Three-dimensional chiral micro-nano structure and preparation method thereof

Abstract

The present application relates to the technical field of chiral micro-nano structure, and particularly relates to a three-dimensional chiral micro-nano structure and a preparation method thereof; the present application applies short-side surface film layers and long-side surface film layers attached on a support part to conduct current, builds a current path in a vertical direction, realizes a current path penetrating through a top film layer and a low film layer, and realizes strong circular dichroism. The structure provided by the present application is simple to prepare, can be realized by inclined evaporation coating on the support part, has low requirements on the size of the support part, does not need to be less than a hundred nanometer scale, and can realize resonance mode and circular dichroism in a visible light range. In combination of the above effects, the present application has good application prospect in the technical field of chiral micro-nano structure.

Description

Technical Field

[0001] This invention relates to the field of chiral micro / nanostructure technology, specifically to a three-dimensional chiral micro / nanostructure and its fabrication method. Background Technology

[0002] Chiral micro / nanostructures, also known as chiral metamaterials, are artificially fabricated chiral materials or structures. When circularly polarized light of different polarizations shines on a chiral micro / nanostructure array, the different polarizations of the light interact differently with the array, resulting in different absorption or transmission characteristics, a phenomenon known as circular dichroism. Compared to naturally occurring chiral structures, artificially fabricated chiral metamaterials or chiral micro / nanostructures exhibit stronger chirality and thus stronger circular dichroism. These chiral micro / nanostructures have important applications in areas such as negative optical refraction, manipulation of optical polarization states, beam splitting based on optical circular dichroism, and biomolecular detection based on optical superchiral fields.

[0003] Compared to two-dimensional chiral micro / nanostructures, three-dimensional chiral micro / nanostructures exhibit stronger optical chirality, such as circular dichroism. In three-dimensional chiral micro / nanostructures, chirality can be achieved through bilayer structures: twisting the anisotropic structures in the two layers can achieve circular dichroism, such as the crossbar structure. Although layered chiral micro / nanostructures are easy to fabricate, the chiral signal of layered three-dimensional chiral micro / nanostructures is weak because the coupling between layers is electric field coupling. Generally, three-dimensional helical structures exhibit the strongest circular dichroism. For example, the literature "Gold helix photonic metamaterial as broadband circular polarizer, Science, Vol. 325, pp. 1513, 2009" used laser writing to fabricate a helical structure, achieving approximately 80% circular dichroism. However, the helical size fabricated by laser writing is large, the circular dichroism signal appears in the micrometer wavelength range, and the technology is technically challenging. It is of great significance to fabricate three-dimensional chiral micro / nano structures with a spiral structure using relatively simple methods and low-performance instruments, and to achieve circular dichroism in the visible light band. Summary of the Invention

[0004] To address the above problems, this invention provides a three-dimensional chiral micro / nanostructure and its fabrication method. The strong circular dichroism exhibited by the helical structure is primarily due to the formation of vortex-like currents in the vertical direction. Conventional electron beam lithography and electron beam evaporation deposition methods struggle to create current paths connecting different height sections. The three-dimensional chiral micro / nanostructure provided by this invention achieves a current path in the vertical direction through a film layer attached to the side, ultimately realizing circular dichroism. These results have significant practical value for broadening the fabrication technology of three-dimensional chiral micro / nanostructures.

[0005] The specific contents of this invention are as follows:

[0006] On one hand, this invention provides a three-dimensional chiral micro / nanostructure, which includes a substrate and periodic units placed on the substrate. The periodic units are arranged in a rectangular periodic pattern. The periodic units include cuboid units and a bottom film layer. Specifically, the cuboid unit includes a support portion, a top film layer, short-side side film layers, and long-side side film layers. The support portion is cuboid, and its center coincides with the center of the periodic unit. The top film layer is placed on top of the support portion, and the short-side and long-side side film layers are respectively attached to the short-side and long-side side of the support portion. The bottom film layer is disposed on the surface of the periodic unit excluding the cuboid units. This invention utilizes the short-side and long-side side film layers attached to the support portion to conduct current, establishing a current path in the vertical direction, realizing a current path connecting the top film layer and the bottom film layer, and achieving strong circular dichroism.

[0007] Furthermore, the long side of the support is parallel to the long side of the periodic element; the short side of the support is parallel to the short side of the periodic element.

[0008] Furthermore, the substrate material is silicon dioxide.

[0009] Furthermore, the height of the support is greater than 200 nanometers and less than 400 nanometers.

[0010] Furthermore, the long side of the periodic unit is greater than 1000 nanometers and less than 1200 nanometers; the short side of the periodic unit is greater than 800 nanometers and less than 900 nanometers.

[0011] Furthermore, the long side of the support is greater than 600 nanometers and less than 700 nanometers; the short side of the support is greater than 400 nanometers and less than 500 nanometers.

[0012] Furthermore, the thickness of the bottom film layer is less than 100 nanometers.

[0013] On the other hand, the present invention provides a method for fabricating three-dimensional chiral micro / nano structures, comprising the following steps:

[0014] Step 1: Prepare the support portion on the substrate;

[0015] The second step involves applying electron beam evaporation coating technology to coat the substrate surface, the sides of the support, and the top.

[0016] Furthermore, in the second step, the base is tilted and rotated so that the angles between the short side and the long side of the support and the vertical direction are equal.

[0017] Furthermore, the base is tilted at an angle of 70 degrees.

[0018] The beneficial effects of this invention are:

[0019] (1) The present invention introduces thin films attached to the side of the support portion. These thin films become the path for electrons to vibrate in the vertical direction, making the whole structure resemble a spiral, thereby achieving circular dichroism.

[0020] (2) The structure provided by the present invention is simple to prepare. It can be achieved by tilting the evaporation coating on the support. The size requirement of the support is low, and it does not need to be smaller than 100 nanometers. It can achieve resonance mode and circular dichroism in the visible light range.

[0021] In summary, this invention has promising application prospects in the field of chiral micro / nano structure fabrication technology.

[0022] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0023] Figure 1 This is a side view of a periodic unit cell of a three-dimensional chiral micro / nano structure.

[0024] Figure 2 It is a top view of a cuboid unit.

[0025] Figure 3 This is a current distribution diagram at the resonant mode.

[0026] Figure 4 This is a flowchart of a method for fabricating three-dimensional chiral micro / nano structures.

[0027] Figure 5 This is a scanning electron microscope image of a three-dimensional chiral micro / nano structure.

[0028] Figure 6 This is a circular dichroic spectrum of a three-dimensional chiral micro / nano structure.

[0029] In the figure: 1. Substrate; 2. Bottom membrane layer; 3. Cuboid unit; 31. Support; 32. Top membrane layer; 33. Short side membrane layer; 34. Long side membrane layer. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0031] Example 1

[0032] This invention provides a three-dimensional chiral micro / nanostructure, comprising a substrate 1 and periodic units on the substrate. The periodic units are periodically arranged on the substrate 1. Specifically, the periodic units are arranged in a rectangular periodic pattern on the substrate 1. The substrate 1 is made of silicon dioxide, facilitating the transmission of visible light through the substrate 1 for testing the transmission spectrum of the three-dimensional chiral micro / nanostructure in a spectral system. The long side of the periodic units is greater than 1000 nm and less than 1200 nm; the short side of the periodic units is greater than 800 nm and less than 900 nm. Figure 1 As shown, the periodic unit includes cuboid units 3 and a bottom layer 2. The bottom layer 2 is made of a noble metal, such as gold or silver, to facilitate the formation of strong surface plasmon resonances on the bottom layer 2. The bottom layer 2 can also be made of a semiconductor material, such as germanium, to facilitate the formation of strong resonant modes in the microwave band. Figure 2As shown, the cuboid unit 3 includes a support portion 31, a top film layer 32, a short-side side film layer 33, and a long-side side film layer 34. The support portion 31 is made of photoresist, but can also be made of materials such as silicon dioxide. When the entire three-dimensional chiral micro / nano structure is used to achieve strong circular dichroism in the visible light band, the support portion 31 is made of a visible-light transparent material, such as photoresist or silicon dioxide. When the entire three-dimensional chiral micro / nano structure is used to achieve strong circular dichroism in the microwave band, the support portion 31 is made of a microwave transparent material, such as polymer materials like plastics. This invention mainly aims to achieve strong circular dichroism in the visible light band; therefore, the support portion 31 is made of a visible-light transparent material. Of course, the structure or conclusions of this invention can still be applied to the microwave band. In addition, the support portion 31 can also be made of a photocatalytic material, such as titanium dioxide or zinc oxide. Under different circularly polarized light excitation, the light field intensity at the location of the support portion 31 varies, thereby achieving different degrees of photocatalysis. Therefore, this invention can also be used for polarization-selective photocatalysis, which is of significant importance for the synthesis of chiral molecules such as drugs. In this embodiment, the support portion 31 is a cuboid, with its center coinciding with the center of the periodic unit. The long side of the support portion 31 is parallel to the long side of the periodic unit; the short side of the support portion 31 is parallel to the short side of the periodic unit. Since the periodic unit is rectangular, and the bottom surface of the support portion 31 is also rectangular, as long as the long side of the support portion 31 is parallel to the long side of the periodic unit and the short side of the support portion 31 is parallel to the short side of the periodic unit, even if the center of the bottom surface of the support portion 31 deviates slightly from the center of the periodic unit, it will not affect the final result. Therefore, this invention has low requirements for the preparation process. Slight deviation between the center of the bottom surface of the support portion 31 and the center of the bottom surface of the periodic unit will not affect the result. A top film layer 32 is placed on top of the support portion 31. Short-side side film layers 33 and long-side side film layers 34 are respectively attached to the short-side and long-side sides of the support portion 31. The top film layer 32, short-side side film layer 33, and long-side side film layer 34 are made of precious metals. The materials of the top film layer 32, short-side side film layer 33, and long-side side film layer 34 can be different. For example, the top film layer 32 can be made of gold, and the short-side side film layer 33 and long-side side film layer 34 can be made of silver. Alternatively, the materials of the top film layer 32, short-side side film layer 33, and long-side side film layer 34 can be the same. For example, the top film layer 32, short-side side film layer 33, and long-side side film layer 34 can all be made of silver. Their identical materials facilitate single-stage tilted evaporation deposition; the specific preparation method is described in Example 2. The bottom film layer 2 is disposed on the surface of the periodic unit excluding the cuboid unit 3. In other words, except for the surface of the substrate 1 occupied by the cuboid unit 3, a bottom film layer 2 is provided, and the thickness of the bottom film layer 2 is less than 100 nanometers.In this embodiment, the height of the support portion 31 is greater than 200 nm and less than 400 nm; the long side of the support portion 31 is greater than 600 nm and less than 700 nm; and the short side of the support portion 31 is greater than 400 nm and less than 500 nm, so as to achieve circular dichroism in the visible light range. When it is necessary to adjust the wavelength at which circular dichroism occurs, these dimensions can be changed so that the wavelength at which circular dichroism occurs can be adjusted.

[0033] The inventors used the finite element method to calculate the circular dichroism of the designed structure. The parameters used were: a periodic element length of 1150 nm, a periodic element width of 890 nm, a support height of 270 nm, a cuboid element width of 450 nm, a cuboid element length of 690 nm, a bottom layer thickness of 60 nm, a top layer thickness of 60 nm, a short side layer thickness of 60 nm, and a long side layer thickness of 60 nm. The results showed that strong resonant modes appeared at wavelengths of 700 nm and 800 nm; at the resonant modes, approximately 1% of the circular dichroism signal was observed. Figure 3 The figure shows the current distribution at the resonant mode. As can be seen from the figure, the short-side side film 33 and the long-side side film 34 become vertical current paths, resulting in strong eddy currents throughout the structure and leading to strong circular dichroism.

[0034] Example 2

[0035] This invention also provides a method for preparing three-dimensional chiral micro / nano structures, such as... Figure 4 As shown, it includes the following steps:

[0036] Step 1: Prepare the support portion 31 on the substrate 1.

[0037] The second step is to apply electron beam evaporation coating technology to coat the surface of the substrate 1, the sides and top of the support 31.

[0038] The first step includes: (1) preparing glass sheets: cleaning the cut glass sheets to remove dust and organic matter from the surface of the glass sheets; (2) preparing a support part 31 made of photoresist material on the glass sheets: the material of the support part 31 is photoresist. First, the photoresist is spun on the glass sheet, dried, exposed by electron beam, developed and fixed to form the support part 31; wherein, the rotation speed of the spin coater is controlled so that the thickness of the photoresist is 200 nanometers-300 nanometers.

[0039] The second step includes: placing the glass plate with the support part 31 into the electron beam evaporation coating instrument for coating. The specific steps include: (1) evacuating the vacuum; (2) tilting the substrate 1 at an angle of 70 degrees, that is, the angle between the normal direction of the substrate 1 and the vertical direction is 70 degrees; (3) rotating the substrate 1 so that the angle between the short side and the long side of the support part 31 and the vertical direction is equal, so that the coating thickness on the adjacent two sides of the support part 31 is the same; (4) coating, the electron beam evaporation coating rate is 0.1 nanometers / second, and the coating time is controlled so that the thickness of the bottom film layer 2 is about 60 nanometers.

[0040] Figure 5 SEM images of the three-dimensional chiral micro / nano structures prepared using the above-described method are shown. The images reveal a periodically arranged array of rectangular units. The shading at the upper right corner of the rectangles represents the shadows left during the tilted coating process. After measurement and averaging, the structural parameters obtained are as follows: the length of the periodic unit is 1150 nm, the width of the periodic unit is 890 nm, the width of cuboid unit 3 is 690 nm, the width of cuboid unit 3 is 450 nm, the height of the support 31 is 270 nm (determined based on the spin coater speed), and the thickness of the bottom film layer 2 is 60 nm (determined based on the crystal oscillator reading).

[0041] Figure 6 This is a transmission circular dichroism spectrum measured using a microspectral system. T++ represents right-handed circularly polarized light incident; T— represents left-handed circularly polarized light incident. CD is the transmitted light intensity when right-handed circularly polarized light is incident minus the transmitted light intensity when left-handed circularly polarized light is incident. As can be seen from the figure, the three-dimensional chiral micro / nanostructure prepared in this invention achieves circular dichroism.

[0042] In summary, this invention provides a three-dimensional chiral micro / nanostructure and its fabrication method. Thin films are introduced and attached to the sides of the support portion 31. These films become paths for electron vibration in the vertical direction, making the entire structure resemble a helix, thereby achieving circular dichroism. Furthermore, the structure provided by this invention is simple to fabricate; it can be achieved by tilted evaporation deposition on the support portion 31. The size requirements for the support portion 31 are low, not needing to be smaller than a few hundred nanometers, and resonance modes and circular dichroism can be achieved in the visible light range. These technical advantages make this invention a promising candidate for application in the field of chiral micro / nanostructure fabrication.

[0043] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A three-dimensional chiral micro-nano structure, comprising a substrate and a periodic unit placed on the substrate, the periodic unit being arranged in a rectangular period, the periodic unit comprising a cuboid unit and a bottom film layer, characterized in that, The cuboid unit comprises a support part, a top film layer, a short side film layer, and a long side film layer. The material of the support part is photoresist. The support part is a cuboid. The center of the support part coincides with the center of the periodic unit. The long side of the support part is parallel to the long side of the periodic unit. The short side of the support part is parallel to the short side of the periodic unit. The top film layer is arranged on the top of the support part. The short side film layer and the long side film layer are respectively attached to the short side of the support part and the long side of the support part. The bottom film layer is arranged on the surface of the periodic unit except the part of the cuboid unit. The materials of the bottom film layer, the top film layer, the short side film layer, and the long side film layer are all noble metals. The preparation method of the three-dimensional chiral micro-nano structure comprises the following steps: Firstly, the support part is prepared on the substrate. Secondly, the electron beam evaporation plating film technology is applied to plate films on the surface of the substrate, the side of the support part, and the top of the support part. In the second step, the substrate is inclined and rotated, so that the angle between the short side of the support part and the vertical direction is equal to the angle between the long side of the support part and the vertical direction. The angle of the inclined substrate is 70 degrees.

2. The three-dimensional chiral micro- or nano-structure of claim 1, wherein: The material of the substrate is silicon dioxide.

3. The three-dimensional chiral micro- or nano-structure of claim 1, wherein: The height of the support part is greater than 200 nanometers and less than 400 nanometers.

4. The three-dimensional chiral micro- or nano-structure of claim 3, wherein: The long side of the periodic unit is greater than 1000 nanometers and less than 1200 nanometers. The short side of the periodic unit is greater than 800 nanometers and less than 900 nanometers.

5. The three-dimensional chiral micro- or nanostructure of claim 4, wherein: The long side of the support part is greater than 600 nanometers and less than 700 nanometers. The short side of the support part is greater than 400 nanometers and less than 500 nanometers.

6. The three-dimensional chiral micro- or nanostructure of claim 5, wherein: The thickness of the bottom film layer is less than 100 nanometers.

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