A terahertz beam splitter with adjustable splitting ratio and a manufacturing process thereof

Abstract

The application claims a terahertz beam splitter with adjustable splitting ratio, which is arranged by super-units, each super-unit is composed of 8*8 basic units, 4 basic units are arranged according to left-right phase difference symmetry, each basic unit includes a dielectric layer, an open circular ring type metal pattern layer and a metal reflection layer, the open circular ring type metal pattern layer is located on the dielectric layer, the metal reflection layer is located below the dielectric layer, and the metal reflection layer covers half of the dielectric layer; the metal reflection layer only covers the basic units on the left side of the symmetry axis; the phase difference between the four basic units of the metasurface beam splitter is 90° at 0.7 THz, when the terahertz wave is vertically incident along the-z axis direction, four beam splitting is obtained at 0.7 THz by observing the far field diagram in the simulation software, and the splitting ratio is 1.2:1; when the terahertz wave is vertically incident along the+z axis direction, four beam splitting is obtained at 0.7 THz by observing the far field diagram in the simulation software, and the splitting ratio is 1.9:1.

Description

Technical Field

[0001] This invention pertains to terahertz beam splitter technology, specifically to a terahertz beam splitter with adjustable splitting ratio and its manufacturing process. Background Technology

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 to 10 THz. The long-wavelength terahertz band overlaps with microwaves, and the short-wavelength band overlaps with infrared waves. Compared to other electromagnetic wave bands, terahertz waves, due to their unique location, have broad application prospects in fields such as biomedicine, non-destructive testing, and communications.

[0003] Metasurfaces, as two-dimensional equivalents of metamaterials, are periodic subwavelength artificial electromagnetic media. Due to their negative refractive index, electromagnetically induced transparency, and nonlinear effects, they have wide applications in terahertz beam splitting devices. In 2017, SLL Wendy et al. proposed an ultrathin circularly polarized terahertz beam splitter whose metasurface unit is a coaxial open-ring resonator. When a circularly polarized wave is incident perpendicularly, the reflected wave is split into two beams: a left-handed circularly polarized wave and a right-handed circularly polarized wave, achieving beam splitting of circularly polarized waves in the range of 0.58–1.00 THz with a splitting ratio of 1:1. In 2022, Zhang Changchang et al. proposed a periodic metallic surface composed of a single-layer copper structure that can achieve beam splitting of terahertz waves in the range of 0.8–1.1 THz, with a splitting ratio of 1:1. With the advancement of beam splitter research, beam splitters with a single splitting ratio can no longer meet the needs of many applications; therefore, research is gradually shifting towards beam splitters with tunable splitting ratios. In 2022, Yin S et al. designed an active terahertz beamsplitter based on an open circular ring array. By using pump light to illuminate a metasurface array, they controlled the reflection and absorption of terahertz waves by the metasurface array, thereby controlling the beam splitting ratio of the γ-polarized wave. The beam splitting ratio could vary from 1:1 to 15:1. While the aforementioned terahertz beamsplitters achieved good beam splitting performance, they could only achieve a single beam splitting ratio, and once fabricated, the ratio was fixed. Beamsplitters with tunable beam splitting ratios, being active devices, are relatively expensive, limiting their applications in multi-signal transmission, imaging, and stealth in Hertz communication.

[0004] CN201910348908.4 discloses a terahertz metamaterial beam splitter, whose unit structure consists of a top metal strip, an intermediate dielectric layer, and a bottom metal plate. Four units with a rotation step of 45° are periodically arranged to form a 4×4 phase gradient metasurface. When a terahertz wave is incident perpendicularly on the array surface, the electromagnetic wave is reflected into four beams of equal energy but with different propagation directions, and the reflection angles at different frequencies are different. This beam splitter has the advantages of small size and low cost, and can be applied to terahertz stealth and terahertz imaging.

[0005] First, the incident electromagnetic wave splitting ratio in this patent is 1:1, while the structure proposed in this paper has a splitting ratio of 1.2:1 when the electromagnetic wave is incident along the -z-axis and 1.9:1 when it is incident along the +z-axis. Therefore, the energy of the split beams generated when the electromagnetic waves are incident is not equal. Second, this patent uses a passive device, and its splitting ratio cannot be changed once it is manufactured. The structure proposed in this paper, also a passive device, can generate different splitting ratios by incident electromagnetic waves in different directions. This paper can achieve two different splitting ratios. Summary of the Invention

[0006] This invention aims to solve the problems of the prior art mentioned above. It proposes a terahertz beam splitter with adjustable beam splitting ratio and its manufacturing process. The technical solution of this invention is as follows:

[0007] A terahertz beamsplitter with adjustable beam splitting ratio is composed of supercells arranged in an array. Each supercell consists of 8*8 basic cells, four of which are arranged symmetrically according to their phase difference. Four types of basic cells are formed by changing the inner diameter and opening size of the open annular metal pattern. Each basic cell includes a dielectric layer, an open annular metal pattern layer, and a metal reflective layer. The open annular metal pattern layer is located above the dielectric layer, and the metal reflective layer is located below the dielectric layer, covering half of the dielectric layer. The metal reflective layer only covers the basic cell located to the left of the axis of symmetry. The phase difference between the four basic cells of the metasurface beamsplitter is 90° at 0.7 THz. When the terahertz wave is incident perpendicularly along the -z axis, the far-field diagram in the simulation software shows a four-way beam splitting at 0.7 THz with a beam splitting ratio of 1.2:1. When the terahertz wave is incident perpendicularly along the +z axis, the far-field diagram in the simulation software shows a four-way beam splitting at 0.7 THz with a beam splitting ratio of 1.9:1.

[0008] Furthermore, the dielectric layer is polyimide with a dielectric constant of 3.5, a thickness of d=50μm, and dimensions of P=125μm*125μm.

[0009] Furthermore, the metal patterned layer is made of gold, has a thickness of 0.2 μm, and an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m.

[0010] Furthermore, the aforementioned metal reflective layer is made of gold, has a thickness of 0.2 μm, and a conductivity of 4.561 × 10⁻⁶. 7 S / m.

[0011] Furthermore, the specific dimensions of the metal pattern layer of one of the basic units are as follows: inner diameter of the open annulus h1 = 57.5 μm, opening size of the annulus g = 10.0 μm, and dielectric layer thickness d = 50.0 μm. The remaining basic units are constructed by changing h1 and g. The parameters of the other three basic units are: h1 = 52.0 μm, 30.0 μm, and 43.75 μm, and g = 10.0 μm, 25.0 μm, and 10.0 μm.

[0012] Furthermore, the generalized Snell reflection theorem for the beam splitter is: The generalized Snell refraction theorem for beam splitters is: ,in, and Indicates the angle of reflection and the angle of incidence. and λ represents the refractive index of the incident and exiting media, λ0 represents the wavelength of the incident wave, and dΦ / dx represents the phase gradient at the interface.

[0013] Furthermore, the adjustable beam splitting ratio terahertz beam splitter according to claim 1 is characterized in that the structural unit period of the metasurface beam splitter is 120μm*120μm ~130μm*130μm, and the parameters of the above four types of open annular basic unit are achieved by changing the inner diameter of the annulus h1 = 20~60μm and the size of the annulus opening g = 10~50μm.

[0014] A manufacturing process for a terahertz beam splitter with adjustable beam splitting ratio based on any one of the above methods, comprising the following processes:

[0015] Step 1: Design four basic unit structures and arrange them into super units; the surface pattern layer adopts a classic anisotropic open annular structure, which can generate a phase difference condition of 90° in the x-axis direction when terahertz waves are incident, and there is no phase difference in the y-axis direction; the pattern layer adopts an open annular structure and a scheme that combines changing the parameters of the inner diameter and opening size of the annular structure to generate phase difference, thereby realizing the tunability of the arrangement of super units and the beam splitting ratio.

[0016] Step 2: Cleaning the photomask; Place the photomask in an ultrasonic cleaner containing acetone to clean it and remove surface impurities. Then, immerse it in anhydrous ethanol, clean the mask with deionized water, and finally dry it with a nitrogen gun.

[0017] Step 3: Substrate cleaning; Clean the polyimide substrate with acetone and alcohol beforehand, and ensure that the surface of the polyimide substrate is dry;

[0018] Step 4: Gold plating on polyimide film; using a magnetron sputtering machine (FHR), gold with a thickness of 200 nm and titanium with a thickness of 10 nm are deposited on both sides of the polyimide film. Titanium is added to increase the adhesion between the metal and the polyimide.

[0019] Step 5: Coating and pre-baking; Place the sample on a spin coater for spin coating, and then pre-bake the coated sample.

[0020] Step 6: Exposure and Development; The sample is exposed to light using a photolithography machine, and then developed using a developer.

[0021] Step 7: Etching; Etching the top and bottom metal layers of the metasurface using an ion beam etching machine;

[0022] Step 8: Resin removal; Immerse the etched sample in acetone solution, then use a plasma resist remover to remove the photolithography from the sample surface, and finally wash the sample with deionized water and air dry.

[0023] The advantages and beneficial effects of this invention are as follows:

[0024] (1) This invention proposes a simple structure consisting of an array of four types of open-ring basic units, which facilitates fabrication. (2) Compared with existing terahertz beam splitters, this invention allows for adjustment of the beam splitting ratio. (3) This beam splitter can achieve two different beam splitting ratios through front and back incident light, meeting the needs of various scenarios. (4) This beam splitter is a passive device with low cost. (5) In terms of fabrication, since the structure of this device is based on metasurface design, existing fabrication processes are mature, so there is no need to worry about fabrication. (6) This beam splitter can operate normally at room temperature without the need for additional external conditions.

[0025] The four basic units are arranged symmetrically on the left and right sides according to the phase difference, similar to a grating structure. The bottom layer of the super unit composed of these units is only half covered with metal. With this structure, when the incident electromagnetic wave is incident from the -z axis, the electromagnetic wave will be affected by the phase gradient arranged on the left and right sides, thus splitting the electromagnetic wave. However, when the incident electromagnetic wave is incident from the +z axis, the left and right symmetry of the super unit is destroyed because the bottom half is covered with metal. This means that the incident electromagnetic wave is only affected by half of the phase gradient, resulting in a difference in the beam splitting ratio compared to the beam splitting ratio when incident from the -z axis, thus achieving two different beam splitting ratios. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the basic unit design scheme of the beam splitter according to a preferred embodiment of the present invention;

[0027] Figure 2 These are front and side views of the basic unit of the beam splitter.

[0028] Figure 3 These are schematic diagrams of the front and back of the supercell;

[0029] Figure 4 The phase distribution diagram of the basic unit of the beam splitter;

[0030] Figure 5 Far-field radar images of beam splitter incident from the front (a) and from the back (b). Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and thoroughly described below with reference to the accompanying drawings. The described embodiments are merely some embodiments of the present invention.

[0032] The technical solution of the present invention to solve the above-mentioned technical problems is:

[0033] The terahertz beam splitter proposed in this invention is based on an array of four types of metasurface-opening annular basic units, and its structure is as follows: Figures 1-3 As shown. 2. The basic unit structure is a typical "sandwich" structure, consisting of a metal pattern layer, an intermediate dielectric layer, and a metal reflective layer from top to bottom.

[0034] The dielectric layer is polyimide with a dielectric constant of 3.5, a thickness of d=50μm, and dimensions of P=125μm*125μm.

[0035] The top metal pattern layer is an open circular metal structure. The metal pattern layer is in close contact with the intermediate dielectric layer.

[0036] The bottom continuous metal layer is a metal reflective layer, and the metal reflective layer is tightly bonded to the intermediate dielectric layer.

[0037] The metal patterned layer is made of gold, has a thickness of 0.2 μm, and an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m.

[0038] The aforementioned metallic reflective layer is made of gold, has a thickness of 0.2 μm, and an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m.

[0039] The metal patterned layer, such as Figure 2 As shown, the specific dimensions are: h1 = 57.5μm, g = 10.0μm, d = 50.0μm. The remaining basic units are constructed by changing h1 and g. The parameters of the other three basic units are: h1 = 52.0μm, 30.0μm, 43.75μm, g = 10.0μm, 25.0μm, 10.0μm.

[0040] The phase difference between the four basic units of the metasurface beam splitter shown is 90° at 0.7 THz.

[0041] The dielectric layer material is polyimide with a dielectric constant of 3.5 and a thickness d = 40~60μm. Alternatively, silicon or quartz can also be selected.

[0042] The metal backplate covering half of the dielectric layer beneath the dielectric layer is made of metallic gold (Au) with a thickness t = 0.1~0.3 μm.

[0043] The metal pattern layer on the dielectric layer is made of metallic gold (Au) with a thickness of t=0.1~0.3μm. Its shape adopts a typical anisotropic open ring structure for array arrangement. In addition, materials such as Cu and Ag can also be selected.

[0044] The structural unit period of the metasurface beam splitter is P=120μm*120μm ~130μm*130μm. The parameters of the six open annular basic unit above are achieved by changing the inner diameter of the annulus h1=20~60μm and the size of the annulus opening g=10~50μm.

[0045] The phase difference between the four basic units of the metasurface beam splitter shown is 90° at 0.7 THz.

[0046] The supercell consists of 8*8 to 16*16 basic cells, of which 4 basic cells are arranged symmetrically on the left and right sides according to the phase difference. The metal layer under the supercell only covers the basic cells located on the left side of the axis of symmetry.

[0047] The super unit consists of 8*8 basic units, of which 4 basic units are arranged symmetrically on the left and right sides according to the phase difference, and the metal layer under the super unit only covers the basic unit located on the left side of the axis of symmetry.

[0048] 11. The generalized Snell's reflection theorem for beam splitters is: The generalized Snell refraction theorem for beam splitters is: ,in, and Indicates the angle of reflection (refraction) and the angle of incidence. and Let dΦ / dx represent the refractive indices of the incident and exiting media, λ0 represent the wavelength of the incident wave, and dΦ / dx represent the phase gradient at the interface. The structure was simulated using the commercial simulation software CST MICROWAVE STUDIO 2022.

[0049] like Figure 5As shown, when the terahertz wave is incident perpendicularly along the -z axis, the far-field diagram in the simulation software shows that a four-way beam is obtained at 0.7 THz with a beam splitting ratio of 1.2:1. When the terahertz wave is incident perpendicularly along the +z axis, the far-field diagram in the simulation software shows that a four-way beam is obtained at 0.7 THz with a beam splitting ratio of 1.9:1.

[0050] like Figure 5 As shown, two beam splitting ratios were implemented on a single supercell, enabling the beam splitting ratio to be tunable.

[0051] The fabrication of a terahertz beam splitter composed of an array of four metasurface-opening annular basic units was carried out using standard semiconductor micro-nano fabrication technology.

[0052] Step 1: Design four basic unit structures and arrange them into supercells. The surface pattern layer adopts a classic anisotropic open annular structure, which can generate a 90° phase difference in the x-axis direction when terahertz waves are incident, while there is no phase difference in the y-axis direction. Since different phase differences can be generated between metal structures of different sizes, the pattern layer adopts an open annular structure and combines the parameters of changing the inner diameter and opening size of the annular structure to generate phase differences, thus realizing the tunability of the supercell arrangement and beam splitting ratio. The intermediate dielectric layer and the bottom metal reflective layer are both implemented using existing mature design schemes.

[0053] Step 2: Cleaning the photomask. Place the photomask in an ultrasonic cleaner containing acetone to remove surface impurities, then immerse it in anhydrous ethanol, clean the mask with deionized water, and finally dry it with a nitrogen gun.

[0054] Step 3: Substrate cleaning. The polyimide substrate is cleaned with acetone and alcohol beforehand, and the surface of the polyimide substrate must be kept dry. This is to ensure the uniformity of Au plating and the adhesion between the substrate and the metal.

[0055] Step 4: Gold plating on the polyimide film. Using a magnetron sputtering FHR machine, gold with a thickness of 200 nm and titanium with a thickness of 10 nm are deposited on both sides of the polyimide film. Titanium is added to increase the adhesion between the metal and the polyimide.

[0056] Step 5: Coating and Pre-baking. Place the sample on a spin coater for spin coating, and then pre-bake the coated sample.

[0057] Step 6: Exposure and Development. The sample is exposed to light using a photolithography machine, and then developed using a developer.

[0058] Step 7: Etching. The top and bottom metal layers of the metasurface are etched using an ion beam etching machine.

[0059] Step 8: Resin Removal. Immerse the etched sample in acetone solution, then use a plasma resist remover to remove the photolithography material from the sample surface. Finally, wash the sample with deionized water and air dry.

[0060] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0061] The above embodiments should be understood as illustrative only and not as limiting the scope of protection of the present invention. After reading the description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A terahertz beam splitter with adjustable beam splitting ratio, characterized in that, The metasurface beam splitter is composed of supercells arranged in an array. Each supercell consists of 8*8 basic cells. Each basic cell includes a dielectric layer and an open annular metal pattern layer. The open annular metal pattern layer is located above the dielectric layer. By changing the inner diameter and opening size of the open annular metal pattern, four types of basic cells are formed. The phase difference between the four basic cells of the metasurface beam splitter is 90° at 0.7 THz. The supercells are arranged symmetrically on both sides, and the four basic cells are arranged symmetrically on both sides of the axis of symmetry according to the phase difference. The metal reflective layer is located below the dielectric layer and covers half of the dielectric layer of the supercells. The metal reflective layer covers the basic cell located on the left side of the axis of symmetry. When the terahertz wave is incident perpendicularly along the -z axis, the far-field diagram in the simulation software shows a four-way beam split at 0.7 THz with a split ratio of 1.2:

1. When the terahertz wave is incident perpendicularly along the +z axis, the far-field diagram in the simulation software shows a four-way beam split at 0.7 THz with a split ratio of 1.9:

1.

2. The terahertz beam splitter with adjustable beam splitting ratio according to claim 1, characterized in that, The dielectric layer of the basic unit is polyimide with a dielectric constant of 3.5, a thickness of d=50μm, and a size of P=125μm*125μm.

3. The terahertz beam splitter with adjustable beam splitting ratio according to claim 1, characterized in that, The metal patterned layer is made of gold, has a thickness of 0.2 μm, and an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m.

4. The terahertz beam splitter with adjustable beam splitting ratio according to claim 1, characterized in that, The aforementioned metallic reflective layer is made of gold, has a thickness of 0.2 μm, and an electrical conductivity of 4.561 × 10⁻⁶. 7 S / m.

5. The terahertz beam splitter with adjustable beam splitting ratio according to claim 1, characterized in that, The specific dimensions of the metal pattern layer of one of the basic units are as follows: inner diameter of the open annulus h1 = 57.5 μm, annulus opening size g = 10.0 μm, and dielectric layer thickness d = 50.0 μm. The other basic units are constructed by changing h1 and g. The inner diameters of the open annulus of the other three basic units are 52.0 μm, 30.0 μm, and 43.75 μm, and the annulus opening sizes are 10.0 μm, 25.0 μm, and 10.0 μm, respectively.

6. The terahertz beam splitter with adjustable beam splitting ratio according to claim 1, characterized in that, The The generalized Snell reflection theorem for beam splitters is: The generalized Snell refraction theorem for beam splitters is: ,in, and Indicates the angle of reflection and the angle of incidence. and λ represents the refractive index of the incident and exiting media, λ0 represents the wavelength of the incident wave, and dΦ / dx represents the phase gradient at the interface.

7. The terahertz beam splitter with adjustable beam splitting ratio according to claim 1, characterized in that, The basic unit period of the metasurface beam splitter is 120μm*120μm ~130μm*130μm. The parameters of the four types of open annular basic units are achieved by changing the inner diameter of the annulus to 20~60μm and the size of the annulus opening to 10~50μm.

8. A manufacturing process for a terahertz beam splitter with adjustable beam splitting ratio according to any one of claims 1-7, characterized in that, Including the following processes: Step 1: Design four basic unit structures and arrange them into supercells; the metal pattern layer adopts a classic anisotropic open ring structure, which can generate a phase difference condition of 90° in the x-axis direction when the terahertz wave is incident, and there is no phase difference in the y-axis direction; the metal pattern layer adopts an open ring structure and a scheme that combines changing the parameters of the inner diameter and opening size of the ring to generate phase difference, thus realizing the tunability of the arrangement of supercells and the beam splitting ratio. Step 2: Cleaning the photomask; Place the photomask in an ultrasonic cleaner containing acetone to clean it and remove surface impurities. Then, immerse it in anhydrous ethanol, clean the mask with deionized water, and finally dry it with a nitrogen gun. Step 3: Substrate cleaning; Clean the polyimide substrate with acetone and alcohol beforehand, and ensure that the surface of the polyimide substrate is dry; Step 4: Gold plating on polyimide film; using a magnetron sputtering machine (FHR), gold with a thickness of 200 nm and titanium with a thickness of 10 nm are deposited on both sides of the polyimide film. Titanium is added to increase the adhesion between the metal and the polyimide. Step 5: Coating and pre-baking; Place the sample on a spin coater for spin coating, and then pre-bake the coated sample. Step 6: Exposure and Development; The sample is exposed to light using a photolithography machine, and then developed using a developer. Step 7: Etching; Etching the top and bottom metal layers of the metasurface using an ion beam etching machine; Step 8: Resin removal; Immerse the etched sample in acetone solution, then use a plasma resist remover to remove the photolithography from the sample surface, and finally wash the sample with deionized water and air dry.

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