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Abstract
Chiral metasurfaces have many applications in the terahertz (THz) band, but they still lack modulation flexibility and functionality expansion. This paper presents a terahertz chiral metasurface with switchable phase distribution and switchable circular dichroism (CD). The metasurface unit consists of a metallic inner ring embedded in vanadium oxide and a vanadium oxide outer ring, state switching by thermal control of vanadium oxide and a change in the frequency of the incident wave. Based on the switchable phase distribution, we designed a focusing vortex beam generator with adjustable focal lengths through simulation. Based on the switching CD capability, we simulate its mode switching in near-field imaging using numerical simulation, and innovatively propose an optical encryption method. Utilizing the chiral property, we also designed dual-channel switchable holographic imaging in the same frequency band, which combined with the state change of VO2 can realize a total of 4 holograms switching. Our proposed metasurface is expected to provide new ideas for the study of optical encryption and wavefront modulation of dynamics.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metasurfaces are man-made two-dimensional planar materials that can be tuned to control electromagnetic waves by adjusting their microstructure [1–3]. Chirality is the inability of an object to coincide with its mirror image [4]. However, with the development of chiral metasurfaces, symmetry-breaking structures can also cause different responses to different circularly polarized wave, giving rise to phenomena such as circular dichroism (CD) [5] and asymmetric transmission (AT) [6]. These structures are widely used in biomolecular detection, and CD spectroscopy [7,8]. Highly efficient chiral metasurfaces can strongly absorb one of the circularly polarized waves while transmitting (reflecting) the opposite wave, thus enabling independent modulation of LCP or RCP waves. Currently, the applications of chiral metasurfaces are also extended to wavefront manipulation, such as holographic imaging [9,10], vortex beam generation [11–13], and beam splitting [14], and many interesting functions have been developed.
Devices with multifunctionality have great potential in future terahertz communication systems. However, most metasurfaces lack active tunability, which affects their functional expansion. Tunable materials such as graphene [15,16], vanadium dioxide [17,18] provide functional expansion of metasurfaces. Some studies [19,20] have designed graphene-based chiral metasurfaces that can dynamically adjust the amplitude of the metasurface, but only achieve on/off control of a single function. Utilizing the thermal phase transition properties of vanadium dioxide, works [21,22] have also achieved switchable wavefront manipulation between reflective and transmissive modes, but were only able to switch between transmissive and reflective for a single function. Niu et al [23] utilized a double-open ring structure combined with $\textrm{V}{\textrm{O}_\textrm{2}}$ to achieve a switchable phase distribution at the same frequency. Li et al [24] used a similar structure and designed a reconfigurable metasurface based on photosensitive silicon and $\textrm{V}{\textrm{O}_\textrm{2}}$, which can realize switchable phase distributions in three frequency bands, but it is difficult to practically applied because it requires corresponding stimulation of each unit. These two studies [23,24], despite realizing multifunctional switching, are limited to switching the phase distribution, and there is no way to broaden the scope of more functionalities, such as circular dichroism (CD).
In this paper, a switchable chiral metasurface based on $\textrm{V}{\textrm{O}_\textrm{2}}$, which can utilize the thermal phase transition properties of $\textrm{V}{\textrm{O}_\textrm{2}}$ and the electromagnetic response of the metasurface units at different frequencies to switch the already aligned phase distributions, and at the same time switch the circular dichroism (CD). Recently, the vortex phase distribution of the metasurfaces has been experimented with using simple metasources and used for photonic routing and topological excitation [25,26], achieving efficient excitation in the microwave band. However, there is still a lack of stable vortex beam sources in the terahertz band and a lack of tunability. We utilize the property that the proposed metasurface has a switchable arbitrary phase distribution to design a terahertz-band focused vortex beam generator, which can be adjusted to different focal lengths by temperature changes, providing a new idea for tunable vortex beams in the terahertz band. Subsequently, the switchable CD was used to develop a near-field imaging function, where the switching of two different images was achieved by thermal control of $\textrm{V}{\textrm{O}_\textrm{2}}$, which is a reversible process, and to simulate its application in optical encryption. Using the CD of the metasurface and its mirror structure, we realize polarization-multiplexed holography in the same frequency band, which, combined with the state switching of $\textrm{V}{\textrm{O}_\textrm{2}}$, achieves a total of four holograms switching, As shown in Fig. 1. This work is expected to be applicable to a new generation of devices such as optical encryption and message multiplexing communications.
Fig. 1. (a) Chiral metasurface structure. (b) Focused vortex beam switching by phase distribution at different temperatures. (c) Near-field image switching using CD at different temperatures. (d) Switchable Polarization Multiplexing Holography.
2. Theoretical analysis and structural design
Using the classical Jones matrix method [27,28], it is possible to analyze the reflected waves generated by terahertz waves incident on the metasurface from a direction perpendicular to the metasurface. Because the terahertz waves excited by the transmitter are linearly polarized, the reflected electric field generated by a beam of linearly polarized electromagnetic waves irradiating the metasurface is represented as follows:
In order to analyze circularly polarized waves, it is necessary to establish a relationship between circular polarization and line polarization. Here, a fundamental mode matrix U is introduced [29]:
According to the principle that two linearly polarized waves can form a circularly polarized wave, the transmission matrix R of the linearly polarized wave in Eq. (1) can be converted into the transmission matrix R’ of the circularly polarized wave using the fundamental mode matrix U:
Here, L and R denote the LCP and RCP waves, respectively, and ${{r}_{{ij}}}$ represents the transmission coefficient of the i-polarized reflected electric field component of the j-polarized incident electric field of amplitude 1, ${i,j} \in { [x,y,R,L]}$.
The difference in the reflection coefficients between LCP and RCP waves mainly stems from the spin-selective absorption of CP waves by the metasurface. Using the above reflection Jones matrix, the absorption of LCP and RCP waves by the designed metasurface can be expressed as follows:
${{A}_{1}}$ in Eq. (4) denotes the absorption of the metasurface for LCP waves, while ${{A}_{2}}$ denotes the absorption of the metasurface for RCP waves. Circular dichroism (CD) can be characterized by the differential absorption of the metasurface for LCP and RCP waves:
In this work, the designed metasurface structural unit is shown in Fig. 2(a) and consists of three layers of structure. The bottom layer is a gold substrate with a thickness of 2 µm. The intermediate layer is a polyimide dielectric material with a thickness of 30µm and a dielectric constant of 3.5 + 0.00945i [30]. The top layer is made of two metal split rings, the outer ring is made of $\textrm{V}{\textrm{O}_\textrm{2}}$ material with a thickness of 1.5 µm, and the inner ring is made of metal and embedded $\textrm{V}{\textrm{O}_\textrm{2}}$ with a thickness of 1 µm. The other parameters are P = 115 µm, R1 = 32.5 µm, R2 = 52.5 µm, W1 = 5 µm, W2 = 12.5 µm, θ1 = 40°, θ2 = 4°. As the temperature changes from 300 K to the phase transition temperature of 350 K, the change in $\textrm{V}{\textrm{O}_\textrm{2}}$ dielectric constant can be similarly described by the Drude model [31]:
Fig. 2. (a) Schematic of the designed metasurface structured unit. (b) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: metasurface unit. (c) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: co-polarized reflection coefficient. (d) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: metasurface unit. (e) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: co-polarized reflection coefficient.
The proposed metasurfaces are also easy to prepare in practice by utilizing the photolithography process. The general preparation steps are as follows [33]. First, a layer of polyimide was deposited on the metal, and then a VO2 film and photoresist were successively deposited, followed by patterning of the VO 2 film using photolithography with a mask. After a series of standard photolithography processes such as exposure and development, the VO 2 pattern was left on the polyimide layer. The same technique can be used for the preparation of metal inner rings.
Calculation of the electromagnetic response of metasurfaces using the finite difference in time domain (FDTD) approach. Figures 3(a) and (b) show the reflection amplitude spectra of the metasurface structure at different temperatures respectively ${{r}_{{ij}}}$. It is obvious from the figure that at different temperatures, $\textrm{V}{\textrm{O}_\textrm{2}}$ has different frequency band responses when it is in different states. When $\textrm{V}{\textrm{O}_\textrm{2}}$ is metallic, the inner ring is equivalent to being shorted and only the outer ring of $\textrm{V}{\textrm{O}_\textrm{2}}$ is operating, as shown in Fig. 3(c), which exhibits a high polarization selectivity at 0.68 THz, ${CD=}{{A}_{2}}\textrm{{-}}{\textrm{A}_\textrm{1}}\textrm{=0}\textrm{.6}$. In contrast, when ${CD=}{{A}_{2}}\textrm{-}{{A}_\textrm{1}}\textrm{=0}\textrm{.6}$ is in the insulated state, as shown in Fig. 3(d), only the metal-open inner ring is in operation and exhibits great polarization selectivity at 0.87 THz, $\textrm{CD=}{\textrm{A}_\textrm{2}}\textrm{{-}}{\textrm{A}_\textrm{1}}\textrm{= 0}\textrm{.62}$. This means that the response of circularly converted dichroism in two different frequency bands can be switched by the thermal phase transition of $\textrm{V}{\textrm{O}_\textrm{2}}$, and temperature control is the easiest to achieve compared to electric, optical, and magnetic modulation.
Fig. 3. (a) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: amplitude spectra. (b) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: amplitude spectra. (c) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: Absorption spectra. (d) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: absorption spectra.
The Pancharatnam-Berry (PB) theory can often be used to manipulate the phase of CP waves [34,35]. When the metasurface structure is rotated around the z-axis by a certain angle θ, the incident CP wave is co-polarized by the action of the metasurface, and the reflected wave carries an additional phase delay of ±2θ. In this section, the phase gradient of the chiral metasurface structure is simulated when VO2 is in different states, as shown in Fig. 4(a). When $\textrm{V}{\textrm{O}_\textrm{2}}$ is in insulated state, the inner ring rotates every 30° from 0° to 180°. From the figure, we can see that the phase changes linearly, covering 2π, and the amplitudes are all kept constant at 0.8. When $\textrm{V}{\textrm{O}_\textrm{2}}$ is in the metallic state, as shown in Fig. 4(b), it also satisfies the phase 2π coverage, while the amplitude remains constant at 0.78. We selected six units with rotation angles (0°, 30°, 60°, 90°, 120°, 150°) for subsequent phase alignment. We also simulated the change in CD values for different rotations of the inner and outer rings, see Fig. S1 in the Supplement 1, the CD values do not change much as the angle changes.
Fig. 4. (a) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: Phase gradient formed by rotating open metal ring. (b) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: Phase gradient formed by rotating $\textrm{V}{\textrm{O}_\textrm{2}}$ outer ring.
Here, we analyze the generation of switchable CD and switchable phase distributions. As shown in Fig. 5(a) and (b), it can be seen that when the embedded $\textrm{V}{\textrm{O}_\textrm{2}}$ is in the metallic state, the opening of the inner metallic ring is shorted to form a complete circle, thus, the inner ring has almost no electric field distribution. Only the outer ring is operating, with a strong electric field response at 0.68 THz. As shown in Fig. 5(b) and (c), when the embedded $\textrm{V}{\textrm{O}_\textrm{2}}$ is in the insulated state, the reflectance factor is close to 0.9 at 0.87 THz, which indicates that the metal inner ring is not shorted. This further illustrates that by exploiting the thermal phase change properties of $\textrm{V}{\textrm{O}_\textrm{2}}$, switchable CD and switchable phase distributions can be achieved.
Fig. 5. (a) Variation of reflection coefficient of cp wave passing through metasurface when VO2 is in metallic and insulating states, respectively. (b) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: Electric field distribution on the metasurface. (c) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: Electric field distribution on the metasurface.
3. Results and discussions
Our proposed metasurface unit can switch the already aligned phase distributions when $\textrm{V}{\textrm{O}_\textrm{2}}$ is in different states, and also switch the CD. To verify its potential, we design and simulate a focused vortex beam generator with switchable focal length function and a near-field imaging device with switchable imaging modes, and innovatively propose an optical encryption method. In Fig. s2 of the Supplement 1, we also implemented switchable vortex beam and near-field imaging. In order to enhance the potential applications of this metasurface, we have also designed dual-channel switchable holographic imaging in the same frequency band.
3.1 Focused vortex beam generator with switchable focal lengths
To obtain a vortex beam carrying OAM, when lining up the metasurface units, the units at different positions must satisfy the corresponding phase distribution in Eq. (7). Where l is the number of topological charges generating the OAM and (x, y) is the given coordinates in space:
The need to control the focusing of the vortex beam to a certain point, which facilitates the reception of the communication system. At this point, the units at different positions must satisfy the corresponding focusing phase distribution in Eq. (8). where f is the focal length and λ is the wavelength.
To focus the desired vortex beam to a certain point, the phase variation of the unit structure can superimpose the phase distribution of Eq. (7) and (8). According to the PB phase principle, the metasurface cell rotates at an angle of 1/2 of the corresponding phase. Therefore, the phase distribution of the unit structure at the (x, y) position can be converted to the corresponding rotation angle, satisfying Eq. (9) [36].
Based on the above principal analysis of phase distribution, we used 25 × 25 metasurface units for two switchable phase arrangements. Figures 6(a) and (b) show the phase distribution of the vortex beam and the phase distribution of the focused beam, respectively, and Fig. 6(c) represents the phase distribution of the focused vortex beam after the superposition of the vortex beam and the focused beam. To realize the switchable focal lengths of the focused vortex beams, the phase arrangement of the focused vortex beams at different focal lengths for each of the inner and outer rings of the metasurface was determined, as shown in Fig. 6(d) and (e). When the temperature is 300 K and the incident wave frequency is 0.87 THz, $\textrm{V}{\textrm{O}_\textrm{2}}$ is in an insulated state. Set the focal length to 1600 µm and the topological charge number to 1. When the temperature is 350 K and the incident wave frequency is 0.68 THz, $\textrm{V}{\textrm{O}_\textrm{2}}$ is in a metallic state. Set the focal length to 900 µm and the topological charge to 1.
Fig. 6. (a) Phase distribution of the vortex beam. (b) Phase distribution of the focused beam. (c) Phase distribution of the superposition of the vortex and focused beams. (d) Schematic of the metasurface structure. (e) Enlargement of the localized structure.
Figure 7 shows the simulation results when $\textrm{V}{\textrm{O}_\textrm{2}}$ is insulated. As shown in Fig. 7(a), it can be seen that the beam has a good focusing effect. The energy amplitude of the beam has a hollow circle in the center, which is the zero of the electric field. This result proves that a focused vortex beam is produced. As shown in Fig. 7(b), it can be seen that the electric field is strongest at a distance of z = 1530 µm, indicating that this is the focal point, close to the preset focal length value. Figure 7(c) shows the phase distribution of the generated vortex beam. Mode purity is often used to characterize the quality of a vortex beam. Mode purity can be obtained by comparing the power of the main mode with that of all modes. As shown in Fig. 7(d), the mode purity at this point is 0.84. Figure 7(e) shows the distribution of the electric field along the X-axis corresponding to the focal point at a distance of z = 1530 µm. It can be seen that when the points x = 203 µm and x = -193 µm approach 1, the electric field is the maximum value. The full width half maximum (FWHM) of the focal spot center is 613 µm.
Fig. 7. $\textrm{V}{\textrm{O}_\textrm{2}}$ in the insulating state:(a) The electric field in the X-Y plane. (b) The electric field in the X-Z plane. (c) Phase of the X-Y plane. (d) Mode purity. (e) The electric field in the X-axis (z = 1530 µm).
Figure 8 shows the simulation results when $\textrm{V}{\textrm{O}_\textrm{2}}$ is in a metallic state and the $\textrm{V}{\textrm{O}_\textrm{2}}$ outer ring works independently. Similar to Fig. 7(a), Fig. 8(a) also demonstrates the generation of a focused vortex beam. As shown in Fig. 8(b), the maximum electric field intensity is observed at the focal point z = 820 µm, which is close to the present value. Figure 8(c) shows the phase distribution corresponding to the vortex beam. As shown in Fig. 8(d), the mode purity is 0.88 at this point. Figure 8(e) shows the distribution of the electric field on the X-axis corresponding to the focal point at a distance of z = 820 µm. It can be seen that the electric field is maximized when the points x = 186 µm and x = -183 µm approach 1. The full width half maximum (FWHM) of the focal spot center is 605 µm.
Fig. 8. $\textrm{V}{\textrm{O}_\textrm{2}}$ in the metallic state: (a) The electric field in the X-Y plane. (b) The electric field in the X-Z plane. (c) Phase of the X-Y plane. (d) Mode purity. (e) The electric field in the X-axis (z = 820 µm).
When the $\textrm{V}{\textrm{O}_\textrm{2}}$ is in different states, the designed focused vortex generator has excellent performance at 0.68 and 0.87 THz. In addition, the designed metasurface can also realize more functional switching combinations in addition to focusing, such as beam splitting and topological charge number switching, with great regulatory freedom.
3.2 Dual-band switchable near-field imaging
The designed chiral metasurface has high-efficiency CD in two different frequency bands, which can be used for near-field imaging applications without considering the PB phase. According to the previous analysis and verification, the designed unit array composed of metasurface units has great potential in imaging display. Because the structure is chiral, the reverse CD can be obtained by mirroring. As shown in Fig. 9(a) and (b), arrange the unit structure of forward CD and reverse CD as 25 × 25 units. Among them, the $\textrm{V}{\textrm{O}_\textrm{2}}$ outer ring and its mirror image structure are arranged into the letter “H”, and the metal inner ring and its mirror image structure are arranged into the letter “N”, and then the pattern is displayed by detecting the reflection fields of the LCP and RCP.
Fig. 9. (a) The $\textrm{V}{\textrm{O}_\textrm{2}}$ outer ring and its mirror image structure are arranged into the letter “H”. (b) The metal inner ring and its mirror image structure are arranged into the letter “N”. (c) Switching of “H” and “N” at RCP wave incidence. (d) Switching of “H” and “N” at LCP wave incidence.
As shown in Fig. 9(c) and (d), a detector is placed 200µm away from the array element to observe the image formed by the co-polarized reflection waves after the incident waves are RCP and LCP. The images of the letter “H” and the letter “N” can be displayed, and the imaging effect is better. Switching of the two-letter images is achieved by changing the temperature. When the temperature is 300 K and $\textrm{V}{\textrm{O}_\textrm{2}}$ is in the insulated state, the metasurface responds strongly at 0.87 THz, at which time the letter “N” arranged by the metal inner ring is imaged. This means that we encode the original image and the encrypted image to the metasurface according to a specific amplitude configuration. At room temperature and a specific incident wave frequency, the reflected wave field is the original image. Only by changing both the temperature and the frequency of the incident wave can the encrypted image appear, and the entire switching process is reversible.
We also innovatively propose an optical encryption method. As shown in Fig. 10(a), the entire structure consisting of a 30 × 10 metasurface units. The entire array is divided into 12 sub-regions, and the same 5 × 5 units is adopted in each sub-region. The initial and mirror structure of the inner and outer rings are arranged into each sub-region according to the design requirements. The reflected strong electric field is marked as “1”, and the weak electric field is marked as “0”. As can be seen from the results in Fig. 10(c), when the incident frequency of the LCP wave is 0.87 THz, when the temperature is 300 K, and $\textrm{V}{\textrm{O}_\textrm{2}}$ is in an insulated state, the near-field number is displayed as “101010/010101”. When the incident frequency of LCP wave is 0.68 THz, the temperature is 350 K, and $\textrm{V}{\textrm{O}_\textrm{2}}$ is in the metallic state, the near-field number is displayed as “110101/010010”. According to this result, we can express the information that needs to be transmitted by binary coding, in which the metal inner ring is responsible for storing the binary code of the original information, and the $\textrm{V}{\textrm{O}_\textrm{2}}$ outer ring is responsible for storing the binary code of the encrypted information. As shown in Fig. 10(c), the reflected electric field of the inner metal ring is the valid information of “101010/010101”, while the reflected electric field of the outer ring of $\textrm{V}{\textrm{O}_\textrm{2}}$ is the invalid information after encryption of “110101/010010”. The original information and the encrypted information can be set independently and do not interfere with each other. The encryption/decryption process can be realized by changing the temperature and frequency of the incident CP wave. It is believed that the design of the unit structure combined with better processing technology can provide higher security in the encryption/decryption and transmission of information.
Fig. 10. (a) Arrays are made using a 30 × 10 metasurface unit structure and are divided into 12 subregions. (b) Structure of the metasurface unit corresponding to each subregion. (c) Reflected electric field intensity distribution at different temperatures under incident LCP wave. (d) Reflected electric field intensity distribution at different temperatures under incident RCP wave.
3.3 Dual-channel switchable holographic imaging at the same frequency
From Fig. 9(ab) in Subsection 3.2, it can be seen that the initial and mirror structures of the metasurface unit have opposite electromagnetic responses for the incidence of LCP waves. This is a property of chiral structures, where the initial structure responds only to the LCP and the mirror structure responds only to the RCP. Using this feature, we can realize the holographic imaging function of dual-channel switching under the same frequency. From the previous section, it can be seen that the inner and outer rings of the metasurface have independent electromagnetic responses to two different frequency bands in the two states of $\textrm{V}{\textrm{O}_\textrm{2}}$, which indicates that the same-frequency dual-channel switchable holographic imaging can be established in each state, and that a total of four holographic image switching can be achieved. This also requires the phase gradient of the mirror structure to be established as well, see Figure S3 in the Supplement 1.
We constructed holograms of the letters “H”, “T”, “L” and “A”. We would like to get “H” and “T” for LCP and RCP channels respectively at 0.87 THz, and “L “ and “A”. The phase distribution of metasurface holographic imaging is generally obtained by an algorithm for phase retrieval, and this design utilizes the classical G-S algorithm, which uses the Fresnel-Kirchhoff diffraction formula:
We use the partitioned G-S algorithm to obtain the desired phase distribution of the electric field on the metasurface so that the cell distributions of the LCP and RCP channels are arranged alternately. The flow of the partitioned G-S algorithm is shown in Fig. 11, where the initial random phase and the partitioned amplitude distribution M1 (M2) are input. After Fourier transform, the amplitude distribution of the computed image is evaluated. If the image quality is poor, the amplitude of the target image is combined with the computed phase distribution to perform an inverse Fourier transform to obtain a new input. Iterate repeatedly until a more like phase distribution of the electric field is obtained. The phase distributions of the respective cells of the initial and mirror structures were finally obtained.
The simulation results are shown in Fig. 12, and it can be seen that the crosstalk between the two channels is small. As can be seen from Fig. 11(bc), when the temperature is increased and VO2 is in the metallic state, the outer ring responds, and the initial and specular structures of the outer ring are arranged alternately, which allows switching between “H” and “T” holographic imaging at a frequency of 0.68 THz by simply changing the incident circular polarization state. From Fig. 11, it can be seen that when the temperature decreases, the inner loop response, at 0.87 THz, is able to accomplish the switching between “L” and “A”. When the frequency and temperature are changed simultaneously, the holographic imaging map formed by the outer ring and the holographic imaging map formed by the inner ring can also be switched. This greatly improves the switching freedom of this metasurface.
Fig. 12. (a) Schematic diagram of the metasurface. (b) Metallic $\textrm{V}{\textrm{O}_\textrm{2}}$: Outer ring independent work. (c) Letter switching between “H” and “L” at the same frequency. (d) Insulating $\textrm{V}{\textrm{O}_\textrm{2}}$: Inner ring independent work. (e) Letter switching between “H” and “L” at the same frequency.
4. Conclusion
In conclusion, we designed a terahertz chiral metasurface with a switchable phase distribution and switchable circular dichroism (CD). By simulation, the inner and outer rings of this structure have independent phase distributions (covering 2π) and circular dichroism (CD) at frequencies of 0.68 and 0.87 THz. Temperature control of the phase change of $\textrm{V}{\textrm{O}_\textrm{2}}$ and changing the frequency of the incident wave enable different phase arrangements and circular dichroism (CD) switching. Utilizing this property, we design focus-switchable vortex beam generators and near-field imaging switching, and present their application in optical encryption. Using the partitioned phase design, we realized switchable holographic imaging in the same frequency band, which combined with the state change of VO2 can realize the switching of four holograms. The proposed metasurface structure is expected to provide a noteworthy perspective for research in the fields of optical encryption and dynamic wavefront modulation.
Funding
National Key Research and Development Program of China (2022YFA1203500); National Natural Science Foundation of China (12274424, 61988102, 62064004, 62174046, 62274048); Open Fund for Innovation and Entrepreneurship of College Students (RSXH20231165808X, RSXH20231165817X).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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