Magneto-optical imaging system based on metasurface and design method thereof

Abstract

The invention discloses a magneto-optical imaging system based on a metasurface and a design method thereof, and relates to the technical field of optical imaging. The magneto-optical imaging system based on the metasurface sequentially comprises a collimation laser light source, a to-be-measured object with magnetism, a metasurface device and a receiving device in the light path transmission direction, the to-be-measured object is used for rotating collimation laser emitted by the collimation laser light source in a polarization state, and the receiving device is used for receiving the collimation laser. The metasurface device is used for splitting emergent light in a polarization state according to horizontal polarization and vertical polarization, and the receiving device is used for receiving two beams of light with different intensities. According to the technical scheme, the emergent light in the polarization state is subjected to beam splitting through the metasurface device, the polarization separation angle is increased, the distance between the metasurface device and the receiving device does not need to be increased, and therefore the length of the magneto-optical imaging system is shortened; therefore, the size of the magneto-optical imaging system is reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical imaging technology, and more specifically to a magneto-optical imaging system based on a metasurface and a design method thereof. Background Art

[0002] Magneto-optical imaging (MOI) is a high-resolution imaging technique that combines the principles of magnetism and optics. It uses the magneto-optical effect to convert the magnetic field distribution of magnetic materials into optical signals, enabling magnetic field visualization. Its core is the interaction between polarized light and magnetic materials. When linearly polarized light passes through a magneto-optically sensitive medium, the magnetic field changes the polarization direction of the light. By analyzing the change in polarization state, the magnetic domain or vortex structure of the sample can be reconstructed. MOI is widely used in condensed matter physics and materials science, for example, to study the flux vortex dynamics of superconductors, the domain wall behavior of magnetic films, and the magnetization process of micro-nano devices. A magneto-optical imaging system typically includes a polarized light source, a magneto-optically sensitive layer, a microscopic optical component, and a polarization detection module. It offers advantages such as non-contact operation and high spatiotemporal resolution (up to micrometer / nanosecond levels), providing a key tool for studying the dynamic properties of magnetic materials.

[0003] In a magneto-optical imaging system, the core component for analyzing the polarization state of outgoing light is a polarization beam splitter prism, which can separate the incident light into two orthogonal linearly polarized beams. By measuring the intensity of the two linearly polarized beams, the magnetic field distribution of the sample can be reconstructed. However, the polarization beam splitter prism is large, which increases the volume of the entire magneto-optical imaging system. The polarization separation angle achievable by the polarization beam splitter prism is also small. In order to achieve a certain separation of the two orthogonal polarized beams at the receiving device, the distance between the polarization beam splitter prism and the receiving device needs to be increased, thereby increasing the length of the magneto-optical imaging system.

[0004] Therefore, in order to solve the above problems, the present invention provides a magneto-optical imaging system based on a metasurface with a smaller volume and shorter length and a design method thereof. Summary of the Invention

[0005] The present invention provides a metasurface-based magneto-optical imaging system and a design method thereof, aiming to solve the problems of large volume and long length of existing magneto-optical imaging systems.

[0006] In order to achieve the above-mentioned objectives, on the one hand, the present invention provides a magneto-optical imaging system based on a metasurface, which includes, in order of the optical transmission direction: a collimated laser light source, a magnetic object to be measured, a metasurface device and a receiving device, wherein the object to be measured is used to rotate the polarization state of the collimated laser emitted by the collimated laser light source, the metasurface device is used to split the polarized output light into horizontal polarization and vertical polarization, and the receiving device is used to receive two beams of light with different intensities.

[0007] A further technical solution is: the metasurface device includes a substrate and a metasurface unit placed on the substrate.

[0008] Its further technical solution is: the metasurface unit is composed of a microstructure array, the microstructure array includes a plurality of microstructures, and the phase distribution of the metasurface unit is changed by the type, size and arrangement of the microstructures, and the phase change range is Inside.

[0009] A further technical solution is: the microstructure is a structure with birefringence characteristics.

[0010] A further technical solution is that the microstructures are arranged in a tetragonal arrangement or a hexagonal arrangement.

[0011] A further technical solution is: the substrate and the microstructure are both made of materials that are transparent to the wavelength of the collimated laser light source, and the refractive index of the microstructure is greater than the refractive index of the substrate.

[0012] A further technical solution is: the collimated laser light source is a laser that can emit collimated laser light, and the receiving device is a device including an image sensor.

[0013] In order to achieve the above-mentioned purpose, on another aspect, the present invention also provides a design method for a magneto-optical imaging system based on a metasurface, including: determining the relationship between the microstructure phase delay and the microstructure size based on a full-wave simulation algorithm, and obtaining different phase delays by changing the size of the microstructure; designing the phase of the metasurface unit to obtain the target phase according to the beam splitting function to be achieved under horizontal polarization and vertical polarization; and matching the phase delay with the target phase.

[0014] A further technical solution is: the formula for designing the phase of the metasurface unit is as follows: ; Among them, the formula φ x_pol 、 φ y_pol are the target phases that need to be met under horizontal polarization and vertical polarization, respectively. λ is the wavelength of the output light in the polarization state. f is the focal length, which is the position of the focused light spot from the metasurface. (x, y) is the spatial coordinate of the metasurface unit. f is the distance between the two light spots in the y direction.

[0015] A further technical solution is: the formula for matching the phase delay with the target phase is as follows: ; in, φ x 、 φ y It refers to the phase delay that the microstructure can meet under horizontal polarization and vertical polarization. FOM is the sum of phase difference values, and meta represents the coordinate set inside the metasurface unit.

[0016] An embodiment of the present invention provides a metasurface-based magneto-optical imaging system and a design method thereof, wherein the metasurface-based magneto-optical imaging system includes, in order according to the optical transmission direction: a collimated laser light source, a magnetic object to be measured, a metasurface device, and a receiving device, wherein the object to be measured is used to rotate the polarization state of the collimated laser emitted by the collimated laser light source, the metasurface device is used to split the polarized output light into horizontal polarization and vertical polarization, and the receiving device is used to receive two beams of light with different intensities. In an embodiment of the present invention, the polarized output light is split into horizontal polarization and vertical polarization by the metasurface device, thereby increasing the polarization separation angle without increasing the distance between the metasurface device and the receiving device, thereby shortening the length of the magneto-optical imaging system. Moreover, since the metasurface device is smaller in size than the polarization beam splitting prism, the volume of the magneto-optical imaging system is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the structure of the magneto-optical imaging system based on the metasurface of the present invention; Figure 2 A design method for a magneto-optical imaging system based on a metasurface of the present invention; Figure 3 Schematic diagram of the structure of the microstructure in the magneto-optical imaging system based on the metasurface of the present invention; Figure 4 Schematic diagram of microstructure size and microstructure phase delay in the magneto-optical imaging system based on metasurface of the present invention; Figure 5 Schematic diagram comparing the beam splitting intensities of the metasurface unit under different polarizations in the magneto-optical imaging system based on the metasurface of the present invention; Reference numerals: 1. Collimated laser light source; 2. Magnetic object to be measured; 3. Metasurface device; 4. Receiving device. DETAILED DESCRIPTION

[0018] The following will be combined with the accompanying drawings of the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments. Similar component numbers in the drawings represent similar components. Obviously, the embodiments described below are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0019] See also Figure 1 , Figure 1 FIG. 1 is a schematic structural diagram of a magneto-optical imaging system based on a metasurface according to the present invention. Figure 1 As shown, the metasurface-based magneto-optical imaging system includes, in order of optical transmission direction, a collimated laser light source 1, a magnetic object to be measured 2, a metasurface device 3, and a receiving device 4. The object to be measured is used to rotate the polarization state of the collimated laser light emitted by the collimated laser light source 1, the metasurface device 3 is used to split the polarized output light into horizontal polarization and vertical polarization, and the receiving device 4 is used to receive two beams of light with different intensities. It should be noted that in this embodiment, the object to be measured is required to be magnetic so that it can reflect the collimated laser light emitted by the collimated laser light source 1 and convert the polarization state of the collimated laser light using its own magnetism. It should also be noted that in this embodiment of the present invention, the polarization state of the output light is split into horizontal polarization and vertical polarization by the metasurface device 3, which increases the polarization separation angle and eliminates the need to increase the distance between the metasurface device 3 and the receiving device 4, thereby shortening the length of the magneto-optical imaging system. Furthermore, since the metasurface device 3 is smaller than the polarization beam splitting prism, the volume of the magneto-optical imaging system is reduced.

[0020] In some embodiments, such as the present embodiment, the object to be measured is located downstream of the collimated laser light source 1, the metasurface device 3 is located downstream of the object to be measured, and the metasurface device 3 includes a substrate and a metasurface unit disposed on the substrate, wherein the metasurface unit is composed of a microstructure array, and the microstructure array includes a plurality of microstructures, and the phase distribution of the metasurface unit is changed by the type, size and arrangement of the microstructures, and the phase change range is within It should be noted that, in this embodiment, the metasurface unit can split the polarization state of the outgoing light reflected by the object to be measured into horizontal polarization (0°) and vertical polarization (90°). When different polarization states are incident, the energies of the two split beams are different. Therefore, the phase design of the metasurface unit can achieve beam collimation and / or shaping functions.

[0021] In certain embodiments, such as the present embodiment, both the substrate and the microstructure are made of materials that are transparent to the wavelength of the collimated laser light source 1, and the refractive index of the microstructure is greater than that of the substrate. It should also be noted that in this embodiment, the shape of the microstructure must be anisotropic, and the microstructure is a structure with birefringence, such as rectangular nanorods or nanopores; the microstructure is arranged in a tetragonal or hexagonal pattern.

[0022] In some embodiments, such as the present embodiment, the collimated laser light source 1 is a laser that can emit collimated laser, for example, a conductor laser, a fiber laser or other types of lasers, and its emitted light needs to have a specific polarization, such as horizontal linear polarization, vertical linear polarization, etc. The receiving device 4 is a device including an image sensor, and the image sensor can be a CCD sensor, a CMOS sensor, etc. The receiving device 4 can be, for example, a camera.

[0023] See also Figure 2 , Figure 2 The present invention is a design method of a magneto-optical imaging system based on a metasurface, and the design method is applied to the magneto-optical imaging system based on a metasurface as described above. Figure 2 As shown, the design method includes the following steps S110-S130: S110. Determine the relationship between the microstructure phase delay and the microstructure size based on a full-wave simulation algorithm, and obtain different phase delays by changing the size of the microstructure.

[0024] In this embodiment, for ease of understanding, the technical principle of the metasurface-based magneto-optical imaging system is first explained: a collimated laser light source emits a collimated laser with a specific linear polarization and irradiates it onto a magnetically attracted object to be measured. Due to the magnetic field of the magnetic object, the collimated laser undergoes a Kerr effect, and the polarization state of the reflected light rotates. The polarized outgoing light is received by the metasurface device. The metasurface device treats the polarized outgoing light as incident light and separates it into two positions in space according to the horizontal and vertical polarization components. Different polarization states are divided into two beams of light with different intensities. The two beams are ultimately received by a receiving device. By analyzing the intensities of the two beams, the change in polarization state can be inverted, and the magnetic field information of the magnetic object to be measured can be calculated based on the Kerr effect.

[0025] After understanding the technical principle of the metasurface-based magneto-optical imaging system, the relationship between the microstructure phase delay and the microstructure size is determined based on a full-wave simulation algorithm, wherein the full-wave simulation algorithm is any one of the time-domain finite-difference method and the rigorous coupled wave analysis method. It should be noted that in this embodiment, the microstructure is a structure with birefringence characteristics, specifically Figure 3As shown, taking the microstructure as a rectangular nanocolumn as an example, different phase delays can be obtained by changing the length L and width W of the rectangular nanocolumn under fixed period P and height H.

[0026] S120. Design the phase of the metasurface unit to obtain the target phase according to the beam splitting function to be achieved under horizontal polarization and vertical polarization.

[0027] In this embodiment, in order to achieve the beam splitting function, the metasurface unit needs to meet a specific phase distribution under horizontal polarization and vertical polarization, that is, the microstructure needs to be arranged in a specific manner. The formula for designing the phase of the metasurface unit is as follows: ; Among them, the above formula φ x_pol 、 φ y_pol are the target phases that need to be met under horizontal polarization and vertical polarization, respectively. λ is the wavelength of the output light in the polarization state. f is the focal length, which is the position of the focused light spot from the metasurface. (x, y) is the spatial coordinate of the metasurface unit. f is the distance between the two light spots in the y direction. Figure 4 The relationship between phase delay and microstructure under horizontal polarization (x polarization) and vertical polarization (y polarization) is given, such as Figure 4 It can be seen that the x-coordinate and y-coordinate correspond to the length and width of the microstructure respectively. The grayscale of each area represents the phase delay. The lighter the color, the greater the phase delay. The phase change range under horizontal polarization and vertical polarization is Inside.

[0028] S130: Match the phase delay with the target phase.

[0029] In this embodiment, the formula for matching the phase delay with the target phase is as follows: ; in, φ x 、 φ y It refers to the phase delay that the microstructure can meet under horizontal polarization and vertical polarization. FOM is the sum of phase difference values, and meta represents the coordinate set inside the metasurface unit. FOM The meaning is the sum of the differences between the phase retardation of orthogonal polarizations provided by a microstructure with a specific size and the required phase distribution. FOM When the minimum is reached, the appropriate microstructure is found. The entire metasurface unit can be obtained by splicing the microstructures at various locations on the metasurface.

[0030] In some embodiments, such as the present embodiment, Figure 5 The figure shows the intensity distribution of two orthogonally polarized light spots at 45° and 60° polarization. According to Malus's law, at 45° polarization, the two spot intensities are equal—that is, the intensity of the horizontally polarized light spot is equal to the intensity of the vertically polarized light spot. At 60° polarization, the intensity of the horizontally polarized light spot is twice that of the vertically polarized light spot. Based on the detected spot intensities, the polarization state of the light reflected from the magnetic object can be inverted and calculated. By comparing the polarization state of the light emitted by the collimated laser light source with the polarization state of the light reflected from the object, the Kerr effect can be used to recover the magnetic field information of the object.

[0031] To summarize, the metasurface-based magneto-optical imaging system provided in this embodiment replaces the polarization beam splitting prism in the magneto-optical imaging system with a metasurface device, and uses the metasurface device to split the polarized output light into horizontal polarization and vertical polarization, thereby increasing the polarization separation angle. This eliminates the need to increase the distance between the metasurface device and the receiving device, thereby shortening the length of the magneto-optical imaging system. Moreover, since the metasurface device is smaller in size than the polarization beam splitting prism, the volume of the entire magneto-optical imaging system is reduced.

[0032] The present invention has been described above in conjunction with the best embodiments, but the present invention is not limited to the embodiments disclosed above, but should cover various modifications and equivalent combinations based on the essence of the present invention.

Claims

1. A magneto-optical imaging system based on a metasurface, characterized in that: According to the direction of optical transmission, the system includes: a collimated laser light source, a magnetic object to be measured, a metasurface device and a receiving device, wherein the object to be measured is used to rotate the polarization state of the collimated laser emitted by the collimated laser light source, the metasurface device is used to split the polarized output light into horizontal polarization and vertical polarization, and the receiving device is used to receive two beams of light with different intensities.

2. The metasurface-based magneto-optical imaging system according to claim 1, wherein: The metasurface device includes a substrate and a metasurface unit disposed on the substrate.

3. The metasurface-based magneto-optical imaging system according to claim 2, wherein: The metasurface unit is composed of a microstructure array, which includes a plurality of microstructures. The phase distribution of the metasurface unit is changed by the type, size and arrangement of the microstructures. The phase change range is Inside.

4. The metasurface-based magneto-optical imaging system according to claim 3, wherein: The microstructure is a structure with birefringence characteristics.

5. The metasurface-based magneto-optical imaging system according to claim 3, wherein: The microstructures are arranged in a tetragonal or hexagonal manner.

6. The metasurface-based magneto-optical imaging system according to claim 3, wherein: The substrate and the microstructure are both made of materials that are transparent to the wavelength of the collimated laser light source, and the refractive index of the microstructure is greater than the refractive index of the substrate.

7. The metasurface-based magneto-optical imaging system according to any one of claims 1 to 6, wherein: The collimated laser light source is a laser that can emit collimated laser light, and the receiving device is a device including an image sensor.

8. A design method for a metasurface-based magneto-optical imaging system, applied to the metasurface-based magneto-optical imaging system according to any one of claims 1 to 7, characterized in that: The method comprises: The relationship between microstructure phase delay and microstructure size is determined based on the full-wave simulation algorithm, and different phase delays are obtained by changing the size of the microstructure. According to the beam splitting function to be achieved under horizontal polarization and vertical polarization, the phase of the metasurface unit is designed to obtain the target phase; The phase delay is matched to the target phase.

9. The design method of the metasurface-based magneto-optical imaging system according to claim 8, wherein: The formula for designing the phase of the metasurface unit is as follows: ； Among them, the formula φ x_pol 、 φ y_pol are the target phases that need to be met under horizontal polarization and vertical polarization, respectively. λ is the wavelength of the output light in the polarization state. f is the focal length, which is the position of the focused light spot from the metasurface. (x, y) is the spatial coordinate of the metasurface unit. f is the distance between the two light spots in the y direction.

10. The method for designing a metasurface-based magneto-optical imaging system according to claim 8, wherein: The formula for matching the phase delay to the target phase is as follows: ； in, φ x 、 φ y It refers to the phase delay that the microstructure can meet under horizontal polarization and vertical polarization. FOM is the sum of the phase difference values, and meta represents the coordinate set inside the metasurface unit.

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