A method for inducing a super-diffraction-limited magnetization field in any orientation

Abstract

The present invention belongs to the technical field of magnetization field customization, and in particular provides a method for inducing a super-diffraction-limited magnetization field in any orientation. The present invention only utilizes the radiation field of a single magnetic dipole antenna and superimposes a first-order corresponding spatial spiral phase thereon, and then the analytical expression of the vector optical field at the pupil plane of an optical tight focusing system can be obtained reversely; the Deby vector diffraction integral theory is used to quantitatively evaluate the tight focusing field of the vector optical field, and based on the inverse Faraday effect, the characteristics of the target magnetization field induced by this tight focusing field in a magnetic material are evaluated; the method of the present invention does not require a complex optimization process, and constructs a purely transverse circularly polarized near-diffraction-limited optical focal spot and induces a super-diffraction-limited magnetization field with a specified spatial orientation. The optical focal field customized by this method and the induced magnetization field have broad application prospects in the fields of optical tweezers, optical processing, all-optical magnetic storage, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetization field customization, and particularly to a method for inducing a super-diffraction-limited magnetization field in any orientation. Background Art

[0002] Magneto-optical recording technology relies on the interaction between the optical focal field and magneto-optical materials, integrating the dual advantages of magnetic storage and optical storage, and is expected to become one of the important solutions to meet the huge data transmission, processing, and storage requirements in the big data era. To achieve the goals of large storage capacity and high storage density, the core prerequisite of magneto-optical storage technology lies in the high-resolution magnetization field generated by light based on the inverse Faraday effect. Facing this problem, in 2013, the research team of Min Gu tightly focused the azimuthally polarized vortex light and applied it to the isotropic magneto-optical material, successfully creating a sub-diffraction pure longitudinal magnetization field with a size of 0.508λ in the focal region, and realizing the efficient flipping of the magnetization field by adjusting the phase direction of the vortex light. In 2014, this scientific research team introduced a double-ring vortex binary filter with a π phase difference into the tight focusing system to modulate the azimuthally polarized light wavefront, thereby inducing the generation of an ultra-long pure longitudinal magnetic needle with a transverse resolution of 0.38λ and a length of 7.48λ. In 2015, the research team of Jiannong Chen used a special annular phase filter with adjustable radial and azimuthal directions to modulate the radially polarized vortex light, and the length and transverse full width at half maximum (FWHM) of the obtained magnetic needle reached 28λ and 0.27λ respectively. Subsequently, researchers have successively obtained better longitudinal magnetization fields by optimizing the design of the entrance pupil field.

[0003] Analyzing the above publicly reported content, it can be seen that although these pure longitudinal magnetization fields provide the possibility for two-dimensional high-density all-optical magnetic recording and large-capacity magneto-optical storage, the magnetic needles with transverse super-resolution only extend along the longitudinal direction, with a single direction, and cannot achieve three-dimensional high-density super-resolution magneto-optical recording and storage. Moreover, generally, complex optimization designs are required to obtain a complex entrance pupil vector light field. In 2018, the research group of Xiangping Li used the radiation fields of a pair of orthogonal dipoles to inversely construct a complex entrance pupil field without complex optimization to induce the generation of a magnetization field with a specified orientation. However, the size of the FWHM of this magnetization field is 0.56λ×0.56λ×1.31λ, which does not exceed the diffraction limit. In view of the above existing problems, the inventor of this case conducted in-depth research on this problem, and thus this case was generated. Summary of the Invention

[0004] In view of the above problems, the present invention provides a method for inducing a super-diffraction-limited magnetization field in any direction. By only using the radiation field of a single magnetic dipole antenna and ingeniously superimposing the corresponding spatial spiral phase, the analytical expression of the required pupil light field can be reversely obtained, and a purely transverse circularly polarized near-diffraction-limited light spot can be constructed, and a super-diffraction-limited magnetization field with a specified spatial orientation can be induced.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is: a method for inducing a super-diffraction-limited magnetization field in any direction, the method comprising the following steps:

[0006] Establish an optical tight focusing system by two objective lenses with a confocal region;

[0007] Place a magnetic dipole antenna in the confocal region of the optical tight focusing system;

[0008] Calculate the radiation field of the magnetic dipole antenna, and after superimposing a spiral phase factor with the same spatial orientation on it, the radiation field radiates out from the confocal region of the optical tight focusing system, propagates outward to the outside of the optical tight focusing system after being collimated by two identical high numerical aperture objective lenses, and the radiation field at the pupil plane of the objective lens is obtained according to the bending effect of the lens on the radiation light;

[0009] Reverse the radiation field at the pupil plane of the objective lens at this time through time reversal technology, propagate it in the reverse direction, and focus it on the origin of the confocal region of the two objective lenses to form a purely transverse circularly polarized near-diffraction-limited light focal field relative to the spatial orientation of the magnetic dipole antenna; wherein, the incident field phases of the two pupil planes on both sides of the optical tight focusing system differ by 180 degrees;

[0010] Based on the inverse Faraday effect, place a magneto-optical material in the confocal region of the 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 in it.

[0011] Furthermore, the optical tight focusing system is composed of two identical high numerical aperture objective lenses placed confocal symmetrically along the Z axis;

[0012] 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 on the Z axis, and f is the focal length of the objective lens.

[0013] Furthermore, the magnetic dipole antenna is a single magnetic dipole antenna arranged at the origin of the optical tight focusing system and carrying a uniform in-phase high-frequency oscillating magnetic current;

[0014] The spatial orientation of the magnetic dipole antenna is ; is the polar angle, and is the angle between the orientation of the magnetic dipole antenna and the Z axis; is the azimuth angle, which is the angle between the projection of the orientation of the magnetic dipole antenna on the XOY plane and the positive direction of the X-axis; through the parameter the orientation of the magnetic dipole antenna in three-dimensional space can be determined.

[0015] Furthermore, the calculation method of the total radiation field of the magnetic dipole antenna is specifically as follows:

[0016] According to the electromagnetic radiation theory, the radiation fields of the magnetic dipole antennas located on the X, Y, and Z axes are:

[0017] (1)

[0018] Spatial orientation of the magnetic dipole antenna radiation field is composed of the coherent superposition of the radiation fields of the magnetic dipole antennas located on the X, Y, and Z axes respectively, and their radiation weight factors are:

[0019] (2)

[0020] The radiation field of the magnetic dipole antenna with spatial orientation can be obtained as:

[0021] (3)

[0022] where the constant coefficient of the radiation field , the coefficient of the component of the radiation field and the coefficient of the component of the radiation field are specifically expressed as shown in equations (4), (5), and (6):

[0023] (4)

[0024] (5)

[0025] (6)

[0026] where is the imaginary unit, is the angular frequency of oscillation of the magnetic current on the magnetic dipole antenna, is the permittivity 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 respectively, is the spatial spherical coordinate of the radiation field, and are the unit vectors of the spherical coordinates.

[0027] Furthermore, along the spatial orientation The calculation process of the spiral phase factor is as follows: 1) Taking the XYZ global coordinate system as the rotation whole, with the origin O of the reference coordinate as the rotation fulcrum, the Z-axis rotates in one step along the plane where the magnetic dipole antenna oscillates and the OZ-axis to the direction, forming the rotated axis; 2) The X-axis and Y-axis of the global coordinate system rotate synchronously to the and axes, , and being the three main axes of the rotated local coordinate system; 3) It is derived that the spiral phase factor in the spatial direction is:

[0028] (7)

[0029] where, is the topological charge number, is the azimuth angle of the plane of the local coordinate system; To induce the target magnetization field, the topological charge number must take the value of ±1;

[0030] is the incident field required to obtain the induced target magnetization field. The radiation field of the magnetic dipole antenna located at the origin of coordinates and spatially oriented is superimposed with the spiral phase factor in the same spatial orientation as shown in formula (7) to obtain the expression of the total radiation field after superposition as shown in formula (8).

[0031] (8)

[0032] Furthermore, due to the bending of the radiation rays 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 plane of the objective lens can be obtained as:

[0033] (9)

[0034] where, are the polar coordinates of the entrance pupil plane, is the distance between the observation point on the entrance pupil plane and the center of the entrance pupil, is the azimuth angle of the observation point on the entrance pupil plane. The component of the entrance pupil is bent from the component of the radiation field.

[0035] Further, based on time-reversal technology, the radiation field at the pupil plane of the objective lens is reversed, propagated backward, and focused towards the origin of the confocal region of the two objective lenses, forming a target optical focal field in the confocal region; at this time, the phase difference of the incident fields on the pupil planes on both sides of the optical tight focusing system is 180 degrees; the focal field distribution in the confocal region can be calculated and quantitatively evaluated by the Deby vector diffraction integral theory, and the calculation formula is:

[0036] (10)

[0037] Among them, is the spherical wavefront after the pupil field is bent by the objective lens are the cylindrical coordinates of the focal field; the optical focal field constructed in this application is a pure transverse circularly polarized near-diffraction-limited optical focal field, and here the pure transverse is relative to the spatial orientation of the magnetic dipole antenna in terms of.

[0038] Further, based on the inverse Faraday effect, a magneto-optical material is arranged at the confocal region of the two objective lenses. From the highly localized transverse circularly polarized light field, a photoinduced magnetization field that is the same as or opposite to the spatial orientation can be induced on the magneto-optical material, and this magnetization field is calculated by Equation (11):

[0039] (11)

[0040] Among them, and are the electric field vector of the focal field and its conjugate vector, is the coupling coefficient related to the magneto-optical material.

[0041] 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:

[0042] The present invention only uses the radiation field of one magnetic dipole antenna and superimposes a first-order corresponding spatial spiral phase on it, and then the analytical expression of the vector light field at the pupil plane of the optical tight focusing system can be obtained reversely; 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 focal spot and induces a super-diffraction-limited magnetization field with a specified spatial orientation. The optical focal field customized by this 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

[0043] The drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0044] Figure 1 Schematic diagram of the optical tight focusing system of the present invention;

[0045] Figure 2 For Example 1 at the constructed optical focal field;

[0046] Figure 3 For Example 1 at the induced pure longitudinal magnetization field;

[0047] Figure 4 For Example 2 at the constructed optical focal field;

[0048] Figure 5 For Example 2 at the induced pure transverse magnetization field;

[0049] Figure 6 For Example 3 at the constructed optical focal field;

[0050] Figure 7 For Example 3 at the induced magnetization field with a specified orientation. Detailed implementation manners

[0051] 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 of the specification and specific implementation manners.

[0052] As Figure 1 shown, a method for inducing an arbitrarily oriented super-diffraction-limited magnetization field, the method comprising the following steps: establishing an optical tight focusing system by two objective lenses having 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, and after superimposing a spiral phase factor with the same spatial orientation thereon, the radiation field radiates outwards from the confocal region of the optical tight focusing system, is collimated by two identical high numerical aperture objective lenses and propagates outside the optical tight focusing system, and the radiation field at the entrance pupil plane of the objective lens is obtained according to the bending effect of the lens on the radiation light; reversing the radiation field at the entrance pupil plane of the objective lens at this time through time reversal technology, propagating in the reverse direction, and focusing towards the origin of the confocal region of the two objective lenses to form a pure transverse circularly polarized near-diffraction-limited optical focal field relative to the spatial orientation of the magnetic dipole antenna in the confocal region; wherein, the phase difference of the incident fields on the entrance pupil planes on both sides of the optical tight focusing system is 180 degrees; based on the inverse Faraday effect, a magneto-optical material is placed in the confocal region of the two objective lenses, and the pure transverse circularly polarized near-diffraction-limited optical focal field can induce a super-diffraction-limited magnetization field with a specified spatial orientation therein.

[0053] The present invention only utilizes the radiation field of a single magnetic dipole antenna and superimposes a first-order corresponding spatial helical phase thereon, and then an analytical expression of the vector optical field at the entrance pupil plane of the optical tight focusing system can be obtained reversely; the Deby vector diffraction integral theory is adopted to quantitatively evaluate the tight focusing field of the vector optical 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 optical focal spot and induces a super-diffraction-limited magnetization field with a specified spatial orientation. The optical focal field customized by this method and the induced magnetization field have broad application prospects in the fields of optical tweezers, optical processing, all-optical magnetic storage, etc.

[0054] Now, the specific implementation steps of the method of the present invention will be introduced in detail:

[0055] Step 1: Build an optical tight focusing system:

[0056] In this application, two identical high numerical aperture objective lenses are symmetrically placed confocal along the Z-axis to form an optical tight focusing system, as Figure 1 shown. Taking the confocal point as the origin of the rectangular coordinate system and the confocal plane as the XOY plane, at this time, the two objective lenses are respectively located at ±f on the Z-axis, and f is the focal length of the objective lens.

[0057] Step 2: Configure a magnetic dipole antenna and solve its radiation field:

[0058] A magnetic dipole antenna carrying a uniform in-phase high-frequency oscillating magnetic current is configured at the origin of the optical tight focusing system (represented by a short thick line at the origin), and its spatial orientation is Figure 1 . is the polar angle, which is the angle between the oscillator orientation and the Z-axis; is the azimuth angle, which is the angle between the projection of the oscillator orientation on the XOY plane and the positive direction of the X-axis; through the parameter , the orientation of the magnetic dipole in three-dimensional space can be determined; <0000...

[0059] According to the electromagnetic radiation theory, the radiation fields of the magnetic dipoles located on the X, Y, and Z axes are:

[0060] (1)

[0061] The radiation field with the spatial orientation is composed of the coherent superposition of the radiation fields of the magnetic dipoles located on the X, Y, and Z axes respectively, and their radiation weight factors are:

[0062] (2)

[0063] The radiation field of the magnetic dipole with the spatial orientation can be obtained as:

[0064] (3)

[0065] The constant coefficients of the above radiation field , the radiation field Coefficients of the component and the radiation field Coefficients of the component The specific expressions are shown in Eqs. (4), (5), and (6) respectively:

[0066] (4)

[0067] (5)

[0068] (6)

[0069] Among them, is the imaginary unit, is the oscillation angular frequency of the magnetic current on the magnetic dipole antenna, is the permittivity 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 respectively, is the spherical coordinate of the radiation field in space, and are the unit vectors of the spherical coordinate.

[0070] Step 3: Superimpose the corresponding helical phase factor on the radiation field calculated in Step (2):

[0071] The calculation process of the helical phase factor along the spatial direction is as follows: 1) Taking the XYZ global coordinate system as the rotation whole, with the origin O of the reference coordinate shown in Figure 1 as the rotation fulcrum, the Z-axis rotates in one step along the plane where the magnetic dipole oscillates and the OZ-axis to the direction, forming the rotated axis; 2) The X-axis and Y-axis of the global coordinate system rotate synchronously to the and axes, , and are the three main axes of the rotated local coordinate system; 3) It is deduced that the helical phase factor in the spatial direction is:

[0072] (7)

[0073] Among them, is the topological charge number, is the local coordinate system The azimuth angle of the plane; for inducing the target magnetization field, the topological charge number must take the value of ±1;

[0074] To obtain the incident field required to induce the target magnetization field, the radiation field of the magnetic dipole antenna located at the origin of coordinates and spatially oriented is superimposed with the helical phase factor in the same spatial orientation as shown in the superposition formula (7), and the expression of the total radiation field after superposition is as shown in formula (8).

[0075] (8)

[0076] Step Four: Derive the vector light field at the pupil based on the total radiation field in Step Three:

[0077] The radiation field after superposition calculated in Step Three is radiated outwards from the confocal region and propagates outwards 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 plane of the objective lens can be obtained as:

[0078] (9)

[0079] where are the polar coordinates of the entrance pupil plane, is the distance between the observation point on the entrance pupil plane and the center of the entrance pupil, is the azimuth angle of the observation point on the entrance pupil plane. It should be noted that the component of the entrance pupil is bent from the component of the radiation field.

[0080] Step Five: Reverse the optical path, focus in the reverse direction, and calculate the light field distribution in the focal region:

[0081] Based on the time reversal technology, reverse the radiation field at the entrance pupil plane of the objective lens, propagate it in the reverse direction, and focus it towards the origin of the confocal region of the two objective lenses to form the target optical focal field at the confocal region; at this time, the phase difference between the incident fields on the entrance pupil planes on both sides of the optical tight focusing system is 180 degrees; the focal field distribution in the confocal region can be calculated and quantitatively evaluated by the Deby vector diffraction integral theory, and the calculation formula is:

[0082] (10)

[0083] where is the spherical wavefront after the entrance pupil field is bent by the objective lens, are the cylindrical coordinates of the focal field; the optical focal field constructed in this application is a pure transverse circularly polarized near-diffraction-limited optical focal field, and here the pure transverse is relative to the spatial orientation of the magnetic dipole antenna in terms of.

[0084] Step 6. Calculate the optically induced magnetization field induced by the tightly focused light field in step 5 on the magneto-optical material:

[0085] Based on the inverse Faraday effect, at the confocal region position of Figure 1 a magneto-optical material is set. The highly localized transverse circularly polarized light field constructed in step 5 can induce an optically induced magnetization field on the magneto-optical material that is the same as or opposite to the spatial orientation (depending on the sign of the topological charge ). This magnetization field is calculated by Equation (11):

[0086] (11)

[0087] where and are the electric field vectors of the focal field and their conjugate vectors, and is the coupling coefficient related to the magneto-optical material; the pure transverse circularly polarized near-diffraction-limited optical focal field constructed in this application can induce an optically induced magnetization field with a super-diffraction limit in the spatial orientation .

[0088] The following gives several specific embodiments to verify the effectiveness of the method proposed in the present invention.

[0089] For simplicity of calculation, the parameters C and γ that are independent of the shape and polarization of the optical focal field and the induced magnetization field in the listed embodiments are normalized, that is, C = 1 and γ = 1 are taken; in order to converge and superimpose all the radiation fields of the magnetic dipole with the helical phase factor, a high numerical aperture objective lens convergence angle is taken, that is, ; when the signs of the topological charges are different, the directions of the induced magnetization fields are opposite. In the following embodiments, = 1 is taken; an objective lens that satisfies the sine condition is used as the objective lens in the embodiments of the present invention.

[0090] Embodiment 1. Induce a pure longitudinal super-diffraction limit optically induced magnetization field:

[0091] Set the spatial orientation of the magnetic dipole at the origin, and calculate the electric field distribution in the focal region using formula (10). Figure 2 and Figure 3 are the normalized light intensity and polarization distribution diagrams of the constructed optical focal field and the magnetization field induced by it in the magneto-optical material in the three principal planes, respectively. From Figure 2 and Figure 3 , it can be seen that they are both short ellipsoids along the Z-axis. From Figure 2It can be seen that the main lobe region of the optical focal field is purely circularly polarized in the XY plane, and is linearly polarized in the X and Y directions in the XZ and YZ planes respectively. The measured full width at half maximum (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. From Figure 3 It can be seen that Figure 2 the magnetization field induced by the optical focal field of

[0092] has only a Z component, and is linearly polarized in the Z direction in both the XZ and YZ planes, which is a purely longitudinal magnetization field. The measured FWHM of this magnetization field along the X, Y, and Z axes are 0.29λ, 0.29λ, and 0.37λ respectively, which is a super diffraction-limited magnetization field.

[0093] Example 2: Inducing a purely transverse super diffraction-limited optically-induced magnetization field: To induce a purely transverse super diffraction-limited optically-induced magnetization field, let the polar angle of the spatial orientation of the magnetic dipole , that is, the magnetic dipole is placed horizontally in the XY plane. In this example, the azimuth angle

[0094] Figure 4 and Figure 5 are the normalized light intensity and polarization distribution diagrams of the constructed optical focal field and the magnetization field induced by it in the magneto-optical material in the three principal planes respectively. From Figure 4 and Figure 5 it can be seen that they are both short ellipsoids along the X axis. From Figure 4 it can be seen that the main lobe region of the optical focal field is purely circularly polarized in the YZ plane, and is linearly polarized in the Z and Y directions in the XZ and XY planes respectively. 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. From Figure 5 it can be seen that Figure 4 the magnetization field induced by the optical focal field of is linearly polarized in the X direction in both the XZ and XY planes, which is a purely X-axis magnetization field. The measured FWHM of this magnetization field along the X, Y, and Z axes are 0.37λ, 0.29λ, and 0.29λ respectively, which is a super diffraction-limited magnetization field. In this example, if the azimuth angle of the magnetic dipole, the azimuth of the polarization direction of the induced magnetization field in the focal plane will be adjusted to

[0095] Example 3: Inducing a super diffraction-limited optically-induced magnetization field with an arbitrarily specified orientation:

[0096] The above-mentioned Example 1 and Example 2 demonstrate the effectiveness of the method proposed by the present invention in realizing the induction of purely longitudinal and purely transverse super-diffraction-limited magneto-optical fields. To demonstrate the effectiveness of this method in inducing magneto-optical fields in any orientation, without loss of generality, taking the pointing parameter as an example, the focal optical field in the focal region is calculated using formula (10), and the target magneto-optical field induced by it is calculated using formula (11). Their characteristics are as shown in Figure 6 and Figure 7 .

[0097] Figure 6 and Figure 7 are the normalized optical intensity and polarization distribution diagrams of the constructed focal optical field and the magneto-optical field induced by it in the magneto-optical material in the three principal planes, respectively. It can be seen from Figure 6 and Figure 7 that the long axis directions of the short ellipsoidal intensity profiles presented by the focal spot and the magneto-optical field are not located on the Z-axis or the XY plane. After measurement, their spatial orientations are the same as that of the magnetic dipole. It can be seen from Figure 6 that the main lobe of the focal optical field is elliptically polarized in the three principal planes. It can be seen from Figure 7 that Figure 6 the magneto-optical field induced by the focal optical field is linearly polarized in the three principal planes, and after measurement, its polarization orientation is also the same as that of the magnetic dipole.

[0098] The effectiveness of the method proposed by the present invention is proved by the above Example 1, Example 2 and Example 3. By setting the spatial orientation parameter of the magnetic dipole and superimposing the first-order helical phase factor in the corresponding orientation, a purely transverse circularly polarized near-diffraction-limited focal optical field can be constructed, and a super-diffraction-limited magneto-optical field with a spatial orientation of can be induced in the magnetic material.

[0099] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for inducing a super-diffraction-limited magnetization field in any orientation, characterized in that, The method includes the following steps: Establish an optical tight focusing system with two objective lenses having a confocal region; Place a magnetic dipole antenna in the confocal region of the optical tight focusing system; Calculate the radiation field of the magnetic dipole antenna. After superimposing a spiral phase factor with the same spatial orientation thereon, the radiation field radiates outward from the confocal region of the optical tight focusing system, propagates outward of the optical tight focusing system after being collimated by two identical high numerical aperture objective lenses, and obtain the radiation field at the entrance pupil plane of the objective lens according to the bending effect of the lens on the radiation light; Reverse the radiation field at the entrance pupil plane of the objective lens at this time through time reversal technology, propagate in the reverse direction, and focus on the origin of the confocal region of the two objective lenses to form a pure transverse circularly polarized near diffraction limit optical focal field 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, place a magneto-optical material in the confocal region of the two objective lenses, and the pure transverse circularly polarized near diffraction limit optical focal field can induce a super diffraction limit magnetization field with a specified spatial orientation therein; The specific calculation method of the total radiation field of the magnetic dipole antenna is as follows: According to the electromagnetic radiation theory, the radiation fields of the magnetic dipole antennas located on the X, Y, and Z axes are: (1) Spatial orientation The radiation field of the magnetic dipole antenna is formed by the coherent superposition of the radiation fields of the magnetic dipole antennas located on the X, Y, and Z axes respectively, and their radiation weight factors are as follows: (2) Can obtain the spatial orientation The radiation field of the magnetic dipole antenna is as follows: (3) Among them, the constant coefficient of the radiation field , the coefficient of the component of the radiation field and the coefficient of the component of the radiation field The specific expressions are shown in Eqs. (4), (5), and (6) respectively: (4) (5) (6) wherein, is the imaginary unit, is the oscillation angular frequency of the magnetic current on the magnetic dipole antenna, is the permittivity of vacuum, is the wave impedance of vacuum, and are respectively the magnitude and geometric length of the oscillating magnetic current of the magnetic dipole antenna, is the spherical coordinate in space of the radiation field, and are the unit vectors of the spherical coordinate.

2. The method for inducing a super-diffraction-limited magnetization field in any orientation according to claim 1, wherein: The optical tight focusing system is composed of two identical high numerical aperture objective lenses placed symmetrically in confocal 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 on the Z axis, and f is the focal length of the objective lens.

3. A method for inducing a super-diffraction-limited magnetization field in any orientation 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 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 orientation of the magnetic dipole antenna and the Z-axis; is the azimuth angle, which is the angle between the projection of the orientation 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 by the parameter .

4. A method for inducing a super-diffraction-limited magnetization field in any orientation according to claim 3, characterized in that: Along the spatial orientation The calculation process of the helical phase factor is as follows: 1) Taking the XYZ global coordinate system as the overall rotation, with the origin O of the reference coordinate as the rotation fulcrum, the Z-axis rotates in one step along the plane where the magnetic dipole antenna oscillates direction and the OZ-axis to direction, forming the rotated axis; 2) The X-axis and Y-axis of the global coordinate system rotate synchronously to and axis, , and are the three main axes of the rotated local coordinate system; 3) It is derived that the helical phase factor in the spatial direction is: (7) Among them, is the topological charge number, is the azimuth angle of the local coordinate system plane; to induce the target magnetization field, the topological charge number must take the value of ±1; To obtain the incident field required to induce the target magnetization field, the radiation field of a magnetic dipole antenna located at the origin of coordinates and oriented in space is superimposed with a helical phase factor having the same spatial orientation as shown in formula (7), and the expression for the total radiation field after superposition is as shown in formula (8): (8)。 5. A method for inducing a super-diffraction-limited magnetization field in any orientation according to claim 4, 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 plane of the objective lens can be obtained as follows: (9) Among them, is the polar coordinate of the entrance pupil plane, is the distance between the observation point on the entrance pupil plane and the entrance pupil center, is the azimuth angle of the observation point on the entrance pupil plane. The component of the entrance pupil is bent from the component of the radiation field.

6. A method for inducing a super-diffraction-limited magnetization field in any orientation according to claim 5, characterized in that: Based on the time reversal technology, reverse the radiation field at the entrance pupil plane of the objective lens, propagate in the reverse direction, and focus on the origin of the confocal region of the two objective lenses to form a target optical focal field; 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 region can be calculated and quantitatively evaluated by the Deby vector diffraction integral theory, and the calculation formula is: (10) Among them, 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 pure transverse circularly polarized near-diffraction-limited focal field, and the pure transverse here is relative to the spatial orientation of the magnetic dipole antenna in terms of.

7. A method for inducing a super-diffraction-limited magnetization field in an arbitrary orientation according to claim 6, characterized in that: Based on the inverse Faraday effect, a magneto-optical material is set at the confocal region of two objective lenses. A highly localized transverse circularly polarized light field can induce a photoinduced magnetization field on the magneto-optical material that is the same as or opposite to the spatial orientation The same or opposite photoinduced magnetization field, which is calculated by Equation (11): (11) Among them, and are the electric field vectors of the focal field and their conjugate vectors, is the coupling coefficient related to the magneto-optical material.

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