Metasurfaces, as ultra-thin two-dimensional structures with subwavelength patterns, can flexibly control optical parameters such as beam phase, amplitude, and polarization, shaping the light wavefront ideally. Through the development of various miniaturized functional components, metasurfaces have been successfully applied in fields such as optical stealth, holographic display, and image recognition. Among them, metasurface holography technology has demonstrated significant potential in optical encryption, 3D display, augmented reality, and virtual reality due to its advantages of high resolution, wide field of view, and miniaturization.

The Janus metasurface is a research platform for studying non-reciprocal transmission optical phenomena, which enables the control of electromagnetic waves on both sides to achieve asymmetric reflection and transmission control. The holographic technology based on Janus metasurface shows great potential for applications in advanced bidirectional displays, data storage, and information encryption. However, the proposed Janus metasurface primarily utilizes optical parameters such as propagation direction and polarization to statically regulate the optical field, thereby limiting the further expansion of information capacity. Furthermore, most Janus metasurfaces exhibit defects such as complex structural sizes and difficulties in manufacturing. Therefore, achieving dynamically tunable holographic displays using the Janus metasurface without increasing manufacturing complexity remains a significant challenge.

To address the aforementioned issues, the team proposed a nonvolatile and reconfigurable Janus metasurface that utilizes the phase change material Sb₂S₃. By leveraging the reconfigurability of Sb₂S₃ and the directional dependence of the Janus metasurface, they achieved a novel dynamic and tunable asymmetric color holographic display in the visible spectrum. By controlling the wavelength of the light wave, the direction of incidence, and the level of crystallization in Sb₂S₃, precise manipulation of a tunable color holographic image display can be achieved. Additionally, the metasurface is made up of a single-layer structure of nanorod arrays, which significantly reduces manufacturing complexity and cost compared to traditional multi-layer designs. Relevant research results were recently published in Photonics Research, Volume 12, No. 2, 2024[Huan Yuan, Zheqiang Zhong, and Bin Zhang. Visible-frequency nonvolatile reconfigurable Janus metasurfaces for dual-wavelength-switched and spin-asymmetric holograms[J]. Photonics Research, 2024, 12(2): 356.]

Figure 1(a) The schematic diagram illustrates the process of illuminating the nonvolatile and reconfigurable Janus metasurface with incident light waves of different wavelengths and reconstructing holographic images at various levels of crystallization. (b) Three optical decoding keys used to reconstruct different holographic images, listed from top to bottom, include the propagation direction, phase transition temperature, and wavelength.

To enhance the holographic multiplexing information capacity of the designed metasurface, the team employed an modified Gerchberg-Saxton algorithm to encode multiple distinct holograms into a single-layer metasurface. The design targets included the wavelength of incident light, the propagation direction, and the crystallization temperature of the phase change material. Furthermore, the team optimized the Sb₂S₃ square nanorods using a particle swarm optimization algorithm to enable the independent display of holographic images across various wavelengths. As a result, each Sb₂S₃ nanorod was designed to exhibit narrowband filtering properties with wavelength independence at different crystallization levels, thereby reducing transmission crosstalk between wavelength channels. The process of dynamically switching color holograms based on a nonvolatile reconfigurable Janus metasurface is depicted in Figure 1 (a). Here, the team simulated asymmetric holographic imaging of the Janus metasurface using green light (550 nm) and red light (650 nm) as target wavelengths. Figure 1 (b) depicts the schematic diagram of the three decoding keys. In the amorphous state, when left-circularly polarized light is incident in both forward and backward directions on the metasurface, holographic images featuring a "lion dance" pattern and a "loong dance" pattern are reconstructed in the far field on opposite sides of the metasurface. In the crystalline state, under the same incident conditions as mentioned earlier, red holographic images depicting another part of the "lion dance" and "loong dance" patterns are reconstructed in the far field on each side of the metasurface. By adjusting to the semi-crystalline state at an appropriate phase transition temperature, full-color holographic images of complete "lion dance" and "loong dance" patterns can be reconstructed in the far field on both sides of the metasurface.

In summary, the metasurface can achieve asymmetric transmission and dynamic switching of color holographic images without increasing processing difficulty or manufacturing cost. The joint control of different optical decryption keys can effectively enhance the information storage capacity of the metasurface. It is worth noting that with the multi-level crystallization of phase change material Sb₂S₃, the metasurface can achieve camouflage display of information in both crystal and amorphous states. It can hide the correct color image information under specific crystallization levels, greatly enhancing the security level of metasurface information encryption. Benefiting from the reconfigurable properties of phase change materials, the Janus metasurface is expected to be applied in related fields such as super-large capacity holographic storage, holographic encryption, and dynamic color holographic display.

The excellent properties of the nonvolatile and reconfigurable Janus metasurface enable precise dynamic control of light fields in the visible spectrum, making it possible to achieve a dynamic display of full-color holographic images in ultra-compact and highly integrated optical components. Building upon this work, our team will further explore more efficient phase change material structural optimization strategies that cover the entire visible spectrum, in conjunction with novel intelligent algorithms. This will lay the foundation for advanced bidirectional display applications capable of dynamically playing colorful holographic images.