In recent years, metasurfaces have garnered significant attention owing to their flexible manipulation of light and remarkable properties such as being ultralightweight and ultrathin [1–5]. Metasurfaces comprising artificially designed subwavelength units can provide a new paradigm for designing various ultracompact optical components. Owing to rapid advancements in micro-nano manufacturing and computer technologies, metasurfaces have been applied in various miniaturized optical fields, including holographic displays [6–9], optical encryption [10–15], nanoprinting [16–20], edge detection [21,22], diffraction neural networks [23,24], and optical stealth [25]. Deng et al. combined the PB and detour phases to design a metasurface with multiple degrees of freedom to achieve a full-color complex amplitude vector hologram [26]. Ni et al. developed a multidimensional light-field encoder based on a tunable metasurface that enables the simultaneous measurement of the polarization and spectrum of near-infrared light [27]. However, most of the aforementioned metasurface-based optical elements exhibit symmetric transmission characteristics, where both ends of the metasurface perform the same operation under identical incident conditions [Fig. 1(a)]. Using a focusing lens as an example, the focal length of the incident light is compared along the forward/backward propagation direction. Although this is beneficial for certain applications, it limits the potential for designing advanced multichannel and multifunctional components. With increasing demand for compact and versatile optical components in various fields, it is crucial to break the symmetrical transmission of metasurfaces for a wide range of photon applications.

A crucial property of electromagnetic waves, propagation direction, is often overlooked in comparison to commonly used parameters such as polarization, phase, incident angle, and wavelength. The Janus metasurface, which serves as a research platform for studying optical phenomena such as nonreciprocal transmission, can control the electromagnetic wavefront on both sides, enabling asymmetric reflection/transmission control [Fig. 1(b)] [28,29]. This metasurface has the potential to be a substitute for engineered multifunctional devices such as asymmetric metals and beam shaping [30,31]. However, traditional Janus metasurfaces rely heavily on complex multihierarchical structures, making them difficult to manufacture and expensive. To address these issues, single-layer metasurface structures based on both the propagation and geometric phases have been developed and utilized for asymmetric transmission [32–35]. For instance, Naveed et al. proposed a metasurface comprising hydrogenated amorphous silicon (a-Si:H) and demonstrated asymmetric light transmission in the visible region. Wan et al. designed an asymmetric bidirectional metasurface to achieve a wideband bidirectional holographic display. However, the electromagnetic functions of the previously proposed asymmetrically transmitted metasurfaces are statically regulated, limiting their capacity for further expansion.

Fortunately, optical phase-change materials (O-PCMs) are highly desirable for designing tunable optical components owing to their reconfigurable properties [36–42]. O-PCMs typically exist in two primary states, crystalline and amorphous (insulated or metallic). The conversion between these two crystallinity states alters the arrangement of chemical bonds in the material, causing significant changes in its complex refractive index [43]. This enables the introduction of substantial phase and amplitude modulation in compact micro and nano sizes. The crystallinity of the O-PCMs can be altered by applying an external stimulus (such as light, electricity, or heat) [38,44,45], and the reversible switching between two stable states has a high durability of up to 1012 cycles [46]. However, classic O-PCMs such as GexSbyTez alloys and VO2 exhibit high absorption losses at different crystallinity levels and cannot satisfy the requirements for high performance. Additionally, the volatility of VO2 limits its feasibility in the design of high-performance nonvolatile reconfigurable devices. For instance, Chen et al. proposed a terahertz Janus metasurface based on an array of O-PCMs, VO2, which can achieve asymmetric transport switching [Fig. 1(c)] [47]. When the O-PCMs are in different states, the transmission characteristics of the incident light along the forward and backward propagation directions vary (F1f≠F1b,…,Fnf≠Fnb). However, this structure has a complex multihierarchical structure and suffers from the drawbacks of volatile regulation. Furthermore, these metasurfaces can achieve asymmetric transmission only at specific wavelengths and cannot simultaneously regulate different wavelengths, resulting in limited optical possibilities. Recently, two novel antimony-based nonvolatile O-PCMs, Sb2S3 (Fig. 2) and Sb2Se3, were reported to exhibit significantly low absorption losses at different crystallinity levels [38]. In particular, Sb2S3 exhibits low absorption loss in the visible band, enabling the design of devices with adjustable color display functions. For instance, Hemmatyar et al. used Sb2S3 to fabricate dynamic metapixels that were switchable, highly saturated, efficient, and of high resolution [46]. Therefore, we explored the strategy of using a simple structure, a nonvolatile Janus metasurface based on Sb2S3 with multiple degrees of freedom, to achieve dynamically switched asymmetric color holographic displays.

In this study, we propose a novel nonvolatile reconfigurable single-layer Janus metasurface to achieve asymmetric wavefront generation and dynamically switched bidirectional holography. The Janus metasurface utilizes the reconfigurability of the nonvolatile O-PCM Sb2S3 and the incident direction dependence. To design switchable asymmetric holograms on single-sized nanostructured metasurfaces, we developed a modified Gerchberg–Saxton (GS) algorithm by considering both the wavelength channels and propagation directions as the design objectives. In comparison with previous complex multilayer cascade designs, this reconfigurable metasurface can integrate multiple functions into a single phase-change element without introducing any additional loss, providing greater flexibility for regulating the light field. By changing the crystallinity of Sb2S3, these functions can be actively switched. In this study, we demonstrated a bidirectional asymmetric color-holographic display at three different crystallization levels. This scheme not only addresses the limitation of statically switching asymmetric holograms but also achieves multifunction integration on an ultrathin single-size nanostructure, significantly reducing manufacturing complexity and cost. The proposed scheme provides a versatile platform for information multiplexing and is promising for various practical applications, including mass data storage and optical encryption.

Figure 3(a) illustrates the concept of a switchable asymmetric hologram based on a nonvolatile reconfigurable Janus metasurface. The scheme allows for the integration of four independent holographic images onto a single metasurface. In the amorphous state, left-circularly polarized (LCP) light is incident forward and backward onto the metasurface, generating green hologram images of a portion of the “lion dance” and a portion of the “dragon” structure in the far field, respectively. In the crystalline state, red hologram images of another portion of the “lion dance” and “dragon” structures are generated in the far field, respectively. In the semi-crystalline state, LCP light is incident forward and backward onto the metasurface, generating complete color holographic images of the “lion dance” and “dragon” structures in the far field. The Sb2S3 array, acting as meta-atoms on the metasurface, serves as a half-wave plate to convert circularly polarized (CP) light and enables asymmetric transmission of circularly polarized light. By utilizing the modified GS algorithm, the desired target function can be achieved by calculating the phase distribution on both sides of the metasurface and further arranging the orientation angles of the meta-atoms. Here, a single-phase profile of the meta-atoms array serves as the phase contribution function, encompassing the information required for different wavelength channels and transmission directions. The metasurface can switch between different functions by reversibly changing its crystalline and amorphous states as needed. Figure 3(b) illustrates the decoding keys required for decoding a holographic image and the associated optical properties for various key combinations.

To encode four different hologram images in the far field, we have developed a modified GS algorithm to determine the orientation angle of each meta-atom, as shown in Fig. 4. It is important to note that our geometric phase manipulation technique arises from the inherent property of the meta-atom, which provides two opposite phase distributions for both forward and backward incident CP light. Leveraging this property, we encode the desired phase onto the metasurface and reconstruct two different holographic images simply by changing the direction of incident light. Therefore, the design process of a multifunctional metasurface based on the modified GS algorithm can be divided into three steps [Fig. 4(a)]. (1) Using the GS algorithm, we design the phase required for forward image 1 and backward image 1 at the wavelength of 550 nm. The total geometric phase profile required is then calculated based on the phase merging formula. (2) Similarly, the GS algorithm is used to design the phase required for forward image 2 and backward image 2 at the wavelength of 650 nm. The total geometric phase profile required is then calculated based on the phase merging formula. When two beams of the same CP light are incident from opposite directions, the total geometric phase profile can be expressed as follows: φ550/650=arg[exp(iφforward)+exp(−iφbackward)],(1)where φforward and φbackward represent the geometrical phases required for forward incidence and backward incidence, respectively. The complete phase profile at wavelengths of 550 nm and 650 nm can be obtained by solving Eq. (1), respectively. (3) The required phase exhibits the same profile under two different crystallinity conditions. To integrate the phase profiles corresponding to both states, we develop a formula for merging phases to obtain the final geometric phase, i.e., φt=arg[exp(iφ550)+exp(iφ650)],(2)where φ550 and φ650 represent the geometrical phases required for the amorphous and crystalline states, respectively. In Fig. 4(a), the right side provides a closer look and an overall view of the resulting phase delay distribution when LCP light is incident on the metasurface from either the forward or backward direction. The selection of the meta-atom array is then determined under the principle that the orientation angle of the meta-atom is twice the phase, allowing us to achieve the desired metasurface structure according to the specific requirements.

The specific flow chart of this modified algorithm is depicted in Fig. 4(b). The intensity distributions of the four images are denoted as A1, A2, A3, and A4, respectively. The propagation process between the metasurface plane and the target plane is constructed using forward Fresnel diffraction (FFrD) and inverse Fresnel (IFrD) diffraction. The modified GS algorithm differs from the traditional one in that it considers both positive and negative values of the initial random phase before performing the Fresnel diffraction calculation. During the iterative process, the phase enhancement calculation is carried out according to Eqs. (1) and (2), simultaneously fulfilling the requirements of four-channel holographic images. After sufficient iterations, the optimized phase contours φt_forward and φt_backward are obtained. The distance between the metasurface and the two target planes is set as z1=D1=150μm and z2=−D2=−150μm, respectively. The propagation formulas for both FFrD and IFrD can be expressed as Hz(x,y)=exp(ikz)ikzexp[ik2z(x2+y2)]×FFT{H0(x0,y0)×exp[ik2z(x02+y02)]},(3)H0′(x0,y0)=exp(ikz)ikzexp[ik2z(x02+y02)]×FFT{Hz*(x,y)×exp[ik2z(x2+y2)]},(4)where * represents complex conjugation, H0(x0,y0) denotes the input light field, and Hz(x,y) is the complex amplitude distribution at the propagation distance z on the observation screen.

To achieve this desired versatility, we have developed a novel single-layer metasurface structure that breaks symmetrical transmission. By optimizing the structural parameters of the meta-atom, we aimed to meet multiple requirements: (1) the spectral responses of meta-atoms should generate distinct colors in different states; (2) the transmission cross talk between different color channels should be minimized to prevent holographic image cross talk; (3) the circular polarization conversion efficiency should be maximized to ensure clear holographic images. To fulfill these requirements, we employed the particle swarm optimization (PSO) algorithm to optimize Sb2S3 square nanorods. Here, we have taken 550 nm (green light) and 650 nm (red light) as the typical examples for the target wavelengths. Therefore, the target spectrum should exhibit narrowband filtering at both target wavelengths and a higher polarization conversion efficiency. The cell period of the elementary meta-atom is fixed at 380 nm (P=380nm), as shown in Fig. 5(a). The optimized structural parameters include length L, width W, and height H of the meta-atoms. It is worth noting that most O-PCMs have a high extinction coefficient in the visible region, which greatly limits their optical transmission efficiency. In our design, we utilize the nonvolatile O-PCM, Sb2S3, as the nanorods and optimize the structure size accordingly. The energy loss is effectively avoided in the visible region due to its low extinction coefficient (the complex refractive index at 633 nm: A-Sb2S3,3.148+0i; C-Sb2S3,3.989+0.276i for visible light) (Fig. 2). The PSO algorithm yields the optimized structure parameters: L=115nm, W=80nm, and H=500nm. Fortunately, these parameters are compatible with current advanced micro and nano machining technology. The transmission efficiency of the structure is simulated using the commercial software Lumerical finite-difference time-domain (FDTD) solutions. The intensity transmission curve for a single meta-atom under periodic boundary conditions is depicted in Fig. 5(b). The transmission spectrum of incident light can meet the functional requirements mentioned above when Sb2S3 is in either its crystalline or amorphous state. Notably, both states exhibit distinct transmission peaks for LCP light at 550 nm and 650 nm, with peak polarization conversion efficiencies of 28.8% and 15.1%, respectively. Additionally, the cross-talk values for the two target wavelength channels are 0.65% and 1.03%, respectively. The cross talk between the two target wavelength channels is negligible. It is worth noting that the peak polarization conversion efficiency at a wavelength of 550 nm is higher than that at 660 nm, due to the lower extinction coefficient of A-Sb2S3 compared to that of C-Sb2S3 (see Fig. 2). To satisfy the required hologram phase, the PB phase principle is adopted to further select the orientation angle of the meta-atoms, assuming the identical sizes of the meta-atoms. The influence of the meta-atom’s orientation angle on the polarization conversion efficiency and PB phase in two states is shown in Fig. 5(c). The polarization conversion efficiency remains almost constant at the two target wavelengths, while the PB phase varies twice with the orientation angle (φ=2θ). Thus, the designed structure can meet the target function of the meta-atoms. The diffraction efficiency (DE), defined as the energy ratio between the transmitted and incident beam, is presented in Fig. 5(d) for various structural dimensions L and W, with a thickness of 500 nm. The spectral responses of cross-polarization transmittance in both states are converted into CIE 1931 color space chromaticity plots to quantitatively evaluate the obtained structural colors, as depicted by white solid dots in Fig. 5(e).

In Fig. 5(f), we utilized FDTD solutions to perform numerical calculations to demonstrate the independence of this meta-atom in two different states. By deliberately arranging a periodic array of meta-atoms with varying orientation angles, we intentionally create a complete 2π gradient phase along the interface. This arrangement enables us to prove the abnormal refraction that occurs when two identical light beams are incident on the metasurface from the opposite sides. As shown in Fig. 5(f), in both states, when the same incident circularly-polarized light is incident on the metasurface from both sides, the outgoing light exhibits opposite beam steering phenomena. Consequently, the wavefronts of the incident light in both forward and backward directions confirm the desired asymmetric transmission performance.

Next, we employed this reconfigurable meta-atom to design a switchable Janus metasurface capable of generating direction-dependent, wavelength-dependent, and crystallinity-dependent holographic images in four channels. To achieve four independent holographic image channels on the metasurface, we utilized the meta-atoms as half-wave plates and rotated the orientation angle of meta-atoms by using geometric phase. Figure 6(a) provides an overview of the metasurface configuration, including top, zoomed, and angled views. The entire metasurface has a size of 38μm×38μm, and it consists of a total of 100×100 meta-atoms. Figure 6(b) depicts the flow diagram of the reconstructed image using LCP light of different wavelengths. When Sb2S3 is in the amorphous state, both forward and backward incident LCP green light passes through the metasurface, resulting in different encrypted images in the far field. Similarly, when Sb2S3 is in the crystalline state, both forward and backward incident LCP red light passes through the metasurface, displaying different encrypted images in the far field. Lastly, when Sb2S3 is in the semi-crystalline state, both green and red light, from the same forward and backward incidence, passes through the metasurface, generating different encrypted images with different colors in the far field. The reconfigurable Janus metasurface boasts a high level of encryption and exclusively retrieves the corresponding effective holographic image through the incidence of CP light. When using other types of light, such as linearly polarized light, only disordered and invalid image information can be observed. Furthermore, we calculated the broadband transmittance curve of the entire reconfigurable Janus metasurface. Figures 7(a) and 7(b) depict the transmission curves of the metasurface transmission terminal under different crystallinity conditions. The total cross-polarization transmission efficiency at 550 nm in the amorphous state reaches 26.3%, while the corresponding value at 650 nm in the amorphous state reaches 13.5%. Notably, the metasurface exhibits robust transmission performance when utilized for bidirectional dynamic holographic display.

The metasurface enables multichannel image encryption by encoding the incident direction, crystallinity, and wavelength. Figure 8 illustrates the decoding process using different optical keys. The holographic images on the left side of Figs. 8(a) and 8(d) are obtained through numerical calculations using the modified GS algorithm, with each cell containing a total of 800×800 pixels. These images exhibit high-fidelity quality in different channels. Additionally, we employed the commercial software FDTD solutions to simulate the metasurface. Due to limited computation resources, the number of pixels was reduced to 100×100. The simulation results on the right side of Figs. 8(a) and 8(d) align with the predetermined dynamic multifunction design. The images encoded in different holographic channels are associated with the three optical keys mentioned above and can only be decoded using a specific combination of encoding. This novel holographic encryption scheme can significantly enhance information security and holds great potential for optical encryption application. Moreover, Sb2S3 exhibits changes in multiple energy states during its transition from the amorphous state to the crystalline state, allowing the phase transition metasurface to be controlled by a multilevel wavefront. The dielectric constant of the intermediate phases of Sb2S3 can be calculated using the Lorenz–Lorentz relation [48]: εeff−1εeff+2=mεC-Sb2S3−1εC-Sb2S3+2+(1−m)εA-Sb2S3−1εA-Sb2S3+2,(5)where m denotes the crystallinity of Sb2S3, and m=0 and m=1 represent the amorphous and crystalline states, respectively. εA-Sb2S3 and εC-Sb2S3 represent the wavelength-dependent dielectric constants of A-Sb2S3 and C-Sb2S3, respectively. Figure 8(b) shows the complex refractive index curve for semi-crystalline Sb2S3 with m=0.5. Taking the wavelength of 633 nm as an example, the complex refractive index of Sb2S3 is 3.51+0.10i. Additionally, we calculated the transmission spectra of meta-atoms in the semi-crystalline state, as depicted in Fig. 8(c). The meta-atoms in the semi-crystalline state exhibit certain transmission efficiency at wavelengths of 550 nm and 650 nm. By simultaneously illuminating the metasurface with red and green light, a new color image can be displayed at both ends. The color lion dance image and color dragon image are displayed respectively at the far field on both sides of the metasurface, as shown in Fig. 8(d). This unique feature enables the presentation of different holographic images in amorphous, semi-crystalline, and crystalline states, thereby expanding the number of channels and enhancing the level of information encryption. It is worth noting that the maximum number of colors that the reconfigurable Janus metasurface can contain depends on two aspects: (1) the number of narrowband filtering channels varying with the crystallinity of Sb2S3 nanorods, and (2) the number of hologram wavelength channels optimized using the GS algorithm. Therefore, the maximum number of color channels in the design scheme can be further increased by combining intelligent algorithms to design a multifreedom structure.

To address the manufacturing tolerances of this reconfigurable Janus metasurface, we conducted simulations to assess the effects of varying the length, width, and thickness of the top Sb2S3 nanorods on the cross-polarized transmission spectrum, as depicted in Fig. 9. Figures 9(a)–9(c) illustrate the impact of different structural parameters including the width, length, and thickness of A-Sb2S3 nanorods on the efficiency of cross-polarization conversion. In each figure, the peak transmission offset is represented by a black dashed line. Furthermore, Figs. 9(d)–9(f) illustrate the impact of different width, length, and thickness of C-Sb2S3 nanorods on the efficiency of cross-polarization conversion. Figure 9 implies that the reconfigurable Janus metasurface can still maintain good performance within a certain range of structural size.

In summary, we demonstrated the use of a reconfigurable Janus metasurface based on single-sized nanostructures for dynamic switching displays without the need for complex and difficult manufacturing processes. In addition, we proposed a modified holographic optimization strategy to achieve high-fidelity asymmetric holographic imaging. The mechanism underlying this approach combines the geometric phase and crystallinity transitions, resulting in different optical responses to the same CP light when incident from opposite directions. In the visible region, we realized a color-holographic encryption display of wavelength, incident direction, and crystal state using an O-PCM array, which was previously impossible with asymmetric transmission metasurfaces. The results demonstrate that the display of different color holographic patterns can be independently controlled by switching three keys: wavelength, incident direction, and crystal state. Furthermore, by combining this scheme with deep learning methods or other optimization algorithms, the number of image channels encoded within a single metasurface can be further increased. With the advantages of miniaturization, integration, and tunability, reconfigurable metasurfaces hold significant potential for application in next-generation color-optical encryption elements.