1. Introduction

Driven by the revolutionary advancements in digital technologies permeating various aspects of daily life, archive management, and military applications, among other applications, the security and capacity of information storage in digital devices have garnered increasing attention. Given the multiple degrees of freedom that light exhibits in terms of polarization, phase, and amplitude, optical information storage and encryption strategies with specific decryption methods and high capacity have emerged as prevailing solutions [1–3]. However, considering the stringent demands for both security and capacity in information storage devices, traditional optical approaches based on conventional interference and diffraction techniques require large working dimensions [4], which hinders the development of a new generation of miniaturized and compact devices. Moreover, the encoded information is vulnerable to duplication owing to the well-established design and manufacturing processes associated with traditional optical approaches. The emergence and rapid development of planar metasurfaces with subwavelength features in the recent decade have revolutionized the efficient manipulation of light fields [5–7]. Owing to the fascinating and unique optical properties of metasurfaces, they have been applied in diverse applications, including nanoprinting [8], holography [9], light modulation [10], and metalens technologies [11]. Notably, metasurface-enabled nanoprinting and holography can achieve high-accuracy nanoscale image restoration and multiplexing for information storage, encryption, and identification [12–14]. Moreover, the encoded information is robust against duplication in the case of elaborately designed nanostructures, which is crucial for the development of new-generation high-security integrated optical storage devices. However, nanoscale metasurfaces are typically limited to static imaging once they have been designed and manufactured, depending on the geometrical parameters and surrounding media [15–17]. In practical digital applications, it is crucial to design dynamic tunable devices, with functionalities ranging from information concealment to information transmission, aligning with the increasing emphasis on high-capacity storage and information security.

Various tunable mechanisms based on multiplexing strategies, e.g., wavelength multiplexing [18], polarization multiplexing [19], and orbital angular momentum multiplexing [20], have been explored to overcome static limitations. Notably, angular incidence multiplexing has demonstrated significant potential in generating independently controlled multichannel nanoprinting and holographic images, thereby enhancing information security and capacity [21]. For example, Tang et al. proposed an angular multiplexing metasurface encoding two nanoprinting images at different angular incidences to achieve optical information concealment [22]. However, achieving control through the incident angle typically requires separate optical components that are bulky compared with the working dimensions of the complete encryption system. To satisfy the increasing demand for the ultra-compactness and miniaturization of optical encryption platforms, numerous researchers have attempted to develop small and integrated meta-systems with dynamic tunable characteristics, for example, by cascading and coupling multiple layers [23] or incorporating phase change materials [24,25].

In response to this need, this paper proposes a novel optical information encryption method based on LC-infiltrated dual-layered metasurfaces as pixels to generate thermal tuning dual-channel nanoprinting images with enhanced security. By cascading a thermally switchable polarization-dependent beam-steering metasurface with another angular multiplexing nanoprinting metasurface, the proposed optical system avoids the need for additional optical elements to control incident angles. Moreover, this design combines the multiplexing of thermal and angular responses, enabling the realization of metasurface dynamic tunability with an ultracompact structure. In the case of the beam-steering metasurface, the LC molecule transitions from anisotropic to isotropic states when the LC cell is heated, thus switching the incident light from 0° to 10.9°. Furthermore, the angular multiplexing nanoprinting metasurface can be designed by exploring and screening the geometric dimensions of the unit cell to generate independently controlled dual-channel images at two illumination angles of 0° and 11°. Ultimately, the cascaded dual-layered metasurface can actively modulate nanoprinting images through the incorporation of a thermal-driven LC tuning scheme, which can serve as an effective thermal tuning optical encryption platform.

2. Structure and method

Figure 1 presents the conceptual schematic of the proposed LC tunable dual-layered metasurfaces with thermal multiplexing optical encryption functionality. The objective of this design is to generate independently encoded nanoprinting images under different temperature environments by leveraging the thermal sensitivity of LCs. The proposed meta-system is engineered to have a dual-layered structural arrangement with an all-dielectric metasurface. It consists of a dynamic polarization-dependent beam-steering metasurface (metasurface 1) vertically coupled with an angle-selected nanoprinting metasurface (metasurface 2). The two metasurfaces are separated by a 1-µm-thick SiO₂ (n₁ = 1.46) spacer layer, which enables complete switching between independently encrypted pre-designed nanoprinting images of “MKKL” at a temperature of 150 °C and “M = U” at a temperature of 25 °C. When the obtained secret key contains the correct values of the surrounding temperature T and wavelength of incident light λ, both nanoprinting images can be independently exhibited. The ciphertext “USST” can be decrypted from the two exhibition images using the Caesar cipher decryption method, as explained in detail in Note S1 of Supplement 1.

 

Fig. 1. Schematic of optical information storage meta-system based on LC tunable dual-layered metasurfaces implemented with thermal-driven LC platform. By heating the surroundings of the LC cell, its orientation induces beam-deflection switching between 0° and 10.9°, generating two independently controlled nanoprinting images (“MKKL” and “M = U”). The ciphertext “USST” must be fully unlocked using the specific secret key.

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To achieve the functionalities of metasurfaces 1 and 2, all-dielectric metasurfaces with distinct arrangements are established. For metasurface 1 with beam-steering functionality, a twelve-element gradient supercell composed of Si nanoblocks infiltrated with high-birefringence LCs is used. Specifically, phase modulation is exclusively introduced in metasurface resonance while controlling the surrounding refractive index through LCs, which is essential for promoting phase accumulation. Hence, high-birefringence LCs (NC-M-LC101-146) with an extraordinary refractive index n_e = 1.9 and ordinary refractive index n_o = 1.5 at a temperature of 20 °C are selected to achieve a complete 2π phase change. Given their clearing point of 150 °C, the LCs infiltrating metasurface 1 can be thermally tuned through applied heating to 150 °C. Notably, the metasurface is able to maintain its functionality even at high temperatures due to the exceptional thermal stability of Si and SiO₂. For metasurface 2 with angular multiplexing functionality, four silicon blocks with appropriately screened geometric dimensions are designed. The distinct amplitude responses of each array resulting from different excitations of the resonant electric field under two different angular incidences enable the generation of nanoprinting images. Moreover, for the fabrication process of the proposed metasurfaces 1 and 2, both are mainly fabricated by electron-beam lithography (EBL), atomic layer deposition (ALD), and ion beam etching (IBE). (see Note 2 in Supplement 1). The SD1 (Dainippon Ink and Chemicals Inc., Chiba, Japan) is served as an alignment layer to precisely guide the orientation of LC molecules [26].

3. Design and discussion

3.1 Dynamic polarization-dependent beam steering

Figure 2 shows the architectural schematic and optical characterization simulations of the proposed all-dielectric gradient LC-infiltrated metasurface, enabling dynamic polarization-dependent beam steering. The dynamic switching of beam deflection is achieved by controlling the environmental temperature to manipulate the interaction between the oriented LC and metasurface composed of Si nanoblocks, as the orientation of LC molecules changes with the ambient temperature. At room temperature (approximately 25 °C), the LCs exhibit anisotropy and are pre-aligned through a multi-step partly overlapping exposure process (Fig. 2(a) and more details can see Note 3 in the Supporting Information). As the temperature rises to the clearing point of the LC (150 °C), the LC arrangement transitions toward isotropy (Fig. 2(b)). In addition, the thermal response of LCs on a millisecond time scale exhibits remarkable reversibility [27], enabling the repeated switching of LC states by cycling the temperature between 25 °C and 150 °C. When x-polarized light is normally incident onto the beam-steering metasurface at two different ambient temperatures, it either transmits the incident light straight (25 °C) or switches the beam diffraction from the zeroth to the first diffraction order (150 °C). Thus, dynamic beam steering, which involves the switching of beam propagation in different directions, can be achieved by modulating ambient temperatures. In addition, the dynamic beam-steering functionality of the proposed metasurface was enabled by controlling the phase of the incident light via thermal response of LCs on a millisecond time scale.

 

Fig. 2. Schematics of dynamic polarization-dependent beam steering metasurfaces infiltrated with LCs in the (a) anisotropic state and (b) isotropic state. (c) Schematic of the proposed beam-steering metasurface with a single period, composed of twelve rectangular silicon blocks, and illustration of a unit cell of the silicon block with height H = 400 nm and period P_x = P_y = 190 nm. (d) Color maps of phase shift (φ_x) and transmission (T_x) for x-polarized incident light at 633 nm as a function of the length of the silicon block meta-unit along both x (L_x)- and y (L_y)-axes.

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To define the thermal-tuning characteristics of LC-infiltrated metasurface, we first design and simulate the unit cell of a beam-steering metasurface pattern consisting of a subwavelength rectangular silicon block on a SiO₂ substrate. As shown in Fig. 2(c), the beam-steering metasurface of a supercell (12P_x = 2280 nm and P_y = 190 nm) consists of twelve rectangular silicon blocks operating on the Huygens principle [28]. Each rectangular silicon block has a fixed height of H = 400 nm, with a distance of 190 nm between them. In particular, the proposed beam-steering metasurface is expected to achieve a high transmission capacity and complete 2π phase control relative to the orientation of LC molecules in a cell configuration with thickness h_LC= 500 nm. Therefore, as shown in Fig. 2(d), the phase shift and transmission of the transmitted light associated with the x-polarized incident light at 633 nm are calculated as functions of the length of the silicon block along both the x (L_x)- and y (L_y)-axes, ranging from 40 to 160 nm, in the beam-deflection state (the ambient environment is heated to 150 °C, and the LC is in an isotropic state, with a refractive index of 1.7). By meticulously screening the geometry dimensions (L_x × L_y) of each silicon block, including values of 60 nm × 60 nm, 75 nm × 75 nm, 85 nm × 85 nm, 94 nm × 94 nm, 100 nm × 100 nm, 108 nm × 108 nm, 115 nm × 115 nm, 124 nm × 124 nm, 131 nm × 131 nm, 145 nm × 145 nm, 151 nm × 151 nm, and 160 nm × 160 nm, we engineer an LC-infiltrated beam-steering metasurface with continuous 2π phase modulation and high transmission, achieving optimal conversion efficiency. Consequently, the phase of this metasurface can be varied to form a binary blazed grating with twelve phase steps.

In contrast, for the anisotropic state of the LC at room temperature (approximately 25 °C), our design rationale is focused on achieving high-power vertical transmission of incident light in the zeroth diffraction order. When the LC director is rotated from a vertical plane (along the direction of light propagation) to a parallel plane (along the direction of incident light polarization), the phase shift of the metasurface can be modulated through spectral tuning of the meta-unit resonance via an LC-enabled variable surrounding refractive index. At the wavelength of 633 nm, the phase shift and transmission change with variations in the surrounding refractive index (from 1.50 to 1.90) controlled by the LC rotation angle, as shown in Fig. 3(a). By manipulating the surrounding refractive index of each silicon block within a supercell, by pre-orienting the LC director, uniform phase shifts can be achieved across all silicon blocks. The geometric dimensions, surrounding refractive index, and phase shift values of the silicon blocks at both room temperature (25 °C) and clearing point of the LC (150 °C), are shown in Fig. 3(b). According to the periodic diffraction equation, the beam-steering angle θ at diffraction order m of ±1 can be expressed as

 

Fig. 3. (a) Color maps of phase shift (φ_x) and transmission (T_x) for x-polarized incident light at 633 nm as a function of the surrounding refractive index. (b) Geometric dimensions of silicon blocks that constitute a twelve-element gradient supercell of the beam-steering metasurface infiltrated LC with a corresponding surrounding refractive index. Simulated electric field of transmission in xz-plane for the (c) vertical and (d) deflecting transmission states at a working wavelength of 633 nm. The purple arrows show the beam outgoing directions. Polar plots of normalized intensity distributions of transmitted light through the supercell for the (e) vertical and (f) deflecting transmission states.

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The transmittance spectra for various diffraction orders at the wavelength of 633 nm are numerically simulated to demonstrate the switching performance of the proposed dynamic beam-steering metasurface under ambient temperatures of T = 25 °C and 150 °C, as depicted in Fig. 4. Panels a (150 °C) and b (25 °C) in Fig. 4 present numerical values of the light intensity directed into the zeroth (blue dot), first (purple dot), second (yellow dot), and higher (green dot) diffraction orders, along with the total power of the transmitted light (red dot). The transmitted light power is mainly distributed in the first diffraction order, accounting for 95.78% in the isotropic state at 150 °C, whereas 69.13% of the power is directed into the zeroth diffraction order in the anisotropic state at 25 °C. Figure 4(c) shows the intensity redistribution between various diffraction orders by manipulating the ambient temperature of the LC cell (with normalization based on the highest intensity among all orders). Under applied heating, the intensities of the zeroth and first diffraction orders decrease and increase respectively, thereby realizing power conversion among various diffraction orders.

 

Fig. 4. Transmittance spectra for different diffraction orders, numerically simulated in (a) isotropic and (b) anisotropic states of the LC cell. (c) Intensity redistribution among various diffraction orders for different temperature of the LC cell.

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The conversion efficiency of the proposed dynamic beam-steering metasurface is quantitatively assessed using the figure of merit (FOM) [29]. Temperatures of T = 25 °C and 150 °C correspond to the OFF and ON states of the metasurface, respectively. The contrast between the intensities of the zeroth and first diffraction orders in these states can be mathematically expressed as

An FOM value of 1 indicates complete switching from the zeroth to the first diffraction order, whereas a value of -1 indicates complete switching from the first to the zeroth order. The FOM value tends toward zero if minor power conversion occurs between the diffraction orders. Therefore, the FOM reflects the beam-deflection switching efficiency. For the proposed dynamic beam-steering metasurface, the FOM is 0.88, as determined by the numerical simulations results shown in Fig. 4, indicating its excellent switching efficiency.

3.2 Angular multiplexing nanoprinting

To illustrate the angular multiplexing functionality using another side of the dual-layered metasurface, Fig. 5 presents the architectural schematic of the angular multiplexing metasurface and its optical characterization simulation results at two incident angles. As shown in Fig. 5(a), the designed metasurface is composed of sub-diffractive rectangular silicon-block (110 nm thick) arrays and a SiO₂ substrate. By arranging Si nanoblocks in patterns with a screened length w (equal along the x- and y-axes) and screened spacing d, each array exhibits an angle-dependent transmission response wherein the light of 633 nm illuminated at θ₁ and θ₂ undergoes different amplitudes (|A₁| and |A₂|, respectively) as it traverses through the metasurface.

 

Fig. 5. (a) Schematic of the angular multiplexing metasurface composed of an Si nanoblock array structure with a single building-block structure. The rectangular silicon block has a thickness t = 110 nm. Two independently controlled optical responses are excited for angular incidences of θ₁ and θ₂, and each Si nanoblock produces transmitted light with different amplitudes. Color maps of transmission for the illumination angles (b) θ₁ = 0° and (c) θ₂ = 11° at a wavelength of 633 nm as a function of length w and distance d of silicon blocks, both ranging from 100 to 400 nm. (d) Pattern design of each angular multiplexing Si nanoblock and corresponding simulated transmission of the states “00”, “10”, “01”, and “11” at incident angles of 0° (blue line) and 11° (yellow line).

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Therefore, to select specific silicon blocks with the required amplitudes in the patterning space, the transmission of incident light at a wavelength of 633 nm at two illumination angles θ₁ = 0° (Fig. 5(b)) and θ₂ = 11° (Fig. 5(c)) are calculated as functions of the length w and distance d of silicon blocks, both ranging from 100 to 400 nm. Binary digital values “0” (low amplitude with light blocking) and “1” (high amplitude with light transmission) represent the two states of amplitude contrast at a specific wavelength and illumination angle. By encoding the amplitude distribution (“0” or “1”) into each individual unit-cell space of the metasurface, the required binary nanoprinting image can be generated. Using an angular multiplexing metasurface with two distinct angular incidences, we establish two independent encryption channels capable of switching nanoprinting images through the adjustment of illumination angles from θ₁ to θ₂. Then, the geometry dimensions of the angular multiplexing metasurface can be systematically determined by referring to the color map (Figs. 5(b) and 5(c)), representing four optical responses at θ₁ and θ₂. These optical responses correspond to the encoded states of “00”, “10”, “01”, and “11”, pertaining to the geometric shapes of the square, regular pentagon, triangle, and circle shown in Figs. 5(b) and 5(c), respectively. Concretely, the length w of four types of unit cells is respectively 275 nm (states “00”), 345 nm (states “01”), 365 nm (states “10”), and 275 nm (states “11”), while the distance d of four types of unit cells is respectively 215 nm (states “00”), 230 nm (states “01”), 160 nm (states “10”), and 100 nm (states “11”). To illustrate the angle-dependent characteristics, simulated transmission spectra for two illumination angles θ₁ = 0° (blue line) and θ₂ = 11° (yellow line) are presented in Fig. 5(d), corresponding to the states of “00”, “10”, “01”, and “11”. At the wavelength of 633 nm (red pachytene), the transmission intensity of resonant spectra between 0° and 11° exhibits two amplitude contrast states: approaching 1 (high contrast), corresponding to states of “10” and “01”; and approaching 0 (low contrast), corresponding to states of “00” and “11”. Ultimately, any two black-and-white nanoprinting images can be simultaneously encoded into the two angular channels using the pixel combinations of “00”, “10”, “01”, and “11”.

To elucidate the mechanism underlying angle-dependent control and angular spectral variations in the proposed metasurface under incident angles of 0° and 11°, the resonant electric-field intensity distributions at x-y plane for each design state (“00”, “10”, “01”, and “11”) are compared, as shown in Fig. 6. The resonance excitation of each silicon-block unit cell for the proposed all-dielectric metasurface operates within a distinct nanowaveguide or a Mie scatterer. The electric field distributions of states “10” and “01” at an incidence angle of 11° are significantly different from those at 0°, as shown in Figs. 6(c) and 6(d). At normal incidence, only a symmetric electric field distribution is excited, whereas both symmetric and antisymmetric electric field distributions are excited under oblique illumination. This phenomenon plays a crucial role in disrupting the angular correlation of optical responses in the proposed metasurface. Based on the angular-induced distinct resonant modes and electric field distribution, the angular multiplexing functionality can be realized to generate a switchable nanoprinting image.

 

Fig. 6. Designed unit cells for states (a) “00”, (b) “10”, (c) “01”, and (d) “11”. (e)–(h) Electric field intensity contours at the x-y plane of the four unit cells at incident angles of 0° and 11°.

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3.3 Applications of optical information storage

To demonstrate the capability of the proposed LC tunable dual-layered metasurfaces for optical information storage and encryption, Fig. 7 illustrates the design process of a novel optical encryption transmission strategy with enhanced security and dual-channel functionality, achieved through binary encoding of optical information. The two encoded printing images, representing the English letter characters “M” and “U” in pixel types, can be alternatively projected under incidence angles of 0 and 11°, respectively. Figure 7(a) shows the arrangement pattern of the metasurface with the simulated color models (right panel), determined by combining the requirements from “M” (left panel) and “U” (middle panel). The proposed dual-layered metasurface-based optical encryption platform can switch between the two nanoprinting images by manipulating the surrounding temperature of the platform, as illustrated in Fig. 7(b).

 

Fig. 7. Demonstration of optical information encryption with different surrounding temperatures at normal incidence. (a) Binary design of pixel types for switchable images. The left and middle panels show the two switchable images, while the bottom panel shows the arrangement pattern of the metasurface using color models. (b) Schematic of the proposed optical encryption platform. The two distinct nanoprinting images are generated at temperatures of 25 °C and 150 °C. (c)–(h) Ciphertext “USST” is transmitted safely through the enhanced optical encryption platform.

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Furthermore, a novel optical encryption transmission strategy with high security is demonstrated based on the proposed LC tunable dual-layered metasurfaces, as shown in right panel of Fig. 7. Using the Caesar cipher, images of “MKKL” and “M = U” are reverse-designed based on the ciphertext “USST” obtained from a transmitter, corresponding to channels 1 and 2, respectively (Fig. 7(c)). In this encryption scheme, the proposed design scheme of unit cells for states “00”, “10”, “01”, and “11” is used to fabricate a dual-channels metasurface with two independently controlled pre-designed nanoprinting images, serving as a transmitted optical encryption platform. The ciphertext “USST” sent from the transmitter can only be decrypted by the correct receiver possessing three essential components: the proposed dual-layered metasurface-based optical encryption platform (Fig. 7(d)), one Caesar cipher decoder (Fig. 7 (g)), and the key with accurate values of the surrounding temperature T and incident light wavelength λ (Fig. 7(e)). Upon receiving the unique key, the correct nanoprinting images of “MKKL” and “M = U” at temperatures of 150 °C and 25 °C can be generated under x-polarized illumination (λ = 633 nm). Eventually, the intended recipient can securely retrieve the confidential plaintext “USST” (Fig. 7(f)) by employing the Caesar cipher table, as shown in Fig. 7 (g). In addition, Fig. 7 (h) demonstrates the security of the proposed optical encryption platform. The encrypted plaintext can be accurately obtained by the authorized receiver using the designated key, while unauthorized individuals obtain an incorrect plaintext through erroneous means. Thus, the proposed platform can achieve high-security encryption and retrieval through the use of multichannel metasurfaces, underscoring its potential for practical applications.

4. Conclusions

We realize thermally switchable nanoprinting functionality based on LC-incorporated dual-layered metasurfaces. By vertically cascading an LC-infiltrated beam-steering metasurface with another angular multiplexing nanoprinting metasurface separated by a SiO₂ spacer layer, the dynamic tuning characteristics of thermal multiplexing and angular multiplexing can be combined in an ultracompact and lightweight meta-system without additional optical elements. Specifically, for the LC-infiltrated metasurface with beam-steering functionality, we successfully manipulate the wave vector of transmitted light, enabling it to propagate either straight or at a deflection angle of 10.9° by controlling the surrounding temperature of the metasurface. Additionally, two angular-switchable nanoprinting images can be generated based on the states “00”, “01”, “10”, and “11” for the angular multiplexing nanoprinting metasurface, introducing a new degree-of-freedom in angular-dependent control at 0° and 11°. Furthermore, by incorporating the LC cell and leveraging its dynamic tunable characteristics for amplitude modulation, we generate dynamic nanoprinting images through heating to achieve encrypted transmission of optical information. Overall, this work provides valuable insights for a universal design that can be extended to enable enhanced integration and degrees of freedom while preparing dynamically tunable metasurface with multiplexing strategies, with potential applications in new-generation optical encryption platforms, identification system, and multi-functional optical devices.