A 0.18 cubic centimeter 3D meta-holographic zoom micro-projector

Abstract

Meta-holography has become a frontier hot spot of photonics, thanks to the rapid advancement of nanofabrication. Tunable meta-holography significantly enhances information capacity and meets the growing demands of adaptive imaging, offering tremendous application prospects in fields such as optical storage, augmented and virtual realities, and biology. However, the existing meta-holography predominantly relies on 2D projection, with a small and non-adjustable image size. Here, we propose a 3D meta-holographic zoom micro-projector by integrating a high-resolution metasurface with a tailored liquid lens, and the size of the demonstrated micro-projector is only 0.18 cm³. A 3D Fourier meta-hologram generation method is proposed, which overcomes the limitation of the traditional Fourier meta-hologram in realizing 3D projection. By proposing a small-sized zoom liquid lens, the projector achieves 3D zoom projection, with the size and projection distance of the meta-holographic image extending to the decimeter scale, a feat unattainable by previous meta-holographic projection. This flexible and miniaturized 3D fingertip zoom micro-projector is anticipated to have broad applications in portable and wearable devices as well as biomedical apparatus.

Wang et al. propose a 3D meta-holographic zoom micro-projector, integrating a high-resolution metasurface with a tailored liquid lens, resulting in a 3D zoom micro-projector of only 0.18 cm³, small enough to fit on a fingertip.

Introduction

Holographic projection can be used in the fields such as near-eye display, encryption and measurement, which has important research significance^1–4. Lightweight and miniaturized zoom holographic projection has become a research hotspot in the field of holography because of its high application requirements^5,6. For example, in holographic near-eye display, the volume and image size of holographic projection directly determine the quality of near-eye display equipment^7,8. The traditional holographic projection uses spatial light modulator (SLM) to modulate the wavefront information, and uses the lens to focus the imaging, which directly leads to a huge and complex system. In addition, there is a contradiction between zoom and miniaturization. Zoom holographic projection needs to be realized by changing the relative positions between several lenses, which makes the system more bulky. Overcoming long-standing limitations, such as shallow depth, fixed focal length, small image size, and system complexity, has been widely regarded as a critical challenge.

Metamaterials are artificial composite materials that exhibit extraordinary physical properties not found in nature, achieved through the design of artificial micro-nano structures. These materials have been recognized as one of the top ten scientific and technological advancements globally^9–12. As a specialized class of metamaterials, metasurfaces are characterized by their planar structure, ultra-thinness, high resolution and flexible design freedom^13–21. In 2015, researchers combined plasmic resonant metasurface with geometric phase and realized efficient 2D holographic projection using the Gerchberg-Saxton algorithm with a system including polarized optical devices²². In 2016, a holographic projection based on Huygens metasurface was demonstrated with operation spectral bandwidth of near-infrared light, and a 5 mm image was reconstructed at a distance of 10 mm²³. Subsequently, a silicon-based nano-antenna with narrow-band response was designed, enabling color holographic projection using metasurface with monolithic geometric phase control²⁴. In this design, three types of nanoblocks were reused within a subwavelength unit to form a metamolecule, enabling the simultaneous modulation of red, green, and blue light. The reconstruction system uses three lasers with different wavelengths, and the volume of the whole system is cubic decimeter level. In 2018, researchers utilized chemical methods to control the composition of metasurface structural units, achieving dual-channel controllable meta-holographic projection. However, the chemical treatment process is slow, leading to a long response time and making it difficult to meet the demands for rapid adjustment²⁵. In 2024, a method for multi-channel meta-holographic projection based on polarization multiplexing mechanism was proposed²⁶. Holographic images on different planes were projected at distances from 0.3 mm to 0.6 mm, respectively. Despite significant advancements in multifunctional meta-holographic projection, the existing methods predominantly achieve multi-channel display through multiplexing. In this approach, the number of projection layers depends on the specific design of the metasurface’s multi-channel configuration, which complicates 3D projection. More critically, the projector’s large size, along with the fixed and unadjustable distance and the small reconstructed image size, remain significant limitations of the technology.

The core of miniaturization zoom projector is to have ultra-thin wavefront control ability and the ability to dynamically change the focal length^27–29. In recent years, zoom lenses have been extensively developed to enable adaptive adjustment of optical systems^30–34. Among these, electrowetting-based liquid lenses have garnered significant attention due to their broad focal length adjustment range and rapid response speed^35–39. By incorporating liquid lenses into holographic systems, researchers can swiftly obtain the depth information of 3D objects or adjust the viewing angle of holographic display^40,41. However, in electrowetting-based liquid lenses, there is a complex trade-off among interfacial tension, response time, optical power, and stability. As the size of the liquid lens decreases, the influence of liquid-liquid interfacial tension on optical power and other properties becomes increasingly dominant, imposing higher demands on the liquid material used. Currently, the design and fabrication of compact electrowetting-based liquid lens with good performance is still challenging⁴².

Here, we propose a 3D meta-holographic zoom micro-projector, as shown in Fig. 1. By integrating a specially designed metasurface and a liquid lens into a single cavity, we have achieved the world’s smallest 3D zoom micro-projector with dimensions of 6.5 mm × 6.5 mm × 4.2 mm. This demonstration represents a significant advancement in meta-holography for the following reasons. First, this approach effectively realizes a large-depth 3D meta-holographic projection. A 3D object is regarded as a series of 2D layers. By incorporating the fractional Fourier transform (FRFT) into meta-holographic coding, the rotation angle of the Wigner distribution function⁴³ can be changed, thus the depth of each layer of the 3D object is characterized and calculated, and finally a phase meta-hologram containing the depth of the 3D object is generated. The limitation that traditional Fourier-algorithm-based meta-holography can only realize 2D projection has been broken. Second, the proposed micro-projection method addresses the zoom capacity issue in traditional meta-holography. An electrowetting liquid lens with a high optical power and a small effective optical aperture of 2 mm has been developed to dynamically adjust the meta-holography. By formulating special liquid materials, the size and position of holographic images can be easily adjusted over a wide range by varying the driving voltage. As shown in Fig. 1c, when the driving voltage of the liquid lens is U_I and U_II, there are two different projection states, and the 3D scene is reconstructed in different positions with different sizes. Consequently, using the liquid lens and the designed metasurface, the projection distance of the meta-holographic image can reach decimeter level, and the size can achieve over 5 cm × 5 cm. Third, our experiments demonstrate multi-wavelength zoom meta-holography with the use of the wide-band response metasurface and liquid lens. The proposed meta-holographic zoom micro-projection method is expected to find massive applications, including high-security encryption, augmented and virtual realities, wearable and biomedical devices, and automotive displays.

Fig. 1. Concept of the proposed zoom micro-projector.

a The fabricated projector on a fingertip. b The projector in comparison to a penny. c Illustration of the internal optical design and principle of the 3D micro-projector. When the driving voltage of the liquid lens is U_I and U_II, there are two different projection states, and the 3D scene is reconstructed in different positions d with different sizes.

Results

Structure of the proposed zoom micro-projector

The proposed meta-holography-based zoom micro-projector consists of a laser (not shown in Fig. 1a, b), a transmissive metasurface, and a specially designed electrowetting liquid lens, as shown in Fig. 1c. In the design process of the metasurface, a 3D Fourier meta-hologram generation method based on FRFT is proposed to generate the 3D meta-hologram, and then the 3D meta-hologram is encoded on the sub-wavelength structures. The liquid lens is positioned behind the metasurface. Unlike traditional liquid lenses, a lightweight electrowetting liquid lens with an effective optical aperture of 2 mm is developed. This lens features an extremely compact cavity size, enabling fingertip micro-projection in conjunction with the metasurface. To achieve system miniaturization, the metasurface and liquid lens are integrated into a single cavity, as shown in Fig. 1a, b. When the laser irradiates the metasurface, the holographic diffraction image passes through the liquid lens. By changing the driving voltage of the liquid lens, 3D images can be projected at different positions, with their sizes adjustable as desired.

Design of the metasurface

Leveraging the simplicity and excellent performance of geometric phase in phase modulation, a metasurface with a resolution of 2500 × 2500 using Pancharatnam-Berry (PB) geometric phase⁴⁴ is designed for the proposed meta-holography. Each unit cell of the metasurface consists of a rectangular monocrystalline silicon nanorod fabricated on a sapphire substrate. L, W, and H represent the length, width, and height of the monocrystalline silicon, respectively. p is the period, and θ is the rotation angle relative to the x-axis. By adjusting the rotation angles of the unit structures, a phase shift directly proportional to twice the rotation angle is achieved, with minimal impact on the magnitude.

Specifically, for rectangular nanorod structures with C₂ symmetry, the PB phase shift value is directly proportional to twice the rotation angle¹⁰. This phase modulation method leverages the geometric nature of the PB phase, enabling precise control over the phase distribution without altering the physical dimensions of the structures. More importantly, the geometric phase is wavelength-independent, allowing this phase relationship to be universally applicable for different wavelengths.

Design of the liquid lens

The designed liquid lens consists of a transparent conductive liquid and an insulating liquid encapsulated between the upper and lower electrodes, as shown in Fig. 2a. Under the effect of interfacial tension, a liquid interface is formed between the conductive liquid and the insulating liquid. Based on the electrowetting effect, the contact angle of the conductive liquid can be changed with driving voltage, and the focal length control can be achieved through voltage control (Supplementary materials S1). To ensure that the liquid lens has a high ratio of optical aperture to mechanical diameter, a straight cylindrical cavity is designed to replace the typical sloping cavity. This design reduces the thickness of the liquid lens cavity, enhancing the compactness of the meta-holographic 3D zoom projector. Additionally, to guarantee a sufficient range of optical power variation and good stability, a biphasic liquid composed of a conductive liquid without aqueous solution and an insulating liquid with low surface tension is developed. So, the proposed liquid lens offers a wide operating voltage range and a great variation in optical power.

Fig. 2. Core components and mechanism of the proposed micro-projector.

a Structure of the proposed liquid lens. When a driving voltage is not applied to the liquid lens, it is in a divergent state, and when a voltage is applied, it can be adjusted to a convergent state. b Computational strategy of the meta-hologram.

The compact liquid lens, with its high ratio of optical to mechanical aperture and substantial optical power variation range, is used to adjust the depth of the meta-holographic 3D zoom projection. By tuning the driving voltage of the liquid lens, the wetting properties of the conductive and insulating liquids vary accordingly, resulting in changes in the curvature of the liquid-liquid interface and the optical power. The optical power Φ of the liquid lens can be expressed as:

$$\Phi =\frac{\left(2{\gamma }_{12}\cos {\theta }_0+C{U}^2\right)\mathrm{\Delta }n}{{\gamma }_{12}D}$$ 1

where γ₁₂ is the interfacial tension between the conductive and insulating liquids, θ₀ is the initial contact angle when no voltage is applied, C is the total capacitance per unit area of the dielectric layer coated with a hydrophobic layer, U and D are the driving voltage and effective optical aperture of the liquid lens, respectively, and ∆n is the refractive index difference of the conductive and insulating liquids. The focal length f_{liquid lens} of the liquid lens can be obtained from:

$$f_{\mathrm{liquid}\mathrm{lens}}=\frac{1}{\Phi }.$$ 2

Principle of the 3D Fourier meta-hologram generation method based on FRFT

In the metasurface design process, a 3D Fourier meta-hologram generation method based on FRFT is proposed, as shown in Fig. 2b. The 3D object is layered according to the depth information. The FRFT and inverse Fractional Fourier transform (IFRFT) are introduced into the algorithm, with iterative constraints added in the fractional domain and spatial domain, respectively, to obtain a convergent final phase distribution and generate a 3D meta-hologram. By encoding the 3D meta-hologram on the sub-wavelength structures, a metasurface is designed and fabricated. Different from the traditional metasurface-based holographic algorithm, the proposed 3D Fourier transform algorithm achieves 3D meta-holographic reconstruction through a transformation order (noted as a) (Supplementary material S2). Meanwhile, the position and depth of the reconstructed images are closely related to the focal length of the liquid lens, enabling efficient 3D zoom projection by adjusting the driving voltage of the liquid lens.

In detail, the 3D object is stratified based on the depth information. For each layer, a random phase distribution is introduced to generate the initial complex amplitude distribution g(x, y; z_a) in the spatial domain. According to the depth cues, an appropriate transformation order a is selected and the complex amplitude distribution G(u, v; z_a) in the fractional domain is then obtained by using FRFT:

$$G\left(u,v;z_a\right)=F_a\left\{g\left(x,y;z_a\right)\right\}=\iint g\left(x,y;z_a\right)\times \exp \frac{j2\pi }{\lambda f_a}\left[\frac{x^2+y^2+u^2+v^2}{2\tan \left(a\pi /2\right)}-\frac{xu+yv}{\sin \left(a\pi /2\right)}\right]\mathrm{d}x\mathrm{d}y$$ 3

where F_a represents the FRFT with transformation order a. (x, y) and (u, v) are the position coordinates of the light field in the spatial domain and fractional domain, respectively, z_a is the depth of each layer. j is the imaginary number, λ is the wavelength of the incident light, and f_a is the arbitrary focal length, which is related to the transformation order a. Inferred from the rotation transformation of the Wigner function, f_a can be expressed as follows:

$$f_a=f\sin \left(\frac{a\pi }{2}\right),$$ 4

where f is the selected normalized focal length in the calculation process.

When the transformation order a is set to 1, the arbitrary focal length f_a equals the normalized focal length. It is evident that introducing the transformation order alters the rotation angle of the Wigner function, which is crucial for extending the light field from the spatial and frequency domains into the fractional domain. This approach breaks the limitation in traditional Fourier coding, where the diffraction distance is solely determined by the focal length. Subsequently, the fractional domain constraint is applied to the complex amplitude distribution G(u, v; z_a), with only the phase information retained:

$$\phi \left(u,v;z_a\right)=\frac{G\left(u,v;z_a\right)}{\mid G\left(u,v;z_a\right)\mid }$$ 5

where |·| denotes taking the module, and φ(u, v; z_a) represents the phase distribution in the fractional domain, corresponding to the phase distribution on the holographic plane. To apply the constraints in the spatial domain, the IFRFT is employed to convert φ(u, v; z_a) into g’(x, y; z_a), which represents the spatial complex amplitude information prior to applying constrains:

$$g^{\prime }\left(x,y;z_a\right)={F_a}^{-1}\left\{\phi \left(u,v;z_a\right)\right\}=\iint \phi \left(u,v;z_a\right)\times \exp \frac{j2\pi }{\lambda f_a}\left[\frac{x^2+y^2+u^2+v^2}{2\tan \left(-a\pi /2\right)}-\frac{xu+yv}{\sin \left(-a\pi /2\right)}\right]\mathrm{d}u\mathrm{d}v$$ 6

where F_a⁻¹ represents the IFRFT. The complex amplitude distribution g₁(x, y; z_a) in the spatial domain is then obtained by retaining the phase distribution and replacing the original amplitude information of the 3D object A₀(x, y; z_a):

$$g_1\left(x,y;z_a\right)=A_0\frac{g^{\prime }\left(x,y;z_a\right)}{\mid g^{\prime }\left(x,y;z_a\right)\mid }$$ 7

The first iteration of the loop is completed, and g₁(x, y; z_a) serves as the input for the next loop. This process continues until the phase distribution in the fractional domain converges to the optimal distribution, culminating in the 3D meta-hologram. The final phase distribution T can be obtained by the following equation:

$$T\left(u,v;z_a\right)=∣\left(\sum angle\left(G\left(u,v;z_a\right)\right),2\pi \right)∣$$ 8

where angle(·) represents the extraction of the phase information from the complex amplitude distribution, and Σ represents the sum of phase distributions at different layers. The phase information T of the 3D meta-hologram is recorded as a matrix (n × n), where each element corresponds to a unit cell of the metasurface. The 3D meta-hologram is encoded onto the sub-wavelength structures, meaning the metasurface is designed based on the matrix T.

Fabrication and performance test of the micro-projector

In the proposed meta-holography-based zoom micro-projector, a laser with the wavelength of 671 nm is used as the reconstruction light to irradiate the metasurface, as shown in Fig. 3a. The distance between the metasurface and the liquid lens is 2 mm. A 3D object composed of the letters ‘N’ and ‘U’ is selected as the recorded object for the metasurface, with both letters having resolutions of 2500 × 2500. The proposed 3D Fourier meta-hologram generation method is employed to generate the 3D meta-hologram, with a normalized focal length f set to 10 cm. The transformation orders for the letters ‘N’ and ‘U’ are set to 1 and 0.2, respectively. The pixel pitch is 0.35 µm, and the period (p), height (H), length (L), and width (W) of the nanorod are 350 nm, 600 nm, 160 nm, and 80 nm, respectively. The scanning electron microscopy (SEM) images of the fabricated sample are shown in Fig. 3c, d.

Fig. 3. Fabrication and performance characterization of the micro-projector.

a Schematic diagram of the optical path of the meta-holographic micro-projection. b Key components of the micro-projector. c Side view and d top view of the metasurface (scalebar: 500 nm). e Optical power range of the liquid lens. f Rise response time and g fall response time of the liquid lens. h–k Simulation results of the 3D meta-holography with different transformation order a.

The liquid lens is positioned directly behind the metasurface for zoom projection, operating with an effective optical aperture of 2 mm and a mechanical diameter of 5 mm (Supplementary material S3). During assembly, the metasurface and the liquid lens are integrated into a cavity to form a projector, with overall dimension of 6.5 mm × 6.5 mm × 4.2 mm, as shown in Fig. 3b. A highly integrated liquid lens controller was developed to drive the liquid lens. Experimental results show that when voltages of 0 V to 70 V are applied to the liquid lens, the corresponding optical power changes from −47.54 m⁻¹ to 18.31 m⁻¹, as shown in Fig. 3e. The testing results demonstrate that the liquid lens exhibits fast response characteristics. When voltage is applied, the optical power can be tuned within ~68 ms, as shown in Fig. 3f, g, indicating the micro-projector can dynamically adjust the projection distance in a swift manner.

Holographic reconstruction

During the generation of 3D meta-hologram, the depth difference is determined by the transformation order a (Supplementary material S4). To validate the effectiveness of the 3D Fourier meta-hologram generation method, a 3D object composed of a ‘bird’ and ‘tree’ was recorded, with a resolution of 2500 × 2500. During the meta-hologram generation, the normalized focal length f was set to 10 cm, and the transformation order for the ‘bird’ (a₁) was set to 1. To explore the impact of transformation order a on the depth difference, the transformation order for the ‘tree’ (a₂) was varied from 0.2 to 1.8, then the depth differences varied accordingly. The 3D reconstruction results with varied depth differences are simulated in Fig. 3h–k, in which the focused ‘bird’ and ‘tree’ are shown in blue and orange boxes, respectively. As a₂ gradually approaches a₁, the blurriness of the ‘tree’ decreases when the ‘bird’ is in focus. Conversely, the ‘bird’ becomes less blurred when the ‘tree’ is in focus. Therefore, for a 3D object, the depth differences between layers can be represented using different transformation orders.

To validate the 3D projection capability of the proposed meta-holography-based zoom micro-projector, the liquid lens was replaced by a solid lens in the experimental system. By moving the receiving screen forward and backward, the letters ‘U’ and ‘N’ were imaged at distances of 0.5 cm and 10 cm, respectively, consistent with the theoretical depth. As shown in Fig. 4a, when the letter ‘N’ is in focus, the letter ‘U’ appears blurred, and vice versa.

Fig. 4. 3D zoom reconstruction results of the micro-projector.

a Results of the 3D meta-holography with a solid lens. b Zoom projection results of the letter ‘N’. (The blue boxes and the green boxes represent the results of voltage off and voltage on, respectively. The detailed information is enlarged in yellow boxes.) c, d 3D zoom projection results when the focal length of the liquid lens is changed from 15 cm to 7 cm.

To evaluate the zoom projection performance of the proposed micro-projector, we compared the imaging results of the letter ‘N’ at different depth planes with and without driving voltage applied to the liquid lens, as shown in Fig. 4b. The first row shows the results without the driving voltage, where the letter ‘N’ appears blurred at depths of 6 cm, 12 cm, 18 cm and 30 cm. The second row presents the results with varied driving voltages applied, demonstrating that the letter ‘N’ can be clearly projected at different depths by adjusting the driving voltage of the liquid lens (Supplementary Video 1). The size of the projected image is positively correlated with the distance of the meta-holographic projection. The micro-projection’s throw ratio (TR), defined as the ratio of projection distance to image size, and the zoom ratio (ZR), defined as the size ratio of the projected image to the metasurface, are analyzed in Supplementary material S5. In addition, based on the parameters of our designed meta-hologram, the field of view of the meta-holographic micro-projection can reach 87.5° at the wavelength of 671 nm, which has obvious advantages in the existing 3D holographic technology.

Furthermore, the 3D zoom projection was also validated, as shown in Fig. 4c, d. When the focal length of the liquid lens is 15 cm, the letter ‘N’ is projected at a depth plane of 15 cm, while the letter ‘U’ is focused at a depth plane of 0.7 cm. As the focal length of the liquid lens decreases, the projection distance becomes closer. As shown in Fig. 4d, when the focal length of the liquid lens is 7 cm, the letter ‘N’ is projected at a depth plane of 7 cm, while the letter ‘U’ is focused at a depth plane of 0.3 cm. Results demonstrate that the proposed meta-holographic micro-projector can readily achieve 3D zoom projection by leveraging the zoom characteristics of liquid lens.

Most reported meta-holographic systems focus on light modulation and multi-channel holographic display based on the polarization selectivity of the metasurface. However, these systems typically achieve 3D reconstruction at a fixed depth, and the inclusion of additional lenses and other optical components complicates integration. An ideal meta-holographic 3D projection requires a continuous zoom function to access a broader depth range. Our proposed micro-projection method addresses this need by enabling continuous 3D zoom projection with a specially designed liquid lens. Additionally, the proposed 3D Fourier transform algorithm based on metasurface can achieve 3D projection without sacrificing other information like polarization and wavelength, allowing the proposed system to realize multi-functional display based on the polarization properties of the metasurface in the future. Table 1 compares the characteristics of the proposed micro-projector with those of state-of-the-art reported methods. The projected image of the liquid lens at its minimum focal length in the experiment was tested, and the minimum image size is 0.3 cm, and the 3D depth is 6.7 cm. Since the image size and depth are positively correlated with the focal length of the liquid lens, increasing the focal length will correspondingly enlarge the image size and 3D depth. In principle, the focal length of the liquid lens can reach infinity, but the projection energy will decrease accordingly when the focal length is too large. In the future, we will continue to explore the relationship between projection energy and distance.

Table 1.

Comparison of characteristics between the proposed micro-projector and the state-of-the-art methods

Devices	Meta-hologram resolution	Zoom function	System volume	Image size	3D depth
This work	2500 × 2500	Yes	~ 0.18 cm3	≥5 cm	≥6.7 cm
Yin et al.26	1200 × 1200	No	> 1 cm3	<0.1 cm	0.02 cm
Huang et al.27	800 × 800	No	> 1 cm3	0.135 cm	0.015 cm
Ren et al.39	2000 × 2000	No	> 1 cm3	0.7 cm	1 cm
Choi et al.48	No hologram	No	> 1 cm3	Large	4.7 m
Kim et al.49	No hologram	No	> 1 cm3	Large	8.4 cm

Discussion

Compared with the existing methods, our proposed method has a different principle and calculation strategies, underscoring the novelty and advantages of our approach. Firstly, the proposed method is based on the FRFT, which is suitable for far-field diffraction, making it ideal for large-depth 3D meta-holography. Secondly, the depth information of 3D object is represented by using transformation orders, breaking through the limiation of the lens’s focal length, and the depth and position of the reconstructed images are related to a and f, which can be flexibly adjusted. Thirdly, in the design of metasurface, only phase information is considered, while polarization information and intensity information are not involved. The designed metasurface has a broadband response, which enables us to verify its holographic reconstruction effect at multiple wavelengths. Consequently, a single metasurface structure can produce reconstructed images at various wavelengths, with the reconstruction depth depending on the focal length of the liquid lens and the image size being related to both the focal length and the wavelength (Supplementary material S5). As shown in Fig. 5a, because both metasurface and liquid lens have the characteristics of wide-band response, when using the proposed meta-holographic micro-projection method with a liquid lens, the depth and size of the reconstructed red, green, and blue images can be adjusted by varying the driving voltage of the liquid lens. Therefore, the proposed micro-projector can be applied to multi-wavelength zoom projection, but it can not eliminate all chromatic aberrations at the same time. In future, we can explore the use of multiple liquid lenses to realize achromatic meta-holography. It is foreseeable that achromatic multi-wavelength 3D projection with adjustable size and position could be realized using fingertip-sized equipment (Supplementary material S6).

Fig. 5. Application verification of the proposed micro-projector.

a Multi-wavelength results of reconstructed images under different wavelengths. b Demonstration of the high-security encryption with three layers of object.

Micro-projector for high-security encryption and large-capacity data storage

Metasurfaces, with their multidimensional control capabilities, offer unique advantages in optical encryption and data storage. By leveraging the flexible zoom feature of the designed liquid lens, the proposed meta-holography-based zoom micro-projector can conveniently control the size and depth of projection images, providing a novel approach for high-security encryption, as shown in Fig. 5b. Three messages ‘Help me’, ‘Thank you’, and ‘Stop’ for different users are encrypted into unique patterns ‘moon’, ‘heart’, and ‘thunder’ located at different depths with parameters a₁, a₂, and a₃, respectively. A 3D meta-hologram with 2500 × 2500 resolution and 0.35 μm pixel pitch is generated by using the proposed 3D Fourier meta-hologram generation method, serving as encryption key I. Simultaneously, the corresponding driving voltages required to project each pattern clearly onto a screen at a fixed distance are pre-calculated based on their depth information, forming encryption Key II. As shown in Fig. 5b, when applying different driving voltages U_n (decrypting from a_n) to the liquid lens, the corresponding patterns are clearly projected onto the receiving screen, with the patterns of other channels being blurred. Consequently, the three messages are successfully decrypted as ‘Help me’, ‘Thank you’, and ‘Stop’, respectively. Notably, successful decryption requires precise alignment of the screen distance, the meta-hologram structure, and the liquid lens driving voltage (Supplementary material S7). Compared to single encryption process, this way significantly enhances information security. Additionally, with a volume of just 0.18 cm³, our proposed micro-projector offers greater portability and concealment for encryption.

Moreover, the meta-hologram is generated by the proposed 3D Fourier transform meta-hologram generation method, which effectively enables multi-plane 3D projection by adjusting the transformation order a. By setting different transformation orders, information can be stored across multiple plane channels, highlighting the potential of our micro-projector for large-capacity data storage. In the future, integrating polarization-controlled or wavelength-controlled structures could further expand data storage capacity.

In our manuscript, we are committed to achieving large image size, large reconstruction depth, and zoom functionality within an ultra-compact, fully integrated 3D projection system. However, the realization of high-quality meta-holography is very important (Supplementary material S2). In future work, we will further focus on refining the 3D Fourier meta-hologram generation method with the goal of realizing crosstalk-free reconstruction for continuous 3D objects. Furthermore, dynamic meta-holographic projection with addressable metasurface has broad application prospects in fields such as AR/VR display, optical sensing, and encryption. In recent years, different design methods for the realization of 2D dynamic meta-holography have been proposed, including electronic control modulation, electrochemical reaction, spatial multiplexing, and temporal multiplexing technologies^45–47. While these advances represent significant progress, they remain limited to dynamic switching between pre-set 2D patterns. In addition, due to the huge amount of information of 3D objects, the technology of real-time refreshing metasurface has not been broken through yet. Dynamic 3D meta-holography has not been realized so far. Although a dynamic metasurface is not the focus of the current work, we elucidate our vision to combine the proposed zoom micro-projector with future developments in dynamically tunable metasurfaces, which could open new possibilities for real-time 3D holography in applications such as AR/VR, encryption, and data storage.

In conclusion, we propose and demonstrate a meta-holography-based zoom micro-projector capable of achieving 3D zoom projection with large size and distance. The micro-projector, primarily composed of a metasurface with a high resolution of 2500 × 2500 and a specially designed liquid lens with an effective optical aperture of 2 mm, has an overall system the volume of only 0.18 cm³. Additionally, the proposed 3D Fourier meta-hologram generation method overcomes the limitation of existing Fourier meta-holograms that restrict the reconstructed image to the focal plane of the lens, thus enabling 3D projection. Such a high-resolution, large projection depth range, and flexible zoom micro-projector opens up broad applications for meta-holography, including high-security encryption, AR/VR, wearable and biomedical devices, automotive displays, and so on.

Methods

Metasurface fabrication

The sample fabrication is performed using silicon on sapphire (SOS) material through a conventional electron beam lithography technique. Firstly, the substrate is cleaned with sequential immersion in N-Methyl-2-pyrrolidone, isopropyl alcohol, and deionized water. Next, a 100 nm-thick layer of hydrogen silsesquioxane is deposited onto the substrate via spin-coating, and subsequently baked on a hot plate at 100 °C for one minute. To enhance the conductivity and reduce electron accumulation, a conductive adhesive layer (AR-PC 5092) is applied by spin-coating. The nano-brick structures are then patterned by electron beam exposure. Following exposure, the development process involves immersing the samples in a 2.38% NMD-3 solution for two minutes. Finally, the structures are transferred from the resist to the SOS material using inductively coupled plasma etching.

Author contributions

D.W., R.N.J., B.H.J., and Q.H.W. conceived the project. D.W., Q.H., C.L. and R.N.J. proposed the idea, performed the simulations and conducted the experiments; C.L., Y.Z. and Y.C.L. designed and fabricated the liquid lens; X.R.Z., X.X., K.S. and S.W.W. fabricated the metasurface and analyzed the data; Y.L.L., F.C.L., Y.W.Z., and W.L. assisted in the verification experiments of meta-holography and manuscript revision. All authors discussed the results and commented on the paper.

Peer review

Peer review information

Nature Communications thanks Raj Kumar and Haoran Ren for their contribution to the peer review of this work. A peer review file is available.

Data availability

All key data that support the findings of this study are included in the article and its Supplementary Information. Additional datasets and raw measurements are available from the corresponding authors upon reasonable request.

Code availability

All relevant codes that support the findings of this work are available. And all have been uploaded to Codeocean. https://codeocean.com/capsule/7486060/tree.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-025-65764-2.