Organic all-medium nonlinear metasurface based on quasi-continuous domain bound state

Abstract

The invention discloses an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state, the organic all-dielectric nonlinear metasurface is formed by periodically arranging a plurality of unit cell units, and each unit cell unit comprises a substrate layer, an intermediate dielectric layer and a resonant unit structure layer which are sequentially arranged from bottom to top, the cross section of the resonance unit structure layer is in an L shape, the L shape is a square surface without a rectangular angle, and the intermediate dielectric layer is made of an organic nonlinear material. By designing the organic all-dielectric nonlinear metasurface structure, the asymmetry degree of the resonance unit structure is changed, perfect absorption of fundamental frequency light and local enhancement of an electric field are achieved, the second harmonic generation efficiency is improved, and the problems that an existing metal nonlinear metasurface is low in damage threshold and low in second-order nonlinear conversion efficiency are solved.

Description

Technical Field

[0001] The present invention relates to the field of nonlinear optics, and in particular to an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state. Background Art

[0002] The research and development of nonlinear metasurfaces has enabled significant progress in nanophotonics, both in fundamental research and applied science. Second-order nonlinear optical effects, in particular, have found increasingly widespread application in laser technology, information and image processing and storage, optical computing, and optical communications. Unlike bulk nonlinear materials, phase matching must be considered during second harmonic generation (SHG) frequency conversion to improve the conversion efficiency of nonlinear signals. However, metasurfaces, due to their subwavelength dimensions, achieve almost negligible phase matching along the direction of light propagation. Currently, nonlinear metasurfaces have become a hot topic in both theoretical and applied research. Summary of the Invention

[0003] The purpose of the present invention is to provide a design method for an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state.

[0004] The present invention provides an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state, comprising: the organic all-dielectric nonlinear metasurface is composed of a plurality of periodically arranged unit cells, each unit cell comprising: a substrate layer, an intermediate dielectric layer and a resonant unit structure layer arranged in sequence from bottom to top, the cross-section of the resonant unit structure layer is L-shaped, the L-shape being a square surface lacking a rectangular corner, and the intermediate dielectric layer being composed of an organic nonlinear material.

[0005] Preferably, the unit cell adopts a sub-wavelength lattice structure.

[0006] Preferably, the organic nonlinear material is 4-(4-dimethylaminophenylvinyl)methylpyridine p-toluenesulfonate.

[0007] Preferably, in the resonant unit structure layer, the offset of the centroid of the square surface relative to the centroid of the L-shaped cross section is used as the asymmetry, and the value range of the asymmetry of the resonant unit structure layer is: greater than 0 and less than 1.

[0008] Preferably, the cross-sections of the substrate layer and the intermediate dielectric layer are square.

[0009] Preferably, the lattice constant of the sub-wavelength lattice structure is 600 nm.

[0010] Preferably, the substrate layer is made of silicon dioxide.

[0011] Preferably, the resonance unit structure layer is made of silicon dioxide.

[0012] Preferably, the operating frequency band of the organic all-dielectric nonlinear metasurface is the near-infrared band of 1150 nm-1170 nm.

[0013] Preferably, the side lengths of the substrate layer and the intermediate dielectric layer are both 600 nm, the height of the substrate layer is 1.2 μm, and the height of the intermediate dielectric layer is 400 nm; the outer side length of the resonant unit structure layer is 300 nm, and the height is 200 nm.

[0014] By adopting the embodiment of the present invention, by designing the cross-section of the resonant unit structure layer into an L-shape and changing the asymmetry of the resonant unit, the quasi-continuous domain bound state can be regulated to achieve perfect absorption of fundamental frequency light and local enhancement of the electric field. Combined with the large second-order nonlinear polarizability of the organic nonlinear material, the conversion efficiency of the second harmonic of the organic all-dielectric nonlinear metasurface is enhanced.

[0015] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it is implemented in accordance with the contents of the specification, and in order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are specifically listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 A schematic structural diagram of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state provided by the present invention.

[0018] Figure 2 Schematic diagram of second harmonic generation of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state provided by the present invention.

[0019] Figure 3 A schematic diagram of the main view structure and dimensions of a single unit cell of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state provided by the present invention.

[0020] Figure 4 A schematic diagram of the top-down cross-sectional dimensions of a single unit cell of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state provided by the present invention.

[0021] Figure 5A schematic diagram of the definition of asymmetry of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state provided by the present invention.

[0022] Figure 6 Schematic diagram of the shape of the resonant unit structure layer under different symmetry degrees of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state provided by the present invention.

[0023] Figure 7 Schematic diagram of the transmittance of six organic all-dielectric nonlinear metasurfaces with different structures provided by the present invention.

[0024] Figure 8 The SHG conversion efficiency and quality factor Q of six organic all-dielectric nonlinear metasurfaces with different structures provided by the present invention are shown.

[0025] Figure 9 The present invention provides a multipole decomposition of a square organic all-dielectric nonlinear metasurface with a=0.

[0026] Figure 10 The invention provides a multipole decomposition of an L-shaped organic all-dielectric nonlinear metasurface with a=0.91.

[0027] Figure 11 The electric field distribution diagrams of the square organic all-dielectric nonlinear metasurface with a=0 and the L-shaped organic all-dielectric nonlinear metasurface with a=0.91 provided by the present invention; (a) electric field distribution diagram of the yz section of the square structure; (b) electric field distribution diagram of the xz section of the square structure; (c) electric field distribution diagram of the xy section of the square structure; (d) electric field distribution diagram of the yz section of the L-shaped structure; (e) electric field distribution diagram of the xz section of the L-shaped structure; (f) electric field distribution diagram of the xy section of the L-shaped structure.

[0028] Figure 12 Magnetic field distribution diagrams of the square organic all-dielectric nonlinear metasurface with a=0 and the L-shaped organic all-dielectric nonlinear metasurface with a=0.91 provided by the present invention; (a) magnetic field distribution diagram of the yz section of the square structure; (b) magnetic field distribution diagram of the xz section of the square structure; (c) magnetic field distribution diagram of the xy section of the square structure; (d) magnetic field distribution diagram of the yz section of the L-shaped structure; (e) magnetic field distribution diagram of the xz section of the L-shaped structure; (f) magnetic field distribution diagram of the xy section of the L-shaped structure.

[0029] Figure 13 Transmission spectra of the L-shaped organic all-dielectric nonlinear metasurface with a=0.91 provided by the present invention at different incident angles.

[0030] Figure 14 The SHG intensity of the L-shaped organic all-dielectric nonlinear metasurface with a=0.91 under different input light intensities provided by the present invention. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] Example According to an embodiment of the present invention, an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state is provided. Figure 1 This is a structural diagram of an organic all-dielectric nonlinear metasurface based on a quasi-continuous domain bound state according to an embodiment of the present invention. The organic all-dielectric nonlinear metasurface is composed of a plurality of periodically arranged unit cells, and the unit cells adopt a subwavelength lattice structure, such as Figure 3 and Figure 4 As shown, each unit cell includes: a substrate layer, an intermediate dielectric layer, and a resonant unit structure layer arranged in sequence from bottom to top, the resonant unit structure layer is arranged at the center above the intermediate dielectric layer, and the cross-section of the resonant unit structure layer is L-shaped. The L-shape is a square surface lacking a rectangular corner, so the resonant unit structure layer has an asymmetric structure; the cross-sections of the substrate layer and the intermediate dielectric layer are square, wherein the substrate layer and the resonant unit structure layer are composed of silicon dioxide, and the intermediate dielectric layer is composed of an organic nonlinear material.

[0033] Organic nonlinear optical materials have the advantages of large second-order nonlinear polarization rate, fast response speed, and easy molecular tailoring. They are widely used in optoelectronic technology, optical information storage, optical imaging and other fields. Among them, 4-(4-dimethylaminophenylvinyl) methylpyridine p-toluenesulfonate (DAST) is the most important organic nonlinear optical material. Studies have shown that DAST crystals have a large second-order nonlinear polarization rate (d 11 1010 pm / V at 1318 nm, 600 pm / V at 1907 nm), large electro-optic coefficient (r 11 50 pm / V at 1313 nm), a large birefringence difference (~0.7) and a low dielectric constant (ɛ=5.2). These unique properties enable DAST to play an important role in many fields such as frequency conversion, electro-optical modulation, electro-optical detection, optical information processing and terahertz wave generation. Therefore, in this embodiment, the intermediate dielectric layer of the organic all-dielectric nonlinear metasurface is composed of DAST. DAST is an anisotropic medium with a refractive index of n along the ordinary optical axis. o =n x = n z =2.46, the refractive index along the extraordinary optical axis is ne =n y =1.70. In this embodiment, by changing the period of the organic all-dielectric nonlinear metasurface and the size of the asymmetric structure of the resonant unit structure layer, the operating band of the organic all-dielectric nonlinear metasurface is set to the near-infrared band of 1150nm-1170nm, and a strong localized electric field is generated, thereby improving the conversion efficiency of the second harmonic. Figure 2 A schematic diagram of second harmonic generation of an organic all-dielectric nonlinear metasurface based on a quasi-continuum bound state is provided in this embodiment. The calculation formula for the second harmonic generation (SHG) conversion rate is as follows: ; Where η represents the conversion efficiency of the second harmonic, and are the incident fundamental frequency light power and the power generated by the second harmonic, respectively.

[0034] In the embodiment of the present invention, the introduction of the resonant unit structure layer is beneficial to the local enhancement of the electric field and the improvement of the conversion efficiency of the second harmonic.

[0035] This embodiment Figure 3 and Figure 4 As shown, the length of the substrate layer and the intermediate dielectric layer are both 600 nm, the height of the substrate layer is 1.2 μm, the height of the intermediate dielectric layer is 400 nm, the outer side length of the cross section of the resonant unit structure layer (i.e., the side length of the square surface) is 300 nm, the area of ​​the missing rectangular corner is S, and the height of the resonant unit structure layer is 200 nm. The size and shape of the resonant unit structure layer can be set according to specific application requirements.

[0036] In this embodiment, the asymmetry of the resonant unit structure layer is defined as the offset of the center of mass o of the square surface relative to the center of mass of the L-shaped cross section. The center of mass offset is calculated as follows: The cross section of the resonant unit structure layer is decomposed into a plurality of regular geometric figures, the centroids of the plurality of regular geometric figures are calculated, and the centroid of the overall asymmetric structure is calculated according to the area and centroid coordinates of each part.

[0037] Taking the L-shaped cross section of the resonant unit structure layer as an example, which is divided into two regular rectangles, A1 and A2 are the areas of the two rectangles, and ,and is the centroid coordinate of the two rectangles, then the centroid coordinate of the cross section of the resonant unit structure layer is , calculated using the following formula: ; Finally, the Euclidean distance between the centroid o coordinate of the square surface and the centroid coordinate of the L-shaped cross section is calculated as the centroid offset.

[0038] In an embodiment of the present invention, an example structure is analyzed, such as Figure 5 As shown, the L-shaped cross-section of the resonant unit structure layer is divided into two upper and lower rectangles along the horizontal line, and the centroids A and B of the upper and lower rectangles are found respectively. The coordinates of the centroid coordinate calculation formula of the cross-section of the entire resonant unit structure layer are calculated using the established coordinate axis. The Euclidean distance from the centroid P to the centroid o of the square surface is calculated as the centroid offset, which is the asymmetry.

[0039] like Figure 6 As shown, the center of mass offset is defined as the asymmetry, represented by a. The cross-section of the resonant unit structure layer is an L-shaped asymmetric structure, and the value of a ranges from (0, 1). In this embodiment, the resonant unit structure layer has five different asymmetries and five different L-shaped cross-sections, with a being 0.14, 0.25, 0.47, 0.67, and 0.91, respectively. a = 0 represents the initial symmetric state, and the cross-section of the resonant unit structure layer is a square. As a control group, the above six resonant unit structure layers are used with the same intermediate dielectric layer and substrate layer to form six different unit cell structures, which are then arranged with the same periodicity to form six different organic all-dielectric nonlinear metasurfaces.

[0040] The transmittance and second harmonic conversion efficiency tests were conducted on six organic all-dielectric nonlinear metasurfaces with different structures. The test results are as follows: Figure 7 and Figure 8 As shown in the figure, it can be found that by changing the asymmetry of the resonant unit structure layer, the position of the resonance peak and the second harmonic conversion efficiency can be effectively adjusted. The resonance peak range can be flexibly adjusted within the range of 1150 nm-1170 nm, and as the asymmetry increases, the second harmonic conversion efficiency increases continuously, and the second harmonic conversion efficiency can reach up to 10 -2 Order of magnitude.

[0041] The multipole decomposition of square and L-shaped (a=0.91) organic all-dielectric nonlinear metasurfaces is carried out, including electric dipole (ED), magnetic dipole (MD), electric quadrupole (EQ), magnetic quadrupole (MQ) and ring dipole (TD). The results are shown in Figure 2. Figure 9 and Figure 10As shown in the figure, for a square-structured organic all-dielectric nonlinear metasurface, at the 1163 nm resonance peak, the contributions of MD and EQ to the total far-field scattered power are 66% and 22%, respectively, while MQ and TD contribute 6%. For the multipole decomposition of an L-shaped organic all-dielectric nonlinear metasurface with a = 0.91, at the 1157 nm resonance peak, the contributions of MD to the total far-field scattered power are 66%, EQ 20%, and both TD and MQ 7%. Therefore, this organic all-dielectric nonlinear metasurface is primarily influenced by MD and EQ.

[0042] The electric and magnetic field analysis of the square organic all-dielectric nonlinear metasurface with a=0 and the L-shaped organic all-dielectric nonlinear metasurface with a=0.91 are shown in the following results. Figure 11 and 12 As shown in the figure, the electric field intensity of the L-shaped organic all-dielectric nonlinear metasurface is approximately 3.5 times that of the square organic all-dielectric nonlinear metasurface, and the magnetic field intensity is approximately 4 times that of the square. It can be seen that the electromagnetic field intensity of the organic all-dielectric nonlinear metasurface with large asymmetry is significantly enhanced, and the second harmonic conversion efficiency can be effectively enhanced with the help of local enhancement of the electric field.

[0043] like Figure 13 As shown in Figure 2, the transmission spectra of an L-shaped organic all-dielectric nonlinear metasurface with a=0.91 were measured at different incident angles. When the incident angle increased from 0° to 60° in 10° steps, the resonant wavelength blue-shifted from 1163 nm to 1100 nm.

[0044] According to the transmission spectrum Figure 13 , we find that there is a quasi-continuum bound state at 1163 nm under normal incidence, which can significantly enhance the interaction between light and matter. Therefore, the optimal incident wavelength is 1163 nm. Under this condition, the SHG intensity of the organic all-dielectric nonlinear metasurface with a=0.91 was tested under different input light intensities. The results are as follows: Figure 14 As shown, the incident power is changed from 0.4 MW / cm 2 Increased to 200 MW / cm 2 Under the action of the quasi-continuous domain bound state, the SHG intensity of the L-shaped organic all-dielectric nonlinear metasurface is significantly enhanced with the continuous increase of the incident power. Taking the logarithm of the SHG intensity and the incident power respectively, the slope of the fitting line is about 2.0, proving that the signal is indeed SHG.

[0045] The beneficial effects of the present invention are as follows: The present invention provides an organic all-dielectric nonlinear metasurface, which, when the incident wavelength is 1163 nm, obtains an organic all-dielectric nonlinear metasurface that enhances the second harmonic conversion efficiency based on the quasi-continuous domain bound state. By designing the resonant unit structural layer into an L-shaped structure and changing the asymmetry of the resonant unit, the quasi-continuous domain bound state can be regulated to achieve perfect absorption of the fundamental frequency light and local enhancement of the electric field. Combined with the large second-order nonlinear polarizability of the organic nonlinear material, the second harmonic conversion efficiency of the organic all-dielectric nonlinear metasurface is further enhanced. At the same time, by adjusting the asymmetry of the resonant unit structural layer, the second harmonic conversion efficiency in the near-infrared band can be significantly enhanced. The present invention has many advantages such as high efficiency and integration, and has broad application prospects in the fields of laser technology, optical computing, optical communications, etc.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. These modifications or replacements of the technical solutions of the embodiments of the present invention do not cause the essence of the corresponding technical solutions to deviate from the scope of this solution.

Claims

1. An organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states, characterized in that: The organic all-dielectric nonlinear metasurface is composed of a plurality of unit cells arranged periodically, each unit cell comprising: a substrate layer, an intermediate dielectric layer and a resonant unit structure layer arranged in sequence from bottom to top, the cross-section of the resonant unit structure layer is L-shaped, the L-shape being a square surface lacking a rectangular corner, and the intermediate dielectric layer being composed of an organic nonlinear material.

2. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 1, characterized in that: The unit cell adopts a sub-wavelength lattice structure.

3. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 1, characterized in that: The organic nonlinear material is 4-(4-dimethylaminophenylvinyl)methylpyridine p-toluenesulfonic acid salt.

4. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 1, characterized in that: In the resonant unit structure layer, the asymmetry is determined by taking the offset of the centroid of the square surface relative to the centroid of the L-shaped cross section as the asymmetry. The asymmetry of the resonant unit structure layer has a value range of greater than 0 and less than 1.

5. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 1, characterized in that: The cross sections of the substrate layer and the intermediate dielectric layer are square.

6. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 2, characterized in that: The lattice constant of the sub-wavelength lattice structure is 600 nm.

7. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 1, characterized in that: The substrate layer is made of silicon dioxide.

8. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 1, characterized in that: The resonance unit structure layer is made of silicon dioxide.

9. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 4, characterized in that: The operating frequency band of the organic all-dielectric nonlinear metasurface is the near-infrared band of 1150 nm-1170 nm.

10. The organic all-dielectric nonlinear metasurface based on quasi-continuous domain bound states according to claim 5, characterized in that: The side lengths of the substrate layer and the intermediate dielectric layer are both 600 nm, the height of the substrate layer is 1.2 μm, and the height of the intermediate dielectric layer is 400 nm; the outer side length of the resonant unit structure layer is 300 nm, and the height is 200 nm.

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