Composite nonlinear metasurface and its second harmonic left-right phase independent control method

Abstract

The present invention discloses a composite nonlinear metasurface and a method for independently controlling the left-right phase of a second harmonic thereof. The metasurface comprises, from bottom to top, a substrate layer; a nonlinear medium layer; and a metasurface structure layer, comprising a plurality of periodically arranged metal metaatoms; the metal metaatoms having triple rotational symmetry, comprising a Y-shaped structure and three arc structures respectively connected to the ends of three arms of the Y-shaped structure; the arc structure having an inner arc and an outer arc, and a preset angle difference exists between the semi-center angle corresponding to the outer arc and the semi-center angle corresponding to the inner arc; the plurality of metal metaatoms having the same or different in-plane rotation angles to introduce spin-related nonlinear geometric phase control, and the arc structures in the plurality of metal metaatoms having the same or different semi-center angles corresponding to the inner arcs to introduce spin-independent transmission phase control. The metasurface can realize independent control of the left-right phase of a second harmonic under the pumping of a circularly polarized fundamental wave, which is of great significance to nonlinear optical systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic wave phase control, and in particular relates to a composite nonlinear metasurface and a method for independently controlling the left-right hand phase of its second harmonic. Background Art

[0002] Metasurfaces are a type of planar optical element composed of spatially varying subwavelength unit structures, which can control multiple parameters of light waves, such as phase, amplitude, polarization, etc., and are of great significance in the fields of imaging, holography, optical encryption, quantum information, etc. However, in existing nonlinear metasurfaces, phase control mostly relies on a single nonlinear geometric phase or transmission phase, which limits the flexibility of wavefront control.

[0003] Analogous to the geometric phase in linear optics, the concept of geometric phase is also introduced in the study of nonlinear metasurfaces. When the metasurface is irradiated with circularly polarized light, the phases carried by the nth harmonics with the same or opposite circular polarization state as the fundamental light are (n-1)σθ and (n+1)σθ, i.e., the nonlinear geometric phase. Among them, σ=±1 represents the left-handed circular polarization state and the right-handed circular polarization state of the incident fundamental light, and θ represents the rotation angle of the meta-atom. It can be seen that when left-handed circularly polarized fundamental light is incident respectively, the geometric phases carried by the harmonics generated on the metasurface are opposite and difficult to reuse; when fundamental light of the same rotation direction is incident, the geometric phases of the harmonics of different rotation directions generated are also correlated and cannot be completely independently controlled. The transmission phase introduced by only relying on the change of the structural size has no spin selectivity. In order to increase the degree of freedom of independent control of the nonlinear light field, multiple meta-atoms are generally required to jointly control. For example, currently only nonlinear metasurfaces that realize vector holography based on nonlinear geometric phase control combine four meta-atoms to form a super unit to independently control the phase, amplitude, polarization and other parameters of nonlinear harmonics, which greatly increases the complexity of structural design and pixel size, and also reduces the control accuracy. Therefore, independent control of the left-right phase of nonlinear harmonics based on a single meta-atom is of great significance for the control of multidimensional nonlinear light fields.

[0004] In linear optical systems, the symmetry of spin-orbit interaction can be broken by combining geometric phase and transmission phase to achieve independent regulation of left-hand and right-hand phases. However, in most cases, in order to reduce the fluctuation of polarization conversion efficiency, the transmission phase can only be discretely regulated by discretely changing the structural size, which reduces the phase regulation accuracy. However, in nonlinear optical systems, changes in the size of meta-atomic structures can easily affect the resonant enhancement conditions of frequency conversion, which in turn makes the harmonic amplitude more sensitive to changes in structural size. Therefore, the coordinated regulation of nonlinear geometric phase and transmission phase is currently difficult to achieve. Summary of the invention

[0005] In response to the above-mentioned problems existing in the coordinated control of nonlinear geometric phase and transmission phase, the present invention provides a composite nonlinear metasurface and a method for independently controlling the left-hand and right-hand phases of its second harmonic. A spin-decoupled phase modulation method is proposed based on the composite nonlinear metasurface, thereby realizing independent control of the left-hand and right-hand phases of the second harmonic.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] One aspect of the present invention provides a composite nonlinear metasurface, comprising from bottom to top:

[0008] A substrate layer; a nonlinear dielectric layer; and a supersurface structure layer, comprising a plurality of periodically arranged metal superatoms; the metal superatoms having a three-fold rotational symmetry, comprising a Y-shaped structure and three arc structures respectively connected to the ends of three arms of the Y-shaped structure; the arc structure having an inner arc and an outer arc, and a preset angle difference exists between the semi-center angle corresponding to the outer arc and the semi-center angle corresponding to the inner arc;

[0009] Among them, multiple metal metaatoms in the metasurface structure layer have the same or different in-plane rotation angles to introduce spin-related nonlinear geometric phase control, and the arc structures in the multiple metal metaatoms have the same or different semi-center angles corresponding to the inner arcs to introduce spin-independent transmission phase control. The composite nonlinear metasurface can achieve independent control of the left-right hand phase of the second harmonic under the pumping of the circularly polarized fundamental wave.

[0010] Furthermore, the metal super-atoms are arranged periodically, including in a hexagonal lattice, with a structural period of p and p<λ0 / 2, wherein λ0 is the wavelength of the fundamental wave of the circular polarization state, which is located in the near-infrared or mid-infrared band.

[0011] Furthermore, the preset angle difference Δα between the semi-center angle corresponding to the outer arc and the semi-center angle corresponding to the inner arc in the arc structure of the metal superatom has a value range of 0≤Δα<π / 9. The semi-center angle corresponding to the outer arc or the inner arc of the arc structure is the angle between the tip of the outer arc or the inner arc and the corresponding arm in the Y-shaped structure.

[0012] Furthermore, the value range of the semi-center angle α1 corresponding to the inner arc in the circular arc structure of the metal superatom is 0<α1<π / 3-Δα; the value range of the semi-center angle α2 corresponding to the outer arc in the circular arc structure of the metal superatom is Δα<α2<π / 3.

[0013] Furthermore, the in-plane rotation angle θ of the metal superatom is in the range of 0≤θ≤2π, where θ is the angle between one arm of the Y-shaped structure and the y-axis.

[0014] Furthermore, the arc structure width w1 of the metal superatom is less than p / 4, and the Y-shaped structure arm length l of the metal superatom is less than p / 4. ＜ p / 4, the arm width of the Y-shaped structure of the metal superatom w2<p / 4.

[0015] Furthermore, the thickness h of the super surface structure layer is in the range of 30nm≤h<150nm, the thickness t of the nonlinear medium layer is in the range of 15nm≤t≤30nm, and the thickness d of the substrate layer is in the range of 0.5mm≤d≤2mm.

[0016] Furthermore, the material of the metal superatoms in the supersurface structure layer includes one of gold, silver and aluminum, the material of the nonlinear medium includes one of indium tin oxide, multiple quantum wells and lithium niobate, and the material of the substrate layer includes one of silicon, silicon dioxide and sapphire.

[0017] Another aspect of the present invention provides a method for independently controlling the left-handed and right-handed phases of the second harmonic based on a composite nonlinear metasurface, wherein the composite nonlinear metasurface is used to independently control the left-handed and right-handed phases of the second harmonic.

[0018] Furthermore, the nonlinear geometric phase is regulated by changing the in-plane rotation angle of the metal metaatoms in the metasurface structure layer, and the transmission phase is regulated by changing the semi-center angle corresponding to the inner arc or the semi-center angle corresponding to the outer arc of the circular arc structure in the metal metaatoms, thereby independently controlling the left-right and right-handed phases of the second harmonic based on the composite nonlinear metasurface under the pumping of the circularly polarized fundamental wave.

[0019] Compared with the prior art, the composite nonlinear metasurface and the method for independently controlling the left-handed and right-handed phases of the second harmonic of the present invention combine the nonlinear geometric phase and transmission phase control through the structural design and optimization of the composite nonlinear metasurface, break the symmetry of the nonlinear spin-orbit interaction of photons, and realize the independent control of the phases of the left-handed and right-handed circular polarization components of the second harmonic, which is of great significance to the realization of ultra-compact multifunctional devices in nonlinear optical systems, as well as multidimensional optical data storage, optical encryption and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly understand the structure and embodiments of the present invention, the necessary drawings will be described below. The following drawings only represent some embodiments of the present invention.

[0021] Figure 1 A schematic diagram of a three-dimensional structure of a unit structure of a composite nonlinear supersurface according to an embodiment of the present disclosure is shown;

[0022] Figure 2A schematic diagram of a top view of a unit structure of a composite nonlinear metasurface according to an embodiment of the present disclosure is shown;

[0023] Figure 3 The unit structure simulation results of the composite nonlinear metasurface designed in accordance with the embodiment of the present disclosure when illuminated by a left-handed circularly polarized fundamental wave and under different rotation angles as the semi-circular center angle corresponding to the inner arc changes, wherein (a) is the normalized intensity simulation result of the second harmonic, and (b) is the phase simulation result of the second harmonic;

[0024] Figure 4a , Figure 4b , Figure 4c , Figure 4d The schematic diagram of the deflector structure and the simulation results based on the nonlinear metasurface design according to the embodiment of the present disclosure are shown schematically, wherein Figure 4a is the schematic diagram of the deflector structure. Figure 4b is the electric field distribution of the second harmonic in the yz plane when the left-handed and right-handed circularly polarized fundamental waves (LCP and RCP) are incident, Figure 4c , Figure 4d They are the corresponding far-field intensity distribution of the second harmonic.

[0025] The meanings of the reference numerals in the figures are as follows:

[0026] 1 is the substrate layer, 2 is the nonlinear medium layer, 3 is the metasurface structure layer, 4 is the Y-shaped structure, and 5 is the arc structure. DETAILED DESCRIPTION

[0027] All features disclosed in this specification, or steps in all methods or processes disclosed, except mutually exclusive features and / or steps, can be combined in any manner.

[0028] Any feature disclosed in this specification, unless otherwise stated, can be replaced by other equivalent or alternative features with similar purposes. That is, unless otherwise stated, each feature is only an example of a series of equivalent or similar features.

[0029] In order to make the contents of the present invention easy to understand, the following will be described in detail with reference to the drawings and specific embodiments. The enumerated embodiments are only part of the embodiments of the present invention, and other combinations are possible without departing from the present invention.

[0030] The present invention will be further described in detail below with reference to the accompanying drawings.

[0031] Figure 1 A schematic diagram of a three-dimensional structure of a unit structure of a composite nonlinear supersurface according to an embodiment of the present disclosure is shown schematically. Figure 2A schematic diagram of a top view of a unit structure of a composite nonlinear metasurface according to an embodiment of the present disclosure is shown schematically.

[0032] The embodiment of the present invention provides a composite nonlinear metasurface. Figure 1 , from bottom to top, it includes: substrate layer 1, nonlinear medium layer 2 and metasurface structure layer 3. Among them, the lower substrate layer 1 is used as a support, the middle nonlinear medium layer 2 is used for frequency conversion, and the upper metasurface structure layer 3 including metal meta-atoms is used to control the light field.

[0033] The super surface structure layer 3 includes a plurality of periodically arranged metal super atoms, see Figure 2 The metal superatom has a three-fold rotational symmetry, including a Y-shaped structure 4 and three arc structures 5 respectively connected to the three arm ends of the Y-shaped structure 4; the arc structure 5 has an inner arc and an outer arc, and there is a preset angle difference between the semi-center angle corresponding to the outer arc and the semi-center angle corresponding to the inner arc.

[0034] Among them, multiple metal metaatoms in the metasurface structure layer 3 have the same or different in-plane rotation angles to introduce spin-related nonlinear geometric phase control, and the arc structures in the multiple metal metaatoms have the same or different semi-center angles corresponding to the inner arcs to introduce spin-independent transmission phase control. The above-mentioned nonlinear metasurface can realize independent control of the left-right hand phase of the second harmonic under the pumping of the circularly polarized fundamental wave.

[0035] Specifically, the present invention controls the spin-related nonlinear geometric phase by rotating the metal meta-atoms or changing the in-plane rotation angle of the metal meta-atoms, and at the same time changes the semi-center angle corresponding to the inner arc of the arc structure in the metal meta-atoms to control the spin-independent transmission phase. Due to the plasmon resonance effect, the z component of the fundamental electric field at the tip of the arc structure is greatly enhanced, and it makes a major contribution to the generation of the second harmonic. When the semi-center angle corresponding to the inner arc or outer arc of the arc structure is changed, although the position of the tip of the arc structure rotates, the arc tip of the metal meta-atom still maintains its original shape, so that the position of the fundamental electric field enhancement changes significantly, while the enhancement amplitude does not change significantly. Therefore, when the semi-center angle corresponding to the inner arc or outer arc in the arc structure of the metal meta-atom is changed, the second harmonic phase is mainly changed, while the intensity does not change significantly, thereby realizing the coordinated regulation of the nonlinear geometric phase and the transmission phase. By optimizing the coverage of the transmission phase, the independent regulation of the left-right rotation phase of the second harmonic can be achieved by a single metal meta-atom. In addition, since the electric field resonance enhancement position changes continuously with the central angle of the inner arc of the circular arc structure, the transmission phase can be continuously adjusted.

[0036] In which, assuming that the entire supersurface corresponds to the xy coordinate system, the in-plane rotation angle θ of the metal superstructure atom is the angle between one arm of the Y-shaped structure 4 and the y-axis; the semi-central angle corresponding to the outer arc or inner arc of the arc structure 5 refers to the angle between the tip of the outer arc or inner arc and the corresponding arm of the Y-shaped structure 4.

[0037] Since the semi-center angle corresponding to the outer arc of the arc structure 5 and the semi-center angle corresponding to the inner arc need to have a preset angle difference to keep the tip shape unchanged, when the semi-center angle corresponding to the inner arc of the arc structure is adjusted, the semi-center angle corresponding to the outer arc also changes accordingly. Of course, the semi-center angle corresponding to the outer arc of the arc structure 5 can also be adjusted, and the center angle corresponding to the semi-inner arc also changes accordingly.

[0038] like Figure 1 , Figure 2 As shown, the thickness of the substrate layer of the composite nonlinear metasurface is d, the thickness of the nonlinear dielectric layer is t, the thickness of the metal superatom in the metasurface structure layer 3 is h, the semi-central angles corresponding to the inner and outer arcs of the arc structure 5 are α1 and α2 respectively, the preset angle difference is Δα, the arc width of the arc structure is w1, the arm length of the Y-shaped structure 4 is l, and the arm width is w2; the period of the metal superatom is p.

[0039] In combination with the above structure, the principle of phase control implemented by the present invention is first described, as follows:

[0040] The present invention realizes independent regulation of the phases of nonlinear second harmonic left-handed and right-handed circular polarization states by combining nonlinear geometric phase and transmission phase.

[0041] First, for metallic metaatoms with three-fold rotational symmetry, the nonlinear geometric phase can be introduced by changing its in-plane rotation angle θ. The geometric phase carried by the second harmonic with a circular polarization state opposite to the fundamental wave is 3σθ, where σ=±1 represents the left- and right-handed circular polarization states of the incident circular polarization state fundamental wave.

[0042] Secondly, the transmission phase can be introduced by changing the semi-center angle α1 corresponding to the inner arc or the semi-center angle α2 corresponding to the outer arc of the metal meta-atom. In addition, due to the non-chiral structure, the intensity and transmission phase of the second harmonic generated when the left-handed and right-handed circularly polarized light illuminate the metasurface are the same. Therefore, under the illumination of the left-handed and right-handed circularly polarized fundamental waves, the regulated phases of the orthogonal circularly polarized second harmonics generated are:

[0043]

[0044] From the above formula, we can see that When the transmission phase When covering 0~π, the left-handed and right-handed phases of the second harmonic can be independently controlled. When designing nonlinear photonic devices, according to the target phase Φ of the second harmonicL-R , Φ R-L (its value range is 0~2π) The required transmission phase can be calculated and the in-plane rotation angle θ of the metal superatom, where Φ L-R is the phase of the right-hand circularly polarized second harmonic generated by the left-hand circularly polarized fundamental wave, Φ R-L The phase of the left-handed circularly polarized second harmonic generated by the right-handed circularly polarized fundamental wave is:

[0045]

[0046] Where m = 0 or 1. When Φ L-R +Φ R-L When it is less than 2π, m is 0; when Φ L-R +Φ R-L When it is greater than 2π, m is set to 1 so that the transmission phase In the designed value range 0~π. When m=1, it is equivalent to setting the target phase Φ L-R Replace with Φ L-R -2π, where the introduction of 2π will not affect the modulation effect of the target light field.

[0047] Among them, the periodic arrangement of metal superstructure atoms can include being arranged in the form of a hexagonal lattice, with a structural period of p and p<λ0 / 2, wherein λ0 is the wavelength of the fundamental wave of the circular polarization state, generally in the near-infrared or mid-infrared band.

[0048] According to the present invention, the value range of the preset angle difference Δα between the semi-center angle α2 corresponding to the outer arc and the semi-center angle α1 corresponding to the inner arc in the circular arc structure of the metal superatom is 0≤Δα<π / 9; the value range of the semi-center angle α1 corresponding to the inner arc in the circular arc structure of the metal superatom is 0<α1<π / 3-Δα; the value range of the semi-center angle α2 corresponding to the outer arc in the circular arc structure of the metal superatom is Δα<α2<π / 3.

[0049] Furthermore, the in-plane rotation angle θ of the metal meta-atom is in the range of 0≤θ≤2π, where the entire meta-surface corresponds to the xy coordinate system, the metal meta-atom corresponds to the uv coordinate system, and the angle between the two coordinate systems is θ. Moreover, the arc structure width w1 of the metal meta-atom is <p / 4, the arm length l of the Y-shaped structure is <p / 4, and the arm width w2 is <p / 4. In addition, the thickness h of the meta-surface structure layer is in the range of 30nm≤h<1S0nm, the thickness t of the nonlinear medium layer is in the range of 15nm≤t≤30nm, and the thickness d of the substrate layer is in the range of 0.5mm≤d≤2mm, which can be adjusted according to actual processing requirements. Among them, each parameter is a standard value based on efficiency optimization, reduced coupling and easy processing.

[0050] Based on the above embodiments, the material of the metal meta-atoms in the metasurface structure layer 3 may include one of gold, silver and aluminum, the material of the nonlinear medium layer 2 may include one of indium tin oxide, multiple quantum wells and lithium niobate, which has a large nonlinear coefficient to achieve high frequency conversion efficiency, and the material of the substrate layer 1 may include one of silicon, silicon dioxide and sapphire.

[0051] The method for independently controlling the left-hand and right-hand phases of the second harmonic based on the composite nonlinear metasurface provided by the present invention uses the above-mentioned composite nonlinear metasurface to independently control the left-hand and right-hand phases of the second harmonic.

[0052] Specifically, the nonlinear geometric phase is regulated by changing the in-plane rotation angle of the metal metaatoms in the surface structure layer, and the transmission phase is regulated by changing the center angle corresponding to the inner arc of the circular arc structure in the metal metaatoms. Then, under the pumping of the circularly polarized fundamental wave, independent regulation of the left-right and right-handed phases of the second harmonic is achieved based on the above-mentioned composite nonlinear metasurface.

[0053] The present invention breaks the symmetry of the nonlinear spin-orbit interaction of photons, realizes the independent and continuous control of the phases of the left-handed and right-handed circular polarization components of the second harmonic, can effectively solve the problem of limited control dimension of nonlinear metasurfaces, and is of great significance to nonlinear optical systems.

[0054] In order to better understand the present invention, further explanation is provided below in conjunction with embodiments.

[0055] Without loss of generality, the material of the metal meta-atoms in the metasurface structure layer of this embodiment is gold, and its dielectric constant can be obtained from the Palik Optical Handbook; the material of the nonlinear dielectric layer is ITO, and its dielectric constant is taken from the experimental test data results. The maximum second-order nonlinear coefficient matrix element is d 33 =0.18pm / V; the material of the substrate layer is S i O2, whose dielectric constant can be obtained from the Palik Handbook of Optics.

[0056] Comsol software was used to simulate and optimize the unit structure. The fundamental wavelength of the circular polarization state of the pump was λ0=1400nm. The optimized unit structure parameters were p=640nm, h=30nm, t=23nm, d=500um, w1=60nm, l=190nm, w2=70nm, Δα=10°, α1=α2-Δα=15°~47°.

[0057] When the metasurface is illuminated by left-handed circularly polarized light, the normalized intensity and phase distribution of the second harmonic obtained by changing the in-plane rotation angle θ of the metal meta-atoms with three-fold rotational symmetry and the semi-circular center angle α1 corresponding to the inner arc of the arc structure is as follows: Figure 3 shown.

[0058] like Figure 3 As can be seen from Figures (a) and (b), for different in-plane rotation angles θ, when the semi-circular center angle α1 corresponding to the inner arc of the arc structure in the metal meta-atom changes from 15° to 47°, the variation range of the nonlinear geometric phase is greater than 180°, and the left-right and right-handed phases of the second harmonic can be completely independently controlled. In addition, the transmission phase The semi-circle center angle α1 corresponding to the inner arc approximately satisfies the linear relationship, and is obtained by fitting According to the fitting formula, the semi-center angle of the metaatom corresponding to the required transmission phase can be calculated.

[0059] Next, a nonlinear metasurface frequency doubling deflection device was designed to test the phase control effect.

[0060] The top view of the deflector structure is shown in Figure 4a As shown, the metal superatoms are arranged in a hexagonal lattice, and each row of metal superatoms has the same in-plane rotation angle θ and the semi-center angle α1 corresponding to the inner arc of the arc structure. The horizontal spacing between two adjacent rows of metal superatoms is Where p is the structural period of the metal superatom.

[0061] The phase differences of left-handed and right-handed second harmonics caused by two adjacent rows of metal superatoms in the deflector are set as follows: According to the generalized Snell's law, the corresponding second harmonic theoretical deflection angles are calculated as follows: and

[0062] When a circularly polarized plane wave is irradiated from the bottom of the composite nonlinear metasurface structure of this embodiment onto the nonlinear metasurface, the generated orthogonal circularly polarized second harmonic will be deflected. The normalized electric field of the second harmonic obtained by the Comsol full-scale simulation with LCP and RCP incident respectively is as follows: Figure 4b As shown in Figure 1, it can be seen that the light is deflected at different angles. Figure 4c and Figure 4d As shown, the electric field is Fourier transformed, and the far-field deflection angles of right-handed and left-handed second harmonics are 3.6° and 7.3° respectively, which is consistent with the theoretical calculation results above. Different from the opposite deflection angles of left-handed and right-handed frequency-doubled light produced by pure nonlinear geometric phase metasurface, the results of this embodiment show that the present invention can achieve independent regulation of the left-handed and right-handed phases of the second harmonic.

[0063] In summary, the composite nonlinear metasurface and the method for independently controlling the left-handed and right-handed phases of its second harmonic of the present invention improve the control dimension of nonlinear harmonics, help to realize multi-channel nonlinear holography, multidimensional optical data storage and complex vector light field generation, and are of great significance to the control of light fields in nonlinear metasurfaces.

[0064] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0065] The present invention is not limited to the above-mentioned specific embodiments, but extends to any new features or any new combination disclosed in this specification, as well as any new method or process steps or any new combination disclosed.

Claims

1. A composite nonlinear metasurface, characterized in that: From bottom to top, this includes: substrate layer; Nonlinear dielectric layer; A metasurface structure layer, comprising a plurality of periodically arranged metal metaatoms; the metal metaatoms having a three-fold rotational symmetry, comprising a Y-shaped structure and three arc structures respectively connected to the ends of three arms of the Y-shaped structure; the arc structure having an inner arc and an outer arc, and a semi-center angle corresponding to the outer arc and a semi-center angle corresponding to the inner arc having a preset angle difference; Among them, multiple metal metaatoms in the metasurface structure layer have the same or different in-plane rotation angles to introduce spin-related nonlinear geometric phase control, and the arc structures in the multiple metal metaatoms have the same or different semi-center angles corresponding to the inner arcs to introduce spin-independent transmission phase control. The composite nonlinear metasurface realizes independent control of the left-right hand phase of the second harmonic under the pumping of the circularly polarized fundamental wave.

2. The composite nonlinear metasurface according to claim 1, characterized in that: The periodic arrangement of the metal super-structure atoms includes an arrangement in the form of a hexagonal lattice, with a structural period of p and p<λ0 / 2, wherein λ0 is the wavelength of the fundamental wave of the circular polarization state, which is located in the near-infrared or mid-infrared band.

3. The composite nonlinear metasurface according to claim 1, characterized in that: The preset angle difference Δα between the semi-center angle corresponding to the outer arc and the semi-center angle corresponding to the inner arc in the arc structure of the metal superatom has a value range of 0≤Δα<π / 9, wherein the semi-center angle corresponding to the outer arc or inner arc of the arc structure is the angle between the tip of the outer arc or inner arc and the corresponding arm in the Y-shaped structure.

4. The composite nonlinear metasurface according to claim 3, characterized in that: The value range of the semi-center angle α1 corresponding to the inner arc in the arc structure of the metal superatom is 0<α1<π / 3-Δα; The value range of the semi-center angle α2 corresponding to the outer arc in the arc structure of the metal superatom is Δα<α2<π / 3.

5. The composite nonlinear metasurface according to claim 1, characterized in that: The in-plane rotation angle θ of the metal superatom is in the range of 0≤θ≤2π, where θ is the angle between one arm of the Y-shaped structure and the y-axis.

6. The composite nonlinear metasurface according to claim 2, characterized in that: The arc structure width w1 of the metal superatom is less than p / 4, the arm length l of the Y-shaped structure of the metal superatom is less than p / 4, and the arm width w2 of the Y-shaped structure of the metal superatom is less than p / 4.

7. The composite nonlinear metasurface according to claim 1, characterized in that: The thickness h of the super surface structure layer ranges from 30nm≤h<150nm, the thickness t of the nonlinear medium layer ranges from 15nm≤t≤30nm, and the thickness d of the substrate layer ranges from 0.5mm≤d≤2mm.

8. The composite nonlinear metasurface according to claim 1, characterized in that: The material of the metal superatoms in the supersurface structure layer includes one of gold, silver and aluminum, the material of the nonlinear medium includes one of indium tin oxide, multiple quantum wells and lithium niobate, and the material of the substrate layer includes one of silicon, silicon dioxide and sapphire.

9. A method for independent control of left-hand and right-hand phases of second harmonic based on composite nonlinear metasurface, characterized in that: The composite nonlinear metasurface described in any one of claims 1 to 8 is used to independently control the left-right hand phase of the second harmonic.

10. The method for independent control of left-hand and right-hand phase of second harmonic based on composite nonlinear metasurface according to claim 9, characterized in that: The nonlinear geometric phase is regulated by changing the in-plane rotation angle of the metal meta-atoms in the metasurface structure layer, and the transmission phase is regulated by changing the semi-center angle corresponding to the inner arc or the semi-center angle corresponding to the outer arc of the circular arc structure in the metal meta-atoms, thereby independently controlling the left-right and right-handed phases of the second harmonic based on the composite nonlinear metasurface under the pumping of the circularly polarized fundamental wave.

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