Ultra-thin high-efficiency mid-infrared transmissive Huygens meta-optics The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Zhang, Li, et al. “Ultra-Thin High-Efficiency Mid-Infrared Transmissive Huygens Meta-Optics.” Nature Communications, vol. 9, no. 1, Dec. 2018. © 2018 The Authors As Published http://dx.doi.org/10.1038/s41467-018-03831-7 Publisher Nature Publishing Group Version Final published version Citable link http://hdl.handle.net/1721.1/118858 Terms of Use Creative Commons Attribution 4.0 International License Detailed Terms http://creativecommons.org/licenses/by/4.0/ ARTICLE Corrected: Author correction DOI: 10.1038/s41467-018-03831-7 OPEN Ultra-thin high-efficiency mid-infrared transmissive Huygens meta-optics Li Zhang1,2, Jun Ding 3,4, Hanyu Zheng1,2, Sensong An4, Hongtao Lin 2, Bowen Zheng4, Qingyang Du2, Gufan Yin2, Jerome Michon2, Yifei Zhang2, Zhuoran Fang2, Mikhail Y. Shalaginov2, Longjiang Deng1, Tian Gu2, Hualiang Zhang4 & Juejun Hu2 The mid-infrared (mid-IR) is a strategically important band for numerous applications ranging 1234567890():,; from night vision to biochemical sensing. Here we theoretically analyzed and experimentally realized a Huygens metasurface platform capable of fulfilling a diverse cross-section of optical functions in the mid-IR. The meta-optical elements were constructed using high-index chalcogenide films deposited on fluoride substrates: the choices of wide-band transparent materials allow the design to be scaled across a broad infrared spectrum. Capitalizing on a two-component Huygens’ meta-atom design, the meta-optical devices feature an ultra-thin profile (λ0/8 in thickness) and measured optical efficiencies up to 75% in transmissive mode for linearly polarized light, representing major improvements over state-of-the-art. We have also demonstrated mid-IR transmissive meta-lenses with diffraction-limited focusing and imaging performance. The projected size, weight and power advantages, coupled with the manufacturing scalability leveraging standard microfabrication technologies, make the Huygens meta-optical devices promising for next-generation mid-IR system applications. 1 State Key Laboratory of Electronic Thin Films and Integrated Devices, National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China. 2 Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 3 School of Information and Science Technology, East China Normal University, Shanghai 200062, China. 4 Department of Electrical & Computer Engineering, University of Massachusetts Lowell, Lowell, MA 01854, USA. These authors contributed equally: Li Zhang, Jun Ding, Hanyu Zheng. Correspondence and requests for materials should be addressed to T.G. (email: [email protected]) or to H.Z. (email: [email protected]) or to J.H. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:1481 | DOI: 10.1038/s41467-018-03831-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03831-7 he mid-infrared (mid-IR) spectral region (spanning 2.5–10 the mid-IR band. The material choice further facilitates scalable μm in wavelength) contains the characteristic vibrational manufacturing of meta-optics: we have already validated large- T ” absorption bands of most molecules as well as two atmo- area (on full 6 wafers), high-throughput (growth rate ~100 nm/ spheric transmission windows, and is therefore of critical min) PbTe film deposition via simple single-source thermal importance to many biomedical, military, and industrial appli- evaporation and wafer-scale lithographic patterning of the fi 23–26 cations such as spectroscopic sensing, thermal imaging, free-space lm , and optical quality CaF2 substrates are now commer- communications, and infrared countermeasures. However, devi- cially available with diameters up to 4”. In addition to the ces operating in the mid-IR band often present a technical material innovation, our work also marks the first experimental challenge for optical engineers. Since most traditional optical demonstration of HMS in the mid-IR, and claims significant materials including silicate glasses and optical polymers become performance improvement over previously demonstrated HMS opaque at wavelength beyond 3 μm, mid-IR optical components devices at optical frequencies leveraging an advanced two- are either made of specialty materials such as chalcogenides or component meta-atom design. Unlike dielectric metasurfaces halides whose processing technologies are less mature, or require based on waveguiding effects which mandate high aspect ratio complicated fabrication methods such as diamond turning (e.g., nanostructures to cover full 2π phase27,28, the concept of Huygens in the cases of silicon or germanium optics). Consequently, unlike metasurfaces, originally derived from the field equivalence prin- visible or near-infrared optical parts which are commonplace and ciple and elaborated in Supplementary Note 1, enables exquisite economically available off the shelf, mid-IR optics are plagued by control of electromagnetic wave propagation in a low-profile much higher costs and often inferior performance compared to surface layer with deep sub-wavelength thickness29. A remarkable their visible or near-infrared counterparts. feature of HMS is that near-unity optical efficiency is possible in Optical metasurfaces, artificial materials with wavelength-scale such a metasurface comprising meta-atoms possessing both – – thicknesses and on-demand electromagnetic responses1 3, pro- electric dipole (ED) and magnetic dipole (MD) resonances30 32. vide a promising solution for cost-effective, high-performance Nevertheless, dielectric Huygens optical metasurfaces experi- infrared optics. These thin-sheet structures can be readily fabri- mentally demonstrated to date, which are made up of a single cated using standard microfabrication technologies, thereby type of circular or rectangular shaped meta-atoms with varying potentially enabling large-area, low-cost manufacturing. Their sizes, suffer from much lower efficiencies around or below – singular electromagnetic properties, custom-tailorable through 50%33 37. Here we show that such single-component meta-atom meta-atom engineering, allow optical designers to manipulate constructions incur an inherent trade-off between phase coverage unconventional optical behavior (e.g., abnormal refraction/ and transmission efficiency, and we overcome this limitation by reflection, dispersion compensation, etc.) inaccessible to natural employing a two-component meta-atom design consisting of materials at the macroscale. In addition, their planar nature is both rectangular and H-shaped meta-atom structures. This conducive to “flat” optics and systems with drastically reduced unique approach significantly boosts the optical transmittance of size, weight, and power (SWaP) relative to their traditional bulk HMS to above 80% with overall optical efficiencies up to 75%, fi λ equivalents. while maintaining a thin pro le with a thickness of 0/8. This So far, a number of metasurface-based functional optical ele- unique combination of judicious material choice and innovative ments have been realized in the mid-IR range, including lenses4, HMS design allows us to demonstrate high-performance trans- – perfect absorbers5 8, polarization controllers9,10, modulators11,12, missive meta-optics operating near the mid-IR wavelength of 5.2 vortex beam generators13, thermal emitters14, nonlinear con- μm. In the following, we first discuss the HMS design rationale verters15, and infrared sensors16. Unfortunately, the devices are followed by elaboration of the material characterization and based on metallic nanostructures whose large optical losses device fabrication protocols. Design and optical characterization restrict their operation to the reflective mode. The limitation is results of three meta-optical devices, a diffractive beam deflector, eliminated in metasurfaces built entirely out of dielectric mate- a cylindrical lens, and an aspheric lens are then presented. – rials17 19, and such transmissive dielectric meta-optics offer several well-established advantages in optical system design including increased alignment tolerance and simplified on-axis Results fi 20 con guration. Recently, chiral metasurfaces made of Si/SiO2 Huygens metasurface design. Figure 1a sketches the structure of and meta-lenses fabricated using Si-on-sapphire21 have been a rectangular meta-atom in the form of a PbTe block sitting on a P μ demonstrated. The former device attained a polarization con- CaF2 substrate. The size of a unit cell, , is 2.5 m along both fi λ version ef ciency up to 50%. The latter Si-on-sapphire meta-lens axes, less than 0/2 to eliminate undesired diffraction orders. exhibited transmission efficiency as high as 79%, although the Total thickness of the PbTe block is fixed at 650 nm or 1/8 of the fi λ = μ optical ef ciency was not reported and optical focusing was not free-space wavelength ( 0 5.2 m), the smallest among dielec- demonstrated either. Furthermore, operation bands of both tric metasurfaces reported to date. material systems are bound to be below 5 μm wavelength due to To ascertain that the rectangular meta-atom indeed supports onset of phonon absorption in SiO2 or sapphire. both electric and magnetic dipole resonances, we simulated the In this article,