Nonlinear optical crystal structure and preparation method therefor, and optical device

Abstract

The present invention relates to a nonlinear optical crystal structure and a preparation method therefor, and an optical device. The nonlinear optical crystal structure comprises a plurality of material layers, wherein the plurality of material layers are stacked in the direction perpendicular to the two-dimensional planes thereof; each material layer is a crystal structure having y-fold rotational symmetry and has a predetermined lattice direction in a direction parallel to the two-dimensional plane; a non-zero deflection angle is formed between adjacent material layers, the deflection angle being an included angle between the predetermined lattice directions of the adjacent material layers; and y is an integer from 1 to 20.

Description

Nonlinear optical crystal structure and preparation method thereof, and optical device This application claims priority to the Chinese patent application filed with the China Patent Office on February 23, 2024, with application number 202410205079.5 and application name “Nonlinear optical crystal structure, preparation method thereof, and optical device”, the entire contents of which are incorporated by reference into this application. Technical Field The present invention relates to the technical field of optical crystals, and in particular to a nonlinear optical crystal structure and a preparation method thereof, and optical equipment. Background Art Research in nonlinear optics truly began in 1961, when Franken used a 694nm laser through a quartz crystal and discovered the generation of frequency-doubled coherent light at a wavelength of 347nm. This marked the first experiment with second harmonic generation in nonlinear optics. Since then, with the maturity of laser technology and the further development of nonlinear optical theory, the field of nonlinear optics has become a hot topic of research. Nonlinear optical crystals are at the heart of laser technologies such as optical frequency conversion, pulse compression, ultra-high-power lasers, and supercontinuum laser generation. They are widely used in fields such as laser communications and lidar. With the advancement of optical technology, exploring new optical crystal systems to meet the demands of high-tech innovation is key to the development of advanced laser technologies and equipment. Currently, mature nonlinear optical crystals include BaB2O4 (BBO), KBeBOF2 (KBBF), and LiNbO3 (LN). When a single-frequency fundamental wave is incident on a nonlinear medium, the coupling effect of higher-order nonlinear electric polarization coefficients generates light waves with frequencies three, four, or even higher than the fundamental frequency. This nonlinear optical phenomenon is called the high-order harmonic effect. High-order harmonics are crucial for the generation of extreme ultraviolet lasers and X-rays, and can be applied to extreme ultraviolet lithography machines and higher-precision detection and resolution micro-nanofabrication methods. Furthermore, high-order harmonics can be used to generate shorter laser pulses. Since attosecond laser generation generally requires the extreme ultraviolet (EUV) band, high-order harmonics are also crucial for attosecond science applications. One nonlinear metasurface technology achieves high-order nonlinear phase control by designing nanoscale metal structures with multiple rotational symmetries, such as triangles and pentagons. By introducing a nonlinear phase-control metasurface, high-order harmonics at different locations experience different polarization and intensity modulation effects. However, these nonlinear metasurfaces fail to achieve phase matching and are very thin, resulting in only limited control of the phase of high-order harmonics and inability to effectively enhance them. Summary of the Invention Based on this, it is necessary to provide a nonlinear optical crystal structure and a preparation method and optical equipment to achieve the enhancement of high-order harmonics. The first aspect of the present invention provides a non-linear optical crystal structure, and the solution is as follows: A non-linear optical crystal structure includes a plurality of material layers, and the plurality of material layers are stacked in a direction perpendicular to their two-dimensional planes; each of the material layers is a crystal structure with y-fold rotational symmetry and has a predetermined lattice direction in a direction parallel to the two-dimensional plane; there is a non-zero deflection angle between adjacent material layers, and the deflection angle is the included angle between the predetermined lattice directions of adjacent material layers; where y is an integer from 1 to 20. In one embodiment, the non-linear optical crystal structure includes N material layers, where the deflection angle θ of the m-th material layer relative to the first material layer Satisfies: Or}] Or, p is greater than y, and p = 2b - 1 + ay, where 1 ≤ b < p, and b is an integer, a is an integer, and a ≠ 0, and Where the first material layer is any one of the material layers on the outermost two sides of the non-linear optical crystal structure, the m-th material layer is adjacent to the (m - 1)-th material layer, t n Is the thickness of the n-th material layer, t m Is the thickness of the m-th material layer, t1 is the thickness of the first material layer, N, n, and m are all integers, N ≥ 2, 1 ≤ n ≤ N, 1 < m ≤ N, Δk is the wave vector mismatch of the non-linear optical effect of the material layer, Where the range of ω is 9.4×10 13 rad / s to 9.4×10 15 rad / s, the range of λ is 200 nm to 20 μm, p is the harmonic order, and p is an integer from 2 to 2000. In one embodiment, the deflection angle θm of the m-th material layer relative to the first material layer satisfies: Or, Or, In one embodiment, the thicknesses of each of the material layers are the same. In one embodiment, each of the material layers has a thickness t, and the deflection angle θ of the m-th material layer relative to the first material layer m Satisfies: or, or, In one embodiment, the deflection angle θ of the mth material layer relative to the first material layer is m satisfy: or, or, In one embodiment, the deflection direction of the m-th material layer relative to the (m-1)-th material layer is the same. In one embodiment, the deflection angles are the same in degree. In one embodiment, the y value of each material layer is the same. In one embodiment, the thickness of each material layer is greater than 5 nm. In one embodiment, adjacent material layers are bonded via van der Waals forces. In one embodiment, y is 1, and the material of the material layer is selected from at least one of silver molybdate, rhenium disulfide, and KP15. In one embodiment, y is 2, and the material of the material layer is selected from at least one of potassium dihydrogen phosphate, potassium titanyl phosphate, barium triborate, germanium arsenide, black phosphorus, germanium selenide, and titanium trisulfide. In one embodiment, y is 3, and the material of the material layer is selected from at least one of 3R-MoS2, BBO, rhombohedral boron nitride, gallium selenide, and KBBF. In one embodiment, y is 4, and the material of the material layer is selected from at least one of perovskite, calcium chloride, sodium chloride, calcium fluoride, and magnesium fluoride. In one embodiment, y is 6, and the material of the material layer is selected from at least one of hexagonal boron nitride and graphene. A second aspect of the present invention provides a method for preparing the above-mentioned nonlinear optical crystal structure, which is as follows: A method for preparing the nonlinear optical crystal structure comprises the following steps: The plurality of material layers are transferred and stacked according to the configuration conditions of the deflection angles. A third aspect of the present invention provides an optical device comprising the nonlinear optical crystal structure according to any one of the above embodiments. Compared with traditional solutions, the above nonlinear optical crystal structure and its preparation method and optical device have the following beneficial effects: The above-mentioned nonlinear optical crystal structure and its preparation method utilize a stack of rotationally symmetric crystals and achieve nonlinear enhancement of the second harmonic and even higher harmonics by controlling the deflection angle between the material layers. This nonlinear optical crystal structure takes advantage of the material layers' greater degrees of freedom (stacking angles and types of stacked materials) and more refined control methods (atomic-level longitudinal stacking accuracy). Therefore, its preparation process is more refined, with higher degrees of control freedom and more precise processing, making the crystal more sensitive to light control. Compared to traditional nonlinear metasurface solutions, it can achieve higher precision, higher operability, and higher efficiency. The above optical device contains the nonlinear optical crystal structure of any of the above embodiments, and thus can achieve corresponding beneficial effects. BRIEF DESCRIPTION OF THE DRAWINGS FIG1A is a schematic structural diagram of a nonlinear optical crystal structure according to some embodiments; FIG1B is a schematic diagram of the deflection angle between different two-dimensional material layers; FIG2A is a schematic structural diagram of a nonlinear optical crystal structure according to some embodiments; FIG2B is a schematic diagram showing simulation results of the second harmonic generation (SHG) enhancement efficiency of nonlinear optical crystal structures according to some embodiments; 3A and 3B are schematic diagrams showing simulation results of second harmonic generation (SHG) enhancement efficiency at different angles under corresponding rotational phase matching conditions of a nonlinear optical crystal structure according to some embodiments of the present disclosure; FIG4 is a schematic diagram showing simulation results of second harmonic generation (SHG) enhancement efficiency at different thicknesses of two-dimensional material layers under corresponding rotational phase matching conditions of a nonlinear optical crystal structure according to some embodiments of the present disclosure; FIG5A is a schematic structural diagram of a nonlinear optical crystal structure according to some embodiments; FIG5B is a schematic diagram showing the thickness of the nonlinear optical crystal structure shown in FIG5A and the deflection angle between adjacent layers; FIG6A is a schematic structural diagram of second harmonic polarization control of a nonlinear optical crystal structure according to some embodiments; FIG6B is a diagram showing the second harmonic polarization experimental results of nonlinear optical crystal structures according to some embodiments; FIG7 is a schematic diagram of a nonlinear optical crystal structure using crystals of different rotational symmetry as material layers to achieve enhanced output of nonlinear light efficiency; FIG8A is a schematic structural diagram of a nonlinear optical crystal structure using a six-fold rotationally symmetric crystal as a material layer in some embodiments; FIG8B is a diagram showing the dependence of the fifth harmonic intensity on the number of material layers when circularly polarized light is incident on the nonlinear optical crystal structure shown in FIG8A ; FIG9A is a schematic structural diagram of a nonlinear optical crystal structure using a four-fold rotationally symmetric crystal as a material layer in some embodiments; FIG9B is a diagram showing the dependence of the third harmonic intensity on the number of material layers when circularly polarized light is incident on the nonlinear optical crystal structure shown in FIG9A . DETAILED DESCRIPTION To make the above-mentioned objects, features, and advantages of the present invention more readily apparent, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. The following description sets forth numerous specific details to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art may make similar modifications without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined. In nonlinear optics, the nonlinear response of a material is described by polarization. Under the action of a light field, the electric polarization in a medium can be described as: P=ε0[χ (1) E+χ (2) E 2 +χ (3) E 3 +...χ (n) E n ], Among them, ε0χ (1) E is a linear term, and other perturbation terms are nonlinear terms. (n) is the n-order nonlinear polarizability. The microscopic mechanisms that produce nonlinear polarization mainly include electron cloud distortion, molecular rotation and vibration, and rearrangement of molecular orientation. The second-order nonlinear optical effect will only occur in crystals with central inversion asymmetry (central inversion symmetry breaking), and the corresponding nonlinear coefficient tensor is χ (2) The second-order polarization intensity can be expressed as: P (2) =∑ m,n ε0χ (2) (-ω,ω m ,ω n ):E(ω m )E(ω n ), Among them, the secondary nonlinear optical effects include second harmonic, sum frequency and difference frequency. Second harmonic generation refers to the generation of frequency-doubled light with a frequency of 2ω when the incident light is monochromatic light with a frequency of ω, called fundamental light, passing through a nonlinear medium (for example, a nonlinear optical crystal). Under the small signal approximation and ignoring the walk-off effect, the intensity of the second harmonic can be written as: Among them I 2ω , I ω are the intensities of the doubled frequency light and the fundamental frequency light, L is the distance the fundamental frequency light propagates in the nonlinear crystal, d eff is the second-order nonlinear coefficient, Δk is the wave vector mismatch, λ is the wavelength of the incident light, and generally Δk≠0, so a phase mismatch effect will occur in traditional nonlinear crystals. When the second harmonic has a phase mismatch, each time it passes through a coherence length The energies of the fundamental and doubled-frequency light are reversed, preventing effective enhancement of the second harmonic. The coherence length lc refers to the characteristic distance over which nonlinear light can be enhanced in a crystal: lc = π / Δk. The wave vector mismatch Δk is related to the wavelength of the light and the type of optical crystal. In the following description, the coherence wavelength lc and the wave vector mismatch Δk have the same meaning and are not repeated here. High-order harmonics refer to the generation of frequency-doubled light with a frequency three, four or even higher than that of the fundamental frequency light due to the coupling effect of the high-order nonlinear electric polarization coefficient when a single-frequency fundamental frequency wave is incident on a nonlinear medium. Similarly, when the high-order harmonics have phase mismatch, each time they pass through a coherence length l c , the energies of the fundamental frequency light and the frequency-doubled light are reversed, and the high-order harmonics cannot be effectively enhanced. For the pth harmonic, the wave vector mismatch is, λ is the wavelength of incident light, and p is the harmonic order. Unlike the second harmonic and other even-order harmonics, the generation of odd-order harmonics such as the third and fifth harmonics does not require the breaking of central inversion symmetry. To overcome the aforementioned problem of frequency-doubled light not being able to be efficiently output due to phase mismatch, a quasi-phase matching approach can be employed. For example, quasi-phase matching can be used to effectively enhance harmonics using periodically poled nonlinear optical crystals. Compared to birefringence phase matching, quasi-phase matching does not require specific polarization and angle conditions. It can also utilize the maximum effective nonlinear coefficient of the nonlinear crystal. Quasi-phase matching only requires introducing a 180° phase compensation every coherence length to achieve effective harmonic enhancement. The introduced polarization reversal period can also be an odd multiple of the coherence length. In nonlinear optics, the phase mismatch compensated by the periodic structure can be described by the reciprocal lattice vector, that is, Where Λ=2l C , which is the length of one period. Constructing a suitable nonlinear crystal structure with polarization period reversal is the key to achieving quasi-phase matching. Traditionally, the crystal materials used for quasi-phase matching are generally lithium niobate (LiNbO3), etc. The pulling growth method or the electric field pattern polarization method can be used to obtain periodically polarized nonlinear optical crystals with a specific stripe structure. Recently, the technology of using laser pulse polarization has also been developed. In quasi-phase matching, the polarization direction of the medium is reversed every time a coherence length is passed, so that the intensity of the harmonics can be continuously enhanced. Quasi-phase matching can also be used to efficiently output sum-frequency light and difference-frequency light. However, preparing periodically poled crystals through quasi-phase matching requires relatively complex equipment and complicated processes, and the polarization units can only be arranged in the positive and negative directions. These all limit the functional applications of traditional periodically poled nonlinear optical crystals. The present invention provides a nonlinear optical crystal structure that can easily achieve high-efficiency output of second harmonics and higher harmonics such as third, fourth, and fifth harmonics. Higher harmonics are crucial for the generation of extreme ultraviolet lasers and X-rays and can be applied to extreme ultraviolet lithography equipment and higher-precision detection and resolution micro-nanofabrication methods. The nonlinear optical crystal structure of the present invention comprises a plurality of material layers. The plurality of material layers are stacked in a direction perpendicular to their two-dimensional planes. Each material layer has a crystal structure with y-fold rotational symmetry and has a predetermined lattice direction parallel to the two-dimensional plane. Adjacent material layers have a non-zero deflection angle, which is the angle between the predetermined lattice directions of adjacent material layers. Wherein, y is an integer between 1 and 20. The above-mentioned nonlinear optical crystal structure uses a stack of rotationally symmetric crystals and achieves nonlinear enhancement of the second harmonic and even higher harmonics by controlling the deflection angle between the material layers. The above-mentioned nonlinear optical crystal structure takes advantage of the material layers' richer degrees of freedom (stacking angle and type of stacking materials) and more precise control methods (atomic-level longitudinal stacking accuracy). Therefore, its preparation process is more refined, with higher control freedom and more precise processing technology, making the crystal more sensitive to light control. Compared with traditional nonlinear metasurface solutions, it can achieve higher precision (nanometer level), higher operability and higher efficiency. Some specific embodiments are provided below to describe the technical solution of the present invention in more detail, so as to make the technical solution of the present invention clearer. FIG1A is a schematic structural diagram of a nonlinear optical crystal structure according to some embodiments of the present invention. FIG1A shows a nonlinear optical crystal structure. As shown in FIG1A , the nonlinear optical crystal structure 100 according to some embodiments of the present invention includes a plurality of material layers 101. For example, each material layer 101 includes a two-dimensional plane extending along the x-direction and the y-direction, and the plurality of material layers 101 are stacked along a direction perpendicular to their two-dimensional planes (z-direction). Each material layer 101 is a crystal structure having y-fold rotational symmetry, and has a predetermined lattice direction parallel to the two-dimensional plane in a direction parallel to the two-dimensional plane of the material layer 101. There is a non-zero deflection angle θ between adjacent material layers 101. The deflection angle θ is the angle between the predetermined lattice directions of adjacent material layers 101 in the same two-dimensional plane. In the above-mentioned nonlinear optical crystal structure, the number of material layers 101 is at least two, and can be two, three, four, or more layers. In some embodiments, the nonlinear optical crystal structure includes 2 to 100 material layers 101, and the specific number of material layers 101 is, for example, 2, 3, 4, 7, 10, 13, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, etc. In the aforementioned nonlinear optical crystal structure, a non-zero deflection angle between adjacent material layers 101 means that there is a certain deflection angle between the crystal structures of adjacent material layers within a plane. The deflection angle is defined by the angle between the predetermined lattice directions of adjacent material layers 101 within the same two-dimensional plane. The predetermined lattice direction here is not limited to a specific lattice direction of the crystal structure. Instead, by selecting the same lattice direction for each material layer, the deflection angle of the crystal structures of the different material layers within a plane can be determined by the angle between the predetermined lattice directions of the different material layers. For example, the predetermined lattice directions of the different material layers 101 can be projected onto a plane parallel to the material layers, and the angle between the projections of the different predetermined lattice directions is the aforementioned deflection angle. For example, for rhombohedral boron nitride structure, its crystal structure

[0001] The crystal direction may roughly coincide with the stacking direction z of the material layer, and the predetermined lattice directions in the two-dimensional plane may be

[0010] ,

[0120] ,

[1100] Any one of the lattice directions in the plane perpendicular to the stacking direction. Of course, the lattice directions listed here are all exemplary. As long as the same lattice direction is selected when evaluating the stacking deflection angle of the material layer, the deflection angle between different material layers can be accurately obtained. Therefore, the lattice direction is referred to as the predetermined lattice direction here, and those skilled in the art should be able to understand its meaning. In addition, the above-mentioned rhombohedral boron nitride structure is also exemplary, and the embodiments of the present invention are not limited to the above-mentioned crystal structure, materials and various lattice directions. In some embodiments, adjacent material layers are bonded via van der Waals forces. In the aforementioned nonlinear optical crystal structure, the van der Waals bonding between adjacent material layers is not limited to the presence of only van der Waals forces between the two bonded surfaces; rather, the bonding can be primarily via van der Waals forces. Van der Waals bonding not only facilitates the stacking process but also maintains the ability to regulate nonlinear light phase matching, thereby improving the output efficiency of nonlinear light. In some embodiments, each material layer is a crystal with a central inversion asymmetric crystal structure. The central inversion asymmetric (or central inversion symmetry-breaking) crystal structure of the material layer is the basis for achieving second harmonics and other even-order harmonics. For example, the material layer itself can be formed by growing or stacking multiple sublayers, wherein the crystal structures of the sublayers in the material layer are identical and parallel in orientation. The multiple sublayers include first-type sublayers and second-type sublayers, wherein the chemical bonds formed between atoms at corresponding lattice positions in the first-type sublayers and the second-type sublayers are in opposite directions, and the number of first-type sublayers is greater than the number of second-type sublayers; or the chemical bonds formed between atoms at corresponding lattice positions in the multiple sublayers are all in the same direction, thereby making the overall crystal structure of the material layer a central inversion asymmetric crystal structure. The term "corresponding lattice position" refers to the lattice position in a sublayer that is shifted along the sublayer stacking direction by an integer multiple of the lattice constant in that direction to another layer, or further shifted along the sublayer surface by an amount less than the maximum dimension within the lattice plane, resulting in a lattice position corresponding to the original lattice position. For example, for two vertex positions on a certain side of a hexagon, after the shift, the two vertex positions on that side become the corresponding positions of the original two vertex positions. For example, if a boron atom and a nitrogen atom distributed at two lattice positions in one sublayer form chemical bonds oriented in a certain direction, and the chemical bonds between the boron atom and the nitrogen atom distributed at two corresponding lattice positions in another sublayer are oriented in opposite directions, then the chemical bonds formed between the atoms at the corresponding lattice positions in the two sublayers are in opposite directions. If a boron atom and a nitrogen atom distributed at two lattice positions in one sublayer form chemical bonds oriented in a certain direction, and the chemical bonds between the boron atom and the nitrogen atom distributed at two corresponding lattice positions in the other sublayer are also oriented in that direction, then the chemical bonds formed between the atoms at the corresponding lattice positions in the two sublayers are in the same direction. It should be noted that the number of first-type sub-layers being greater than the number of second-type sub-layers may be such that the number of first-type sub-layers accounts for more than 60% of the total number of sub-layers, so that the material layer has a more pronounced nonlinear optical effect. For example, the above ratio may be greater than 70%, greater than 80%, or greater than 90%. The above-mentioned sub-layers may be atomic layers, and the above-mentioned sub-layers may be bonded to each other via van der Waals forces. In some other embodiments, the material layers are not limited to crystals with a central inversion asymmetric crystal structure. Unlike second harmonics and other even-order harmonics, the generation of odd-order harmonics, such as third and fifth harmonics, does not require breaking central inversion symmetry. For the generation of odd-order harmonics, the material layers are not limited to crystals with a central inversion asymmetric crystal structure. In the nonlinear optical crystal structure of the present invention, each material layer has a crystal structure with y-fold rotational symmetry, wherein y is 1, 2, 3, 4 or 6. In some embodiments, the y value of each material layer is the same. In other words, each material layer in the nonlinear optical crystal structure has a crystal structure with one-fold rotational symmetry, two-fold rotational symmetry, three-fold rotational symmetry, four-fold rotational symmetry, or six-fold rotational symmetry. When the material layer is a single-fold rotationally symmetric crystal (C1), that is, when y is 1, the material of the material layer can be, but is not limited to, at least one of silver molybdate, rhenium disulfide and two-dimensional material KP15. When the material of the material layer adopts a double rotational symmetry crystal (C2), that is, y is 2, the material of the material layer can be specifically but not limited to at least one of potassium dihydrogen phosphate (KDP), potassium titanyl phosphate (KTP), barium triborate (LBO), germanium arsenide (GeAs), black phosphorus (BP), germanium selenide (GeSe) and titanium trisulfide (TiS3). When the material layer is a three-fold rotationally symmetric crystal (C3), that is, y is 3, the material of the material layer can be, but is not limited to, at least one of 3R-MoS2, BBO, rhombohedral boron nitride, gallium selenide and KBBF. When the material layer is made of a four-fold rotationally symmetric crystal (C4), that is, y is 4, the material of the material layer may be, but is not limited to, at least one of perovskite, calcium chloride, sodium chloride, calcium fluoride, and magnesium fluoride. When the material of the material layer adopts a six-fold rotational symmetry crystal (C6), that is, y is 6, the material of the material layer can specifically be but is not limited to at least one of hexagonal boron nitride and graphene. When y is an integer from 5, 7 to 20, the material of the material layer may be, for example, a quasicrystal. In some embodiments, the material layers have the same y value. The specific materials listed above are merely exemplary, and the materials applicable to the material layer in the present invention are not limited thereto. In the above nonlinear optical crystal structure, the specific material of each material layer may be the same. However, the embodiments of the present invention are not limited thereto, and at least two material layers among the multiple material layers may be made of different specific materials. In some embodiments, the thickness t of each material layer 101 is greater than 5 nm. Furthermore, the thickness t of each material layer 101 is greater than 7 nm. Furthermore, the thickness t of each material layer 101 is greater than 9 nm. In some embodiments, the thickness t of the material layer 101 is between 6 nm and 50 μm, specifically, for example, 6 nm, 10 nm, 30 nm, 50 nm, 60 nm, 80 nm, 300 nm, 500 nm, 800 nm, 1 μm, 5 μm, 10 μm, 20 μm, 30 μm, 50 μm, etc. In order to achieve higher nonlinear light output efficiency, the deflection angle and thickness of each layer of the nonlinear optical crystal structure formed by stacking two-dimensional crystal material layers can be adjusted. When the thickness is very small, the deflection angle will also be very small, and the applicable angle range for enhancing the nonlinear light output efficiency will also be very small. This makes the processing process more difficult, and if the manufacturing precision is not high, the nonlinear light output efficiency may be lost. The inventors found that when the thickness of each material layer is above 5 nm, high-efficiency output and process reliability and convenience can be achieved through stacking of such material layers. It should be noted that the material layers in the above nonlinear optical crystal structure may have the same thickness or different thicknesses. It should be noted that although the light output efficiency can be greatly improved by adjusting the deflection angle and the thickness of each layer, as long as the above-mentioned corner stack has a non-zero deflection angle, it can compensate for the wave vector mismatch of nonlinear light to a certain extent. Therefore, it can improve the output efficiency of nonlinear light to a certain extent. In the embodiment of FIG. 1A , the nonlinear optical crystal structure includes four material layers. However, this number of material layers is merely exemplary. The nonlinear optical crystal structure is not limited to this number of material layers, and may include more or fewer material layers. For example, the structure may include two, three, five, or even more material layers. The above-mentioned deflection angles are further explained below in conjunction with FIG1B to clarify their meaning. For example, the predetermined lattice directions F1, F2, F3, and F4 selected for the four material layers in FIG1A are projected onto the same plane 001 parallel to the material layers. As shown on the left side of FIG1B , F2 is deflected counterclockwise by θ21 relative to F1, F3 is deflected counterclockwise by θ32 relative to F2, F4 is deflected counterclockwise by θ43 relative to F3, and F4 is deflected counterclockwise by θ41 relative to F1. From another perspective, as shown on the right side of FIG1B , F1 is deflected clockwise by θ12 relative to F2, F2 is deflected clockwise by θ23 relative to F3, F3 is deflected clockwise by θ34 relative to F4, and F1 is deflected clockwise by θ14 relative to F4. For example, as shown in FIG1B , the second layer is deflected counterclockwise relative to the first layer, the third layer is deflected counterclockwise relative to the second layer, and the fourth layer is deflected counterclockwise relative to the third layer. For this stacking method, the deflection direction is constant. This stacking method can be called a continuous angle stacking method. Although the above description of the deflection angle is described in conjunction with a clockwise or counterclockwise direction, this is merely exemplary. It can be understood that since one direction can have a 360-degree rotation in a plane, and considering the rotational symmetry (for example, three-fold rotational symmetry) used for nonlinear optical crystals, the above limitation on the rotation direction may be more meaningful at a smaller angle, for example, within a deflection angle range of less than 60 degrees, but the embodiments of the present invention are not limited to this. Although a better technical effect can be achieved in combination with an adjusted deflection angle range under a specific deflection method (which will be explained in more detail in the following embodiments), some embodiments of the present invention do not limit the deflection direction between the above-mentioned adjacent material layers. For example, in the above nonlinear optical crystal structure, the stacking direction of the material layer is parallel to a rotation axis of the crystal structure of the material layer. For example, for rhombic boron nitride, its stacking direction can be parallel to

[0001] Lattice direction. In some embodiments, as shown in FIG2A , the nonlinear optical crystal structure adopts a continuous angle structure and the thickness of each material layer is the same. For example, the nonlinear optical crystal structure includes N material layers, each of which has a thickness of t, and the total thickness T of the nonlinear optical crystal structure is N·t. The deflection angle of the mth layer relative to the first layer is θ m=(m-1)θ, where N is an integer greater than or equal to 2, m is a positive integer greater than or equal to 2 and less than or equal to N, and θ is equal to the deflection angle of the second layer relative to the first layer. It should be noted that the first material layer is any one of the material layers located on the outermost sides of the nonlinear optical crystal structure, and the mth material layer is adjacent to the (m-1)th material layer. The layer numbers of the material layers described here and below are counted sequentially along the stacking direction of the material layers from one side of the nonlinear optical crystal structure. For example, in Figure 1B, they can be counted sequentially from bottom to top, namely the first material layer, the second material layer, the third material layer, and the fourth material layer. Of course, the embodiments of the present invention are not limited to this, for example, Figure 1B can also be counted sequentially from top to bottom. In this embodiment, the thickness of each material layer is the same, and the deflection angle between each two adjacent layers is the same. To understand the effects of different deflection angles θ and layer thicknesses t on secondary nonlinear optical effects such as second harmonic generation (SHG), the inventors conducted research and experiments. The following uses a nonlinear optical crystal structure to achieve SHG enhancement as an example. First, a nonlinear optical crystal structure is selected, in which the deflection angle satisfies the conditions for compensating the wave vector mismatch of the second harmonic, and the total thickness T of the nonlinear optical crystal structure (T is equal to the sum of the thicknesses of the N material layers used to construct the nonlinear optical crystal structure, as an example, T = 8μm) is controlled to be the same. Then, the final enhancement intensity is obtained by simulation and calculation based on the nonlinear coupled wave equations under continuous rotation angle conditions. Figure 2B is a schematic diagram of the simulation results of the second harmonic enhancement efficiency of the above nonlinear optical crystal structure. It can be seen from Figure 2B that when 3θ = Δk·t, the second harmonic output efficiency is enhanced to the maximum. This corresponds to the conditions for angle phase matching enhancement. It can also be seen that the smaller the thickness t, the higher the second harmonic enhancement efficiency. Figures 3A and 3B show schematic diagrams of simulation results of the second harmonic generation (SHG) enhancement efficiency at different angles under the corresponding angle phase matching conditions of the nonlinear optical crystal structure of some embodiments of the present invention. In the embodiment shown in Figure 3A, two layers of rhombic boron nitride (rBN) material with a thickness of coherence length lc are used. By adjusting the deflection angle between the two material layers, the dependence of the second harmonic intensity on the deflection angle is obtained. As shown on the left side of Figure 3A, for rBN with a thickness of coherence length lc, the second harmonic conversion efficiency is highest when the deflection angle is 60 degrees, which corresponds to quasi-phase matching. Figure 3B shows a continuous angle structure of rBN formed by five material layers with a non-60-degree angle. In this embodiment, the corresponding deflection angle is adjusted according to the thickness of the material layer, and the second harmonic enhancement efficiency is simulated. By solving the nonlinear coupled wave equations, the final second harmonic output intensity is obtained, which is between perfect phase matching and quasi-phase matching (the gray area in Figure 3B). This mode can be called angle phase matching. When the thickness t and the rotation angle approach 0, the angle phase matching approaches perfect phase matching. Furthermore, a curve of the second harmonic conversion efficiency and thickness t under angle matching conditions was calculated. At a rotation angle of 30 degrees, the conversion efficiency is twice that of quasi-phase matching, effectively enhancing the second harmonic. Figure 4 shows a schematic diagram of simulation results for the second harmonic generation (SHG) enhancement efficiency of nonlinear optical crystal structures of some embodiments, under the corresponding rotation angle phase matching conditions, for different material layer thicknesses. In the embodiment shown in Figure 4, the total thickness T was adjusted to 3.2 μm, and the relationship between the two-dimensional nonlinear light output efficiency and the deflection angle θ between adjacent material layers was obtained. Figure 4 also shows that as the thickness of the material layer decreases, higher second harmonic generation enhancement efficiency is achieved. Combining the simulation results of Figures 2B, 3A, 3B, and 4, we can see that as the thickness of the material layer decreases and the deflection angle changes, the second harmonic conversion efficiency gradually increases, and the conversion efficiency falls between the conversion efficiency of quasi-phase matching and the conversion efficiency of perfect phase matching. However, under the same thickness conditions, the range of deflection angles θ required to achieve enhanced angular phase matching also gradually decreases as the thickness of the material layer decreases (for example, see the width of the black portion in Figure 2B). As mentioned above, the inventors have also discovered that as the thickness of the material layer decreases, the precision of the deflection angle required for enhanced angular phase matching becomes increasingly stringent. This has implications for both fabrication and performance. First, when the thickness is very small, the range of angles required for coordination and adjustment is also very small, placing high demands on the precision of controlling the material layer thickness and the deflection angle when stacking the material layers. Second, due to these stringent precision requirements, fabrication errors can in turn adversely affect the nonlinear light output efficiency of the fabricated nonlinear optical crystal structure. Through experiments, the inventors have found that stacking material layers with a thickness of 5 nm or more can easily achieve a good enhancement of the second-order nonlinear light output efficiency. For example, the continuous rotation structure of the nonlinear optical crystal structure can achieve rotation phase matching under circularly polarized light excitation. Under the continuous rotation structure, when the condition of 3θ = Δk·t is met, the efficiency of generating second harmonics by circularly polarized light excitation is four times that of linear polarization, and the smaller the rotation angle, the smaller the corresponding layer thickness, and the higher the efficiency of second harmonic, sum frequency or difference frequency conversion. In theory, perfect phase matching conditions can be achieved under continuous rotation conditions, and the efficiency is π of quasi-phase matching. 2 / 4 times. In the above embodiment, the thickness t of each material layer is the same. In other embodiments, the thickness t of each material layer in the nonlinear optical crystal structure may also be different. FIG5A is a schematic diagram illustrating the structure of a nonlinear optical crystal structure according to some embodiments of the present invention; FIG5B is a schematic diagram illustrating some thicknesses and deflection angles between adjacent layers of the nonlinear optical crystal structure shown in FIG5A . As shown in FIG5A , the nonlinear optical crystal structure is also a continuous angle structure. From top to bottom in FIG5A , the thicknesses of the first to fourth material layers are t1, t2, t3, and t4, respectively. As shown in FIG5B , the thickness t1 of the first material layer is 0.8 μm, the thickness t2 of the second material layer is 0.6 μm, the thickness t3 of the third material layer is 0.4 μm, and the thickness t4 of the fourth material layer is 0.3 μm. Therefore, when light is incident from the first material layer, the propagation distances when it reaches the surfaces of the first, second, third, and fourth material layers away from the light incident side are 0.8 μm, 1.4 μm, 1.8 μm, and 2.1 μm, respectively. The deflection angle θ1 between the first and second material layers is 25°, the deflection angle θ2 between the second and third material layers is 42°, and the deflection angle θ3 between the third and fourth material layers is 55°. In this embodiment, by adjusting the deflection angles between material layers of different thicknesses, high output efficiency of second-order nonlinear light can be achieved, which is between quasi-phase matching and perfect phase matching, thereby enhancing the second harmonic output efficiency. As can be seen from Figures 5A and 5B, in the embodiment where material layers of different thicknesses are stacked, the output efficiency of the second harmonic can be enhanced by adjusting the deflection angle between adjacent material layers according to the thickness of the material layers. The inventors have found through experiments that when a nonlinear optical crystal structure adopts a continuous angle structure to stack N layers of material layers, the deflection angle θ of the mth layer relative to the first layer is m satisfy: This can enhance the output efficiency of second-order nonlinear light and achieve angular phase matching. The output efficiency of second-order nonlinear light is between quasi-phase matching and perfect phase matching. Here, t1 is the thickness of the first material layer, and N, n, and m are all integers, with N ≥ 2, 1 ≤ n ≤ N, and 1 < m ≤ N. From the results in Figures 5A and 5B, we can see that compared to the stringent requirements of quasi-phase matching for layer thickness t, here for any layer thickness t m , we can find one or more corresponding θ m , which results in angle phase matching, and the second harmonic efficiency is higher than quasi-phase matching. In the embodiment where the material layers have the same thickness, the relationship between the thickness t of the material layer and the deflection angle θ between adjacent material layers is θ=1 / 3Δk·t. In the embodiment where the material layers have different thicknesses, the relationship between the thickness t of the material layer and the deflection angle θ between adjacent material layers is θ=1 / 3Δk·t. m The relationship between However, for example, as shown in FIG2B , when the thickness of the material layer is above 5 nm, the nonlinear optical crystal structure formed by stacking the material layers has a higher “tolerance” to the deflection angle. In other words, it can achieve angular phase matching within a certain angle range, thereby enhancing the output efficiency of the nonlinear light. The inventors have found that the thickness t of the material layer and the deflection angle θ between adjacent material layers are related to the material layer thickness. m The relationship between When the nonlinear light output efficiency can be effectively enhanced. Wherein, p is the harmonic order, that is, an integer greater than 2. For example, p is an integer from 2 to 2000, and specifically, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100, etc. It should be noted that, The relationship can also be applied to the case where the thickness of each material layer is the same. n , t m If t1 and t2 are set to the same value, we can get Furthermore, the thickness t of the material layer and the deflection angle θ between adjacent material layers m The relationship between Among them, when the outgoing polarized light has the same rotation as the incident polarized light Take "-", for example Another example When the outgoing polarization is opposite to the incident polarization When taking "+", for example Another example Furthermore, when the deflection angle θ of the mth material layer relative to the first material layer is m satisfy: Nonlinear light has the highest output efficiency. When the incident light is elliptically polarized light or linearly polarized light and satisfies: p (harmonic order) is greater than y, and p = 2b - 1 + ay, where 1 ≤ b < p, b is an integer, a is an integer, and a ≠ 0, the thickness t of the material layer and the deflection angle θ between adjacent material layers m The relationship between is such that the output efficiency of the non - linear light can be effectively enhanced. When the thicknesses of each material layer are the same, setting t n 、t m and t1 to the same value, then we can obtain Furthermore, when the deflection angle θ m of the m - th material layer relative to the first material layer satisfies: the output efficiency of the non - linear light is the highest. In some embodiments, each material layer has a crystal structure with three - fold rotational symmetry. In this embodiment, when the thickness t of each material layer is equal to lc and the inter - layer deflection angle is equal to 60 degrees, an enhancement effect equivalent to the second - order non - linear light output efficiency of quasi - phase matching can be obtained. Additionally, in some cases with a crystal structure having three - fold rotational symmetry, when the thickness of each material layer is less than lc and the inter - layer deflection angle is less than 60 degrees, a better enhancement of the second - order non - linear light output efficiency can be achieved. Further, in the case where each material layer has a crystal structure with three - fold rotational symmetry, the thickness is less than lc and the inter - layer deflection angle is less than 60 degrees, adjusting the thickness t of the material layer and the inter - layer deflection angle θ to conform to the relationship between the thickness t of the material layer and the deflection angle between adjacent material layers, then an enhancement of the second - order non - linear light output efficiency between quasi - phase matching and perfect phase matching can be obtained. Figures 6A and 6B illustrate some other embodiments of the present invention. Based on the above embodiments, the total thickness of the nonlinear optical crystal structure can be set to achieve the output of nonlinear light with specific polarization conditions. As shown in Figure 6A, under the condition of setting the above-mentioned continuous rotation angle θ, the total thickness of the nonlinear optical crystal structure is further set to T = c·t = n·lc, where c and n are positive integers and lc is the coherence length. If 3θ = +Δk·t, the emitted second harmonic is right-handed circularly polarized light; if 3θ = -Δk·t, the emitted second harmonic is left-handed circularly polarized light. Figure 6A is a schematic diagram of the input light, output light, and nonlinear optical crystal structure, and Figure 6B is the experimental measurement results of this embodiment. In Figure 6B, the phase difference ΔΦ between Ex and Ey is 86°, and the ellipticity η is 40°. As shown in Figure 6B, by stacking two material layers and maintaining the deflection angle of 3θ = +Δk·t or 3θ = -Δk·t, linearly polarized light can be made to output a circularly polarized second harmonic (the test results in Figure 6B show elliptical polarization close to circular polarization) after passing through the nonlinear optical crystal structure. Furthermore, by controlling the deflection direction of adjacent material layers, the polarization direction of the circularly polarized light can be further controlled. It should be noted that the "+" and "-" signs in the conditions 3θ = +Δk·t and 3θ = -Δk·t described in conjunction with the embodiments of Figures 6A and 6B here represent the direction of deflection, and a positive deflection angle is opposite to a negative deflection angle. For example, a positive deflection angle represents a deflection in the counterclockwise direction, and a negative deflection angle represents a deflection in the clockwise direction; or, a positive deflection angle represents a deflection in the clockwise direction, and a negative deflection angle represents a deflection in the counterclockwise direction. Therefore, as an exemplary continuous angle structure, it can also be considered that the deflection angles are all positive or all negative. However, it should be noted that for the sake of simplicity of description, in the case of a specified deflection direction (for example, the deflection direction is the same) or when the deflection direction is not specified, the angle value or angle range without the "+" and "-" signs can be considered as the absolute value of the deflection angle or the absolute value range of the deflection angle. Therefore, the deflection direction of adjacent material layers can be changed as needed or in conjunction with other conditions. Therefore, the aforementioned conditions for achieving a nonlinear optical crystal structure that converts linear polarization into circular polarization in a specific direction can also be expressed as the total thickness of the nonlinear optical crystal structure, T = c·t = n·lc, and 3θ = Δk·t. Depending on the desired circular polarization direction, the deflection direction of the material layer, or the side of the nonlinear optical crystal structure from which the incident light enters, can be determined. The embodiments shown in Figures 6A and 6B use the same thickness t for each material layer, but the structure that can achieve conversion from linearly polarized light to circularly polarized light is not limited to the structure with the same thickness t. In some embodiments, at least two of the multiple material layers have different thicknesses, and the effect of converting linearly polarized light into circularly polarized light can also be achieved when the following conditions are met:m The relationship satisfies: And the thickness of the material layer satisfies: Where q is an integer greater than or equal to 1 and less than or equal to N, t q is the thickness of the qth material layer. In the above embodiments, the thickness of each material layer may be less than the coherence length lc. However, the nonlinear optical crystal structure is not limited thereto. The thickness of at least one of the multiple material layers included in the nonlinear optical crystal structure may also be greater than the coherence length. In this case, by setting the relationship between the thickness of the material layer and the deflection angle between adjacent material layers, the nonlinear optical crystal structure can also increase the nonlinear light output efficiency. In some embodiments, the coherence length lc of the crystalline material used for the material layer is greater than 0 and less than or equal to 1 mm. In some embodiments, the coherence length may range from 10 nm to 100 μm. As some examples, the wave vector mismatch of the two-dimensional material used to construct the nonlinear optical crystal structure in the second-order nonlinear optical effect is Δk (Δk = k3-k2-k1, that is, the difference between the wave vector of the outgoing light and the wave vector of the incident light). The layer thickness or polarization period of the structure of the above optical crystal when achieving quasi-phase matching to obtain enhanced second-order nonlinear optical effect can be calculated by t = m1*lc = m1*π / Δk, where m1 is an odd number and lc is the coherence length (the coherence length is the characteristic distance at which nonlinear light can be enhanced, lc = π / Δk). Each layer is stacked at a specific deflection angle to achieve quasi-phase matching, thereby achieving effective enhancement of the second harmonic; the above deflection angle is to reverse the polarization direction between them. If the symmetry of the crystal is n-fold rotational symmetry, the deflection angle for achieving quasi-phase matching periodic structure is 180° / n*m2, where m2 is an odd number. In some embodiments, the periodic structure further includes multiple periods and quasi-periods. In some embodiments, the thickness of each material layer is substantially equal to the coherence length lc, and the deflection angle of adjacent material layers is 51 to 69 degrees. Under such conditions, the nonlinear light output efficiency can be enhanced and quasi-phase matching can be achieved. 7 and Table 1, for the above-mentioned nonlinear optical crystal structure, when the material layer uses different symmetry materials (single-fold rotational symmetry crystal C1, double-fold rotational symmetry crystal C2, triple-fold rotational symmetry crystal C3, quadruple-fold rotational symmetry crystal C4 or sextuple-fold rotational symmetry crystal C6), different geometric phases will be introduced. In addition, when circularly polarized light is incident, circularly polarized light of different rotations will introduce different geometric phases. For the p-th harmonic, the geometric phase introduced when emitting circularly polarized light of the same rotation is (p-1)θ, and the geometric phase introduced when emitting circularly polarized light of opposite rotation is (p+1)θ, where p is the harmonic order. In FIG7 and Table 1, "+" represents the same rotation, and "-" represents the opposite rotation. Table 1 For example, as shown in (a) of Figure 7, when circularly polarized light with a frequency of ω is incident on a material layer using a nonlinear optical crystal structure of a double rotational symmetry crystal C2, when the deflection angle θ satisfies the phase matching condition, the output includes circularly polarized light with the same rotation and an introduced geometric phase of 2θ at a frequency of 3ω, circularly polarized light with the opposite rotation and an introduced geometric phase of 4θ at a frequency of 3ω, circularly polarized light with the same rotation and an introduced geometric phase of 4θ at a frequency of 5ω, and circularly polarized light with the opposite rotation and an introduced geometric phase of 6θ at a frequency of 5ω. For example, as shown in (b) of Figure 7, when circularly polarized light with a frequency of ω is incident on a material layer using a nonlinear optical crystal structure of a triple rotational symmetry crystal C3, when the deflection angle θ satisfies the phase matching condition, the output includes circularly polarized light with an opposite rotation and an introduced geometric phase of 3θ at a frequency of 2ω, circularly polarized light with the same rotation and an introduced geometric phase of 3θ at a frequency of 4ω, and circularly polarized light with an opposite rotation and an introduced geometric phase of 6θ at a frequency of 5ω. For example, as shown in (c) of Figure 7, when circularly polarized light with a frequency of ω is incident on a material layer using a nonlinear optical crystal structure of a four-fold rotational symmetry crystal C4, when the deflection angle θ satisfies the phase matching condition, the output includes circularly polarized light with an opposite rotation and an introduced geometric phase of 4θ at a frequency of 3ω and circularly polarized light with the same rotation and an introduced geometric phase of 4θ at a frequency of 5ω. For example, as shown in (d) in Figure 7, when circularly polarized light with a frequency of ω is incident on the material layer using a nonlinear optical crystal structure of a six-fold rotational symmetry crystal C6, when the deflection angle θ satisfies the phase matching condition, the output includes circularly polarized light with an opposite rotation with a frequency of 5ω and an introduced geometric phase of 6θ. FIG8A shows a nonlinear optical crystal structure capable of achieving fifth harmonic phase matching in some embodiments, which is a continuous corner stacking method, each material layer has the same thickness t, and the deflection angle θ between adjacent layers is the same, both In this embodiment, each material layer is made of hexagonal boron nitride (h-BN), a material with six-fold rotational symmetry. According to Table 1, the selection rule is that the outgoing circular polarization light is opposite to the incident circular polarization light, and the introduced geometric phase is 6θ. FIG8B shows the dependence of the fifth harmonic intensity on the number of material layers when left-handed circularly polarized light and right-handed circularly polarized light are incident on the nonlinear optical crystal structure of the embodiment shown in FIG8A. As shown in FIG2B, under the excitation of left-handed circularly polarized light, as the number of material layers increases, the fifth harmonic intensity increases approximately quadratically, that is, fifth harmonic phase matching is achieved, and the conversion efficiency can reach 1.5×10 -5 . FIG9A shows a nonlinear optical crystal structure capable of achieving third harmonic phase matching in some embodiments, which is a continuous corner stacking method, wherein each material layer has the same thickness t, and the deflection angles θ between adjacent layers are the same, both being In this embodiment, each material layer is made of calcium fluoride, a material with four-fold rotational symmetry. According to Table 1, the selection rule is that the outgoing circular polarization light is opposite to the incident circular polarization light, and the introduced geometric phase is 4θ. Figure 9B shows the dependence of the third harmonic intensity on the number of material layers when left-handed circularly polarized light and right-handed circularly polarized light are incident on the nonlinear optical crystal structure of the embodiment shown in Figure 9A. As shown in Figure 3B, under the excitation of left-handed circularly polarized light, the third harmonic intensity increases approximately quadratically with the increase in the number of material layers, thus achieving third harmonic phase matching. In combination with the types of materials that can be used in the material layer of the nonlinear optical crystal and the types of modulated light used, in some embodiments, the wave vector mismatch Δk is within 10 3 ~10 9 m -1 For example, the nonlinear optical crystal of the present invention can be applied to modulate light with a wavelength range of 200 nm to 20 μm, but is not limited thereto. For example, the second-order nonlinear optical coefficient of the two-dimensional optical crystal layer can be greater than or equal to 0.01 pm / V. In this case, a good second-order nonlinear light output efficiency can be achieved. However, the present invention is not limited thereto. The above-mentioned nonlinear optical crystal structure uses a stack of rotationally symmetric crystals and achieves nonlinear enhancement of the second harmonic and even higher harmonics by controlling the deflection angle between the material layers. The above-mentioned nonlinear optical crystal structure takes advantage of the material layers' richer degrees of freedom (stacking angle and type of stacking materials) and more precise control methods (atomic-level longitudinal stacking accuracy). Therefore, its preparation process is more refined, with higher control freedom and more precise processing technology, making the crystal more sensitive to light control. Compared with traditional nonlinear metasurface solutions, it can achieve higher precision (nanometer level), higher operability and higher efficiency. The above scheme can be applied to more material systems, including non-van der Waals materials and van der Waals material systems, not only to two-dimensional materials, but also to traditional optical crystal materials. The present invention further provides a method for preparing the nonlinear optical crystal structure of any of the above embodiments, comprising the following steps: The plurality of material layers are transferred and stacked according to the configuration conditions of the deflection angles. The present invention also provides a nonlinear light modulation method, comprising modulating light using the nonlinear optical crystal structure of any of the aforementioned embodiments: light is incident on the first principal surface of the nonlinear optical crystal structure, and light is emitted from the second principal surface. The nonlinear optical crystal structure may be any of the aforementioned embodiments of the present invention. It should be noted that the nonlinear light modulation method of the present invention can utilize the nonlinear optical crystal structure of any of the aforementioned embodiments to modulate light to output highly efficient nonlinear light. Therefore, the features of any of the aforementioned embodiments can be incorporated into this nonlinear light modulation method and will not be further elaborated upon here. Optionally, the incident light may be linearly polarized light, elliptically polarized light, or circularly polarized light, but is not limited thereto. Optionally, the wavelength range of the above-mentioned incident light can be 200nm~20μm, for example 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, etc. Optionally, the frequency range of the incident light may be 9.4×10 13 rad / s~9.4×10 15 rad / s, for example 9.5×10 13 rad / s, 1×10 14 rad / s、3×10 14 rad / s、6×1014 rad / s、8×10 14 rad / s, 1×10 15 rad / s, 2×10 15 rad / s、3×10 15 rad / s、4×10 15 rad / s、5×10 15 rad / s、6×10 15 rad / s、7×10 15 rad / s、8×10 15 rad / s、9×10 15 rad / s, etc. The present invention also provides an optical device comprising the nonlinear optical crystal structure according to any one of the above embodiments. The above-mentioned optical devices include but are not limited to ultraviolet lasers, lidars, optical chips, polarization modulators, etc. The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A nonlinear optical crystal structure, characterized in that: It comprises a plurality of material layers, wherein the plurality of material layers are stacked in a direction perpendicular to their two-dimensional planes; each of the material layers is a crystal structure with y-fold rotational symmetry and has a predetermined lattice direction in a direction parallel to the two-dimensional plane; adjacent material layers have a non-zero deflection angle, wherein the deflection angle is the angle between the predetermined lattice directions of adjacent material layers; wherein y is an integer between 1 and 20.

2. The nonlinear optical crystal structure according to claim 1, wherein The nonlinear optical crystal structure comprises N material layers, wherein the deflection angle θ of the mth material layer relative to the first material layer is m satisfy: or, or, p is greater than y, and p = 2b - 1 + ay, where 1 ≤ b < p and b is an integer, a is an integer, and a ≠ 0, and The first material layer is any one of the material layers located on the outermost sides of the nonlinear optical crystal structure, the mth material layer is adjacent to the (m-1)th material layer, t n is the thickness of the nth material layer, t m is the thickness of the mth material layer, t1 is the thickness of the first material layer, N, n and m are all integers, N≥2, 1≤n≤N, 1<m≤N, Δk is the wave vector mismatch of the nonlinear optical effect of the material layer, Where λ is the wavelength of the incident light and ω is in the range of 9.4×10 13 rad / s~9.4×10 15 rad / s, λ ranges from 200nm to 20μm, p is the harmonic order, and p is an integer from 2 to 2000.

3. The nonlinear optical crystal structure according to claim 2, wherein: The deflection angle θ of the mth material layer relative to the first material layer m satisfy: or, or, 4. The nonlinear optical crystal structure according to claim 2, wherein: The thickness of each material layer is the same.

5. The nonlinear optical crystal structure according to claim 4, wherein: Each of the material layers has a thickness t, and the deflection angle θ of the mth material layer relative to the first material layer is m satisfy: or, or, 6. The nonlinear optical crystal structure according to claim 4, wherein: The deflection angle θ of the mth material layer relative to the first material layer m satisfy: or, or, 7. The nonlinear optical crystal structure according to any one of claims 2 to 6, wherein: The deflection directions of the m-th material layer relative to the (m-1)-th material layer are all the same.

8. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: The degrees of the deflection angles are the same.

9. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: The y value of each material layer is the same.

10. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: The thickness of each material layer is above 5 nm.

11. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: Adjacent material layers are bonded via van der Waals forces.

12. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: y is 1, and the material of the material layer is selected from at least one of silver molybdate, rhenium disulfide and KP15.

13. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: y is 2, and the material of the material layer is selected from at least one of potassium dihydrogen phosphate, potassium titanyl phosphate, barium triborate, germanium arsenide, black phosphorus, germanium selenide, and titanium trisulfide.

14. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: y is 3, and the material of the material layer is selected from at least one of 3R-MoS2, BBO, rhombohedral boron nitride, gallium selenide and KBBF.

15. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: y is 4, and the material of the material layer is selected from at least one of perovskite, calcium chloride, sodium chloride, calcium fluoride and magnesium fluoride.

16. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: y is 6, and the material of the material layer is selected from at least one of hexagonal boron nitride and graphene.

17. The nonlinear optical crystal structure according to any one of claims 1 to 6, wherein: y is an integer from 5, 7 to 20, and the material of the material layer is a quasicrystal.

18. A method for preparing a nonlinear optical crystal structure according to any one of claims 1 to 17, characterized in that: The following steps are involved: The plurality of material layers are transferred and stacked according to the configuration conditions of the deflection angles.

19. An optical device, characterized in that: A nonlinear optical crystal structure comprising the nonlinear optical crystal structure according to any one of claims 1 to 17.

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