Magneto-optical vector holographic encryption method capable of repeatedly and randomly generating polarization key

Abstract

The invention discloses a magneto-optical vector holographic encryption method capable of repeatedly and randomly generating a polarization key, and belongs to the field of optical holographic encryption. According to the implementation method, the designed metasurface is composed of a ferromagnetic optical rotation layer and a medium microstructure layer, and the microstructure layer is a vector hologram designed through a spin decoupling Fourier holographic method according to an encrypted image. According to the holographic encryption method, the polarization key can be randomly generated, and by means of optical rotation and non-volatility of a ferromagnetic material, a required optical rotation angle can be calculated through a polarization channel where an encrypted image is located and the random polarization key, and is magnetized and stored in a ferromagnetic optical rotation layer. If the secret key is stolen in the sending process, the secret key can be randomly generated again and is magnetized and encrypted again, and at the moment, the original secret key is completely invalid. According to the method, the problem that the key is fixed along with the fixation of the metasurface structure is solved, the risk of key loss is avoided, and the method is a vector holographic encryption technology with remarkable safety.

Description

Technical Field

[0001] The present invention belongs to the field of optical vector holographic encryption, and particularly relates to a vector holographic encryption method based on a non-volatile magneto-optical metasurface and capable of repeated encryption. Background Art

[0002] Vector holographic technology can endow an image with polarization information and generate a complex spatial vector light field. This technology has given rise to a scheme for encrypting output image information using the incident polarization state, that is, vector holographic encryption technology. Since traditional diffraction devices (such as gratings, Fresnel zone plates, etc.) and hologram preparation schemes (such as optical holography, embossed holography, etc.) cannot effectively control polarization, the development of vector holography is greatly restricted. The emergence of metasurfaces provides a feasible and reliable technical solution for vector holographic technology. A metasurface is a two-dimensional optical device composed of artificial microstructures, and each unit has the ability to control the amplitude, phase, and polarization of the light field. Its planarization, integration, multi-control dimensions, aberration compensation, etc. have made it shine in the field of vector holography. However, metasurfaces used for vector holographic encryption always face some problems that cannot be ignored: due to the fixation of the metasurface structure, once the encrypted information is written into the metasurface, it cannot be changed; if the key is lost, the metasurface immediately loses its security; the encrypted information can be deduced from the signal in the reverse channel, that is, it cannot defend against the theft of information by back physical attacks. Therefore, there is an urgent need for a vector holographic encryption method with erasable, non-volatile, and non-reciprocal keys.

[0003] Ferromagnetic materials have non-volatility and can be used as media for storing information, laying the foundation for current information age storage devices. At the same time, some ferromagnetic materials can be used as gyromagnetic magneto-optical materials (such as YIG, etc.), which have the Faraday rotation effect and are active materials with continuously adjustable polarization states. Further, due to their non-volatility and optical activity, the polarization state can be stored as information in this magneto-optical material, providing a convenient and reliable technical path for preparing optical information storage devices in the future optical computing era. In addition, magneto-optical materials have non-reciprocity, so they can be used to prepare unidirectional devices, prevent the theft of information by back physical attacks, endow misleading information to the reverse channel, reduce echo noise, etc. In summary, ferromagnetic magneto-optical materials have great potential in enhancing the security of vector holographic encryption. Summary of the Invention

[0004] To solve the problems existing in the current vector holographic encryption scheme, such as the inability to rewrite key information, the invalidation of the key when lost, and the theft of information by back physical attacks, the present invention provides a vector holographic encryption method with repeated random encryption, which combines a layer of ferromagnetic magneto-optical material and a layer of microstructure layer to form a vector holographic metasurface, and uses this metasurface to perform random repeated vector encryption.

[0005] The technical solution of the present invention is as follows: The metasurface in the method of the present invention is composed of a layer of ferromagnetic magneto-optical material and a layer of microstructure layer. The ferromagnetic magneto-optical material is a layer of uniform and equally thick parallel plate dielectric layer, whose area can cover the entire microstructure layer, and the thickness should ensure an obvious remanent magnetic rotation angle. Preferably, the remanent magnetic rotation angle should be ≥ ±90°. The microstructure layer needs to be designed according to the encrypted image information, and the specific steps are as follows:

[0006] Step 1: Design of the dielectric column structure parameters. Each single dielectric column can act as a half-wave plate, and its front view is an anisotropic pattern. Preferably, such as rectangle, ellipse, rhombus, cross, L shape, T shape. The electromagnetic parameters that can be controlled by a single column include the transmission phase and the geometric phase Combining the two can obtain spin-decoupled phase regulation, that is, regulating the output left-handed light phase and the output right-handed light phase The relationship between them and the transmission phase and the geometric phase is as follows: To make the two spin phases can be designed to any value between 0° and 360°, the transmission phase needs to satisfy a coverage range of 0 to 180°, and the geometric phase needs to satisfy a coverage range of 0 to 360°. Among them, the geometric phase is twice the rotation angle, which naturally satisfies the adjustment of the phase coverage range, and the transmission phase needs to construct a structure library by optimizing the size.

[0007] Step 2: Unit design. Each unit is composed of a pair of dielectric columns. By performing far-field interference on the output light of the two dielectric columns, further spin-decoupled amplitude-phase independent regulation can be achieved, that is, regulating four electromagnetic degrees of freedom (the amplitude A R and phase of the output right-handed light, the amplitude A L and phase of the output left-handed light). The relationship between these four degrees of freedom and the left- and right-handed phases generated by the two dielectric columns is as follows:

[0008]

[0009] Step 3: Calculate the amplitudes and phases of the left- and right-handed lights required at each position of the metasurface. Decompose the designed holographic pattern containing encrypted information into the amplitudes and phases of the left- and right-handed lights, and perform inverse Fourier transform to obtain the complex transmittance of the left- and right-handed lights of each unit. Then, add the quadratic phase factor of the focusing lens to the phase term to obtain the final output complex amplitude of each unit. Then, select the specific structure parameters of each unit according to Step 1 and Step 2.

[0010] The random repeated encryption working process of this magneto-optical vector holographic encryption method is as follows: Encode the encryption information (such as an image) into different polarization channels rot c , design a microstructure layer based on this information, and randomly generate a polarization key rot s ; According to the polarization channel rot c where the encoded information is located and the polarization key rot s , calculate the required optical rotation angle rot e , and the calculation method is: rot e = rot s - rot c . Then magnetize the magneto-optical material according to this optical rotation angle. The magnetized device can decode the encrypted message encoded in the polarization channel rot e through the key rot c . After that, send the key and the metasurface successively. If the key is leaked, the metasurface will no longer be sent, and a new random polarization key rot s ’ will be generated again. And according to the new key rot s ’ and the polarization channel rot c where the encrypted message is located, recalculate the magneto-optical rotation angle rot e ’ and perform magnetization. Repeat this re-encryption process until it is ensured that the key is sent securely, and then send the encrypted metasurface. After decryption, randomly magnetize the metasurface, which can prevent the metasurface and the key from being stolen simultaneously and can be used again. Further, the reverse output signal of the magnetized and encrypted metasurface is no longer the output image of the polarization channel rot c , but the output image of the polarization channel rot c ’ = rot c - 2rot e . Based on this, the echo signal can be suppressed or misleading information can be given to the echo signal to prevent information from being stolen by a back physical attack.

[0011] The beneficial effects and advantages of the present invention are as follows:

[0012] 1. The method of the present invention can generate a completely random key, overcoming the problem that the key is fixed with the fixed metasurface structure. Information leakage in any process before metasurface encryption (such as the metasurface design process, the processing process) will not affect the security of metasurface encryption.

[0013] 2. The method of the present invention can prevent the theft of the key or the metasurface. If the key is stolen or intercepted, the metasurface does not need to be destroyed, and it can be re-magnetized and encrypted. After decryption, randomly magnetize it. At this time, even if the key and the metasurface are stolen simultaneously, no information will be leaked, and it can be used again.

[0014] 3. The method of the present invention can prevent the theft of encrypted messages from backside physical attacks. The echo information of the metasurface is different from that of the encrypted message, and the echo signal can be suppressed or misleading information can be designed.

[0015] 4. The encryption scheme of the present invention is based on vector holographic encryption technology and non-volatile magneto-optical rotation effect, which significantly increases the security and convenience of vector encryption and shows great application value in the fields of optical information encryption, anti-counterfeiting, display, storage, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 (a) is a Poincaré sphere vector holographic pattern provided by an embodiment of the present application;

[0017] Figure 1 (b) are different encrypted patterns generated by different channels of the Poincaré sphere vector holographic metasurface in an embodiment of the present application;

[0018] Figure 2 (a) is a three-dimensional schematic diagram of a unit structure provided by an embodiment of the present application;

[0019] Figure 2 (b) is a top view of a unit structure provided by an embodiment of the present application;

[0020] Figure 3 is the calculation process of the complex transmittance of each unit of the metasurface provided by an embodiment of the present application;

[0021] Figure 4 is the decryption system provided by an embodiment of the present application;

[0022] Figure 5 is a relationship between the remanent magneto-optical rotation angle and the magnetization magnetic field of a ferromagnetic magneto-optical layer BaAl 1.4 Fe 10.6 O 19 provided by an embodiment of the present application;

[0023] Figure 6 is the encryption and re-encryption flow chart provided by an embodiment of the present application;

[0024] Among them, the microstructure unit (1) and the ferromagnetic magneto-optical layer (2). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] To make the above objects, features, and advantages of the present invention more comprehensible, the following describes the specific embodiments of the present invention in detail with reference to the accompanying drawings. Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from this description, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0026] The present invention discloses a magneto - optical vector holographic encryption method for repeatable random generation of polarization keys, and its embodiments are as follows:

[0027] In this embodiment, a Poincaré sphere projection surface is used as the vector holographic pattern, as shown in Figure 1 (a). Based on this vector pattern, different polarization channels transmit different image information. Figure 1 As shown in (b), the output images in different polarization channels are shown. The unit structure of the metasurface processed in this embodiment is as shown in Figure 2 The medium of the microstructure part (1) is silicon. Each unit includes two silicon dielectric columns. Each dielectric column is located within a square period with a side length of 320 μm and a height of 500 μm. The material of the ferromagnetic optically active layer (2) is BaAl 1.4 Fe 10.6 O 19 . Figure 3 The calculation process of the complex transmittance required for each unit of the metasurface: The designed vector pattern is decomposed into the left - and right - handed amplitudes and phases, and then the complex transmittance of the left - and right - handed light of each unit of the metasurface is obtained through inverse Fourier transform and superposition of the phase of the focusing lens.

[0028] Furthermore, according to formula (1), the phases of the left - and right - handed light output from each dielectric column can be obtained through the complex transmittance of the left - and right - handed light of each unit.

[0029] Furthermore, through The required transmission phase and geometric phase of each dielectric column can be obtained according to the left - and right - handed complex transmittance of each unit.

[0030] Furthermore, according to the transmission phase, appropriate dielectric column sizes can be found. The unit sizes come from the constructed database: A group of structure libraries are composed of 12 basic dielectric column units. These 12 units satisfy an arithmetic progression with an interval of 15° from 0° to 180° for the transmission matrix. The lengths (unit: μm) of each dielectric column are in turn: 184, 184, 180, 180, 194, 212, 225, 200, 201, 190, 185, 184; the widths (unit: μm) are in turn: 95, 100, 106, 111, 110, 110, 110, 60, 65, 74, 83, 89.

[0031] Furthermore, according to The rotation angles of each dielectric column can be calculated.

[0032] Further, the designed microstructures are etched: The silicon microstructures are generated by ion beam etching. Before etching, the silicon wafers are cleaned with acetone solution and ultrasonic waves. Then, the adhesion between the silicon wafer and the photoresist is promoted by hexamethyldisilazane (HMDS) vapor. The photoresist AZ4620 is spin-coated to a thickness of 14 μm and baked at a temperature of 110 °C for 20 minutes. After exposure and development, the silicon wafer is etched by Bosch process using passivation gas (C 4 F 8 ) and etching gas (SF 6 ).

[0033] Further, the decryption system is as shown in Figure 4 , where the angle between the analyzer and the polarizer is the polarization key used for decryption.

[0034] Further, hexagonal ferrite BaAl 1.4 Fe 10.6 O 19 is prepared. The specific steps are as follows: First, the mixed powders of Fe 2 O 3 , Al 2 O 3 and BaCO 3 are ball-milled in an ethanol environment. After the grinding process, it is dried and pre-sintered in an air atmosphere at 1100 °C to 1180 °C for 2 hours. This pre-sintering step helps to initially form the required ferrite material. After pre-sintering, the powder undergoes a second round of ball-milling to further refine the particles and improve the uniformity. Subsequently, the ground powder is formed into small aggregates by dry granulation, which improves the particle fluidity in the pressing stage. Then it is pressed into shape and sintered in an oxygen atmosphere at a temperature of 1200 °C to 1250 °C for 4 hours. During the sintering process, the raw materials gradually crystallize into a hexagonal ferrite structure, and finally the required ferromagnetic optical rotation layer is obtained. For the prepared 3-mm ferrite layer, the relationship between the optical rotation angle after magnetization and the required external magnetic field is as shown in Figure 5 .

[0035] An encryption and re-encryption case of this embodiment is as shown in Figure 6 : First, select the polarization channel rot c = +50° as the polarization key, and the designed correct image is as shown in the inset (1); Second, randomly generate the polarization key rot s , here it is 30°, as shown in the inset (2); Third, calculate the optical rotation angle rot e = rot s - rot c = -20°, and magnetize according to this optical rotation angle. At this time, when decrypting with the wrong key, the output image is incorrect. If the wrong key is rot s’ = -70°, that is, the angle between the polarizer and the analyzer is adjusted to -70°, and the decrypted image with error is shown in the illustration (3). When decrypting with the correct polarization key rot s = 30°, that is, when the angle between the polarizer and the analyzer is adjusted to 30°, the image is shown in the illustration (4). If an accident such as key leakage occurs, re-encryption is performed, and the process is the same as the encryption process. At this time, the correct image is still in rot c = +50° polarization channel, and the newly randomly generated polarization key is rot s ” = +80°, and the required optical rotation angle at this time is rot e ’ = rot s ”-rot c = 30°. After re-encryption according to this optical rotation angle, the image decrypted by the new key rot s ” = +80° is shown in the illustration (5). At this time, the original key rot s = 30° becomes invalid. If decrypted with the original polarization key, the obtained image is an incorrect image, as shown in the illustration (6).

[0036] In summary, this embodiment realizes the function of repeatable random generation of polarization keys through the non-volatile magneto-optical effect of ferrite magneto-optical materials, overcomes the problem that the traditional vector holographic encryption key completely depends on the metasurface structure, and avoids the risk caused by key loss.

[0037] In the embodiments provided in this application, it should be understood that the disclosed device / system and method can also be implemented in other ways. For example, the dielectric column structure can be other anisotropic structures, such as rectangular, elliptical, rhombic, cross-shaped, L-shaped, T-shaped, etc.; for example, the dielectric column can be made of other materials, such as silica, germanium, etc.; for example, the working band can be extended to the optical or microwave band; for example, the ferrite can be other non-volatile magneto-optical materials, such as other strontium barium ferrite, YIG crystals mixed with ferromagnetic materials, etc.; for example, the encrypted information can be other design patterns, such as numbers, symbols, characters, etc., or a time series signal can be designed as the ciphertext in the corresponding polarization channel.

[0038] The above description is only an alternative implementation manner of the embodiments of this application, but the protection scope of the embodiments of this application is not limited thereto. Any modification made by those skilled in the art within the technical scope disclosed in the embodiments of this application should be covered within the protection scope of the embodiments of this application.

Claims

1. A magneto-optical vector holographic encryption method with repeatable and random generation of polarization keys, characterized in that: The following steps are included: Step 1: Prepare a ferromagnetic optical rotation layer with non-volatile magnetic optical rotation function, and design a suitable thickness so that it has a significant optical rotation angle under an available magnetic field. Preferably, the optical rotation angle should be greater than 90° under the maximum available magnetic field; Step 2: Design a vector holographic pattern by combining the encrypted information with the polarization channel, and design a dielectric microstructure layer by using the vector holographic pattern. Combine the dielectric microstructure layer with the ferromagnetic optical rotation layer to form a vector holographic encryption device; Step 3: Encryption process. A polarization key is randomly generated according to the polarization channel rot where the encrypted information is located. c With polarization key rot s Calculate the required optical rotation angle rot e , the calculation method is: e =rot s -rot c , and magnetize the device according to the rotation angle; Step 4: Re-encryption process. If the key is lost or intercepted during the transmission process, a random key can be regenerated and e =rot s -rot c , re-magnetize and encrypt the device. For the re-magnetized device, the original key becomes invalid.

2. According to claim 1, a magneto-optical vector holographic encryption method with repeatable and random generation of polarization keys is characterized in that: The method for designing the medium microstructure layer by means of the vector holographic pattern in step 2 is: Step 1: Design of dielectric column structure parameters. A set of anisotropic structure libraries are designed through size. Each structure in the library has a high circular polarization cross-polarization ratio at the designed frequency position. At the same time, the structure library meets the transmission phase coverage of 0 to 180°. Step 2: Hologram calculation. According to the required vector hologram pattern, the complex transmittance of left and right rotation at each unit is calculated by inverse Fourier transform and superposition of the lens quadratic phase factor; Step 3: Specific unit design. The complex transmittance of each unit for right-handed light contains four electromagnetic degrees of freedom: the amplitude A of the output right-handed light R and Phase Output left-handed light amplitude A L and Phase Each unit is composed of two dielectric columns, which can control four electromagnetic variables, namely the left-right rotation phase generated by the two dielectric columns. according to A R , A L , Calculate Afterwards according to Calculate the transmission phase and geometric phase, and find the appropriate size in the structure library through the transmission phase, and the rotation angle (θ) and the geometric phase relationship, The rotation angle of the dielectric column is calculated. At this point, all the structural parameters of each unit are determined.

3. The magneto-optical vector holographic encryption method with repeatable and random generation of polarization keys according to claim 1, characterized in that: The decryption device used is a polarizer and an analyzer, and the angle between the analyzer and the polarizer is the polarization key angle.

4. The magneto-optical vector holographic encryption method with repeatable and random generation of polarization keys according to claim 1, characterized in that: The microstructure layer can be used alone as a vector holographic metasurface. Its materials can be silicon, germanium, silicon dioxide, or good metal conductors. Its processing can be achieved through wet etching, ion beam etching, and laser direct writing.

5. The magneto-optical vector holographic encryption method with repeatable and random generation of polarization keys according to claim 1, characterized in that: The ferromagnetic material layer has the functions of non-volatile magnetic storage and magneto-optical rotation. The material may be ferrite ceramics, which may be processed and prepared by pressing, vapor phase growth, solution growth, and solid phase growth.

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