image: Figure 1 | Design of on-chip metasurface. a, Principle of phase modulation of diatomic structure based on geometric phase and detour phase. Simulated phase shift map of b, left circularly polarized and c, right circularly polarized output channel as a function of rotation angle θ and displacement δx. d, Optical microscope image of the fabricated on-chip metasurface on top of LN waveguide crossing. e, Scanning electron microscope images of the on-chip metasurface. f, Simulated and measured four-channel holographic images under TE0 mode illuminations along x and y directions along with LCP and RCP analyzers. The double-headed arrows correspond to the polarization state of the input TE0 mode and the circular arrows represent the polarization state of the analyzer.
Credit: Zhizhang Wang et al.
Advancements in photonic integration demand high-speed, precise dynamic light field control and large-capacity information processing capability. As subwavelength artificial structures, metasurfaces can be integrated with optical waveguides to couple on-chip signals into free space with multi-dimensional manipulation capability. This offers a new paradigm for miniaturized and multi-functional photonic integrated devices.
Nevertheless, existing on-chip metasurfaces mainly face two key challenges: dynamic tunability and limited information capacity. The optical response of current devices is almost fixed after fabrication, hindering real-time adjustment. While some modulation methods have been explored (e.g., liquid crystals), their response speeds are relatively slow, and they typically offer only global control, lacking the ability to manipulate individual pixels independently. Furthermore, existing multiplexing techniques (e.g., joint phase modulation, spatial multiplexing) provide limited information capacity, falling short of multi-functional, high-throughput optical information processing needs. Achieving large-scale, independently addressable dynamic light field control on a tiny chip remains a significant technical hurdle.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Tao Li from National Laboratory of Solid State Microstructures, Key Laboratory of Intelligent Optical Sensing and Manipulations, Jiangsu Key Laboratory of Artificial Functional Materials, College of Engineering and Applied Sciences, Nanjing University, Nanjing, China, and co-workers have proposed a fast modulation strategy for metasurface with improved multiplexing capability on LNOI platform.
To overcome the limited information capacity of individual metasurface, the research team, building upon their prior work in multi-dimensional light field manipulation (Nat. Commun. 15, 8271 (2024), Nano Lett. 23, 2750 (2023)), developed a diatomic on-chip integrated metasurface as an addressing unit by combining geometric phase and detour phase (Fig. 1). This design enables four-channel multiplexing for both illumination direction and polarization control. Since on-chip metasurfaces are on this basis of a waveguide architecture, they can expand information capacity by constructing metasurface network for multi-channel multiplexing. Meanwhile, waveguide-integrated devices offer flexible on-chip optical signal processing capabilities, particularly with waveguide crossing arrays providing a reliable framework for addressable local light field manipulation. To this end, this team expanded a single metasurface into a 2×2 on-chip metasurface network on a waveguide crossing array (Fig. 2c). By adjusting the incident ports, specific metasurface units within the network can be selectively activated, thereby achieving various holographic display. Notably, this architecture is highly scalable, promising a viable solution for high-density, large-capacity optical information storage and encoding.
The key to solving the dynamic tunability problem lies in seeking for a suitable material platform. With the rapid rise of lithium niobate on insulator (LNOI) technology, thin-film lithium niobate has become a crucial platform for building next-generation photonic integrated chips. In particular, the development of high-performance thin-film lithium niobate electro-optic modulators provides a reliable solution for electro-optical reconfigurable light field manipulation. The research team fully leveraged the excellent electro-optical response of LNOI platform and integrated on-chip metasurface network with lithium niobate electro-optical switches (Figs. 2a-b) to achieve nanosecond-level, high-speed light field modulation. Within this setup, the electro-optical switch, composed of three lithium niobate modulators, acts as a high-speed optical router. It can precisely guide optical signals to specific input ports of the metasurface network based on applied voltages, enabling high-speed activation and switching of different units within the network, thereby dynamically calling up desired holographic images. As a proof-of-concept, the researchers experimentally demonstrated the dynamic switching of eight holographic letters under left and right circular polarization states (Fig. 2d).
This work exemplifies a scalable approach for dynamic manipulation of guided signals and paves the way towards on-chip holographic displays, high-capacity optical communication and integrated photonic information processing.
Journal
Light Science & Applications
Article Title
Dynamic holographic display with addressable on-chip metasurface network based on lithium niobate photonics