Method for inducing magnetizing field in any orientation to super diffraction limit

Abstract

The invention belongs to the technical field of magnetization field customization, and particularly provides a method for inducing a magnetization field in any direction to a super diffraction limit. According to the invention, only a radiation field of one magnetic dipole antenna is utilized, and a first-order corresponding spatial spiral phase is superposed on the radiation field, so that an analytical expression of a vector light field at an entrance pupil plane of the optical tight focusing system can be reversely obtained; quantitatively evaluating a tight focusing field of a vector light field by adopting a Debyy vector diffraction integral theory, and evaluating characteristics of a target magnetization field induced by the tight focusing field in a magnetic material based on an inverse Faraday effect; according to the method disclosed by the invention, a complicated optimization process is not needed, the pure transverse circular polarization near-diffraction limit focal spot is constructed, the super-diffraction limit magnetizing field capable of specifying the spatial orientation is induced, and the focal field customized by the method and the induced magnetizing field have wide application prospects in the fields of optical tweezers, optical processing, all-optomagnetic storage and the like.

Description

Technical Field

[0001] The invention relates to the technical field of magnetization field customization, and in particular to a method for inducing a magnetization field in any direction beyond the diffraction limit. Background Art

[0002] Magneto-optical recording technology relies on the interaction between the optical focal field and magneto-optical materials, and combines the dual advantages of magnetic storage and optical storage. It is expected to become one of the important solutions to meet the huge data transmission, processing and storage needs in the era of big data. In order to achieve the goal of large storage capacity and high storage density, the core premise of magneto-optical storage technology is the high-resolution magnetization field generated by light based on the inverse Faraday effect. Faced with this problem, in 2013, the research team of Min Gu focused the diagonally polarized vortex light tightly and acted on isotropic magneto-optical materials, successfully creating a sub-diffraction pure longitudinal magnetization field with a size of 0.508λ in the focal area, and achieved efficient reversal of the magnetization field by adjusting the phase direction of the vortex light. In 2014, the research team introduced a double-ring vortex binary filter with a π phase difference in the tight focusing system to modulate the diagonally polarized light wavefront, thereby inducing the generation of an ultra-long pure longitudinal magnetic needle with a lateral resolution of 0.38λ and a length of 7.48λ. In 2015, JiannongChen's research team used a special annular phase filter with adjustable radial and angular directions to modulate radially polarized vortex light, and the length and transverse full width at half maximum (FWHM) of the resulting magnetic needle reached 28λ and 0.27λ respectively. After that, the researchers successively obtained a longitudinal magnetization field with better performance by optimizing the design of the entrance pupil field.

[0003] Analysis of the above public reports shows that although these pure longitudinal magnetization fields provide possibilities for two-dimensional high-density all-optical magnetic recording and large-capacity magneto-optical storage, the lateral super-resolution magnetic needle is only along the longitudinal direction, with a single direction, and cannot achieve three-dimensional high-density super-resolution magneto-optical recording and storage, and generally requires complex optimization design to obtain a complex entrance pupil vector light field. In 2018, Xiangping Li's research group used the radiation field of a pair of orthogonal dipoles to reversely construct a complex entrance pupil field without complex optimization to induce a magnetization field with a predeterminable direction. However, the FWHM size of this magnetization field is 0.56λ´0.56λ´1.31λ, which does not exceed the diffraction limit. In view of the above-mentioned problems, the inventor of this case conducted an in-depth study on the problem, which led to the creation of this case. Summary of the invention

[0004] In response to the above problems, the present invention provides a method for inducing a super-diffraction limit magnetization field in any orientation. This method only utilizes the radiation field of a magnetic dipole antenna and cleverly superimposes the corresponding spatial spiral phase to obtain the analytical expression of the required entrance pupil light field in reverse, and construct a pure transverse circularly polarized near-diffraction limit light focus, and induce a super-diffraction limit magnetization field with a specified spatial orientation.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for inducing a super-diffraction limit magnetization field in any direction, the method comprising the following steps: An optical tight focusing system is established by two objective lenses with a confocal area; placing a magnetic dipole antenna in the confocal region of the optical tight focusing system; The radiation field of the magnetic dipole antenna is calculated. After the spiral phase factor with the same spatial orientation is superimposed on it, the radiation field radiates outward from the confocal area of ​​the optical tight focusing system, and then propagates outward from the optical tight focusing system after being collimated by two identical high numerical aperture objective lenses. The radiation field at the entrance pupil plane of the objective lens is obtained based on the bending effect of the lens on the radiation light. The radiation field at the entrance pupil plane of the objective lens is reversed by time reversal technology, propagated in the opposite direction, and focused toward the origin of the confocal area of ​​the two objective lenses, so as to form a purely transverse circularly polarized near-diffraction-limited optical focal field in the confocal area relative to the spatial orientation of the magnetic dipole antenna; wherein, the incident field phases of the entrance pupil planes on both sides of the optical tight focusing system differ by 180 degrees; Based on the inverse Faraday effect, magneto-optical materials are placed in the confocal area of ​​two objective lenses, and the purely transverse circularly polarized near-diffraction-limited light focal field can induce a super-diffraction-limited magnetization field with a specified spatial orientation therein.

[0006] Furthermore, the optical tight focusing system is composed of two identical high numerical aperture objective lenses placed confocally and symmetrically along the Z axis; In the optical tight focusing system, the confocal point is taken as the origin of the rectangular coordinate system, and the confocal plane is taken as the XOY plane. At this time, the two objective lenses are respectively located at ±f of the Z axis, where f is the focal length of the objective lens.

[0007] Furthermore, the magnetic dipole antenna is a magnetic dipole antenna arranged at the origin of the optical tight focusing system and carrying a uniform and in-phase high-frequency oscillating magnetic current; The spatial orientation of the magnetic dipole antenna is ; is the polar angle, which is the angle between the magnetic dipole antenna and the Z axis; is the azimuth, which is the angle between the projection of the magnetic dipole antenna on the XOY plane and the positive direction of the X axis; The orientation of the magnetic dipole antenna in three-dimensional space can be determined.

[0008] Furthermore, the total radiation field of the magnetic dipole antenna is calculated as follows: According to electromagnetic radiation theory, the radiation field of the magnetic dipole antenna located on the X, Y and Z axes is: (1) Space Orientation The magnetic dipole antenna radiation field is composed of the coherent superposition of the magnetic dipole antenna radiation fields located on the X, Y and Z axes respectively, and their radiation weight factors are: (2) Can get space orientation The radiation field of the magnetic dipole antenna is: (3) Among them, the constant coefficient of the radiation field , Radiation Field Coefficient of component and radiation field Coefficient of component The specific expressions are shown in equations (4), (5) and (6) respectively: (4) (5) (6) in, is an imaginary unit, is the oscillation angular frequency of the magnetic current on the magnetic dipole antenna, is the dielectric constant of vacuum, is the wave impedance of vacuum, and are the magnitude and geometric length of the oscillating magnetic current of the magnetic dipole antenna, is the spatial spherical coordinate of the radiation field, and is the unit vector in spherical coordinates.

[0009] Furthermore, along the space The calculation process of the spiral phase factor is as follows: 1) Taking the XYZ global coordinate system as the rotating whole, the origin O of the reference coordinate is the rotation fulcrum, and the Z axis is rotated one step along the oscillation direction of the magnetic dipole antenna and the plane where the OZ axis is located to direction, forming a rotated Axis; 2) The X-axis and Y-axis of the global coordinate system rotate synchronously to and axis, , and are the three principal axes of the local coordinate system after rotation; 3) the spatial direction is derived The spiral phase factor is:

[0010] (7) in, is the topological charge number, is the local coordinate system The azimuth of the plane; to induce the target magnetic field, the topological charge The value must be ±1; In order to obtain the incident field required to induce the target magnetization field, the coordinate origin and the spatial orientation The radiation field of the magnetic dipole antenna is superimposed with the spiral phase factor of the same spatial orientation shown in equation (7), and the expression of the total radiation field after superposition is obtained as shown in equation (8).

[0011] (8) Furthermore, due to the bending of the radiation light by the lens, an objective lens satisfying the sine condition is selected to form an optical tight focusing system, and its apodization function is , then the radiation field at the entrance pupil of the objective can be obtained as: (9) in, is the polar coordinate of the entrance pupil surface, is the distance between the entrance pupil observation point and the entrance pupil center, is the azimuth of the observation point on the entrance pupil plane, The component of the radiation field The weight is bent.

[0012] Furthermore, based on the time reversal technology, the radiation field at the entrance pupil plane of the objective is reversed, propagated in the opposite direction, and focused to the origin of the confocal area of ​​the two objective lenses, forming the target light focal field in the confocal area; at this time, the incident field phases of the entrance pupil planes on both sides of the optical tight focusing system differ by 180 degrees; the focal field distribution in the confocal area can be calculated and quantitatively evaluated by the Deby vector diffraction integral theory, and the calculation formula is: (10) in, is the spherical wavefront after the entrance pupil field is bent by the objective lens is the cylindrical coordinate of the focal field; the optical focal field constructed in this application is a purely transverse circularly polarized near-diffraction-limited optical focal field, where the pure transverse direction is relative to the spatial orientation of the magnetic dipole antenna In terms of.

[0013] Furthermore, based on the inverse Faraday effect, magneto-optical materials are set at the confocal area of ​​the two objective lenses. The highly localized transverse circularly polarized light field can induce a spatial orientation on the magneto-optical material. The same or opposite photoinduced magnetization field, which is calculated by equation (11): (11) in, and is the electric field vector of the focal field and its conjugate vector, is the coupling coefficient related to the magneto-optical material.

[0014] From the above description of the structure of the present invention, it can be seen that compared with the prior art, the present invention has the following advantages: The present invention utilizes only the radiation field of a magnetic dipole antenna and superimposes a first-order corresponding spatial spiral phase thereon, so as to reversely obtain an analytical expression of the vector light field at the entrance pupil plane of the optical tight focusing system; the Deby vector diffraction integral theory is used to quantitatively evaluate the tight focusing field of the vector light field, and based on the inverse Faraday effect, the characteristics of the target magnetization field induced by this tight focusing field in the magnetic material are evaluated; the method of the present invention does not require a complex optimization process, and constructs a pure transverse circularly polarized near-diffraction-limited light focal spot, and induces a super-diffraction-limited magnetization field with a specified spatial orientation. The light focal field customized according to the method and the induced magnetization field have broad application prospects in the fields of optical tweezers, optical processing, all-optical magnetic storage, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 is a schematic diagram of an optical tight focusing system of the present invention; Figure 2 For Example 1 When , the constructed optical focal field; Figure 3 For Example 1 When , the induced pure longitudinal magnetization field; Figure 4 For Example 2 When , the constructed optical focal field; Figure 5 For Example 2 When , the induced pure transverse magnetization field; Figure 6 For Example 3 When , the constructed optical focal field; Figure 7 For Example 3 When , the induced regulation direction is the magnetizing field. DETAILED DESCRIPTION

[0016] In order to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0017] like Figure 1 As shown, a method for inducing a super-diffraction-limited magnetization field in an arbitrary direction comprises the following steps: establishing an optical tight focusing system by two objective lenses with a confocal region; placing a magnetic dipole antenna in the confocal region of the optical tight focusing system; calculating the radiation field of the magnetic dipole antenna, superimposing a spiral phase factor of the same spatial orientation on the radiation field, radiating the radiation field outward from the confocal region of the optical tight focusing system, and propagating to the outside of the optical tight focusing system after being collimated by two identical high numerical aperture objective lenses, and obtaining the object according to the bending effect of the lens on the radiated light. The radiation field at the entrance pupil plane of the objective lens is reversed by time reversal technology, propagated in the opposite direction, and focused to the origin of the confocal area of ​​the two objective lenses, so as to form a purely transverse circularly polarized near-diffraction-limited optical focal field relative to the spatial orientation of the magnetic dipole antenna in the confocal area; wherein, the incident field phases of the entrance pupil planes on both sides of the optical tight focusing system differ by 180 degrees; based on the inverse Faraday effect, magneto-optical materials are placed in the confocal area of ​​the two objective lenses, and the purely transverse circularly polarized near-diffraction-limited optical focal field can induce a super-diffraction-limited magnetization field with a specified spatial orientation therein.

[0018] The present invention utilizes only the radiation field of a magnetic dipole antenna and superimposes a first-order corresponding spatial spiral phase thereon, so as to reversely obtain an analytical expression of the vector light field at the entrance pupil plane of the optical tight focusing system; the Deby vector diffraction integral theory is used to quantitatively evaluate the tight focusing field of the vector light field, and based on the inverse Faraday effect, the characteristics of the target magnetization field induced by this tight focusing field in the magnetic material are evaluated; the method of the present invention does not require a complex optimization process, and constructs a pure transverse circularly polarized near-diffraction-limited light focal spot, and induces a super-diffraction-limited magnetization field with a specified spatial orientation. The light focal field customized according to the method and the induced magnetization field have broad application prospects in the fields of optical tweezers, optical processing, all-optical magnetic storage, etc.

[0019] The specific implementation steps of the method of the present invention are now introduced in detail: Step 1: Build an optical tight focusing system: The present application uses two identical high numerical aperture objective lenses placed confocally and symmetrically along the Z axis to form an optical tight focusing system, such as Figure 1 As shown. With the confocal point as the origin of the rectangular coordinate system and the confocal plane as the XOY plane, the two objective lenses are located at ±f of the Z axis respectively, and f is the focal length of the objective lens.

[0020] Step 2: Configure the magnetic dipole antenna and solve its radiation field: A magnetic dipole antenna (with a uniform and in-phase high-frequency oscillating magnetic flux) is arranged at the origin of the optical tight focusing system. Figure 1The origin is represented by a short thick line), and its spatial orientation is . is the polar angle, which is the angle between the oscillator direction and the Z axis; is the azimuth, which is the angle between the projection of the vibrator direction on the XOY plane and the positive direction of the X axis; It can determine the orientation of magnetic dipoles in three-dimensional space; According to electromagnetic radiation theory, the radiation field of the magnetic dipole located on the X, Y and Z axes is: (1) Space Orientation The magnetic dipole radiation field of is composed of the coherent superposition of the magnetic dipole radiation fields located on the X, Y and Z axes, and their radiation weight factors are: (2) Can get space orientation The magnetic dipole radiation field is: (3) The constant coefficient of the radiation field in the above formula is , Radiation Field Coefficient of component and radiation field Coefficient of component The specific expressions are shown in equations (4), (5) and (6) respectively: (4) (5) (6) in, is an imaginary unit, is the oscillation angular frequency of the magnetic current on the magnetic dipole antenna, is the dielectric constant of vacuum, is the wave impedance of vacuum, and are the magnitude and geometric length of the oscillating magnetic current of the magnetic dipole antenna, is the spatial spherical coordinate of the radiation field, and is the unit vector in spherical coordinates.

[0021] Step 3: Superimpose the corresponding spiral phase factor on the radiation field calculated in step 2: Along the space direction The calculation process of the spiral phase factor is as follows: 1) Take the XYZ global coordinate system as the rotation system, Figure 1 The origin O of the reference coordinates shown is the rotation fulcrum, and the Z axis rotates along the oscillation direction of the magnetic dipole and the plane where the OZ axis is located to direction, forming a rotated Axis; 2) The X-axis and Y-axis of the global coordinate system rotate synchronously to and axis, , and are the three principal axes of the local coordinate system after rotation; 3) the spatial direction is derived The spiral phase factor is: (7) in, is the topological charge number, is the local coordinate system The azimuth of the plane; to induce the target magnetic field, the topological charge The value must be ±1; In order to obtain the incident field required to induce the target magnetization field, the coordinate origin and the spatial orientation The radiation field of the magnetic dipole antenna is superimposed with the spiral phase factor of the same spatial orientation shown in equation (7), and the expression of the total radiation field after superposition is obtained as shown in equation (8).

[0022] (8) Step 4: Derivation of the vector light field at the pupil based on the total radiation field in step 3: The radiation field calculated in step 3 is superimposed and radiated outward from the confocal area, and then propagated outward after being collimated by two identical high numerical aperture objective lenses. Due to the bending of the radiation light by the lens, an objective lens that satisfies the sine condition is selected to form an optical tight focusing system, and its apodization function is: , then the radiation field at the entrance pupil of the objective can be obtained as: (9) in, is the polar coordinate of the entrance pupil surface, is the distance between the entrance pupil observation point and the entrance pupil center, is the azimuth of the observation point on the entrance pupil. It should be noted that The component of the radiation field The weight is bent.

[0023] Step 5: Reverse the light path, focus in the opposite direction, and calculate the light field distribution in the focal area: Based on the time reversal technology, the radiation field at the entrance pupil of the objective is reversed, propagated in the opposite direction, and focused to the origin of the confocal area of ​​the two objective lenses, forming the target light focal field in the confocal area; at this time, the incident field phase of the entrance pupil on both sides of the optical tight focusing system differs by 180 degrees; the focal field distribution in the confocal area can be calculated and quantitatively evaluated by the Deby vector diffraction integral theory, and the calculation formula is: (10) in, is the spherical wavefront of the entrance pupil field after being bent by the objective lens, is the cylindrical coordinate of the focal field; the optical focal field constructed in this application is a purely transverse circularly polarized near-diffraction-limited optical focal field, where the pure transverse direction is relative to the spatial orientation of the magnetic dipole antenna In terms of.

[0024] Step 6: Calculate the photoinduced magnetization field on the magneto-optical material based on the tightly focused light field in step 5: Based on the inverse Faraday effect, Figure 1 The magneto-optical material is set at the confocal area of ​​the magneto-optical material. The highly localized transverse circularly polarized light field constructed in step 5 can induce a polarization in the magneto-optical material that is consistent with the spatial orientation. The same or opposite photoinduced magnetization field (depending on the topological charge The magnetization field is calculated by equation (11): (11) In the formula and is the electric field vector of the focal field and its conjugate vector, is the coupling coefficient related to the magneto-optical material; the pure transverse circular polarization near diffraction limit light focal field constructed by step 5 in this application can induce a spatial orientation Super-diffraction-limited photoinduced magnetization field.

[0025] Several specific examples are given below to demonstrate the effectiveness of the method proposed by the present invention.

[0026] To simplify the calculation, the parameters C and γ that are independent of the shape and polarization of the optical focal field and the induced magnetization field are normalized in the examples listed, that is, C=1, γ=1; to converge the entire radiation field of the magnetic dipole with the superimposed spiral phase factor, the high numerical aperture objective lens convergence angle is taken as ,Right now ; Topological charge The signs of are different, and the induced magnetic field directions are opposite. =1; the objective lens that satisfies the sine condition is used as the objective lens used in the embodiment of the present invention.

[0027] Example 1: Inducing a pure longitudinal super-diffraction-limited photoinduced magnetization field: Set the spatial orientation of the magnetic dipole at the origin , the electric field distribution in the focal area is calculated using formula (10). Figure 2 and Figure 3 They are the normalized intensity and polarization distribution diagrams of the constructed optical focal field and the magnetization field induced in the magneto-optical material in three principal planes. Figure 2 and Figure 3It can be seen that they are all short ellipsoids along the Z axis. Figure 2 It can be seen that the main lobe area of ​​the optical focal field is pure circular polarization in the XY plane, and the XZ and YZ planes are respectively distributed in X- and Y-linear polarization. The measured FWHM of the optical focal spot along the X, Y, and Z axes are 0.43λ, 0.43λ, and 0.51λ, respectively, which is a near-diffraction-limited optical focal spot. Figure 3 It can be seen that Figure 2 The magnetic field induced by the optical focal field has only Z component, and is distributed in Z linear polarization in both XZ and YZ planes, which is a pure longitudinal magnetic field. The FWHM of this magnetic field along the X, Y and Z axes are measured to be 0.29λ, 0.29λ and 0.37λ respectively, which is a super diffraction limited magnetic field.

[0028] Example 2: Inducing a pure transverse super-diffraction-limited photoinduced magnetization field: In order to induce a pure transverse super-diffraction-limited photoinduced magnetization field, the polar angle of the magnetic dipole space is , that is, the magnetic dipole is placed horizontally in the XY plane. In this example, the azimuth angle is As an example, a method similar to that in Example (1) is used to calculate the light focal field distribution of the focal area using formula (10).

[0029] Figure 4 and Figure 5 They are the normalized intensity and polarization distribution diagrams of the constructed optical focal field and the magnetization field induced in the magneto-optical material in three principal planes. Figure 4 and Figure 5 It can be seen that they are all short ellipsoids along the X axis. Figure 4 It can be seen that the main lobe area of ​​the optical focal field is a pure circular polarization distribution in the YZ plane, and the XZ and XY planes are respectively Z-linear polarization and Y-linear polarization distribution. The measured FWHM of the optical focal spot along the X, Y and Z axes are 0.51λ, 0.43λ and 0.43λ respectively, which is a near-diffraction-limited optical focal spot. Figure 5 It can be seen that Figure 4 The magnetic field induced by the focal field of the light is X-polarized in both the XZ and XY planes, which is a pure X-axial magnetic field. The FWHM of this magnetic field along the X, Y and Z axes is measured to be 0.37λ, 0.29λ and 0.29λ respectively, which is a super-diffraction-limited magnetic field. In this example, if the azimuth angle of the magnetic dipole is , then the induced magnetic field polarization direction in the focal plane will be adjusted to direction.

[0030] Embodiment 3: Inducing an arbitrary prescribed direction to a photoinduced magnetization field beyond the diffraction limit: The above-mentioned Examples 1 and 2 demonstrate the effectiveness of the method proposed by the present invention in inducing pure longitudinal and pure transverse photoinduced magnetization fields beyond the diffraction limit. In order to demonstrate the effectiveness of the method inducing magnetization fields in arbitrary directions, without loss of generality, the pointing parameter For example, the focal field of the focal area light is calculated using formula (10), and the induced target magnetization field is calculated using formula (11). Their characteristics are as follows: Figure 6 and Figure 7 shown.

[0031] Figure 6 and Figure 7 They are the normalized intensity and polarization distribution diagrams of the constructed optical focal field and the magnetization field induced in the magneto-optical material in three principal planes. Figure 6 and Figure 7 It can be seen that the long axis direction of the short ellipsoid intensity profile presented by the light focus and the magnetization field is not located on the Z axis or XY plane. Its spatial orientation is measured to be the same as that of the magnetic dipole. Figure 6 It can be seen that the main lobe of the optical focal field is elliptically polarized in the three main planes, as shown by Figure 7 It can be seen that Figure 6 The magnetic field induced by the optical focal field is linearly polarized in the three main planes, and its polarization direction is measured to be the same as that of the magnetic dipole.

[0032] The effectiveness of the method proposed by the present invention is demonstrated by the above examples 1, 2 and 3. By setting the spatial orientation parameter of the magnetic dipole By superimposing the first-order spiral phase factor of the corresponding orientation, a purely transverse circularly polarized near-diffraction-limited optical focal field can be constructed, and a spatial orientation can be induced in the magnetic material. Super-diffraction-limited photoinduced magnetization field.

[0033] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for inducing a magnetization field in any direction beyond the diffraction limit, characterized in that: The method comprises the following steps: An optical tight focusing system is established by two objective lenses with a confocal area; placing a magnetic dipole antenna in the confocal region of the optical tight focusing system; The radiation field of the magnetic dipole antenna is calculated. After the spiral phase factor with the same spatial orientation is superimposed on it, the radiation field radiates outward from the confocal area of ​​the optical tight focusing system, and then propagates outward from the optical tight focusing system after being collimated by two identical high numerical aperture objective lenses. The radiation field at the entrance pupil plane of the objective lens is obtained based on the bending effect of the lens on the radiation light. The radiation field at the entrance pupil plane of the objective lens is reversed by time reversal technology, propagated in the opposite direction, and focused toward the origin of the confocal area of ​​the two objective lenses, so as to form a purely transverse circularly polarized near-diffraction-limited optical focal field in the confocal area relative to the spatial orientation of the magnetic dipole antenna; wherein, the incident field phases of the entrance pupil planes on both sides of the optical tight focusing system differ by 180 degrees; Based on the inverse Faraday effect, magneto-optical materials are placed in the confocal area of ​​two objective lenses, and the purely transverse circularly polarized near-diffraction-limited light focal field can induce a super-diffraction-limited magnetization field with a specified spatial orientation therein.

2. The method for inducing a super-diffraction-limited magnetization field in any direction according to claim 1, characterized in that: The optical tight focusing system is composed of two identical high numerical aperture objective lenses placed confocally and symmetrically along the Z axis; In the optical tight focusing system, the confocal point is taken as the origin of the rectangular coordinate system, and the confocal plane is taken as the XOY plane. At this time, the two objective lenses are respectively located at ±f of the Z axis, where f is the focal length of the objective lens.

3. The method for inducing a super-diffraction-limited magnetization field in any direction according to claim 2, characterized in that: The magnetic dipole antenna is a magnetic dipole antenna arranged at the origin of the optical tight focusing system and carrying a uniform and in-phase high-frequency oscillating magnetic current; The spatial orientation of the magnetic dipole antenna is ; is the polar angle, which is the angle between the magnetic dipole antenna and the Z axis; is the azimuth, which is the angle between the projection of the magnetic dipole antenna on the XOY plane and the positive direction of the X axis; The orientation of the magnetic dipole antenna in three-dimensional space can be determined.

4. The method for inducing a super-diffraction-limited magnetization field in any direction according to claim 3, characterized in that: The total radiation field of the magnetic dipole antenna is calculated as follows: According to electromagnetic radiation theory, the radiation field of the magnetic dipole antenna located on the X, Y and Z axes is: （1） Space Orientation The magnetic dipole antenna radiation field is composed of the coherent superposition of the magnetic dipole antenna radiation fields located on the X, Y and Z axes respectively, and their radiation weight factors are: （2） Can get space orientation The radiation field of the magnetic dipole antenna is: （3） Among them, the constant coefficient of the radiation field , Radiation Field Coefficient of component and radiation field Coefficient of component The specific expressions are shown in equations (4), (5) and (6) respectively: （4） （5） （6） in, is an imaginary unit, is the oscillation angular frequency of the magnetic current on the magnetic dipole antenna, is the dielectric constant of vacuum, is the wave impedance of vacuum, and are the magnitude and geometric length of the oscillating magnetic current of the magnetic dipole antenna, is the spatial spherical coordinate of the radiation field, and is the unit vector in spherical coordinates.

5. The method for inducing a super-diffraction-limited magnetization field in any direction according to claim 4, characterized in that: Along the space direction The calculation process of the spiral phase factor is as follows: 1) Taking the XYZ global coordinate system as the rotating whole, the origin O of the reference coordinate is the rotation fulcrum, and the Z axis is rotated one step along the oscillation direction of the magnetic dipole antenna and the plane where the OZ axis is located to direction, forming a rotated Axis; 2) The X-axis and Y-axis of the global coordinate system rotate synchronously to and axis, , and are the three principal axes of the local coordinate system after rotation; 3) the spatial direction is derived The spiral phase factor is: （7） in, is the topological charge number, is the local coordinate system The azimuth of the plane; to induce the target magnetic field, the topological charge The value must be ±1; In order to obtain the incident field required to induce the target magnetization field, the coordinate origin and the spatial orientation The radiation field of the magnetic dipole antenna is superimposed with the spiral phase factor of the same spatial orientation shown in formula (7), and the expression of the total radiation field after superposition is obtained as shown in formula (8): （8）。 6. The method for inducing a super-diffraction-limited magnetization field in any direction according to claim 5, characterized in that: Due to the bending of the radiation light by the lens, an objective lens that satisfies the sine condition is selected to form an optical tight focusing system, and its apodization function is , then the radiation field at the entrance pupil of the objective can be obtained as: （9） in, is the polar coordinate of the entrance pupil surface, is the distance between the entrance pupil observation point and the entrance pupil center, is the azimuth of the observation point on the entrance pupil plane, The component of the radiation field The weight is bent.

7. The method for inducing a super-diffraction-limited magnetization field in any direction according to claim 6, characterized in that: Based on the time reversal technology, the radiation field at the entrance pupil of the objective is reversed, propagated in the opposite direction, and focused to the origin of the confocal area of ​​the two objective lenses, forming the target light focal field in the confocal area; at this time, the incident field phase of the entrance pupil on both sides of the optical tight focusing system differs by 180 degrees; the focal field distribution in the confocal area can be calculated and quantitatively evaluated by the Deby vector diffraction integral theory, and the calculation formula is: 　（10） in, is the spherical wavefront of the entrance pupil field after being bent by the objective lens, is the cylindrical coordinate of the focal field; the optical focal field constructed in this application is a purely transverse circularly polarized near-diffraction-limited optical focal field, where the pure transverse direction is relative to the spatial orientation of the magnetic dipole antenna In terms of.

8. The method of inducing a super-diffraction-limited magnetization field in any direction according to claim 7, characterized in that: Based on the inverse Faraday effect, magneto-optical materials are placed at the confocal area of ​​the two objective lenses. The highly localized transverse circularly polarized light field can induce a spatial orientation-related polarization on the magneto-optical material. The same or opposite photoinduced magnetization field, which is calculated by equation (11): （11） in, and is the electric field vector of the focal field and its conjugate vector, is the coupling coefficient related to the magneto-optical material.

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