Terahertz electromagnetic wave transmission structure design method for high-compactness on-chip integration

Abstract

The invention belongs to the technical field of terahertz integrated chips, and particularly relates to a terahertz electromagnetic wave transmission structure and a preparation method thereof. The transmission structure comprises a substrate, a graphene layer arranged on the surface of the substrate, a gap layer located on one side of the graphene layer, and a silicon tube waveguide located on one side of the gap layer. The gap layer has a low dielectric constant while the silicon tube waveguide has a high dielectric constant. According to the transmission structure, the mixed low-dielectric-constant gap layer is formed between the silicon tube waveguide and the two-dimensional graphite layer, a coupling mode of a high-dielectric-constant tube waveguide and a polariton mode of two-dimensional graphene is generated, local mode limitation is achieved in a gap area, the diffraction limit of transmission focusing of a sub-wavelength structure is broken through, and the transmission efficiency is improved. Sub-diffraction mode focusing of terahertz wave propagation is realized, and the design problem of on-chip terahertz wave transmission structural members is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz integrated chips, and specifically relates to a terahertz electromagnetic wave transmission structure and a preparation method thereof, in particular to a terahertz electromagnetic wave transmission structure and a preparation method thereof for highly compact on-chip integration. Background Art

[0002] With the development of the fifth generation (5G) communication technology, emerging applications such as digital twins, augmented reality, and edge computing have put forward higher requirements for real-time communication rates. To meet this demand, it is urgent to increase the carrier frequency of radar and communication systems. Terahertz waves (THz) refer to electromagnetic radiation with a wavelength between 0.03mm and 3mm. They can provide ultra-large bandwidths of tens of gigahertz and have ultra-high-speed data transmission capabilities of terabits per second, which can meet the bandwidth requirements of many emerging businesses.

[0003] Due to the lack of efficient on-chip terahertz wave transmission structures, research on terahertz technology has been slow, and this frequency band is called the "terahertz gap". In engineering, it is often necessary to transmit electromagnetic waves from one location to another in a directional manner. This requirement requires an electromagnetic wave transmission structure to be implemented. Although electromagnetic wave transmission structures such as metal wires have been widely used, there have been few reports on the design of on-chip terahertz wave transmission structures so far. This is because due to the limitations of the diffraction limit, the size of traditional transmission structures is usually much larger than the wavelength, making it difficult to achieve on-chip miniaturization integration. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a terahertz electromagnetic wave transmission structure and a preparation method thereof, aiming to provide a better terahertz electromagnetic wave transmission solution.

[0005] The first aspect of the present invention is to provide a terahertz electromagnetic wave transmission structure, comprising a substrate, a graphene layer disposed on the surface of the substrate, a gap layer located on one side of the graphene layer, and a silicon tube waveguide located on one side of the gap layer; the gap layer has a low dielectric constant, and the silicon tube waveguide has a high dielectric constant. The device integrates the silicon tube waveguide with graphene, and through the effective coupling of the two-dimensional graphene and silicon tube waveguide modes, the device has sub-wavelength transmission mode characteristics, and realizes low-threshold terahertz wave excitation by increasing the quantum spontaneous emission rate.

[0006] Among them, two-dimensional graphene can excite surface plasmon modes to couple with silicon tube waveguides to form special hybrid plasmon modes. Traditional surface plasmons are a type of surface wave that can be transmitted on the metal-dielectric interface, which can confine electromagnetic waves to subwavelength scales and break through the diffraction limit. Compared with surface plasmons in metal-dielectric interfaces, graphene plasmons have strong electromagnetic field confinement and low loss characteristics.

[0007] As an optimization solution for terahertz electromagnetic wave transmission structure, the silicon tube waveguide material is silicon, and the relative dielectric constant is 11.5 to 13.5.

[0008] As an optimization scheme for terahertz electromagnetic wave transmission structure, the outer diameter of the silicon tube waveguide is d, the inner diameter is di, the tube empty ratio γ=di / d, and the tube empty ratio of the silicon tube waveguide is adjustable, thereby presenting different transmission characteristics. Preferably, the tube empty ratio γ of the silicon tube waveguide is 0.4-0.7.

[0009] As an optimization scheme for terahertz electromagnetic wave transmission structure, the relative dielectric constant of the gap layer is 1.35 to 2.45.

[0010] As an optimization solution for terahertz electromagnetic wave transmission structure, the base material is silicon dioxide with a relative dielectric constant of 1.35 to 2.45.

[0011] As an optimization scheme for the terahertz electromagnetic wave transmission structure, the operating wavelength of the transmission structure is 0.03 mm to 3 mm.

[0012] As an optimization scheme for the terahertz electromagnetic wave transmission structure, a graphene layer is deposited and bonded on the surface of a substrate, a gap layer is deposited on the surface of the graphene layer, and a silicon tube waveguide is deposited on the surface of the gap layer.

[0013] As an optimization scheme for terahertz electromagnetic wave transmission structure, the width of the gap layer is the same as the outer diameter of the silicon tube waveguide. The device characteristics are controlled by controlling the diameter of the silicon tube waveguide and the height of the low dielectric constant gap layer.

[0014] As an optimization scheme for terahertz electromagnetic wave transmission structure, the cross section of the transmission structure is slightly scanned in space to form a preset transmission structure spatial shape, and the spatial shape is selected from a straight-line type, a curved type, and a cross type.

[0015] The second aspect of the present invention is to provide a method for manufacturing a terahertz electromagnetic wave transmission structure, comprising the following steps: depositing a positive photoresist film on a silicon dioxide substrate, exposing the positive photoresist film through a mask and then developing it, removing the positive photoresist covering the area where graphene needs to be deposited, then depositing graphene on the area, and then depositing a silicon microtube waveguide on the graphene to obtain a terahertz electromagnetic wave transmission structure.

[0016] Beneficial Effects

[0017] Compared with the prior art, the present invention forms a hybrid low dielectric constant gap layer between the silicon tube waveguide and the two-dimensional graphene layer by designing a unique transmission structure, generating a coupling mode of the polariton mode of the high dielectric constant tube waveguide and the two-dimensional graphene, leading to local mode confinement in the gap region, breaking through the diffraction limit of sub-wavelength structure transmission focusing, achieving sub-diffraction mode focusing of terahertz wave propagation, and breaking through the design problem of on-chip terahertz wave transmission structure. This miniaturized transmission structure will not only play a core role in the construction of information infrastructure, but also the breakthrough of electromagnetic wave transmission technology beyond the wavelength scale has opened up new opportunities for studying wave-matter interactions in extreme local electromagnetic fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a cross-sectional schematic diagram of the transmission structure of the present invention.

[0019] Figure 2 This is the electric field profile distribution diagram of the transmission structure of the present invention.

[0020] Figure 3 The figure is a simplified flowchart of the preparation process of the transmission structure of the present invention.

[0021] Figure 4 It is a transmission loss characteristic curve diagram of the transmission structure of the present invention.

[0022] Figure 5 It is a transmission mode regional factor curve diagram of the transmission structure of the present invention. DETAILED DESCRIPTION

[0023] The present invention is further illustrated by specific examples below. These examples are exemplary and are intended to illustrate the problem and explain the present invention, but are not intended to be limiting.

[0024] The present invention provides a terahertz electromagnetic wave transmission structure for highly compact on-chip integration, forming a hybrid low dielectric constant gap layer between a silicon tube waveguide and two-dimensional graphene, generating a coupling mode of a high dielectric constant waveguide and a two-dimensional graphene surface plasmon mode, leading to local electromagnetic energy limitation in the gap region, breaking through the diffraction limit of sub-wavelength structure transmission focusing, and realizing sub-diffraction ultra-compact focusing of terahertz wave propagation. The transmission structure is composed of a gap resonant cavity and a waveguide gain material, and its principle includes the excitation of the gain material and the radiation and amplification of the plasmon. Under the excitation of the pump energy provided by the outside world, the gain material produces a population inversion, and the electrons in the excited state transition to a low energy level and radiate to the plasmon mode, which is then amplified by stimulated radiation of the gain material to form a terahertz wave focused transmission.

[0025] Example 1

[0026] Figure 1The schematic diagram of the terahertz electromagnetic wave transmission structure for highly compact on-chip integration is shown in FIG. The transmission structure includes a silicon tube waveguide 10, a low dielectric constant gap layer 20, a two-dimensional graphene layer 30 and a silicon dioxide substrate 40 from top to bottom.

[0027] For example, the outer diameter d of the silicon tube waveguide 10 is 30 μm, the width of the low dielectric constant gap layer 20 is equal to it, also 30 μm, and the height g of the low dielectric constant gap layer 20 is 5 μm. The inner diameter of the tube waveguide is di, and γ=di / d is defined as the ratio of the inner diameter to the outer diameter of the hollow tube, which is called the tube hollowness. The larger the tube hollowness γ, the higher the hollowness of the tube waveguide.

[0028] When operating at a terahertz wavelength of λ=100μm, the relative dielectric constants of SiO2 and Si are measured to be 1.5 and 3.5, respectively.

[0029] The equivalent dielectric constant of graphene can be characterized as

[0030]

[0031] Wherein, η0 is air impedance, k0 is wave number. In the example structure, the thickness of the two-dimensional graphene is Δ=0.5 nm.

[0032] The electromagnetic properties of single-layer graphene can be measured using the surface conductivity σ g , can be represented by Kubo theory as

[0033] σ g =σ intra +σ inter

[0034]

[0035] In the formula, σ intra represents intraband electron-photon scattering, σ inter represents the interband electron transition, ω is the angular frequency of the incident electromagnetic wave, μ c =0.5eV is the chemical potential, is the electron relaxation time, μ=4m 2 / (V·s) is the carrier mobility of graphene. f =10 6 m / s is the Fermi velocity, T=300K is the temperature, is the reduced Planck constant, k B is the Boltzmann constant, and e is the electron charge constant.

[0036] In the specific design calculation, the single-layer graphene can be represented by a sub-nanometer thickness geometric thin layer, and the grid is divided. However, the thickness of graphene is much smaller than the size of the overall structure. Therefore, in order to ensure the accuracy of electromagnetic calculation, it is bound to require a small grid size and a high grid density. The resulting data volume increases dramatically, the calculation time becomes longer, and the requirements for computer hardware increase. The calculation in this embodiment uses equivalent boundary conditions to replace the graphene layer. Since the graphene thickness is very thin, the equivalent boundary condition (Transition Boundary Condition) can be used to approximate it, thereby avoiding very fine grid division of the graphene layer, thereby significantly improving the design efficiency.

[0037] In order to study the subwavelength transmission performance of the graphene-dielectric nanowire plasmon transmission structure, it is assumed that the plasmon mode propagates along the out-of-plane direction (denoted as the z direction) and its time correlation is exp(-iωt). Then the transverse mode field The two-dimensional Helmholtz equation should be satisfied

[0038]

[0039] Where ε and μ are the dielectric constant and magnetic permeability, n eff is the complex effective mode index. β=k0n eff is the complex propagation constant, k0 = 2π / λ0, where λ0 is the wavelength in free space. In the formula, the propagation constant β is obtained by calculation, and the effective refractive index n of the mode can be obtained. eff =β / k0.

[0040] The full vector finite element method is used to perform mode solving on the transmission structure in this embodiment, and the mode field distribution and device characteristics of the transmission mode are calculated.

[0041] In numerical analysis using the finite element method, convergence testing is required to ensure that the numerical boundaries and meshing do not affect the exact solution.

[0042] Figure 2 The fundamental mode electric field intensity distribution diagram of the transmission structure of the present invention. The mode electric field intensity of the transmission structure has an obvious mode confinement enhancement effect in the low dielectric constant gap layer 20 region, realizing an ultra-compact transmission mode that breaks through the diffraction limit at a deep sub-wavelength scale.

[0043] The transmission structure utilizes the dielectric mode of the silicon tube waveguide 10 to couple with the surface plasmon of graphene, and the formed specific hybrid plasmon has ultra-compact mode confinement.

[0044] Silicon microtubes, which are used as silicon tube waveguides 10, are a new one-dimensional micro-nano material with unique physical properties compared to carbon microtubes and silicon microwires. Studies have shown that because one-dimensional silicon micromaterials have excellent compatibility with existing silicon technology, silicon microtubes will be more promising than carbon microtubes in the application of micro-nano technology and devices. In addition, the structure of silicon microtubes has the characteristics of hollow adjustability, high surface area and low conductivity, which makes it more unique than the widely used silicon nanowires.

[0045] The fabrication of the transmission structure can be achieved using standard thin film deposition techniques and photolithography, while graphene can be produced using chemical vapor deposition. For microtube waveguides, taking silicon microtubes as an example, a simple preparation method is silicon shell deposition technology.

[0046] Preparation process Figure 3 As shown, the following steps are included:

[0047] Step (a): depositing a positive photoresist (PR) film on a silicon dioxide (SiO2) substrate, the thickness of which is consistent with the thickness of the graphene layer;

[0048] Step (b): placing a mask above the PR film, wherein the light-transmitting portion of the mask corresponds to the portion on the substrate where the graphene layer needs to be deposited, exposing the PR film to ultraviolet light through the mask. After the exposure is completed, using a developer to remove the exposed portion of the PR film to expose the position for depositing the graphene layer;

[0049] Step (c): Prepare a two-dimensional graphene layer by chemical vapor deposition (CVD), transfer the two-dimensional graphene layer to the developed area on the substrate by wet method, and then perform PR demolding. Usually, the CVD method needs to be carried out on a metal substrate such as copper. Therefore, the graphene must be transferred intact from the metal growth substrate to the target substrate. Taking the auxiliary method using polymethyl methacrylate (PMMA) polymer film as a carrier as an example, the transfer process is described as follows. (1) Spin-coat a polyvinyl alcohol (PVA) water-soluble polymer film on the graphene / metal substrate laminate sample, then spin-coat PMMA on the surface of the water-soluble film, heat and dry it, and peel off the two-dimensional graphene on the surface of the PMMA polymer film; (2) Slowly immerse the entire substrate into a container filled with deionized water. The water-soluble polymer will gradually dissolve under the action of the solvent, and the hydrophobic PMMA polymer film carrying the two-dimensional graphene sample will float on the surface of the solution; (3) Adsorb and fix the PMMA film on a micromanipulation platform, accurately align the graphene layer to be transferred on the target substrate under an optical microscope, and then slowly lower the film height to make it contact with the substrate; (3) Heat the target substrate to remove the moisture adsorbed on the sample surface and promote the full adsorption of the PMMA polymer film and the substrate, soak the substrate in an acetone solution to remove the PMMA polymer film, and then rinse with isopropanol and dry it to complete the transfer.

[0050] Step (d): forming a microtube waveguide by a silicon shell deposition method. The method first prepares magnesium oxide microrods by a hydrothermal method, then forms a uniform thin coating of silicon on the magnesium oxide microrod template by thermal cracking of silane, and finally dissolves the magnesium oxide microrods by acid treatment to obtain a hollow silicon microtube. In this process, the outer diameter of the silicon tube waveguide can be adjusted by controlling the diameter of the magnesium oxide microrods, and the outer diameter of the silicon tube waveguide is controlled by electron beam lithography, thereby achieving the control of the tube hollowness.

[0051] Figure 4 The terahertz wave transmission loss curve of the device. Propagation distance L p =1 / k0αeff=λ / [4πIm(n eff )] is a measure of the propagation loss of a single transmission structure. It is the distance the mode propagates when the amplitude is attenuated to 1 / e of the original value. eff is the transmission structure propagation loss, including graphene loss and radiation loss caused by the coupling between the tube waveguide and graphene, n effis the complex effective mode index of the mode. It can be seen that as the tube hollowness γ increases, the propagation distance of the mode decreases. This phenomenon means that appropriately reducing the hollowness of the tube waveguide is helpful to reduce the device loss.

[0052] Figure 5 This is the curve diagram of the transmission mode regional factor regulation. In order to measure the breakthrough relative to the traditional diffraction limit energy region, the normalized transmission mode regional factor A is introduced m It is defined as the ratio of the total modal energy to the peak energy density, then normalized to the diffraction-limited energy

[0053]

[0054] In the formula, is the diffraction-limited energy. By calculating the normalized transmission mode area factor, the energy distribution of the transmission mode or the magnitude of the local field enhancement capability can be evaluated. It is not difficult to conclude that the transmission mode area is inversely proportional to the energy localization capability, which is one of the important indicators for measuring whether the transmission structure can be applied to a highly integrated on-chip system.

[0055] from Figure 5 It can be seen that when the tube empty rate γ is 0.5, the transmission mode area factor A m The minimum is close to 0.002. This shows that compared with the diffraction limit, the energy localization performance has been improved by 3 orders of magnitude, achieving deep subwavelength energy localization. In addition, as the transmission structure tube empty ratio γ decreases, the transmission mode area factor A m It decreases first and then increases. This means that by adjusting the geometric dimensions of the silicon tube waveguide 10, the local performance of energy transmission can be optimized. The preferred tube empty ratio γ is 0.4 to 0.7.

[0056] The transmission structure of the present invention realizes ultra-compact electromagnetic energy transmission, laying a foundation for terahertz wave transmission of highly integrated on-chip circuits.

[0057] The above embodiments are exemplary, and their purpose is to illustrate the technical concept and features of the present invention so that people familiar with the technology in this field can understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A terahertz electromagnetic wave transmission structure, characterized in that: The invention comprises a substrate, a graphene layer arranged on the surface of the substrate, a gap layer located on one side of the graphene layer, and a silicon tube waveguide located on one side of the gap layer, wherein the gap layer has a lower relative dielectric constant than that of the silicon tube waveguide.

2. The terahertz electromagnetic wave transmission structure according to claim 1, characterized in that: The silicon tube waveguide material is silicon, and the relative dielectric constant is 11.5-13.

5.

3. The terahertz electromagnetic wave transmission structure according to claim 1, characterized in that: The outer diameter of the silicon tube waveguide is d, the inner diameter is di, and the tube hollowness ratio γ=di / d, then the tube hollowness ratio γ of the silicon tube waveguide is 0.4~0.

7.

4. The terahertz electromagnetic wave transmission structure according to claim 2, characterized in that: The relative dielectric constant of the gap layer is 1.35-2.

45.

5. The terahertz electromagnetic wave transmission structure according to claim 1, characterized in that: The substrate material is silicon dioxide, and the relative dielectric constant is 1.35-2.

45.

6. The terahertz electromagnetic wave transmission structure according to any one of claims 1 to 5, characterized in that: The operating wavelength of the transmission structure is 0.03 mm to 3 mm.

7. The terahertz electromagnetic wave transmission structure according to claim 6, characterized in that: The graphene layer is deposited and bonded on the surface of the substrate, the gap layer is deposited on the surface of the graphene layer, and the silicon tube waveguide is deposited on the surface of the gap layer.

8. The terahertz electromagnetic wave transmission structure according to claim 6, characterized in that: The width of the gap layer is the same as the outer diameter of the silicon tube waveguide.

9. The terahertz electromagnetic wave transmission structure according to claim 6, characterized in that: The cross section of the transmission structure is slightly swept in space to form a preset transmission structure spatial shape, and the spatial shape is selected from a straight-line type, a curved type, and a cross type.

10. A method for manufacturing a terahertz electromagnetic wave transmission structure, characterized in that: The following steps are involved: A positive photoresist film is deposited on a silicon dioxide substrate, exposed through a mask and then developed to remove the positive photoresist covering the area where graphene needs to be deposited, and then graphene is deposited on the area, and then a silicon microtube waveguide is deposited on the graphene to obtain the terahertz electromagnetic wave transmission structure.

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