Metalenses: Advances and Applications

PROGRESS REPORT Metalenses www.advopticalmat.de Metalenses: Advances and Applications Ming Lun Tseng, Hui-Hsin Hsiao, Cheng Hung Chu, Mu Ku Chen, Greg Sun, Ai-Qun Liu, and Din Ping Tsai* and optoelectronics. Metasurfaces as 2D Metasurfaces, the 2D counterpart of artificial metamaterials, have attracted artificial photonic structures are com- much attention because of their exceptional ability to manipulate the electro- posed of subwavelength resonators (often [1–8] magnetic wave such as amplitude, phase, polarization, propagation direc- referred to as meta-atoms). The optical responses/functionalities of metasurfaces tion, and so on. Different from conventional lenses, metalenses based on the can be tailored by choosing the proper metasurface optics are truly flat and compact and exhibit superior perfor- geometrical parameters of these meta- mance. In this report, recent progress in the development of metalenses is atoms.[9–14] They have been applied to explored. First, the working principle and characteristics of metalenses are realize ultrathin invisible cloaking,[15] [16–20] discussed. Then, it is described how the dispersion aberration in metalenses metahologram, surface plasmon [21–23] can be eliminated to make them suitable for being employed in a range of launchers, compact nonlinear devices,[24–28] nanolasers,[29–34] and many applications that are difficult or impossible for traditional lenses. In addition, novel compact photonic devices.[11,35–57] various metalens-based applications are introduced, including imaging, high Among all the promising applications spectral resolution spectroscopy, and multiplex color routing. Furthermore, of metasurfaces, planar metalenses with a survey of reconfigurable and tunable metalenses is conducted. Finally, the superior performance and functionalities report concludes by addressing future prospects of metalenses. over their conventional counterparts are definitely one of the most exciting and important research areas. This is because metasurface lenses (metalenses) can not 1. Introduction only show better optical functionalities but also allow for much more compact device design than the conventional high-end In recent years, optical metasurfaces have attracted much objective lenses. In this article, we will review the progress in attention due to their great potential to advance photonics the development of metalenses. This article is organized into three major parts. First, the working mechanisms of chromatic Dr. M. L. Tseng, Dr. C. H. Chu, M. K. Chen, Prof. D. P. Tsai and achromatic metalenses will be introduced. The design Research Center for Applied Sciences principle of integrated-resonant units for correcting chromatic Academia Sinica aberration of a metalens will be discussed. Second, novel appli- Taipei 11529, Taiwan E-mail: [email protected], [email protected] cations of metalenses mentioned in the first part for full color Dr. M. L. Tseng, Dr. C. H. Chu, M. K. Chen, Prof. D. P. Tsai imaging, sensing, and spectroscopy will be reviewed. After the Department of Physics discussion of the passive metalenses, the emerging tunable National Taiwan University metalenses and their applications will be surveyed, followed by Taipei 10617, Taiwan concluding remarks on future prospects. Prof. H.-H. Hsiao Institute of Electro-Optical Science and Technology National Taiwan Normal University Taipei 11677, Taiwan 2. The Origin of Chromatic Aberration Prof. H.-H. Hsiao Graduate Institute of Biomedical Optomechatronics Conventional refractive lenses rely on gradual phase accumu- Taipei Medical University lation along the optical path traveled by a light beam. This is Taipei 11031, Taiwan accomplished either by controlling the surface topography or Prof. G. Sun by varying the spatial profile of the refractive index. The chro- Department of Engineering University of Massachusetts at Boston matic aberration in refractive optical components arises from Boston, MA 02125, USA material dispersion. In a material with normal dispersion, the Prof. A.-Q. Liu refractive index decreases with increasing wavelength so that School of Electrical and Electronic Engineering a refractive lens made of the same material has larger focal Nanyang Technological University length in red than in blue. Diffractive optical devices, on the Singapore 639798, Singapore other hand, operate as the interference of light through an The ORCID identification number(s) for the author(s) of this article amplitude or phase mask. The deflection angle of a diffrac- can be found under https://doi.org/10.1002/adom.201800554. tive grating varies with wavelength, where the rate of change DOI: 10.1002/adom.201800554 depends on the pitch of the grating. Such dispersion is opposite Adv. Optical Mater. 2018, 1800554 1800554 (1 of 16) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advopticalmat.de to the standard refractive optical elements (known as “nega- tive dispersion”),[58] where longer focal length corresponds to a Greg Sun received his Ph.D. shorter wavelength. In order to eliminate chromatic aberration, in Electrical Engineering from early effort has endeavored to achieve refractive or diffractive Johns Hopkins University, achromatic lenses by integrating or cascading them in the form Baltimore, USA in 1993. He of doublets and triplets to obtain the same focal length for mul- is now professor in engi- tiple wavelengths of interest. However, the composite configu- neering and physics at the ration makes the optical system bulky, complicated, and costly. University of Massachusetts Metasurfaces are thin optical elements comprised of spatially Boston where he also serves varying optical resonators with subwavelength separations that as the founding chair of the provide an entirely new mechanism for light control. In order Engineering Department. His to convert incident planar wavefront into a spherical one, one research interests are in sem- should impose the following phase profile[59] iconductor optoelectronics, silicon photonics, and nanophotonics. He is an associate 2π 22 2 editor for the Journal of Lightwave Technology. ϕλ(,xy,)−=ϕλ(0xy,0==,) −+xy+−ff (1) λ () Ai-Qun Liu received his where x and y are the spatial coordinates with respect to Ph.D. degree in Applied the center of lens (x = y = 0), and f is the focal length of the Mechanics from National designed lens. Since the required phase profile has a depend- University of Singapore (NUS) ence on wavelength, a metalens must exhibit independent in 1994. Currently, he is a pro- phase control at each desired wavelength to achieve achromatic fessor at School of Electrical focusing. However, if the desired phase coverage of 0−2π for & Electronic Engineering, metasurface-based devices can only be obtained by either var- Nanyang Technological ying the geometric dimensions of one set of meta-atoms or University (NTU). He serves by rotating their orientations, the outcome is that this one set as an editor and editorial of meta-atoms can only satisfy the required phase for a given board member of several jour- wavelength but not at other wavelengths which leads to chro- nals. Prof. Liu is interested in matic aberration. This causes the demonstrated devices in early the research fields of MEMS, nanophotonics, optofluidics, times suffer from strong chromatic aberration even though nano/microfluidics, and sensors for water and environ- metasurface-based optical elements can be designed to operate mental monitoring. He is the author of over 160 publica- over a broadband wavelength range. The dependence of the tions including peer-reviewed journal papers and two books. deflection angle or focal length on wavelength significantly Also, he is OSA Fellow, RCS Fellow, and SPIE Fellow. compromises their performance in full-color optical applications such as imaging and displaying. Din Ping Tsai received his Ph.D. degree in Physics from University of Cincinnati, USA 3. Metalens: Control of Chromatic Dispersion in 1990. He is the Director and Distinguished Research 3.1. Multiwavelength Achromatic Metalenses Fellow of Research Center for Applied Sciences, Academia Up to now much effort has focused on multiwavelength metade- Sinica since 2012. He is also vices aimed at improving the lens behavior at a discrete set of the Distinguished Professor wavelengths.[60–63] Cappasso and co-workers utilized an aperi- of Department of Physics at odic arrangement of coupled rectangular dielectric resonators National Taiwan University. He to provide the phase coverage at three wavelengths (Figure 1a). is a fellow of AAAS, APS, IEEE, This design allows the incident near-infrared light to have the OSA, SPIE, and EMA. His current research interests are same focal length at λ = 1300, 1550, and 1800 nm with meas- nanophotonics, near-field optics, plasmonics, metamaterials, ured efficiencies of 15%, 10%, and 21%, respectively.[64] green photonics, quantum photonics, and their applications. Composite metalenses in cascading or spatially multiplexing configurations have also been demonstrated, comprising of several sublenses with each one designed to fulfill the required frequency band.[65] Arbabi et al. utilized spatially multiplexing phase profile at a specific working wavelength. Avayu et al. high contrast dielectric metaslenses to achieve light focusing used a vertical stacking of three layers to demonstrate a triply at wavelengths of 915 and 1550 nm to the same focal plane red (650 nm), green (550 nm), and blue (450 nm) achromatic (Figure 1c). Two metalenses comprised of

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