Introduction

The terahertz spectral region serves as a bridge between electronics and optics, and it is anticipated to play a crucial role in advancing wireless communication and imaging system1. It is envisioned that sixth-generation (6G) wireless network will utilize terahertz wave to provide faster and more reliable communication2. This frequency band, ranging from 0.1 to 10.0 THz, is situated between millimeter wave and infrared light. While electromagnetic waves in infrared and microwave bands have been widely used, the terahertz band has been relatively little explored and researched, and thus the related materials and devices are not yet mature3. The main reasons for the lack of progress in this area are that there are very few available materials in nature to directly respond for terahertz wave. Besides, these devices are bulky making them expensive to build, which substantially limits the application of terahertz wave. In order to overcome the above limitations, research teams gradually turn their attention to artificially fabricated structures.

The emergence of metamaterials as a kind of artificial structures has played an important role in boosting the field of electromagnetic materials over the past few years4,5. However, metamaterials have some drawbacks such as large size and complexity of fabrication restricting its development to some degree6. Metasurfaces, as two-dimensional forms of metamaterials, effectively overcome shortcomings that exist in metamaterials. In addition, they display a remarkable ability to manipulate wave amplitude, phase, and polarization at a local level in terahertz band7. In general, the ultrathin nature and simple fabrication processes make metasurfaces easy to integrate into devices. In recent years, integrating multiple optical controls into a single metasurface for optical integration and multifunctionality has become a novel and ingenious approach particularly in imaging8. This capability can facilitate the development of multifunctional complex devices for compact optical engineering9. In 2018, Yue et al. experimentally verified high-resolution near-field grayscale image based on Malus’s law10. In 2020, Deng et al. proposed and experimentally validated the Malus metasurface, which utilizes a single structure to achieve simultaneous control of phase and intensity of light leading to single near-field and single far-field images11. In 2021, Ren et al. demonstrated that polarization-multiplexed metasurfaces were capable of utilizing three non-orthogonal states to achieve three-channel imaging12. Besides, a compact and effective dual-atom metasurface platform based on the Jones matrix design to effectively realize multichannel near-field and far-field modulations is very popular among scholars in recent years13,14. However, a drawback of their works is that the designed features will be fixed once metasurfaces are completely designed. Such metasurfaces containing fixed phase information hinder a variety of applications15. For example, realization of dynamic modulation of terahertz wave is very essential for the successful operation on terahertz devices16.

Phase change materials (PCMs) provide a truly promising avenue for realizing active metasurfaces and large refractive index contrasts17. In other words, phase transition leads to a change in the local dielectric environment by integrating PCM, which consequently causes a change in optical properties as well. Thus, it is possible to utilize PCMs for reconfigurable metasurfaces. In 2022, Liu et al. demonstrated that a phase-change metasurface platform can actively modulate amplitude and phase to achieve dynamic nanoprinting and holographic image displays18. Nevertheless, the shortcoming of their work is that only one state works and the result disappear after state switching, which reduces the channel capacity to a certain extent. In other areas, PCMs also facilitate the development of dynamic devices in the terahertz frequency range and allow active modulation for electromagnetic properties by thermal, electrical, and optical modulators19. Among a variety of temperature-sensitive materials, VO2 is one of the most favored materials by scholars, and it suffers a dielectric-to-metal phase transition upon electric control and heating20,21. Threshold transition temperature of VO2 from insulator to metal is about 68 °C, which is not only easy to achieve but also suitable for a wide spectral range22,23,24. Moreover, the whole phase transition process is reversible25. Utilizing these advantages of VO2, a significant number of studies have been conducted by researchers in dynamic modulation for electromagnetic wave. In 2019, Liu et al. experimentally proposed a wideband meta-holography technique that enables dynamic holographic imaging with temperature variation26. In 2023, Chen et al. presented directional terahertz holography based on a VO2 reconfigurable Janus metasurface exhibiting four independent electromagnetic functions throughout space27. Consequently, VO2 will play an important role in the dynamic modulation of multichannel imaging in terms of future development.

In this study, a novel single-cell bilayer structure is designed based on VO2 to achieve simultaneous amplitude and phase control. Unlike previous interleaved structures28,29,30,31, the designed metasurface on the basis of this novel structure innovatively combines geometric phase, propagation phase, and Malus’s law to realize six-channel images in terahertz band. To explain our work more clearly, the functional schematic diagram of metasurface is depicted in Fig. 1. VO2 turns into metal as temperature is higher than 68 °C. Due to the presence of an intermediate VO2 film, VO2 patches play an absolute role in electromagnetic modulation without taking into account the crosstalk of gold patches. Two independent holographic images of letters S and P with square are projected into the far field when metasurface is illuminated by left-handed circularly polarized (LCP) wave and right-handed circularly polarized (RCP) wave. Meanwhile, a continuous grayscale image for number 1 appears on the surface of metasurface when the designed metasurface is inserted into an orthogonally linearly polarized (LP) optical path. Similarly, VO2 acts as an insulator and gold patches within the VO2 film are responsible for controlling electromagnetic wave when temperature falls below 68 °C. Far-field holographic letters I and N with circle are generated in the LCP channel and RCP channel. The near field around metasurface produces a grayscale image of number 0 in an orthogonal LP optical path. The designed structure has many advantages such as wide bandwidth and low crosstalk. For one thing, low crosstalk between two VO2 states means that the designed bilayer structure allows the number of independent channels to be doubled within the acceptable range of errors. There is an urgent need for multiplexing technology to improve system capacity and the proposed design provides a new solution in this regard.

Fig. 1
figure 1

Schematic of six-channel imaging in a single-cell bilayer metasurface. When the temperature is above 68 °C, holographic letters S and P are produced in the far field under two CP incidences and a grayscale image 1 is produced on the surface of the metasurface under LP incidence. When the temperature is below 68 °C, the metasurface produces holographic letters I and N in the far field under CP incidences and a grayscale image 0 appears on the surface of metasurface under the incidence of LP wave.

Full size image

Principle and design

Geometric phase is the phase of the additional scattered wave triggered by the rotating scatterer, and it is related to the orientation of meta-atom but independent of the resonance. Thus, geometric-phase metasurfaces have the potential to be extended for broadband applications of CP wave32. The propagation phase is controlled by the dimension of meta-atom, which can break the spin-locking restriction of geometrical phase to accomplish spin-decoupled multifunctional metasurfaces33. Based on the spin-decoupling law, propagation phase Φ and rotation angle θ can be expressed as

$$\:\begin{array}{c}\varPhi\:=\frac{{\phi\:}_{RCP}+{\phi\:}_{LCP}}{2}\end{array}$$
(1)
$$\:\begin{array}{c}\theta\:=\frac{{\phi\:}_{RCP}-{\phi\:}_{LCP}}{4}\end{array}$$
(2)

where φLCP = Φ-2θ and φRCP = Φ + 2θ represent full-wave phase distributions under LCP and RCP incidences, and their phases need to cover 360°. In order to reduce the difficulty of the design, the rotation angle θ is utilized to compensate for propagation phase Φ. The corresponding equation is given as

$$\:\begin{array}{c}\varPhi\:=\left\{\begin{array}{c}\varPhi\:\:\:\:\:,\:\:\:\varPhi\:<180^\circ\:\\\:\varPhi\:-180^\circ\:,\:\:\:\varPhi\:\ge\:180^\circ\:\end{array}\right.\end{array}$$
(3)
$$\:\begin{array}{c}\theta\:=\left\{\begin{array}{c}\theta\:\:\:\:,\:\:\:\varPhi\:<180^\circ\:\\\:\theta\:+90^\circ\:,\:\:\:\varPhi\:\ge\:180^\circ\:\end{array}\right.\end{array}$$
(4)

Propagation phase only needs to cover 180° on the basis of Eqs. (3) and (4). Further, propagation phase is necessary to cover 135° when it is discretized to 2 bits (0°, 45°, 90°, 135°).

When a LP wave is incident on a meta-atom, the polarized direction of the output wave can be controlled arbitrarily and the intensity is determined by the Malus’s law. By placing meta-atoms in a device consisting of a bulk polarizer and an analyzer, the reflected wave is given as

$$\:\begin{array}{c}{E}_{out}=\:\left[\begin{array}{cc}{cos}^{2}{\phi\:}_{2}&\:sin{\phi\:}_{2}cos{\phi\:}_{2}\\\:sin{\phi\:}_{2}cos{\phi\:}_{2}&\:{cos}^{2}{\phi\:}_{2}\end{array}\right]J\left(\lambda\:,\theta\:\right)\left[\begin{array}{c}cos{\phi\:}_{1}\\\:sin{\phi\:}_{1}\end{array}\right]\end{array}$$
(5)

where \(\:J\left(\lambda\:,\:\theta\:\right)=\left[\begin{array}{cc}cos\theta\:&\:-sin\theta\:\\\:sin\theta\:&\:cos\theta\:\end{array}\right]\left[\begin{array}{cc}{r}_{l}\left(\lambda\:\right)&\:0\\\:0&\:{r}_{s}\left(\lambda\:\right)\end{array}\right]\left[\begin{array}{cc}cos\theta\:&\:sin\theta\:\\\:-sin\theta\:&\:cos\theta\:\end{array}\right]\). Thus, Eq. (5) can be further expressed as

$$\:\begin{array}{c}\begin{array}{c}{E}_{out}=\:\frac{1}{2}\left\{\left[{r}_{l}\left(\lambda\:\right)+{r}_{s}\left(\lambda\:\right)\right]\text{cos}\left({\phi\:}_{1}-{\phi\:}_{2}\right)+\left[{r}_{l}\left(\lambda\:\right)-{r}_{s}\left(\lambda\:\right)\right]\text{cos}\left(2\theta\:-{\phi\:}_{1}-{\phi\:}_{2}\right)\right\}\left[\begin{array}{c}cos{\phi\:}_{2}\\\:sin{\phi\:}_{2}\end{array}\right]\end{array}\end{array}$$
(6)

where \(\:{\phi\:}_{1}\) represents the direction of y axis with respect to transmission axis of bulk polarizer and \(\:{\phi\:}_{2}\) represents the direction of y axis in relation to transmission axis of analyzer. \(\:\theta\:\) represents the direction of meta-atom’s long-axis with respect to y axis, \(\:\lambda\:\) denotes the working wavelength, and \(\:{r}_{l}\left(\lambda\:\right)\) and \(\:{r}_{s}\left(\lambda\:\right)\) are reflection coefficients along long axis and short axis of the meta-atom. As \(\:{\phi\:}_{1}\) is equal to 0° and \(\:{\phi\:}_{2}\) is equal to 90°, intensity of the reflected wave is expressed as

$$\:\begin{array}{c}{I}_{out}=\:{E}_{out}^{2}=\:\frac{1}{4}\times\:{M}_{in}\times\:{\left|{r}_{l}\left(\lambda\:\right)-{r}_{s}\left(\lambda\:\right)\right|}^{2}{\text{sin}}^{2}2\theta\:\end{array}$$
(7)

where \(\:{M}_{in}\) signifies the amplitude of the incident wave when crossing the bulk polarizer. It can be observed from the above expression that intensity is simply stated by

$$\:\begin{array}{c}I=\:{I}_{0}{\text{sin}}^{2}2\theta\:\end{array}$$
(8)

where \(\:{I}_{0}\) is equal to \(\:\frac{1}{4}\times\:{M}_{in}\times\:{\left|{r}_{l}\left(\lambda\:\right)-{r}_{s}\left(\lambda\:\right)\right|}^{2}\) representing the conversion efficiency of meta-atom34.

The schematic of the designed metasurface realizing six-channel independent imaging is given in Fig. 2(a), and the corresponding meta-atom is displayed in Fig. 2(b). The structure consists of six parts containing a VO2 patch, SiO2 spacer, a VO2 film, a gold patch inserted in a VO2 film, SiO2 spacer, and a gold substrate from top to bottom. Figure 2(c, d) describe top views of a VO2 patch and a gold patch to display details of the meta-atom more clearly. Thicknesses of VO2 patch, gold patch, and gold substrate are 1 μm, 0.9 μm, and 1 μm. For the clarity of expression, remaining structural parameters are shown in Table 1. Propagation phase is determined by lengths l1 and l2, and geometric phase and intensity of near field based on Malus’s law are decided by rotation angles θ1 and θ2. Dielectric constant of VO2 is expressed by Drude model \(\:\epsilon\:\left(\omega\:\right)={\epsilon\:}_{inf}-\frac{{\omega\:}_{p}^{2}\left(\sigma\:\right)}{{\omega\:}^{2}+i\gamma\:\omega\:}\), where εinf and γ are 12 and 5.75 × 1013 s−1. ωp(σ) is a plasma frequency related to conductivity, which is represented as \(\:{\omega\:}_{p}^{2}\left(\sigma\:\right)=\:\frac{\sigma\:}{{\sigma\:}_{0}}{\omega\:}_{p}^{2}\left({\sigma\:}_{0}\right)\). σ0 is 3 × 105 S/m and ωp(σ0) is 1.4 × 1015 rad/s. As temperature is lower than 68 °C, VO2 exhibits a conductivity σ of 200 S/m and the plasma frequency ωp(σ) is 3.6148 × 1013 rad/s. When the temperature rises above 68 °C, σ adjusts to 3 × 105 S/m and ωp(σ) is 1.4 × 1015 rad/s at this point35,36. Finite element software is used to simulate meta-atoms based on different states of VO2. Furthermore, periodic boundaries are set along x and y directions, and perfect matching layers (PMLs) are imposed along z direction.

Fig. 2
figure 2

Three-dimensional structure schematic of metasurface (a) and meta-atom (b). Top view of VO2 patch (c) and gold patch (d). The optimized geometry parameters are shown in Table 1.

Full size image
Table 1 Geometry parameters of meta-atoms.
Full size table

First of all, full-wave phase needs to cover 360° while the propagation phase only needs to cover 135° for 2-bit meta-atoms37. Figure 3(a, b) depict amplitude and phase of VO2 patches as temperature is above 68 °C. It is noticed that amplitude is uniformly distributed around 0.8 and the phase covers about 135° within 0.8–1.4 THz, which proves a nice broadband effect. Black circles represent the selected meta-atoms at 1.2 THz and the corresponding structures are shown at the top of Fig. 3(a). As VO2 becomes the insulator, previous work has successfully demonstrated that the influence of VO2 patches on gold patches can be almost ignored, and there is no need to take into account the dimensional interference of VO2 patches37,38. Thus, the design difficulty of gold meta-atoms is significantly reduced. Figure 3(c, d) describe amplitude and phase of gold patches. Due to the existence of the upper SiO2 spacer and VO2 patches, there is a certain degree of decrease in the quality of gold meta-atoms, but the design requirements of Eq. (3) are still satisfied. The chosen four meta-atoms at the target frequency is shown at the top of Fig. 3(c).

Fig. 3
figure 3

Reflection amplitude (a) and phase (b) with length l1 as VO2 is the metal. The black circles depict the optimal VO2 meta-atoms that fulfill the design requirements. Reflection amplitude (c) and phase (d) with length l2 of gold patches as VO2 is the insulator. The black circles correspond to the selected gold meta-atoms.

Full size image

Amplitudes of eight different meta-atoms are around 0.8 and can be roughly considered as half-wave plates. The polarization direction of incident wave can be altered by utilizing a half-wave plate, and a meta-atom as a half-wave plate is able to control the intensity of incident wave at the pixel level. Thus, it is possible to form a continuous near-field grayscale image by simply changing the orientation of the meta-atom. Specifically, the intensity of the output wave can be given as Eq. (8) after inserting the metasurface into the orthogonal path. As shown in the inset of Fig. 4, the orientation degeneracy of Malus’s law leads to four orientation angles that yield identical near-field intensity. The existence of this interesting “one-to-many mapping” relationship between direction and intensity provides the additional flexibility for manipulating the geometrical phase of incident wave.

Fig. 4
figure 4

Flowchart for six-channel simultaneous holographic and grayscale imaging. Channels 1–3 represent imaging as VO2 is metallic and channels 4–6 represent imaging as VO2 is insulating. Phase distributions φ1 and φ2 for independent far-field imaging are derived using the GSA. Then, Eqs. (1)–(4) are used to find propagation phase Φ and rotation angle θf that satisfy the design requirements, and Φ determines the structure of the meta-atom. At the same time, rotation angles θn are obtained based on Eq. (8). A one-to-four mapping between near-field intensity and geometric phase is seen in the bottom right figure. Angles θf and θn are mixed with a reasonable weight to obtain proper angles θ. Eventually, combining θ and structures of meta-atoms completes the design of metasurface.

Full size image

Figure 4 illustrates a detailed flowchart of simultaneous near-field and far-field imaging based on Malus’s law and phase modulation. Firstly, one of VO2 states is selected, such as the metallic state. Phases φ1 and φ2 are obtained based on Gerchberg-Saxton algorithm (GSA)39. Next step is to calculate propagation phase Φ and rotation angle θf by Eqs. (1)-(4). At the same time, different angles θn corresponding to different intensities in the near field are calculated based on Eq. (8). Since there is orientation degeneracy between near field and far field, a single intensity is selected for four different rotation angles θm, \(\:\frac{\pi\:}{2}\:\)- θm, \(\:\frac{\pi\:}{2}\:\)+ θm, \(\:\pi\:\:\)- θm where m is either 1 or 2 representing two kinds of meta-atoms. Since the intensity rather than the geometric phase is more sensitive to angle, the final choice of the rotation angle of meta-atom is favored by θn. In this work, the weight ratio between θn and θf is set as 0.99 and 0.01. In the next step, suitable meta-atoms are selected according to propagation phase. Finally, the selected meta-atoms are combined with the rotation angles θn to design a single-cell bilayer metasurface. Through the above design, four completely independent far-field holographic images can be obtained under LCP and RCP incidences. Meanwhile, two different gray-scale images are generated in the near field under LP incidences.

Results and discussions

A terahertz six-channel metasurface is designed and simulated according to the above principle. Our metasurface includes 80 × 80 meta-atoms and boundary conditions along x, y, and z directions are set to PMLs. Root means square error (RMSE) is generally utilized to assess the deviation between the target and the simulated intensity. RMSE is very sensitive to outliers in the data, so it accurately reflects the magnitude of the prediction error. The closer the normalized values of RMSE are to 0, the closer the simulated images are to the target images40. The closer the simulated image is to the target image, the lower the crosstalk is. RMSE is defined as

$$\:RMSE\:=\sqrt{\frac{1}{N}\sum\:_{i=1}^{N}{({Y}_{i}-f({x}_{i}\left)\right)}^{2}}$$
(9)

where \(\:{Y}_{i}\) represents simulation results and \(\:f\left({x}_{i}\right)\) represents original data. N is the amount of data. Holographic efficiency reflects the ability to modulate electromagnetic wave and it is defined as

$$\:\begin{array}{c}\eta\:=\frac{{\int\:}_{0}^{2\pi\:}{\int\:}_{0}^{\pi\:}{\left|{E}_{r}\left(\theta\:,\phi\:\right)\right|}^{2}{sin}\theta\:d\theta\:d\phi\:}{{\int\:}_{0}^{2\pi\:}{\int\:}_{0}^{\pi\:}{\left|{E}_{m}\left(\theta\:,\phi\:\right)\right|}^{2}{sin}\theta\:d\theta\:d\phi\:}\end{array}$$
(10)

where \(\:{E}_{r}(\theta\:,\phi\:)\) and \(\:{E}_{m}(\theta\:,\phi\:)\) are far-field scattering patterns of metasurface and a metal mirror with the identical area41.

As VO2 is the metal, Fig. 5(a, b) show the arrangement of VO2 patches and angles of rotation at the corresponding positions. Left side of Fig. 5(c, d) represents target image, while right side represents simulated far-field holograms “S” and “P” under LCP and RCP incidences at 1.2 THz. The normalized RMSEs between the simulated and target images are 0.11 and 0.11, and imaging efficiencies are 48.9% and 48.6%. Figure 5(e) depicts the target grayscale image and simulated grayscale image “1” generated in the near field under LP incidence and its RMSE is 0.22. As the state of VO2 is altered, gold patches modulate electromagnetic wave. Similarly, Fig. 5(f, g) exhibit the arrangement of gold meta-atoms and the corresponding rotation angles. At this moment, Fig. 5(h, i) depict the target and simulation holograms “I” and “N” under LCP and RCP incidences. Their efficiencies are 64.4% and 66.3%, and the normalized RMSEs are 0.12 and 0.12. Figure 5(j) shows the near-field grayscale image “0” under LP incidence and it has a RMSE of 0.27. Clear images can be seen in the near field for both VO2 states, which indicates that the near-field amplitude modulation works well. The noise speckles in images may be caused by near-field coupling between neighboring meta-atoms.

Fig. 5
figure 5

Arrangement positions (a) and corresponding rotation angles θ1 (b) when VO2 is metallic and codes “1”, “2”, “3”, “4” represent VO2 meta-atoms. Simulated far-field holograms as well as grayscale maps and their target maps are demonstrated under LCP incidence (c), RCP incidence (d), and LP incidence (e). Positions (f) and rotation angles θ2 (g) of gold patches as VO2 is insulating. Holograms and grayscale maps are exhibited for LCP wave (h), RCP wave (i), and LP wave (j).

Full size image

To study characteristics of the six-channel metasurface over a broad frequency spectrum, Figs. 6 and 7 demonstrate far-field holograms and near-field grayscale images at frequencies from 0.9 THz to 1.4 THz. Figure 6(a–c) demonstrates 3-channel imaging results as the conductivity of VO2 is 3 × 105 S/m. From these results, it can be seen that the imaging performance is satisfying. Besides, the size of two spin-decoupled holograms decreases with the increasing frequency, while the size of grayscale images generated in the near field do not change. Figure 7(a–c) present broadband effects of the remaining three channels as the conductivity of VO2 is 200 S/m. Imaging results show good quality for three channels from 0.9 THz to 1.2 THz. However, imaging quality of these channels becomes poor at 1.3 THz and the near-field grayscale map is almost drowned in background noise at 1.4 THz. As observed in Fig. 3(c, d), this is due to the resonant phenomenon of meta-atoms near 1.3 THz, where amplitude and phase are completely destroyed. Therefore, the quality of meta-atoms decreases significantly over 1.3 THz, which is the fundamental reason of the imaging deterioration.

Fig. 6
figure 6

Broadband display from three of six-channels within 0.9–1.4 THz as VO2 is metallic. Two different channels are available for LCP wave (a) and RCP wave (b), and another channel is available for LP wave (c).

Full size image
Fig. 7
figure 7

Broadband display from three of six-channels within 0.9–1.4 THz as VO2 is insulating. Two different channels are available for LCP wave (a) and RCP wave (b), and another channel is available for LP wave (c).

Full size image

Possible fabrication of sample and its measurement

Feasible manufacturing and measurement in experiment are important for an ideal model. Herein, some possible manufacturing procedures involving lithography and deposition are succinctly described42,43. Figure 8(a, b) demonstrate deposition of a gold film and a SiO2 layer using electron beam evaporation. In Fig. 8(c), the photoresist is spin-coated onto the SiO2 layer with a predefined shape. Figure 8(d) depicts the step of depositing the gold film. In Fig. 8(e), the excess gold layer is removed by dissolving the photoresist. The above steps are repeated in Fig. 8(f–j) to achieve the fabrication of sample. The possible measurement setup is described in Fig. 9. The whole measurement setup consists of a source, three lenses, an objective lens, a beam splitter (BS), a quarter waveplate, two linear polarizers, and a probe. For measuring far-field holograms, terahertz wave is emitted from a terahertz source and conditioned into a LP signal by a linear polarizer 1. LP wave is converted to CP wave by a quarter waveplate. The polarity of CP wave can be controlled by the angle between the quarter waveplate and the linear polarizer. The lens 1 and the back plane of the objective lens are confocal, thereby realizing the incidence of CP wave on the sample. The reflected wave is detected by the polarizer 2, thus allowing the cross-polarized wave to pass. The cross-polarized wave is incident on the terahertz detector by the confocal lenses 2 and 3, thus enabling the detection of holograms in the far field (k-space). For measuring near-field grayscale images, we remove quarter waveplate and lens 2 to achieve imaging of the sample in real space. Similarly, terahertz wave is emitted from the terahertz source and conditioned to a Y-polarized wave by the first linear polarizer. After passing through the lens 1 and the objective lens, terahertz wave is converted into Y-polarized wave, and it is incident normally on the sample. The reflected signal is detected by the polarizer 2 so as to allow a signal with X polarization to pass. Finally, the X-polarized wave is focused into the terahertz detector by the lens 3 to get grayscale image in real space.

Fig. 8
figure 8

The possible fabrication steps of sample.

Full size image
Fig. 9
figure 9

The possible experimental measurement setup.

Full size image

Conclusion

To summarize, a single-cell bilayer structure is proposed based on phase change characteristics of VO2 to realize the six-channel independent imaging. By merging amplitude, propagation phase, and geometric phase modulations within a single-cell design instead of the traditional subarray approach, it is possible to realize one near-field grayscale image and simultaneously produce two far-field holograms. Two states of VO2 results in doubling the number of channels. Compared with previous works about multi-channel imaging in Table 2, their designs work in the visible region while our metasurface focuses on the multi-channel design for terahertz wave. Our scheme not only achieves six imaging channels, but also realizes dynamic switching between different channels. The proposed method ensures maximum design freedom and independence among different channels without increasing the design complexity due to the introduction of phase and amplitude optimization algorithms. Moreover, phase change in VO2 is so complete that the crosstalk between different states is negligible. The metasurface proposed in this work has some advantages of low crosstalk, ultra-compactness, and flexibility in designing image types. Our design has a promising application in information encryption and high-density imaging.

Table 2 Comparison between some published works and our work.
Full size table