An optical switch based on a full-dielectric intrinsic chiral metasurface and a preparation method thereof

Abstract

This invention belongs to the field of optical switch technology, specifically disclosing an optical switch based on an all-dielectric intrinsic chiral metasurface and its fabrication method. The switch includes a substrate and an array of metasurface unit structures distributed on the substrate. The metasurface unit structures are cubic prism-shaped silicon blocks with a central hole, the hole being moved along the y-axis to break structural symmetry. The silicon blocks are tilted along both the negative x-axis and y-axis directions to obtain a metasurface unit structure with intrinsic chirality in a quasi-BIC mode. Compared with existing technologies, the metasurface of this invention's optical switch based on an all-dielectric intrinsic chiral metasurface exhibits a strong intrinsic chiral response. The transmission spectrum can be adjusted by changing the rotation direction of circularly polarized light incident perpendicularly to the metasurface to achieve a high on / off ratio, and the operating wavelength can be easily extended. This optical switch is a compact passive device that does not require an additional active pump field, and is expected to enrich applications in optical communication and all-optical computing devices.

Description

Technical Field

[0001] This invention relates to the field of optical switch technology, and in particular to an optical switch based on an all-dielectric intrinsic chiral metasurface and its fabrication method. Background Technology

[0002] Traditional optical switches are based on the thermo-optical effect and the plasmonic dispersion effect, and mainly come in three forms: thermo-optical, electro-optical, and all-optical. For all-optical switches, the Kerr effect is one of the most important mechanisms, describing a phenomenon where the refractive index of a material depends on the intensity of the pump light. Currently, on-chip interferometers, photonic crystals, plasmonic waveguides, waveguide ring resonators, and metasurfaces based on the Kerr effect are widely used in the design of various all-optical switches.

[0003] Optical chirality manifests as the different responses of a medium to left-handed and right-handed circularly polarized light; this difference in response is measured by circular dichroism (CD). The weak chiral optical interactions in natural materials limit their applications. Metasurfaces, due to their superior optical field manipulation and localization capabilities, can achieve large CD values. Specifically, under perpendicular incidence conditions, chiral metasurfaces can achieve spin-selective transmission of incident light. Intrinsic chiral metasurfaces do not involve polarization conversion losses, allowing for even higher CD values ​​in practical applications. The spin-selective transmission properties of intrinsic chiral metasurfaces make them suitable for high-modulation-depth optical switch designs.

[0004] However, there is currently a lack of designs for intrinsic chiral metasurfaces in optical switch applications. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides an optical switch based on an all-dielectric intrinsic chiral metasurface and its fabrication method.

[0006] To achieve the above objectives, the present invention is implemented according to the following technical solution:

[0007] One of the technical solutions of the present invention is an optical switch based on an intrinsically chiral metasurface, comprising a substrate and a metasurface unit structure arrayed on the substrate. The metasurface unit structure is a cubic columnar silicon block with a hole in the center, and the hole is moved in the y-axis direction to break the structural symmetry. The silicon block is tilted in the negative x-axis direction and the y-axis direction respectively to obtain a metasurface unit structure with intrinsically chiral quasi-BIC mode.

[0008] Furthermore, the silicon block has a width of 450 nm, a thickness of 300 nm, and a radius of 80 nm for the hole at the center of the silicon block.

[0009] Furthermore, the metasurface unit structure has a period P = 800 nm.

[0010] Preferably, the distance the hole moves in the y-axis direction is 10 nm, the angle at which the silicon block tilts in the negative x-axis direction is a = 0.11°, and the angle at which it tilts in the y-axis direction is b = 0.019°.

[0011] Preferably, the substrate is made of silicon dioxide.

[0012] The second technical solution of the present invention is a method for fabricating an optical switch based on an all-dielectric intrinsic chiral metasurface, comprising the following steps:

[0013] S1. Design the above-mentioned metasurface unit structure with intrinsic chiral quasi-BIC mode;

[0014] S2. Silicon atoms are deposited onto a substrate to form a metasurface unit structure with intrinsic chiral quasi-BIC mode, resulting in an optical switch based on an all-dielectric intrinsic chiral metasurface.

[0015] Compared with the prior art, the optical switch based on the all-dielectric intrinsic chiral metasurface of the present invention has a strong intrinsic chiral response. The transmission spectrum can be adjusted by changing the rotation direction of the circularly polarized light perpendicularly incident on the metasurface to make the switch have a high on / off ratio, and the operating band can be easily extended. This optical switch is a compact passive device that does not require the addition of an additional active pump field, and is expected to enrich the application of optical communication and all-optical computing devices. Attached Figure Description

[0016] Figure 1 The model diagram of the optical switch based on an intrinsically chiral metasurface of all dielectric material, established using COMSOL Multiphysics commercial multiphysics simulation software, is as follows: (a) is the overall structure diagram; (b) is the distance the central hole moves along the y-axis; (c) is the angle at which the silicon block tilts towards the negative x-axis; and (d) is the angle at which the silicon block tilts towards the y-axis.

[0017] Figure 2 The chiral response of the metasurface under normal incidence is given by: (a) the transmission spectra of left-handed and right-handed circularly polarized light and the transitions between them; and (b) the CD values ​​at the resonance positions.

[0018] Figure 3 This describes the ON / OFF state of the optical switch operating in the telecommunications U-band (1625–1675 nm) under normal incidence of left-handed and right-handed circularly polarized light.

[0019] Figure 4 The position of the minimum CD value varies with the parameter k. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0021] In this embodiment, a model of an optical switch based on an all-dielectric intrinsic chiral metasurface is first established using COMSOL Multiphysics commercial multiphysics simulation software, such as... Figure 1 As shown, a cubic prism-shaped silicon block with a centrally located hole is first designed as the initial structure. The width of the silicon block is 450 nm, the thickness is 300 nm, and the radius of the central hole is 80 nm. To obtain a symmetry-protected quasi-BIC, the central hole is moved a distance d along the y-axis to break the fourfold rotational symmetry of the structure. Finally, by tilting the silicon block structure along the negative x-axis and the y-axis, one of the C points in momentum space is harmonicized to the Γ point, resulting in a metasurface unit structure with an intrinsically chiral quasi-BIC mode. For example, the angle of tilting the silicon block in the negative x-axis direction is a = 0.11°, and the angle of tilting in the y-axis direction is b = 0.019°. For example, electron beam lithography can be used to draw the desired pattern on the substrate, and deep reactive ion etching can be used to etch the tilted hole. To form a tilted surface, a thin layer of material can be deposited on the sidewalls of the hole using corner deposition technology. After deposition, subsequent processing is performed to form a metasurface unit structure with intrinsic chiral quasi-BIC mode, resulting in an optical switch based on an all-dielectric intrinsic chiral metasurface with a metasurface unit structure period P = 800 nm.

[0022] In this embodiment, when the silicon block structure remains perfectly symmetrical, a BIC mode is obtained. This mode has a polarization singularity at point Γ in momentum space, meaning that no perpendicularly incident light can excite this mode. When the aperture is moved along the y-axis, the structural symmetry is broken, and the polarization singularity splits into two circularly polarized points (C points). Point C is another polarization singularity that cannot be excited by far-field incident light with opposite circular polarization. The evolution paths of the two C points in momentum space can be controlled by an out-of-plane perturbation such as tilting the silicon block structure. That is, by harmonicizing one of the C points to point Γ, an intrinsically chiral quasi-BIC mode is obtained. At this point, only one of the left-hand and right-hand circularly polarized light incident perpendicularly to the metasurface can be transmitted, while the other is completely reflected. We can control the direction of the incident light to determine whether the optical switch is on or off, thus obtaining an optical switch with high modulation depth and a large on / off ratio.

[0023] Since the designed structure operates in the communication band (1260-1625nm), which falls within the near-infrared range, we chose silicon, which has a high refractive index and low light absorption in the near-infrared band, as the material for the unit structure. Additionally, we selected silicon dioxide, which also has low light absorption in the near-infrared band but a lower refractive index, as the substrate material. Based on this approach, we optimized the structural parameters using numerical simulation software, obtaining a strong intrinsic chiral response.

[0024] The optical switch based on an all-dielectric intrinsic chiral metasurface was simulated in the wavelength domain using COMSOL Multiphysics commercial multiphysics simulation software. The metasurface was perpendicularly illuminated with left-handed and right-handed circularly polarized light, and the transmission spectra of the left-handed, right-handed, left-handed-to-right-handed, and right-handed-to-left-handed circularly polarized light were measured. Based on the data, the CD value was calculated using the formula:

[0025]

[0026] In equation (1): T rr T rl T ll T lr These represent the transmittance of right-handed circularly polarized light when incident with right-handed circularly polarized light, the transmittance of right-handed circularly polarized light when incident with left-handed circularly polarized light, the transmittance of left-handed circularly polarized light when incident with left-handed circularly polarized light, and the transmittance of left-handed circularly polarized light when incident with right-handed circularly polarized light. The chiral response of the metasurface under normal incidence is as follows: Figure 2 As shown, Figure 2 As shown in (a), a spin-selective resonance induced by chiral quasi-BIC exists at 1667 nm. Right-handed circularly polarized light exhibits very low transmittance due to resonance coupling with intrinsic modes in the structure, while the transmittance of left-handed circularly polarized light is enhanced to almost 1. This significant difference in transmittance reflects the strong intrinsic chiral response of the metasurface. We calculated the CD value, as follows... Figure 2 As shown in (b), the CD value can reach -0.96. (From...) Figure 2 It is known that the metasurface of the optical switch based on the all-dielectric intrinsic chiral metasurface of the present invention has a strong intrinsic chiral response.

[0027] Then, the metasurface was perpendicularly illuminated with left-handed and right-handed circularly polarized light, respectively, and the transmission spectra of the left-handed and right-handed circularly polarized light were measured to find the resonance position and calculate the on / off ratio:

[0028]

[0029] In equation (2): η is the on / off ratio, T ON T OFFThese represent the minimum and maximum transmittance of left-handed or right-handed circularly polarized light at the resonant position. This formula shows that changing the rotation direction of the circularly polarized light controls the on or off state of the optical switch.

[0030] When left-handed and right-handed circularly polarized light are incident normally, the optical switch operates in the telecommunications U-band (1625–1675 nm). The ON / OFF states are as follows: Figure 3 As shown, by Figure 3 It can be seen that for a center wavelength of 1667nm {belonging to the telecommunications U-band (1625-1675nm)}, their transmission spectrum can be adjusted by changing the rotation direction of the incident circularly polarized light, where the maximum and minimum transmittance values ​​can be set to the ON and OFF states of an optical switch, respectively. Figure 3 It can be seen that when the incident light is left-handed circularly polarized light, the optical switch is in the ON state, and the transmittance is T. ON =0.97959. Simultaneously, when the incident light is right-handed circularly polarized, the optical switch is triggered to the OFF state, and the transmittance is T. OFF =0.0206, the on / off ratio η is 16.8dB.

[0031] Simultaneously, this embodiment also utilizes COMSOL Multiphysics commercial multiphysics simulation software to change the geometric parameters of the metasurface unit structure and calculate the CD value accordingly. The method of changing the geometric parameters is as follows: the new CD value is obtained by multiplying the silicon block thickness t, period P, silicon block width L, central hole radius r, and hole movement distance d along the y-axis with the scaling parameter k. Figure 4 As shown, the CD values ​​were calculated for the proportionality parameter k when it takes values ​​of 0.8, 0.85, 0.9, and 1. Figure 4 It is understood that the operating band of the optical switch of the present invention can cover the entire telecommunications band.

[0032] In summary, the optical switch based on intrinsic chiral metasurface designed in this invention has a high on / off ratio and its operating wavelength can be easily extended.

[0033] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. An optical switch based on an all-dielectric intrinsic chiral metasurface, comprising a substrate and a metasurface unit structure arrayed on the substrate, characterized in that: The metasurface unit structure is a cubic columnar silicon block with a hole in the center, and the hole is moved in the y-axis direction to break the structural symmetry. The silicon block is tilted in the negative x-axis direction and the y-axis direction respectively to obtain a metasurface unit structure with intrinsic chirality quasi-BIC mode. The silicon block has a width of 450nm, a thickness of 300nm, and a radius of 80nm for the hole at its center. The hole moves 10nm along the y-axis, and the silicon block is tilted at an angle of a=0.11° in the negative x-axis direction and at an angle of b=0.019° in the y-axis direction.

2. The all-dielectric intrinsic chiral metasurface based optical switch of claim 1, wherein: The metasurface unit structure has a period of P=800nm.

3. The all-dielectric intrinsic chiral metasurface based optical switch of claim 1, wherein: The substrate is made of silicon dioxide.

4. A method for preparing an optical switch based on a full dielectric intrinsic chiral metasurface, characterized in that, Includes the following steps: S1. Design a metasurface unit structure with intrinsic chiral quasi-BIC mode as described in any one of claims 1-3; S2. Silicon atoms are deposited onto a substrate to form a metasurface unit structure with intrinsic chiral quasi-BIC mode, resulting in an optical switch based on an all-dielectric intrinsic chiral metasurface.

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