A new publication from Opto-Electronic Sciences; DOI 10.29026/oea.2024.240086 , discusses multiplexing infinite functionalities by single metasurface based on coherent wave interferences.

With the development of optical sciences and applications, there is an increasing demand on multifunctional optical devices capable of multiplexing as many as possible wave-control functionalities in one single ultra-compact system. Optical devices made by conventional dielectrics, however, reply on propagation phases of light, thereby inevitably suffer from bulky device sizes (compared to wavelengths) and/or low efficiencies (due to lack of magnetic responses). Moreover, without additional degrees of freedom to manipulate light, it is hard to employ conventional dielectrics to realize compact optical devices with multiple functionalities, being highly unfavourable for optical integration.

Metasurfaces, ultra-thin metamaterials composed of planar subwavelength microstructures, possessing tailored optic responses arranged in certain pre-designed sequences, exhibit extraordinary capabilities to control light waves and thus have attracted tremendous attention recently. Through designing both metaatoms and their arranging sequences, scientists have realized various metasurfaces that can control locally scattered waves in terms of both phase and polarization, thus forming tailored light beams in the far-field based on Huygens’ principle. Many fascinating wave-manipulation functionalities have been realized based on metasurfaces, even bi/multifunctional ones with one single meta-device. Regarding these bi/multi-functional meta-devices, scientists have combined multiple-mechanism generated interface phases to obtain dual/multi-functional light field control, for example, simultaneously utilizing polarization-dependent geometric phases induced by structural rotations and polarization-independent structural resonance phases, realizing different functionalities within one single subwavelength ultra-thin device, greatly advancing the development of integrated optics. However, existing multi-functional light field control devices mostly require simultaneous variation of multiple different characteristics of the incident light, while employing solely the variation of the incident light polarization can only exhibit no more than two distinct wave-control functionalities, dictated by the number of independent incident polarizations. To further increase the number of functionalities multiplexed by one single metasurface device, new design strategies need to be developed to overcome the limitation imposed by the number of independent polarization states on the number of independent functionalities.

The authors of this article propose an approach to design metadevices exhibiting (in principle) infinite number of wave-control functionalities based on coherent wave interferences tuned by continuously varying the polarization state of incident light, and experimentally verify the concept in the telecom wavelength regime (1550 nm). This work is published with the title “Functionality multiplexing in high-efficiency metasurfaces based on coherent wave interferences” on Opto-Electronic Advance.

It is proposed that the incident polarization can be projected to the bases of left circular polarization (LCP) and right circular polarization (RCP), i.e. σ0=A+++A-- , ± denoting LCP and RCP components respectively. The wave scattered by the metasurface can be denoted by the linear decomposition of the LCP and RCP wave-fronts: A+⋅F+(r)σ+(r)+A-⋅F-(r)σ-(r) , F±(r) denoting the LCP and RCP wave-fronts respectively. Independently design these two wave-fronts of components possessing opposite helicities, the continuous tune of incident polarization, i.e. the ratio of LCP and RCP components, can effectively modulate the wave-front and local polarization of the total field obtained through coherent interference denoted Ff(r)σf(r)=A+⋅F+(r)σ+(r)+A-⋅F-(r)σ-(r) , and thus multiplex (in principle) infinite number of wave-control functionalities.

After designing a series of metaatoms with tailored reflection phases and polarization-conversion capabilities, two functional metadevices were constructed and their wave-control functionalities were experimentally calculated under illuminations of light with polarization continuously tuned along a certain path on the Poincare’s sphere. The experiments show that the first device can generate two non-overlapping yet distinct vortex beams with continuously varying strengths, while the second one can generate a single vectorial vortex beam carrying orbital angular momentum (OAM) and/or local polarization distributions (LPDs) continuously tuned by varying the incident polarization. Experimental results are in excellent agreement with numerical simulations and theoretical expectations.

These findings can find numerous applications in practice and can stimulate many future studies. For example, extensions to near-field and far-field complexing and/or transmissive systems are interesting future projects, and using vectorial beams as the incident light can further enrich the wave-manipulation functionalities of the metadevices.

Keywords: metasurface / coherent wave interferences / vectorial vortex beam / functionality multiplexing / orbital angular momentum / local polarization distributions / polarization-dependent

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The co-first authors of this paper are Ms. Yuejiao Zhou and Dr. Tong Liu. Ms. Yuejiao Zhou is a PhD candidate in the Department of Physics, Fudan University, her research interest is controlling light fields with metasurfaces. Dr. Tong Liu is currently a postdoctoral fellow in the Department of Physics, Hong Kong University of Science and Technology. He obtained his Bachelor's and Ph.D. degrees from Fudan University in 2016 and 2022, respectively, and was awarded the Outstanding Graduate of Shanghai in 2022. His research interests are electromagnetic metamaterials, electromagnetic metasurfaces, and plasmonics.

The corresponding authors of this paper are Dr. Dongyi Wang and Professor Lei Zhou. Dr. Dongyi Wang is currently a research associate at the Hong Kong Baptist University. She received her Bachelor's and Ph.D. degrees from Fudan University in 2017 and 2022, respectively. Her research interests include electromagnetic metasurfaces and acoustic topolpogy. She has published multiple papers as a first / co-first / corresponding author in journals including PRL, AOP, LSA, PhotoniX and LPR. She’s also currently working as the editor of Nanophotonics. Professor Zhou Lei is the Vice President of Fudan University, the Director of the Shanghai Key Laboratory of Metasurface Light Field Control, and the Xie Xide chair professor in the Department of Physics, Fudan University. He won “the National Science Fund for Distinguished Young Scholars” in 2007, “Changjiang Scholar” (appointed by the Ministry of Education) in 2010, and special allowance from the State Council in 2011. He was selected as the “leading talent in the National High-level Talents Special Support Plan” in 2017, the Fellow of the American Optical Society in 2019, and the globally highly cited researcher by the Web of Science ESI list from 2019 to 2023. He currently serves as the co-founding-editor of the journal Photonics Insights and the executive editor of the journal Nanophotonics. He was awarded the first prize of natural science by the Shanghai Municipal Government in 2016 and the second prize of national natural science in 2019.
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Zhou YJ, Liu T, Dai CH et al. Functionality multiplexing in high-efficiency metasurfaces based on coherent wave interferences. Opto-Electron Adv 7, 240086 (2024). doi: 10.29026/oea.2024.240086