In a new publication from Opto-Electronic Advances; DOI 10.29026/oea.2021.210013 , Yechuan Zhu, Xiaolin Chen, Weizheng Yuan, Zhiqin Chu, Kwok-yin Wong, Dangyuan Lei and Yiting Yu from Northwestern Polytechnical University, Xi’an, China, Xi’an Technological University, Xi’an, China, Hefei University of Technology, Hefei, China, The Hong Kong Polytechnic University, Hong Kong, China, The University of Hong Kong, Hong Kong, China and City University of Hong Kong, Hong Kong, China discuss waveguide metasurface based quasi-far-field transverse-electric superlenses.
As a key component for optical focusing and imaging, lenses have been widely used in many important technologies such as microscopic imaging and micro-nano manufacturing. In the imaging process with a traditional optical lens, only propagating waves are involved in the image formation, while the high-frequency evanescent waves carrying the fine information of an object are usually not evoked, which results in the imaging resolution being limited by the diffraction of light to about 0.61λ/NA. This resolution limit imposes a fundamental restriction on the physical scale of the corresponding focal spot and the ability of the lens to observe the fine details of an object, and also hinders the development of optical imaging, nanolithography and other related technologies. Therefore, it is of great significance to break the diffraction limit and achieve super-resolution focusing and imaging.
In the last two decades, many theoretical and experimental efforts have been devoted to overcoming the above-mentioned hurdle. In 2000, Prof. Sir John Pendry from Imperial College London conceptually proposed a perfect lens formed by a slab of a negative refractive index to focus all Fourier components stemming from strongly enhanced evanescent waves across the lens. Inspired by this theoretical proposal, Prof. Xiang Zhang from UC Berkeley developed an ultrathin silver slab-based superlens that showed an imaging resolution down to λ/6 through the excitation of surface plasmon polaritons (SPPs) to recover and enhance evanescent waves. Moreover, taking the advantage of surface plasmons, super-resolution focusing of light was also realized with various metallic nanostructures. However, the working distances of these devices are only tens of nanometers away from themselves, namely operating in the near-field regime, which have severely limited their prospect of practical applications. In recent years, many metasurface-based flat lenses, called metalenses, have been proposed and demonstrated with various metallic and dielectric nanostructures, which can achieve far-field focusing with many new functionalities. Nevertheless, none of these existing metalenses can realize quasi-far-field or far-field super-resolution focusing.
To simultaneously overcome the limitations of superlens and metalens, the research groups of Professor Yiting Yu from Northwestern Polytechnical University and Dr Dangyuan Lei from City University of Hong Kong report a waveguide metasurface based quasi-far-field transverse-electric (TE) superlens that can focus the visible to ultraviolet light into a focal spot as small as λ/4.13 (at the illumination wavelength 405 nm) at a focal distance of about 1.5 μm. Moreover, such super-resolution focusing capability can be further improved by operating the device in a high-index dielectric environment, leading to the truly deep-subwavelength focusing of light. The superlens is formed by an array of metal-dielectric-metal (MIM) waveguides under TE-polarized light illumination. In contrast to the previous transverse-magnetic (TM) waveguide modes, the phase delay of the TE modes increases with the MIM waveguide width, which allows a significant reduction in the aspect ratio of the MIM waveguide and thus benefits the device fabrication. We emphasize again that our waveguide metasurface-based TE superlens can realize super-resolution focusing of light in the quasi-far-field zone, which cannot otherwise be achieved with the previous TM superlens in which the high-spatial-frequency surface waves like SPPs are localized in the near-field zone and decay rapidly from the surface.
More importantly, the authors of this article present a clear theoretical understanding of the underlying physical mechanism and provide corresponding experimental verification of the metasurface superlens. They show that exciting the TE waveguide modes not only modulate their optical phase but also evoke evanescent waves. As a result, some high-spatial-frequency waves can contribute to the focusing of the device, leading to the quasi-far-field super-resolution focusing of light. In order to experimentally demonstrate the super-resolution focusing performance, a metasurface superlens immersed in cedar oil was fabricated using focused ion beam (FIB). Optical measurements showed that the fabricated device has a focus spot of 98 nm (i.e. λ/4.13) at a focal distance of 1.49 μm (i.e. 3.68λ) under the light illumination at 405 nm, substantially breaking the diffraction limit of λ/2.38 in the quasi-far field regime. The experimental measurements on the fabricated superlens showed an excellent agreement with both numerical simulations and theoretical predictions. The demonstrated metasurface-based superlens with such extraordinary super-resolution capabilities promises an exciting avenue for developing ultrahigh-resolution nanolithography and ultra-small optoelectronic devices.
Article reference: Zhu YC, Chen XL, Yuan WZ, Chu ZQ, Wong KY et al. A waveguide metasurface based quasi-far-field transverse-electric superlens. Opto-Electron Adv 4, 210013 (2021) . doi: 10.29026/oea.2021.210013
Keywords: superlens / metasurface / waveguide / quasi-far-field super-resolution focusing / breaking the diffraction limit
Yiting Yu is a Professor of the Key Laboratory of Aerospace Micro-nano System, Ministry of Education, Northwestern Polytechnical University, China. His research centers on micro/nano optical imaging and sensing and the development of high performance, low cost and ultra-compact micro/nano optoelectronic functional chips for the use in aerospace and related fields. He has published 89 research papers, including 44 SCI papers as the first author or corresponding author (including JMEMS, JMM, OE, OL, Nanoscale, AOM), and has been granted 17 Chinese invention patents. He has presided over more than 10 projects such as National Natural Science Foundation of China (Excellent Youth), and received several important awards and honors, including:
1. The Second Prize of National Technical Invention
2. National Hundred Excellent Doctoral Dissertation Award by the Ministry of Education.
Dangyuan Lei is an Associate Professor at the Department of Materials Science and Engineering in the City University of Hong Kong. His research interest centers on plasmonics and nanophotonics of low-dimensional materials and structures, with particular interest in plasmon-enhanced light-matter interaction at the nanoscale and its applications in efficient light harvesting, advanced biosensing and bioimaging. He has published more than 155 scientific papers in prestigious academic journals, including 1 Nature Photonics, 3 Nature Communications, 1 Science Advances, 3 Light: Science & Applications, 3 Physical Review Letters, 13 Advanced (Functional) Materials, 17 Nano Letters and ACS Nano, and 3 Angewandte Chemie, and received 6120 citations and an h-index of 46. He has been granted 1 US invention patent. He has also received several important awards and honors, including:
1. Nano Research Young Innovator Award of Springer Nature Nano Research (NR45), 2021
2. NSFC Excellent Young Scientists Fund (Hong Kong and Macau), 2020
3. Anne Thorne PhD Thesis Prize and Deputy Rector’s Award, Imperial College London, 2012 & 2018-2011
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