Developing cost-effective and easy-to-operate dynamic optical control methods is a long-term objective for the advancement of next-generation intelligent display devices. Traditional gratings have been extensively employed in daily applications, such as optical filtering [1–4], spectral dispersion, and beam steering [5–8], owing to their effective optical control capabilities and affordability. However, traditional gratings lack the ability to accurately control the amplitude or phase at pixel-level, thus limiting their potential for multi-function optical modulation or holographic display.

To achieve precise pixel-level optical modulation, researchers have turned their attention to the metasurface, which exhibits unique electromagnetic wave manipulation capabilities [9–12]. In the past decade, dynamic meta-optical manipulations have been achieved through active tuning of the metasurfaces triggered by electrical methods, thermal methods, chemical methods, etc. [13–18]. An alternative approach involves multiplexing the fundamental parameters of the incident light field for optical encoding and decoding, including amplitude [19,20], phase [21], polarization [22–27], wavelength [28–31], and orbital angular momentum (OAM) [32–35]. Furthermore, a prototype of cascaded metasurface holography has been demonstrated, enhancing the security of dynamic information transmission [36,37]. While metasurface multiplexing technology has been extensively researched, its integration with traditional optical devices has not been fully explored. Previous approaches to amplitude/phase multiplexing often involved using digital micromirror devices (DMDs) [19,38,39], spatial light modulators (SLMs) [21,35,40], or cascaded metasurfaces [36,37]. However, the implementation of such solutions is associated with high costs and bulkiness, presenting challenges for practical application and compact integration. Hence, it remains a valuable endeavor to find an economically viable and facilely operable modulation technique.

Here, we ingeniously propose a metasurface-assisted grating-modulation system in which traditional gratings can be extended to precise beam control at pixel-level, thereby enabling a programmable meta-hologram. Our proposed system utilizes common grating to regulate the amplitude of the incident light field and modulates its phase via metasurface, eventually realizing the pixel-level control of the programmable holographic image displays with two, four, and up to eight channels. Compared with the previous related work [36], our modulation system cascades inexpensive traditional gratings with an individual metasurface, which significantly reduces the manufacturing cost and system complexity. Such a cost-effective strategy serves as a valuable endeavor to find an economically viable and facile modulation technique and facilitate the meta-device potential for practical applications. Moreover, it also reduces the difficulty of alignment operations and increases the robustness of the modulation system. Overall, the grating modulation scheme offers cost-effective convenience, strong encoding freedom, and facile operation, which may open new avenues for the next-generation intelligent display, optical information anti-counterfeit/storage, and optical sensors.

As a low-cost diffractive optical element, traditional gratings can achieve specific optical manipulating functions. As shown in the top part of Fig. 1, on the one hand, amplitude-modulated binary grating and phase-modulated sinusoidal grating can achieve basic beam separation and steering. However, due to the lack of pixel-level modulation capability, it is difficult for traditional grating to realize more complex functionalities. On the other hand, the invention of the static metasurface itself cannot provide multi-fold image information and encoding. It necessitates modulation of the incident light field. In this research, we incorporate the metasurface to address the critical drawback of conventional gratings lacking pixel-level controlling capability and propose a metasurface-assisted grating-modulation (MAGM) system. Through combining the conventional grating with the metasurface and tuning their relative positions/states, the MAGM system enables the capability to independently present and tune the multi-fold holographic images and dynamics that cannot be exhibited by either the grating or the metasurface alone. The schematics and mechanism of the MAGM system are illustrated in the lower part of Fig. 1, showing that the holographic meta-display system comprises two integral components: the incident light field modulation system and the metasurface-assisted device. By combining various time-dependent grating frames to cooperate with the metasurface and modulating their relative positions/states, dynamic holographic image displays and even video presentations can be exhibited and realized.

Figure 2(a) illustrates the process of calculating and optimizing the metasurface pattern with the phase profile through the gradient descent algorithm. The iteration begins with given amplitude matrices of the gratings and a random initial phase matrix φ. Fast Fourier transformation (FFT) is then applied to obtain the complex amplitude distributions of all holographic images. To quantify the dissimilarities between the target and generated images, we compute the average root mean square error (RMSE) and employ it as the loss function for the gradient descent algorithm. The resulting optimal phase profile is shown in Fig. 2(b), and the convergence graph of the figure of merit according to the number of iterations is illustrated in Fig. 2(c). The phase matrix undergoes evolutions, and the loss is quantified as the root mean square error, formulated as loss=1m∑jm1PQ∑p,q[Ij′(p,q)−Ij(p,q)]2,(1)where Ij represents the intensity of the target image, Ij′ is the intensity of the reconstructed image obtained through fast Fourier transformation after the jth iteration, and m denotes the number of channels. The nanopillars are designed with dimensions of 70 nm in width, 140 nm in length, and 380 nm in height, with in-plane rotation characterized by an orientation angle θ. Figure 2(d) exhibits a portion of the scanning electron microscopic images of the metasurface. The sample comprises 1000×1000pixels, with each pixel covering an area of 400nm×400nm.

To further validate the proposed grating manipulation scheme, the MAGM system is optically characterized with the setup in Fig. 3(a). Circularly polarized light is generated using a laser (633 nm) in conjunction with a linear polarizer followed by a quarter-wave plate. The light is subsequently modulated by the grating, and the stripe pattern of the grating is focused onto the metasurface sample via a 4f optical system. The desired holographic images are projected onto a screen in the far field and subsequently captured with a camera. In this experimental configuration, the grating-modulation states are determined by manipulating the position and orientation of the grating to switch among the eight holographic scenes.

To verify the switching capability of parallel movement, we first designed a two-channel scheme to switch between two holographic images solely by moving the grating along the vertical direction to the grating lines. The transmittance function of the grating can be expressed as T(x,y)=12sgn{sin[2π(xsinθ+ycosθ+ΔD)P]}+12,(2)where θ is the orientation angle of the grating lines, ΔD is the displacement of the grating, and P is the grating period. The amplitude distribution of the incident beam is modulated by these parameters. Figure 3(b) displays the working schematics of the reconstructed images for this two-channel scheme. Right circularly polarized light composed of 633 nm laser beams is incident, with a grating period of 30 μm, and the orientation angle is 0°. Placing the metasurface at an appropriate relative position with the grating leads to the generation of a holographic image depicting a “Heart” image. Furthermore, by vertically displacing the grating by half of a grating period (ΔD=15μm), the holographic image is immediately switched to a “Briefcase.” The experimental results for the reconstructed images are shown in Fig. 3(c). Notably, the two images demonstrate high fidelity to the intended designs, with negligible crosstalk and high contrast between the two channels. These results validate the feasibility of our design and the quality of the generated images. It is worth noting that the image quality is strongly influenced by the accurate alignment of pixels between the grating and the metasurface, which would be impacted with a minor translational shift and result in transitions between the “Heart” and “Briefcase” images (for details, see Visualization 1).

To further enhance information density, adding holograms manipulated by gratings of different periods and orientations could be potentially considered and applied. Figure 4 shows the simulated and experimental results of a four-channel scheme. When the grating period is set to 50 μm (ΔD=25μm) with the orientation angle of 0°, lateral movement of the grating alternates between displaying “Gear” and “Star” holographic images. Shifting the grating period to 20 μm (ΔD=10μm) with the orientation angle of 90° introduces two additional image channels, enabling dynamic tuning between “Heart” and “Briefcase” holographic projections, as shown in Fig. 4(a). Nevertheless, as depicted in Fig. 4(b), the observed images exhibit inevitable crosstalk, notably in the form of lower-brightness images from two other channels with different grating periods and orientations (for details, see Visualization 2). The crosstalk phenomenon exists because the gratings in orthogonal directions inherently possess overlapping groove regions, leading to a partial reconstruction of holographic images in unintended directions. Overall, for both fabricated samples, experimentally captured holographic images are visually clear, making it easy to identify the designed shape and distinguish from other channel images, which proves that the simultaneous storage and encryption of multi-fold images can be achieved using conventional gratings with different periods or tuning the relative positions.

In order to further expand the modulation channels, we investigate and introduce additional rotation angles for placing the grating relative to the metasurface. The above grating amplitude distributions are set to be confined in the orthogonal directions, which minimizes the overlap between different amplitude distributions and thus facilitates its satisfactory image quality. Eventually, an eight-channel scheme is designed by placing the grating at varied angles of 0°, 45°, 90°, and 135° to generate different amplitude distributions. The grating period is 50 μm. By switching the relative positions of the grating, each distinct image among the eight holographic channels would alternatively display and switch when modulating the grating orientation and the respective defined displacement. The complete details are presented in Visualization 3.

Note that the information encryption functionality is achieved by employing specific-period gratings to locate at a specific position with the metasurface and modulating the incident light with a particular amplitude distribution. The pre-modulation of the metasurface by gratings results in switching among different images. Therefore, the encrypted information enjoys high security because only under accurate conditions can the correct image be extracted. To successfully decrypt the target image, both keys of (i) the incident light amplitude distribution (grating parameters and its relative locations) and (ii) the designed metasurface pattern must be acquired accurately. Even if the metasurface itself were stolen, it would remain unfeasible to obtain the correct image among multi-fold channels without the key of incident light amplitude distribution. Specifically, when an unmodulated beam directly illuminates the metasurface sample, all channel information simultaneously appears, which causes the encrypted information to be submerged by a large amount of interfering information and makes it difficult to be accurately acquired.

Furthermore, as a proof-of-concept demonstration, a holographic dynamic modulation is simulated by engaging more grating-modulation states (period/orientation). To create the metasurface hologram, we extracted image frames with 800×800 pixels from a video depicting a plane flying in circles with a framerate of 30framess−1. We used the gradient descent algorithm to optimize and obtain the final metasurface phase profile. The corresponding holographic video is reconstructed at the same framerate as the original video (for details, see Visualization 4). The dynamic image tuning further indicates the potential of the proposed MAGM scheme in information storage capacity and dynamic modulating capability, which promise the application of multiplexing optical displays, information storage, and next-generation intelligent displays.

In summary, we proposed and experimentally demonstrated a metasurface-assisted grating-modulation (MAGM) scheme for the encryption and storage of multi-channel holographic images and dynamic switching functionality. Through introducing the precise light control capabilities of metasurfaces to cooperate with gratings, traditional gratings are empowered to manipulate and modulate with more complex light field generation for holographic display. By dynamically adjusting the grating period, relative position, and rotational orientation, multi-fold (2–8 channels) programmable meta-holograms are realized to be dynamically exhibited and switched. In comparison to previous works utilizing SLM or DMD tools, the MAGM system offers convenience in practice and strong encoding freedom, as well as simplicity in optical system configuration and significant cost reduction, thus promising critical progress toward practical applications. We believe that the proposed scheme can facilitate the development of programmable holographic dynamic display technology with promising applications in next-generation intelligent displays, optical information storage/encryption, etc.

Acknowledgment. The work is supported by the Center for Nanoscience and Nanotechnology at Wuhan University.