Reconfigurable terahertz encryption chiral metasurface

Abstract

This invention provides a reconfigurable terahertz encryption chiral metasurface, comprising a substrate dielectric layer, a phase change material layer, and a metal pattern layer stacked sequentially from bottom to top. The substrate dielectric layer is made of polyimide; the phase change material layer contains elongated vanadium dioxide strips distributed horizontally; and the metal pattern layer is made of gold, containing a short central metal strip and two longitudinal metal strips with "Γ"-shaped grooves on either side, forming a chiral configuration in conjunction with the phase change material layer. One side of the metal pattern layer serves as the input port for terahertz waves, and the other side of the substrate dielectric layer serves as the output port. Through laser doping and dedoping techniques, reversible switching between the metallic and insulating states of vanadium dioxide can be achieved, thereby dynamically controlling the transmission amplitude of left-handed / right-handed circularly polarized waves in dual-band operation, forming a four-state reconfigurable polarization-selective response. This design is suitable for multi-dimensional dynamic encryption and possesses programmable characteristics.

Description

Technical Field

[0001] This invention relates to the field of terahertz device technology, and in particular to a reconfigurable terahertz encrypted chiral metasurface. Background Technology

[0002] In recent years, with the rapid development of information technologies such as cloud computing, the Internet of Things, and big data, the global data scale has grown exponentially, and information security issues have become increasingly serious. Developing "hardware-software integrated" encryption technologies that combine physical anti-cloning properties with algorithmic complexity has become a key focus for both academia and industry. The essence of information encryption is to encode data using algorithms and keys; plaintext can only be deciphered if the key is correct. Electromagnetic waves, as the core carrier of information transmission, have multiple degrees of freedom, such as amplitude, phase, polarization, and wavelength, which can all be used as key dimensions. Combining multi-dimensional keys can significantly increase the difficulty and cost of decryption.

[0003] Terahertz waves (0.1–10 THz), located between microwaves and infrared, possess unique electromagnetic properties such as broadband, low energy, strong penetration, and transient response. They have shown great promise in fields such as 6G communication, biomedical detection, and non-destructive testing, and are gradually entering the research scope of encryption technology. Currently, one of the mainstream methods for controlling them is the use of electromagnetic metasurfaces.

[0004] Metamaterials are artificial composite materials composed of periodically arranged subwavelength unit structures. Through artificial design and arrangement, they can achieve electromagnetic responses not found in natural materials, such as negative refraction and perfect absorption. Metasurfaces are two-dimensional forms of metamaterials, with thicknesses only a fraction of the operating wavelength, allowing for precise control of key parameters such as amplitude, phase, and polarization of incident electromagnetic waves. Compared to traditional bulk optical devices, metasurfaces offer advantages such as compact structure, low loss, and ease of integration, making them highly promising for electromagnetic wave manipulation and information encryption. Especially in image encryption, holographic anti-counterfeiting, and multidimensional information encoding, metasurfaces, with their unique and complex physical structure, can act as "hardware keys," providing a new dimension of protection for information security.

[0005] However, current research on encryption based on metasurfaces is mostly focused on the visible light band, where signals are easily captured by conventional detectors, posing a potential risk of interception and cracking. Moreover, the design strategies are mostly static structures, and once fabricated, their electromagnetic response is permanently fixed and cannot be dynamically reconstructed or reprogrammed by external stimuli. Summary of the Invention

[0006] In view of this, to overcome several technical defects of existing terahertz metasurfaces, such as functional rigidity, single encryption dimension, and lack of dynamic control capability, this invention provides a reconfigurable terahertz encrypted chiral metasurface based on the dynamic control of the phase change material vanadium dioxide. The metasurface is characterized by comprising multiple periodically arranged structural units; each structural unit includes, from bottom to top, a dielectric substrate layer, a phase change material layer, and a metal pattern layer; one side of the metal pattern layer serves as the input port for terahertz waves, and the other side of the dielectric substrate layer serves as the output port for terahertz waves.

[0007] Furthermore, the material of the dielectric substrate layer is polyimide, which is a rectangular sheet with a square cross-section, a thickness of 16.7 μm, and a cross-sectional width of 140 μm.

[0008] Furthermore, the phase change material layer is made of vanadium dioxide (VO2), which comprises two rectangular strips placed on the left and right sides. The two rectangular strips are placed on the dielectric substrate layer and are distributed in a mirror-symmetric manner along the midline of the plane. Each rectangular strip has a length of 140 μm and a width of 32 μm.

[0009] Furthermore, the material of the metal pattern layer is gold (Au), which comprises two parts: the first part consists of two rectangular strips placed on the left and right sides, which cover the phase change material layer, and each rectangular strip has a symmetrical "Γ" shaped groove; the second part consists of a rectangular strip placed along the center line of the plane, which has a length of 102μm and a width of 32μm.

[0010] Furthermore, the phase state modulation of the phase change material layer is achieved through laser doping / dedoping technology, that is, by controlling the insertion and extraction of hydrogen atoms (H⁺) in the vanadium dioxide lattice, H⁺ is induced to form a phase change material layer. x Reversible, localized transformation between the VO2 metallic phase and the VO2 insulating phase.

[0011] Furthermore, based on the phase state of vanadium dioxide within the "Γ"-shaped grooves on the left and right sides of each structural unit, the structural unit can exhibit four phase state combinations, defined as states B1, B2, B3, and B4, respectively. Among them, state B1 is characterized by the left vanadium dioxide region being in a metallic state and the right vanadium dioxide region being in an insulating state; state B2 is characterized by the left vanadium dioxide region being in an insulating state and the right vanadium dioxide region being in a metallic state; state B3 is characterized by both left and right vanadium dioxide regions being in an insulating state; and state B4 is characterized by both left and right vanadium dioxide regions being in a metallic state.

[0012] Furthermore, when the incident circularly polarized wave is right-handed circularly polarized, its transmittance is defined as the ratio of the amplitude of the left-handed circularly polarized outgoing wave to the amplitude of the right-handed circularly polarized incident wave; when the incident circularly polarized wave is left-handed circularly polarized, its transmittance is defined as the ratio of the amplitude of the right-handed circularly polarized outgoing wave to the amplitude of the left-handed circularly polarized incident wave. Based on the above four phase states, this metasurface exhibits the following response characteristics:

[0013] In state B1: For right-hand circularly polarized incident waves, the transmittance is high at 0.657 THz and low at 0.872 THz; for left-hand circularly polarized incident waves, the response characteristics are completely opposite to those of right-hand circularly polarized incident waves.

[0014] In state B2: For right-hand circularly polarized incident waves, the transmittance is low at 0.657 THz and high at 0.872 THz; for left-hand circularly polarized incident waves, the response characteristics are completely opposite to those of right-hand circularly polarized incident waves.

[0015] In state B3: For right-hand circularly polarized incident waves, the transmittance is low at 0.657 THz and high at 0.872 THz; for left-hand circularly polarized incident waves, the response characteristics are exactly the same as those for right-hand circularly polarized incident waves.

[0016] In state B4: For right-hand circularly polarized incident waves, the transmittance is high at 0.657 THz and low at 0.872 THz; for left-hand circularly polarized incident waves, the response characteristics are exactly the same as those for right-hand circularly polarized incident waves.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] This invention features outstanding reconfigurability and dynamic tunability: it creatively introduces vanadium dioxide as a phase change material and combines it with laser doping / dedoping technology to achieve non-contact, reversible, and precise dynamic control of the phase state of local regions of the metasurface. This allows a single device to freely switch between four distinct chiral response states—B1, B2, B3, and B4—endowing the device with unprecedented functional programmability and environmental adaptability, overcoming the limitations of fixed functions in traditional devices.

[0019] This invention features a highly efficient non-volatile phase transition mechanism: the phase transition mechanism based on hydrogen ion (H⁺) insertion and extraction, compared to traditional thermally induced phase transitions, offers faster response speeds, lower energy consumption, and the ability to maintain a stable non-volatile phase transition state under non-operating conditions. This characteristic ensures extremely high reliability of the device in terahertz systems requiring long-term stable operation.

[0020] This invention exhibits excellent chiral control performance and a compact structure: through a meticulously designed "sandwich" layered structure and a metal pattern with "Γ"-shaped grooves, it achieves efficient and strong-response control of the chiral characteristics of terahertz waves at the subwavelength scale. At the two characteristic frequency points of 0.657 THz and 0.872 THz, the device can exhibit diametrically opposed transmission characteristics based on phase state combinations, providing a high-performance solution for dynamic filtering, polarization conversion, and signal routing of terahertz waves.

[0021] This invention has broad application prospects: the reconfigurable terahertz encrypted chiral metasurface proposed in this invention has a relatively simple structure, its fabrication process is compatible with existing micro-nano fabrication technologies, and it is easy to achieve large-scale array production. This device has enormous application potential and market value in fields such as switchable terahertz polarizers, dynamic waveplates, high-speed terahertz modulators, high-sensitivity biomolecular sensors, and next-generation terahertz wireless communication systems. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the reconfigurable chiral metasurface provided in an embodiment of the present invention.

[0023] Figure 2 The diagram shows the relationship between transmittance and frequency of the reconfigurable metasurface in state B1 when left-handed light is incident, as provided in an embodiment of the present invention.

[0024] Figure 3 The diagram shows the relationship between transmittance and frequency of the reconfigurable metasurface in the B1 state when right-handed light is incident, as provided in an embodiment of the present invention.

[0025] Figure 4 The diagram shows the relationship between transmittance and frequency of a reconfigurable metasurface in the B2 state when left-handed light is incident, as provided in an embodiment of the present invention.

[0026] Figure 5 The graph shows the relationship between transmittance and frequency of the reconfigurable metasurface in the B2 state when right-handed light is incident, as provided in an embodiment of the present invention.

[0027] Figure 6 The graph shows the relationship between transmittance and frequency of the reconfigurable metasurface in the B3 state when left-handed light is incident, as provided in an embodiment of the present invention.

[0028] Figure 7 The graph shows the relationship between transmittance and frequency of the reconfigurable metasurface in the B3 state when right-handed light is incident, as provided in an embodiment of the present invention.

[0029] Figure 8 The diagram shows the relationship between transmittance and frequency of the reconfigurable metasurface in the B4 state when left-handed light is incident, as provided in an embodiment of the present invention.

[0030] Figure 9The graph shows the relationship between transmittance and frequency of the reconfigurable metasurface in the B4 state when right-handed light is incident, as provided in the embodiments of the present invention.

[0031] Figure 10 This is a schematic diagram illustrating how a reconfigurable metasurface can achieve four state transitions using laser doping and dedoping techniques, according to an embodiment of the present invention.

[0032] Figure 11 The reconfigurable metasurface of this invention utilizes laser doping and dedoping techniques to achieve four states of output response to four different incident electromagnetic waves.

[0033] Figure 12 This is a simulation effect diagram of the target image steganography encryption achieved by the reconfigurable metasurface according to an embodiment of the present invention. Figure 13 This is a schematic diagram of the decryption process of the reconfigurable metasurface according to an embodiment of the present invention, which intuitively presents the entire process from the incident terahertz wave to the reconstruction of the target image information. Detailed Implementation

[0034] The reconfigurable terahertz encryption chiral metasurface of this invention is based on the dynamic control principle of the phase change material vanadium dioxide. Its core working mechanism relies on the asymmetric electromagnetic response of the chiral structure to circularly polarized waves. For example... Figure 1 As shown, the metasurface adopts a three-layer "sandwich" structure, consisting of a dielectric substrate layer, a phase change material layer, and a metal pattern layer from bottom to top, forming a complete electromagnetic wave control unit.

[0035] Specifically, metasurface units break spatial mirror symmetry through the design of asymmetric geometric configurations, which is the structural basis for achieving the circular dichroism effect. For example... Figure 1 As shown, the "T"-shaped groove metal structure used in this invention exhibits significant geometric asymmetry on the horizontal plane, meaning that the projected length of the structure along a specific crystal axis differs significantly from that in the vertical direction, resulting in the structure lacking any mirror symmetry plane. When terahertz waves are incident perpendicularly on this metasurface, left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) waves undergo differentiated electromagnetic coupling with the edges, corners, and dielectric interfaces of the metal pattern at the subwavelength scale.

[0036] This effect can be rigorously explained theoretically using a multiple interference model. From a wave optics perspective, the metasurface can be equivalent to a Fabry-Perot resonant cavity structure. The incident terahertz wave undergoes multiple reflections and transmissions between the top metallic pattern and the bottom dielectric substrate, forming multi-beam interference. Each reflection or transmission introduces a phase delay determined by the structural geometry. For LCP and RCP incident waves, due to the phase difference between the surface plasmon modes and magnetic dipole resonance modes excited in the asymmetric structure, the phase factors accumulated within the cavity are drastically different.

[0037] Mathematically, for RCP and LCP incident waves, their transmitted field components can be described as the superposition result of multiple cyclic interferences.

[0038] In this model, matrices A, B, C, and D are defined by the metasurface's scattering parameters (S-parameters), which incorporate comprehensive information about the conductivity of the metal layer, the dielectric constant of the dielectric layer, and the complex reflection coefficient of the air-metal interface. By precisely controlling the unit geometry parameters (such as groove arm length, spacing, and depth), the eigenvalues ​​of the transmission matrices corresponding to LCP and RCP waves can be optimized, thereby achieving a transmittance difference of up to 20 dB at the target frequency (such as 0.657 THz or 0.872 THz). This physical mechanism based on phase accumulation difference ensures the metasurface's selective response to terahertz waves of different chiralities, providing a solid physical basis for subsequent dynamic reconstruction and refinement.

[0039] In terms of fabrication technology, the metasurface is achieved using multilayer thin-film deposition and micro / nano fabrication techniques. The dielectric substrate layer is made of polyimide, and a 16.7 μm thick film is formed through spin coating. The vanadium dioxide phase change material layer consists of two rectangular strips placed side-by-side, each measuring 140 μm × 32 μm. The metallic pattern layer is made of gold and contains a complex structure with symmetrical "T"-shaped grooves.

[0040] In this embodiment of the invention, phase state control of the phase change material layer is achieved through laser doping / dedoping technology, which is the core technical path to realize the dynamic reconstruction function of metasurfaces. This technology is based on the controllable insertion and extraction mechanism of hydrogen atoms in the vanadium dioxide lattice, and achieves reversible, localized transformation between the HxVO2 metallic phase and the VO2 insulating phase by precisely controlling the stoichiometry of local regions.

[0041] like Figure 10As shown, this technology involves two reversible physicochemical processes. During doping, when a pulsed laser with a wavelength of 1064 nm is focused onto the surface of the vanadium dioxide thin film, the temperature in the localized area rapidly rises above the phase transition critical temperature (approximately 68°C) under the influence of the laser thermal effect. In a hydrogen atmosphere (pressure 0.1 MPa), hydrogen atoms gain sufficient kinetic energy and embed themselves into the interstitial positions of the VO2 lattice, forming a hydrogen-doped HxVO2 compound. Due to the contribution of hydrogen atoms, the electronic structure of the material undergoes a significant change, transforming from the original insulating phase to a conductive phase with metallic properties. The dedoping process involves irradiating the doped region with a lower-energy laser, causing hydrogen atoms to escape from the interstitial positions, and vanadium dioxide regains its intrinsic insulating phase properties. This laser-based doping / dedoping phase transition modulation technology provides a key technological foundation for terahertz reconfigurable metasurfaces, enabling spatial localization and patterning capabilities.

[0042] The electromagnetic response characteristics of metasurfaces in the B1 state are as follows: Figure 2 and Figure 3 As shown, when the incident wave is a left-handed circularly polarized wave, the transmittance is low at the 0.657 THz frequency point and high at the 0.872 THz frequency point; while when the incident wave is a right-handed circularly polarized wave, its response characteristics are completely opposite.

[0043] The response characteristics of state B2 are as follows: Figure 4 and Figure 5 As shown, for right-hand circularly polarized incident waves, the transmittance is low at 0.657 THz and high at 0.872 THz; the response characteristics of left-hand circularly polarized incident waves are completely opposite to those of right-hand circularly polarized incident waves.

[0044] Figure 6 and Figure 7 The electromagnetic response characteristics of state B3 are shown. In this state, for right-hand circularly polarized incident waves, the transmittance is low at 0.657 THz and high at 0.872 THz; the response characteristics of left-hand circularly polarized incident waves are exactly the same as those of right-hand circularly polarized incident waves, exhibiting a unique symmetrical response.

[0045] The response characteristics of state B4 are as follows: Figure 8 and Figure 9 As shown, for right-hand circularly polarized incident waves, the transmittance is high at 0.657 THz and low at 0.872 THz; the response characteristics of left-hand circularly polarized incident waves are exactly the same as those of right-hand circularly polarized incident waves, again exhibiting symmetrical response characteristics.

[0046] like Figure 11As shown, the four states exhibit regular transmission responses to different incident electromagnetic waves. This table systematically summarizes the transmission response patterns of left-handed and right-handed circularly polarized incident waves at the two characteristic frequency points of 0.657THz and 0.872THz under the four states B1-B4, providing a theoretical basis for the design of encryption schemes.

[0047] In terms of encryption applications, this invention realizes QR code steganography encryption based on a reconfigurable terahertz-based chiral metasurface. The core feature of this scheme is that the four phase states (B1, B2, B3, and B4) of the metasurface cannot be directly parsed into QR code information; specific logical discrimination rules are required to reconstruct the valid data. This design concept originates from the fundamental principles of physical layer security, namely, utilizing the inherent electromagnetic response characteristics of the metasurface to construct complex mapping relationships.

[0048] like Figure 12 As shown, the encryption process employs a distributed coding strategy. The state settings of each metasurface unit do not have a simple one-to-one correspondence with the pixel values ​​of the final QR code. For example, state B1 (left metallic phase / right insulating phase) exhibits high transmittance for RCP incident waves at 0.657 THz, but low transmittance for LCP incident waves at 0.872 THz. This frequency- and polarization-dependent characteristic causes a single state to exhibit different response characteristics under different detection conditions.

[0049] The specific coding rules are based on the synergistic effect of multiple physical parameters. For example... Figure 11 As shown, the four states exhibit regular differences in response to LCP and RCP incident waves at the two characteristic frequencies of 0.657THz and 0.872THz. For example, in state B1, the RCP transmission at 0.657THz is "high" and the LCP transmission is "low"; while in state B2, this response characteristic is completely reversed. The system utilizes these response differences to construct a multi-dimensional coding space, distributing and mapping the QR code information to different state combinations.

[0050] The decryption process must strictly follow the preset multi-level discrimination rules. The overall process is illustrated as follows: Figure 13 As shown, the receiver first needs to simultaneously illuminate the metasurface with terahertz sources at two frequencies, 0.657 THz and 0.872 THz, using LCP and RCP polarization states respectively. Then, the transmission response of each element is detected under four detection conditions to obtain the complete response spectrum. Finally, according to a pre-agreed decoding algorithm, the multidimensional response data is converted into binary information.

[0051] The effectiveness of this indirect encoding method is achieved through... Figure 12The simulation results were verified. The simulation showed that by using the correct decoding rules, a clear QR code image can be reconstructed, while any incorrect decoding attempt will result in an unrecognizable image. This mechanism ensures that even if an attacker obtains the physical metasurface, they cannot extract useful information without mastering the specific decoding rules.

[0052] The innovation of this scheme lies in its deep integration of traditional digital encryption with physical layer characteristics. By utilizing the complex physical process of the interaction between terahertz waves and metasurfaces, a significant expansion of the key space is achieved. Simultaneously, the reconfigurable properties based on the vanadium dioxide phase transition enable the system to support dynamic updates to encoding rules, further enhancing the system's security and practicality.

[0053] It should be clarified that the present invention is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown. Those skilled in the art can make various changes, modifications, and additions, or change the order of steps, after understanding the spirit of the present invention.

[0054] In this invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or in place of features of other embodiments.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations of the embodiments of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A reconfigurable terahertz encrypted chiral metasurface, characterized in that, The metasurface comprises multiple periodically arranged structural units, each of which, from bottom to top, comprises a dielectric substrate layer, a phase change material layer, and a metal pattern layer. One side of the metal patterned layer serves as the input port for terahertz waves, and the other side of the substrate dielectric layer serves as the output port for terahertz waves.

2. The reconfigurable terahertz encrypted chiral metasurface according to claim 1, characterized in that, The substrate layer is made of polyimide, and is shaped as a rectangular sheet with a square cross-section, a thickness of 16.7 μm, and a cross-section width of 140 μm.

3. The reconfigurable terahertz encrypted chiral metasurface according to claim 1, characterized in that, The phase change material layer is made of vanadium dioxide and consists of two rectangular strips placed on the left and right sides. The strips are placed on the dielectric substrate layer and are mirror-symmetrical along the midline of the plane. The strips are 140 μm long and 32 μm wide.

4. The reconfigurable terahertz encrypted chiral metasurface according to claim 1, characterized in that, The metal pattern layer is made of gold and consists of two parts. Part one consists of two rectangular strips placed on the left and right sides, which cover the phase change material layer and have symmetrical "Γ" shaped grooves. Part two consists of a rectangular strip placed along the center line of the plane, with a length of 102 μm and a width of 32 μm.

5. The reconfigurable terahertz encrypted chiral metasurface according to claim 1, characterized in that, Phase state modulation of the phase change material layer is achieved through laser doping / dedoping technology, by controlling the insertion and extraction of hydrogen atoms in the vanadium dioxide lattice to achieve H x Reversible, localized transformation between the VO2 metallic phase and the VO2 insulating phase.

6. The reconfigurable terahertz encrypted chiral metasurface according to claim 1, characterized in that, The structural unit can present four phase state combinations based on the phase state of vanadium dioxide in the "Γ"-shaped grooves on the left and right sides, which are defined as states B1, B2, B3, and B4 respectively; wherein state B1 is that the vanadium dioxide region on the left is in a metallic state and the right is in an insulating state; state B2 is that the left is in an insulating state and the right is in a metallic state; state B3 is that both sides are in an insulating state; and state B4 is that both sides are in a metallic state.

7. When the incident circularly polarized wave is right-handed, the transmittance is defined as the ratio of the amplitude of the left-handed circularly polarized outgoing wave to the amplitude of the right-handed circularly polarized incident wave; when the incident circularly polarized wave is left-handed, the transmittance is defined as the ratio of the amplitude of the right-handed circularly polarized outgoing wave to the amplitude of the left-handed circularly polarized incident wave. The reconfigurable terahertz encrypted chiral metasurface according to claim 6 is characterized in that... In state B1, for right-hand circularly polarized incident waves, the transmittance is high at 0.657 THz and low at 0.872 THz; for left-hand circularly polarized incident waves, the response characteristics are the opposite.

8. The reconfigurable terahertz encrypted chiral metasurface according to claim 6, characterized in that, In state B2, for right-hand circularly polarized incident waves, the transmittance is low at the 0.657 THz frequency point and high at the 0.872 THz frequency point; for left-hand circularly polarized incident waves, the response characteristics are the opposite.

9. The reconfigurable terahertz encrypted chiral metasurface according to claim 6, characterized in that, In state B3, for right-hand circularly polarized incident waves, the transmittance is low at the 0.657 THz frequency point and high at the 0.872 THz frequency point; for left-hand circularly polarized incident waves, the response characteristics are the same.

10. The reconfigurable terahertz encrypted chiral metasurface according to claim 6, characterized in that, In state B4, for right-hand circularly polarized incident waves, the transmittance is high at the 0.657 THz frequency point and low at the 0.872 THz frequency point; for left-hand circularly polarized incident waves, the response characteristics are the same.

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