017年“事件视界望远镜”项目通过遍布全球的射电望远镜网络形成的合成孔径，捕获并生成了史上第一张黑洞图像。在过去的几十年里，合成孔径方法也被引入光学领域，在遥感、显微成像等领域得到了应用。
近日，清华大学杨原牧副教授课题组联合航天工程大学陈向宁教授课题组，探索采用合成孔径的方法来解决单一超透镜孔径较小的问题。该团队通过实验证明：在计算图像重建的辅助下，由多个相对较小孔径的超透镜组成的合成孔径超透镜，其成像分辨率可与具有同等大孔径的传统透镜相媲美。
该团队将合成孔径超透镜与近红外相机进行集成，利用自然阳光对目标进行照明，通过户外成像实验验证了这一概念的有效性。相关研究成果发表于Photonics Research 2021年第12期(Feng Zhao, Zicheng Shen, Decheng Wang, Bijie Xu, Xiangning Chen, Yuanmu Yang. Synthetic aperture metalens[J]. Photonics Research, 2021, 9(12): 12002388)。
在这项工作中，该团队采用合成孔径方法突破了单一超透镜的孔径尺寸限制问题。实验结果表明：通过将多个孔径较小的超透镜组合在一起，可以将合成孔径超透镜的调制传递函数（MTF）的最大截止频率提高到与等效大孔径透镜相当的水平。
对于常规的单孔径超透镜，可以通过目标物体与超透镜的点扩展函数（PSF）卷积得到图像。而如果考虑由两个子孔径组成的合成孔径超透镜，由于其PSF中附加的旁瓣以及系统MTF的中频响应的降低，会导致系统直接获得的图像变得模糊。
然而，由于合成孔径超透镜的MTF的最大截止频率被提高到了与子孔径的外接圆孔径相当的水平，进而在经过合成孔径超透镜成像系统卷积所得到的信号中，保留了目标物体的精细特征。因此，研究者可以在其后使用维纳滤波或理查德-露西反卷积等图像恢复算法，从系统直接获得的图像中恢复出更清晰的图像。
该团队探索了多种超透镜子孔径的排列方式，并通过实验证明：在计算重建的辅助下，由3个或4个子孔径组成的合成孔径超透镜，其成像分辨率可与孔径面积是其所有子孔径之和的两倍的传统透镜相比。他们进一步将超透镜镜头与近红外相机集成，演示了在户外使用自然阳光对目标进行照明的情况下，合成孔径方法对于超透镜成像效果的提升(图1)。
在户外成像实验中，研究团队利用自然阳光对约7m远处的清华大学校徽图案进行照明。为了证明合成孔径超透镜的分辨率提升效果，他们使用与三子孔径超透镜的子孔径和外接圆孔径尺寸相当的传统透镜分别拍摄了两幅图像作为对比，从三子孔径超透镜中恢复的图像可以分辨出目标中的细线，分辨率与传统的外接圆孔径透镜相当，相比传统的子孔径透镜有了很大的提高。
通过使用平面超透镜，可以在合成孔径系统中实现更加灵活的子孔径结构排布。由于制造方面的限制，这些结构排布很难由传统透镜实现。通过合成孔径的方法，将更多毫米或厘米尺寸的超透镜组合在一起，人们有望在未来构建更大（例如米量级）口径的平面超透镜，从而为基于超透镜技术的众多激动人心的应用开辟了新的可能性。