Optical switch based on all-dielectric intrinsic chiral metasurface and preparation method thereof

Abstract

The invention belongs to the technical field of optical switches, and particularly discloses an optical switch based on an all-dielectric intrinsic chiral metasurface and a preparation method of the optical switch based on the all-dielectric intrinsic chiral metasurface. The metasurface unit structure is a cubic column type silicon block with a hole in the center, the hole moves towards the y-axis direction to break structural symmetry, and the silicon block inclines towards the x-axis negative direction and the y-axis direction to obtain the metasurface unit structure with the intrinsic chiral quasi-BIC mode. Compared with the prior art, the metasurface of the optical switch based on the all-dielectric intrinsic chiral metasurface has strong intrinsic chiral response, the transmission spectrum can be adjusted by changing the rotation direction of circularly polarized light perpendicular to the incident metasurface so that the switch can have a high on / off ratio, the working wave band can be easily expanded, and the optical switch can be applied to the field of optical devices. The optical switch is a compact passive device, an additional active pumping light field does not need to be added, and the application of optical communication and all-optical computing equipment is expected to be enriched.

Description

Technical Field

[0001] The present invention relates to the technical field of optical switches, and in particular to an optical switch based on an all-dielectric intrinsic chiral metasurface and a preparation method thereof. Background Art

[0002] Traditional optical switches are based on thermo-optical effect and plasma dispersion effect, and there are three main forms: thermo-optical, electro-optical and all-optical. For all-optical switches, the Kerr effect is one of the most important mechanisms, which describes a phenomenon in which the refractive index of a material depends on the intensity of the pump light. At present, on-chip interferometers, photonic crystals, plasmon waveguides, waveguide ring resonators and metasurfaces based on the Kerr effect are widely used to design a variety of all-optical switches.

[0003] Optical chirality is manifested as the different responses of the medium to left-handed circularly polarized light and right-handed circularly polarized light. This response difference is measured by circular dichroism (CD). The weak chiral optical interaction in natural materials limits their application. Due to its superior light field control and localization capabilities, metasurfaces can obtain large CD value responses. Specifically, under the condition of vertical light incidence, chiral metasurfaces can achieve spin-selective transmission of incident light. Among them, intrinsic chiral metasurfaces do not involve polarization conversion losses, and higher CD values ​​can be obtained in practical applications. Intrinsic chiral metasurfaces can be used in the design of optical switches with high modulation depth due to the characteristics of spin-selective transmission.

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

[0005] In order to solve the above technical problems, the present invention provides an optical switch based on an all-dielectric intrinsic chiral metasurface and a preparation method thereof.

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

[0007] One of the technical solutions of the present invention is an optical switch based on an all-dielectric intrinsic chiral metasurface, comprising a substrate and a metasurface unit structure arrayed on the substrate, wherein 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, and the silicon block is tilted in the negative direction of the x-axis and the y-axis direction respectively to obtain a metasurface unit structure with an intrinsic chiral quasi-BIC mode.

[0008] Furthermore, the width of the silicon block is 450 nm, the thickness of the silicon block is 300 nm, and the radius of the hole in the center of the silicon block is 80 nm.

[0009] Furthermore, the period P of the metasurface unit structure is 800 nm.

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

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

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

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

[0014] S2. Depositing silicon atoms onto a substrate to form a metasurface unit structure with an intrinsic chiral quasi-BIC mode, thereby obtaining 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, and the transmission spectrum can be adjusted by changing the handedness of the circularly polarized light perpendicularly incident on the metasurface to make the switch have a high on / off ratio, and the working band can be easily expanded. This optical switch is a compact passive device that does not require the addition of additional active pump light fields, and is expected to enrich the applications of optical communications and all-optical computing devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Figure 2. Model diagram of an optical switch based on an all-dielectric intrinsic chiral metasurface established using the commercial multi-physics simulation software COMSOL Multiphysics: (a) is the overall structure diagram; (b) is the distance the hole at the center moves along the y-axis; (c) is the angle at which the silicon block tilts toward the negative direction of the x-axis; (d) is the angle at which the silicon block tilts toward the y-axis.

[0017] Figure 2 Figure 2 shows the chiral response of the metasurface under normal incidence: (a) the transmission spectra of left-handed and right-handed circularly polarized light and the conversion between them; (b) the calculated CD value of the resonance position.

[0018] Figure 3 It shows the ON / OFF status of the optical switch working in the telecommunication U band (1625-1675nm) under normal incidence of left-handed circularly polarized light and right-handed circularly polarized light.

[0019] Figure 4 The position of the minimum CD value changes with the parameter k. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the invention.

[0021] In this embodiment, the COMSOL Multiphysics commercial multi-physics simulation software is first used to establish a model of an optical switch based on an all-dielectric intrinsic chiral metasurface. Figure 1 As shown, a cubic columnar silicon block with a hole in the center is first designed as the initial structure, the width of the silicon block is 450nm, the thickness of the silicon block is 300nm, and the radius of the hole in the center of the silicon block is 80nm; in order to obtain a symmetry-protected quasi-BIC, the hole in the center is moved along the y-axis direction by a distance d to break the four-fold rotational symmetry of the structure; finally, by tilting the silicon block structure along the negative x-axis direction and the y-axis direction, one of the C points in the momentum space is tuned to the Γ point, and a metasurface unit structure with an intrinsic chiral quasi-BIC mode is obtained. For example, the angle of the silicon block tilting in the negative x-axis direction is a=0.11°, and the angle of the silicon block tilting in the y-axis direction is b=0.019°. For example, the desired pattern can be drawn on the substrate using electron beam lithography technology, and the tilted hole can be etched using deep reactive ion etching technology. In order to form an inclined surface, an angle deposition technique can be used to deposit a thin layer of material on the side wall of the hole. After the deposition is completed, subsequent processing is performed to form a metasurface unit structure with an intrinsic chiral quasi-BIC mode, thereby obtaining an optical switch based on an all-dielectric intrinsic chiral metasurface, and the metasurface unit structure period P = 800nm.

[0022] In this embodiment, when the structure of the silicon block remains completely symmetrical, a BIC mode will be obtained, and the Γ point of this mode in the momentum space is a polarization singularity, and no vertically incident light is allowed to excite the mode. When the hole is moved along the y-axis, the symmetry of the structure is broken, and the polarization singularity will split into two circular polarization points (Circular Polarization Points, C points). Point C is another polarization singularity, which cannot be excited by far-field incident with opposite circular polarization. The evolution path of the two C points in momentum space can be controlled by the out-of-plane perturbation of the tilted silicon block structure. In other words, one of the C points is tuned to the Γ point, so that the intrinsic chiral quasi-BIC mode is obtained. At this time, only one of the left-handed and right-handed circularly polarized light incident vertically on the metasurface can be transmitted and the other is completely reflected. We can control the handedness of the incident light to determine whether the optical switch is on or off, thereby obtaining an optical switch with high modulation depth and large on / off ratio.

[0023] Since the designed structure works in the communication band (1260-1625nm), which belongs to the near-infrared band, we choose silicon with high refractive index and low light absorption rate in the near-infrared band as the material of the unit structure. In addition, we choose silicon dioxide with low light absorption rate but lower refractive index in the near-infrared band as the substrate material. Based on the above ideas, we use numerical simulation software to optimize the structural parameters and obtain a strong intrinsic chiral response.

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

[0025]

[0026] In formula (1): T rr , T rl , T ll , T lr They are the transmittance of right circularly polarized light when right circularly polarized light is incident, the transmittance of right circularly polarized light when left circularly polarized light is incident, the transmittance of left circularly polarized light when left circularly polarized light is incident, and the transmittance of left circularly polarized light when right circularly polarized light is incident. The chiral response results of the metasurface under normal incidence are shown in Figure 2 As shown, Figure 2 As shown in (a), there is a spin-selective resonance caused by the chiral quasi-BIC at 1667nm. The right-handed circularly polarized light resonates with the intrinsic mode in the structure, resulting in a very low transmittance, while the transmission of the left-handed circularly polarized light is enhanced to almost 1. This huge transmission difference reflects the strong intrinsic chiral response of the metasurface. We calculated the CD value, as shown in Figure 2 As shown in (b), the CD value can reach -0.96. Figure 2 It can be seen 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] Furthermore, left-handed circularly polarized light and right-handed circularly polarized light are used to vertically illuminate the metasurface, respectively, and the transmission spectra of left-handed circularly polarized light and right-handed circularly polarized light are measured to find the resonance position and calculate the on / off ratio:

[0028]

[0029] In formula (2): η is the on / off ratio, T ON , T OFFis the minimum and maximum transmittance of left-handed circularly polarized light or right-handed circularly polarized light at the resonance position. From this formula, it can be seen that changing the hand direction of circularly polarized light controls the on or off state of the optical switch.

[0030] Under normal incidence of left-handed circularly polarized light and right-handed circularly polarized light, the optical switch operates in the telecommunication U band (1625-1675nm), and the ON / OFF conditions are as follows: Figure 3 As shown by Figure 3 It can be seen that for a central wavelength of 1667nm {belonging to the telecommunication U band (1625-1675nm)}, their transmission spectra can be adjusted by changing the handedness of the incident circularly polarized light, where the maximum and minimum values ​​of the transmittance can be set as the ON and OFF states of the 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. At the same time, when the incident light is right-handed circularly polarized light, 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] At the same time, this embodiment also uses COMSOL Multiphysics commercial multi-physics field simulation software to change the geometric parameters of the metasurface unit structure and calculate the CD value respectively. 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, center hole radius r, and the distance d of the hole moving along the y-axis with the proportional parameter k. Figure 4 As shown in the figure, the CD values ​​when the scale parameter k is 0.8, 0.85, 0.9, and 1 are calculated. Figure 4 It can be seen that the working band of the optical switch of the present invention can cover the entire telecommunication band.

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

[0033] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution 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, and the silicon block is tilted in the negative x-axis direction and the y-axis direction respectively to obtain a metasurface unit structure with an intrinsic chiral quasi-BIC mode.

2. The optical switch based on the all-dielectric intrinsic chiral metasurface according to claim 1, characterized in that: The width of the silicon block is 450 nm, the thickness of the silicon block is 300 nm, and the radius of the hole in the center of the silicon block is 80 nm.

3. The optical switch based on the all-dielectric intrinsic chiral metasurface according to claim 1, characterized in that: The period of the supersurface unit structure P=800nm.

4. The optical switch based on the all-dielectric intrinsic chiral metasurface according to claim 1, characterized in that: The distance that the hole moves in the y-axis direction is 10 nm, the angle of inclination of the silicon block in the negative direction of the x-axis is a=0.11°, and the angle of inclination in the y-axis direction is b=0.019°.

5. The optical switch based on the all-dielectric intrinsic chiral metasurface according to claim 1, characterized in that: The material of the substrate is silicon dioxide.

6. A method for preparing an optical switch based on an all-dielectric intrinsic chiral metasurface, characterized in that: The following steps are involved: S1. Designing a metasurface unit structure having an intrinsic chiral quasi-BIC mode as claimed in any one of claims 1 to 5; S2. Depositing silicon atoms onto a substrate to form a metasurface unit structure with an intrinsic chiral quasi-BIC mode, thereby obtaining an optical switch based on an all-dielectric intrinsic chiral metasurface.

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