High-order Bessel beam generation method and metasurface axial cone device thereof

Abstract

The invention provides a high-order Bessel beam generation method and a metasurface axicon device thereof, the metasurface axicon device comprises a reflecting layer, a dielectric layer and an antenna array layer, and the antenna array layer is formed by arranging a plurality of nano columns in a rectangular array. The azimuth angle of each nanorod is coded based on the phase distribution of the high-order Bessel beam to be generated; during use, a high-order Bessel beam is directly generated based on the reflective metasurface, and the scheme has the advantages of small size, integration, wide spectrum and the like; compared with a traditional method, the method has the advantages that the size of the device can be greatly reduced while the same function is realized, so that the application of the high-order Bessel beam in integrated optics is promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of metasurface light field control, and specifically relates to a high-order Bessel light beam generation method and a metasurface axicon device thereof. Background Art

[0002] When propagating in a free light path, the transverse light field of the Bessel beam remains unchanged and has a non-diffraction characteristic. The Bessel beam also has the characteristic of self-healing. When it is blocked by an obstacle and scattered, the original light field distribution can be reconstructed after propagating for a certain distance, which is impossible for an ordinary Gaussian beam. The above characteristics make Bessel beams have broad application prospects in fields such as deep imaging and light manipulation. The ideal transverse light field of a Bessel beam consists of countless rings and requires infinite energy. Therefore, only approximate Bessel beams can be generated in experiments. In experiments, a conical lens is often used to generate zero-order Bessel beams. This lens is called an axicon. Bessel beams carrying orbital angular momentum (OAM) are called high-order Bessel beams. They usually require a combination of an axicon and a phase modulation device to generate them. This method has the disadvantages of large size, complex processing, single function, and dispersion, which limits the application of Bessel beams in integrated optics.

[0003] Metasurface is a two-dimensional artificial material composed of an array of subwavelength structures. It has many properties that do not exist in natural materials, such as negative refraction and perfect absorption of electromagnetic waves. Through the careful design of the metasurface unit structure, arbitrary modulation of the phase, amplitude, polarization and other parameters of the incident light can be achieved. The unit structure period of the metasurface must be smaller than the wavelength of the incident light to ensure that the incident light will not diffract after passing through the metasurface. Metasurfaces have been widely used in the fields of beam focusing, holographic imaging, polarization conversion and spatial beam generation. Therefore, how to generate high-order Bessel beams based on metasurfaces is a potentially feasible research route. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a high-order Bessel beam generation method and a metasurface axicon device thereof, so as to solve the problems of large size, high cost, complex processing and the like of traditional devices, and facilitate large-scale and low-cost production.

[0005] The present invention achieves the above technical objectives through the following technical means.

[0006] A metasurface axicon device comprises a reflective layer, a dielectric layer and an antenna array layer, wherein the antenna array layer is composed of a plurality of nanocolumns arranged in a rectangular array, and the azimuth angle of each nanocolumn is encoded based on the phase distribution of a high-order Bessel light beam to be generated.

[0007] Furthermore, the phase distribution of the high-order Bessel beam to be generated is produced by the linear superposition of the conical wavefront of the zero-order Bessel beam and the spiral wavefront of the orbital angular momentum.

[0008] Furthermore, the azimuth angle of each nanorod is:

[0009]

[0010] In the formula, (x i ,y i ) represents the position coordinates of each nanocolumn, λ is the wavelength of the incident light, NA is the numerical aperture, l is the orbital angular momentum topological charge, and is also the order of the Bessel beam to be generated.

[0011] Furthermore, the reflective layer is made of metal.

[0012] Furthermore, the reflective layer is made of Au with a thickness of 100 nm; the dielectric layer is made of SiO2 with a thickness of 85 nm; the nanocolumn is made of Au, and is a rectangular column structure with a length, width and height of 200 nm×100 nm×30 nm.

[0013] Furthermore, the array period of the nanorods is p×p, where p=400 nm.

[0014] Furthermore, the reflective layer is arranged on a substrate, and the substrate is made of SiO2.

[0015] Furthermore, the plurality of nanorods are arranged in a circular area.

[0016] A method for generating a high-order Bessel beam: using the above-mentioned metasurface axicon device to generate a Bessel beam.

[0017] Furthermore, circularly polarized light is injected into the metasurface axicon device, and a Bessel beam is obtained by reflection; when encoding the azimuth angle of the nanorod array, the value of l is adjusted to obtain a Bessel beam of a corresponding order.

[0018] The beneficial effects of the present invention are:

[0019] (1) The present invention provides a method for generating a high-order Bessel beam and a metasurface axicon device thereof, wherein a reflective metasurface is used to directly generate a high-order Bessel beam. This scheme has the advantages of small size, integrability, and a wide spectrum. Compared with the traditional method, the present invention can greatly reduce the size of the device while achieving the same function, thereby promoting the application of high-order Bessel beams in integrated optics.

[0020] (2) The present invention has a simple structure and can achieve the design purpose with only one metasurface structure, thus solving the problems of large size, complex structure, and high processing precision requirements of traditional devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1( a ) is a schematic diagram of a unit structure of a metasurface axicon device of the present invention;

[0022] FIG1( b ) is a schematic diagram of the distribution of the metasurface antenna array in the metasurface axicon device of the present invention;

[0023] Figure 2 Phase modulation diagram of the metasurface used to generate the first-order Bessel beam in Test 1;

[0024] Figure 3 Propagation diagram of the Bessel beam generated in test 1, where (a) is the propagation direction and (b) is the cross section;

[0025] Figure 4 The detection diagram of the first-order Bessel beam generated for test 1;

[0026] Figure 5 Propagation diagram of the third-order Bessel beam generated for test 2 along the propagation direction;

[0027] Figure 6 Detection diagram of the third-order Bessel beam generated for Test 2. DETAILED DESCRIPTION

[0028] Embodiments of the present invention are described in detail below, examples of the illustrated embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0029] 1. Solution

[0030] As shown in Figures 1(a) and 1(b), the metasurface axicon device proposed by the present invention is a reflective metasurface, which includes a transparent substrate, a metal reflective layer, a dielectric layer, and an antenna array layer from bottom to top. In this embodiment, the substrate and the dielectric layer are both made of silicon dioxide (SiO2), wherein the dielectric layer has a thickness of 85nm; the reflective layer is made of gold (Au) with a thickness of 100nm, which is used for reflection; the antenna array layer is composed of a number of nanocolumns, and the nanocolumns in this embodiment are also made of gold (Au). The operating wavelength of the above-mentioned metasurface axicon device is 800nm.

[0031] The cross section of the nanocolumn is a rectangular structure, with a length, width and height of 200nm×100nm×30nm respectively. Several nanocolumns are arranged in a circular area in the form of a rectangular array, and the array period is p×p. The period of the nanocolumn array (the center distance between two adjacent unit structures in the metasurface) should be smaller than the wavelength of the incident light to avoid high-order diffraction, but it should not be too small to avoid coupling between adjacent nanocolumns. In this embodiment, the period of the nanocolumn array is set to p=400nm.

[0032] When the phase introduced by the antenna array layer satisfies the phase distribution of the high-order Bessel beam, a Bessel beam of any order can be directly generated by reflection. The wavefront of the high-order Bessel beam is generated by the linear superposition of the conical wavefront of the zero-order Bessel beam and the spiral wavefront of the orbital angular momentum; therefore, the corresponding phase distribution can be encoded into the metasurface antenna array according to the geometric phase principle:

[0033] 1) The phase distribution of the zero-order Bessel beam is:

[0034]

[0035] In the formula, (x i ,y i ) represents the position coordinates of each unit structure in the metasurface, λ is the wavelength of incident light, and NA is the numerical aperture;

[0036] 2) The phase distribution of orbital angular momentum is:

[0037]

[0038] Where l is the orbital angular momentum topological charge, and also represents the order of the Bessel beam;

[0039] 3) In summary, the phase distribution of the high-order Bessel beam to be generated is:

[0040] Φ(x i ,y i )=Φ1(x i ,y i )+Φ2(x i ,y i )

[0041] 4) According to the geometric phase principle, the corresponding relationship between the azimuth angle of each unit structure in the metasurface (the azimuth angle of each nanopillar) θ and its phase modulation amount Φ for the incident light is θ=Φ / 2; therefore, the azimuth angle distribution of each unit structure (each nanopillar) in the metasurface is:

[0042]

[0043] 5) Since the above-mentioned metasurface axicon device is dependent on the rotation direction of the incident circularly polarized light, when encoding the metasurface according to 4), only left-handed circularly polarized light can be used to generate a Bessel beam; if right-handed circularly polarized light is used as the incident light, the phase generated by the outgoing left-handed circularly polarized component is the conjugate phase of the Bessel beam, and the Bessel beam cannot be generated. Therefore, the formula of 4) above is modified as follows:

[0044]

[0045] According to θ(x i ,y i ) is used to encode the metasurface by changing the size of l; then, under the incidence of corresponding circularly polarized light, the metasurface axicon device can directly generate a Bessel beam of corresponding order.

[0046] For incident circularly polarized light:

[0047]

[0048] In the formula, i is an imaginary unit, and the positive and negative signs are used to indicate whether the incident circularly polarized light is left-handed or right-handed;

[0049] The outgoing light after reflection by the metasurface axicon device of the present invention is:

[0050]

[0051] Where t0 and t e They represent the transmission coefficients of the outgoing light when the polarization of the incident light is parallel to the two principal axes of the nanorod.

[0052] 2. Testing

[0053] The following tests will be conducted using finite time-domain difference (FDTD) simulation software, where the incident light is set to be left-handed circularly polarized light with a wavelength of 800nm, and the right-handed component in the outgoing light is detected. When right-handed circularly polarized light is incident, the phase generated by the outgoing left-handed circularly polarized component is the conjugate phase of the Bessel beam, and the Bessel beam cannot be generated. This is because this Bessel beam generator has a circular polarization-dependent characteristic.

[0054] Test 1

[0055] When l is 1, a first-order Bessel beam is generated. The phase modulation generated by the corresponding metasurface is Figure 2 As shown, it can be seen that the number of spiral lines is 1, which represents the topological charge of the orbital angular momentum, that is, the order of the Bessel beam is 1. At the same time, the phase changes periodically, uniformly and continuously in the radial direction, which is consistent with the phase characteristics of the Bessel beam.

[0056] like Figure 3Shown is the propagation diagram of the generated first-order Bessel beam, where (a) is the propagation direction and (b) is the light spot on the cross section. Figure 3 It can be seen from Figure (a) that the light beam maintains strong energy in the range of 5μm to 18μm, showing a non-diffraction characteristic. Figure 3 It can be seen from Figure (b) that the light spot is composed of multiple concentric rings. From the center to the edge, the energy of the rings gradually decays, and the light intensity at the center of the light spot is zero, which is consistent with the characteristics of a high-order Bessel beam.

[0057] Figure 4 The figure shows the order detection of the generated high-order Bessel beam. Interference method is often used to detect beams carrying orbital angular momentum; any linearly polarized incident light can be decomposed into two beams of left-handed circularly polarized light and right-handed circularly polarized light with the same frequency and constant phase difference. When they pass through the metasurface, they will be converted into right-handed light carrying orbital angular momentum with a topological charge of +l and left-handed transmitted light with a topological charge of -l, respectively. The light spot obtained after the interference of the two is distributed in a spiral shape. Specifically, Figure 4 The number of spirals shown in is twice the orbital angular momentum topological charge. Therefore, by counting the number of spirals, the orbital angular momentum topological charge, that is, the order of the high-order Bessel beam, can be obtained.

[0058] Test 2

[0059] When l is 3, a third-order Bessel beam is generated. The corresponding third-order Bessel beam is as follows: Figure 5 As shown; compared with Test 1, the propagation distance of the third-order Bessel beam is comparable to that of the first-order Bessel beam, but the radius of its central bright ring is larger, which is one of the basic characteristics of a beam carrying orbital angular momentum. Figure 6 Shown is the detection diagram of the generated third-order Bessel beam. There are 6 spiral lines in the figure, which is twice the orbital angular momentum topological charge number 3, verifying that a third-order Bessel beam is generated.

[0060] In the description of the present invention, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are 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 a limitation on the present invention.

[0061] The present invention is not limited to the above-mentioned embodiments. Any obvious improvement, substitution or deformation that can be made by those skilled in the art without departing from the essential content of the present invention belongs to the protection scope of the present invention.

Claims

1. A metasurface axicon device, characterized in that: It includes a reflective layer, a dielectric layer, and an antenna array layer, wherein the antenna array layer is composed of a plurality of nanocolumns arranged in a rectangular array, and the azimuth angle of each nanocolumn is encoded based on the phase distribution of the high-order Bessel light beam to be generated.

2. The metasurface axicon device according to claim 1, characterized in that: The phase distribution of the high-order Bessel beam to be generated is produced by the linear superposition of the conical wavefront of the zero-order Bessel beam and the spiral wavefront of the orbital angular momentum.

3. The metasurface axicon device according to claim 2, characterized in that: The azimuth angle of each nanorod is: In the formula, (x i ,y i ) represents the position coordinates of each nanocolumn, λ is the wavelength of the incident light, NA is the numerical aperture, l is the orbital angular momentum topological charge, and is also the order of the Bessel beam to be generated.

4. The metasurface axicon device according to claim 1, characterized in that: The reflective layer is made of metal.

5. The metasurface axicon device according to claim 1, characterized in that: The reflective layer is made of Au with a thickness of 100 nm; the dielectric layer is made of SiO2 with a thickness of 85 nm; the nanocolumn is made of Au and has a rectangular column structure with a length, width and height of 200 nm×100 nm×30 nm.

6. The metasurface axicon device according to claim 5, characterized in that: The array period of the nanorods is p×p, where p=400 nm.

7. The metasurface axicon device according to claim 5, characterized in that: The reflective layer is arranged on a substrate, and the substrate is made of SiO2.

8. The metasurface axicon device according to claim 1, characterized in that: The plurality of nanorods are arranged in a circular area.

9. A method for generating a high-order Bessel beam, characterized in that: A Bessel beam is generated using the metasurface axicon device as described in any one of claims 1 to 8.

10. The method for generating a high-order Bessel beam according to claim 9, characterized in that: Circularly polarized light is injected into the metasurface axicon device, and a Bessel beam is obtained by reflection; when encoding the azimuth angle of the nanocolumn array, the value of l is adjusted to obtain a Bessel beam of a corresponding order.

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