Terahertz Three-Dimensional Unidirectional Transmission Waveguide Based on Magnetic Surface Plasmon

Abstract

The present invention discloses a terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmons, which is composed of a unidirectional transmission region and two artificial magnetic conductor layers with the same structure and opposite directions spliced on the left and right sides of the unidirectional transmission region; the unidirectional transmission region is an electrically opaque medium-magnetized semiconductor structure, and the artificial magnetic conductor is composed of a structure in which square metal rods are periodically extended in a dielectric background and placed on a metal floor; the artificial magnetic conductor can replace the ideal magnetic conductor boundary condition to support three-dimensional unidirectional transmission; the waveguide structure provided by the present invention realizes three-dimensional unidirectional transmission based on magnetic surface plasmons in the terahertz band, provides a solution for expanding such two-dimensional unidirectional waveguides to three-dimensional unidirectional waveguides, and has great application prospects in the miniaturization of non-reciprocal devices.

Description

Technical Field

[0001] The present invention relates to the field of terahertz unidirectional waveguides, and in particular, to a terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmons. Background Art

[0002] Terahertz waves refer to electromagnetic radiation with frequencies in the range of 0.1 - 10 THz. In the electromagnetic spectrum, the infrared and microwave radiation technologies on both sides of the terahertz band are very mature. However, the terahertz technology is extremely disproportionate to the mature technologies in the adjacent bands, forming a so-called "terahertz gap", resulting in a serious lag in the development of terahertz technology. Terahertz waves have the advantage of causing less damage to substances and the human body. This band covers the characteristic spectra of substances such as semiconductors, plasmas, and biological macromolecules, and can promote the understanding of basic scientific issues in physics, chemistry, life sciences, etc. Terahertz communication integrates the advantages of microwave communication and optical communication, and has many characteristics such as high transmission rate, large capacity, strong directivity, high security, and good penetrability. It has particularly attractive prospects in military communication applications and has become a hot spot for exploration and development in various countries. In recent years, terahertz radiation sources and terahertz detection technologies have developed rapidly. In contrast, the development of terahertz wave manipulation technologies and related photonic devices has progressed slowly.

[0003] Surface plasmon polaritons (SPPs) are a hybrid excited state of electrons and photons at the metal-dielectric interface, and they can propagate like waves. SPPs can break through the diffraction limit and have the ability to confine the electromagnetic field within a range much smaller than the wavelength, achieving so-called sub-wavelength confinement, which provides an excellent way for the miniaturization of photonic devices and the high integration of optical systems. However, the plasma frequency of metals is usually in the ultraviolet band, so SPPs on the metal surface are only suitable for the visible light band; in the terahertz band, there are actually corresponding plasma materials, namely semiconductors. The plasma frequency of semiconductors is in the terahertz band. Therefore, the propagation behavior of terahertz SPPs on the semiconductor surface is similar to that of (visible light band) SPPs on the metal surface. Using semiconductors, the SPP manipulation technology in the visible light band can be directly analogized to the terahertz band. Particularly importantly, semiconductors are also a kind of gyrotropic electro-optic material. Under the action of an external magnetic field, the time-reversal symmetry of the system is broken, and non-reciprocal SPPs, namely so-called surface magneto plasmons (SMP), can be supported. Based on SMP, terahertz non-reciprocal photonic devices can be constructed. Such terahertz non-reciprocal devices have the advantages of miniaturization, simple structure, and broadband. However, such devices rely on ideal magnetic conductor boundaries to achieve, and actually can only be regarded as two-dimensional devices. Therefore, it is of great significance to expand such devices to three dimensions.

[0004] Non-reciprocal devices are usually an indispensable and important element in optical systems. Ideal non-reciprocal devices can ensure that signals are transmitted in one direction in the system. In addition to eliminating destructive feedback to the emission source, they can also greatly simplify the design of optical systems by suppressing multi-path reflections between devices, and greatly improve the stability of the system to manufacturing defects and environmental changes. With the rapid development of optical information technology, the high demand for miniaturization of non-reciprocal photonic devices has led to the emergence of electromagnetic unidirectional modes, which is a new physical concept that has emerged in this century. Unidirectional electromagnetic modes refer to electromagnetic wave modes that can only propagate in one direction. They are immune to backscattering caused by defects (or bends) because there is no back-propagation mode in the system. Unidirectional electromagnetic modes (Unidirectional Surface Magneto Plasmon, USMP) based on SMP are not only simple in configuration, but also easy to obtain broadband, and non-reciprocal devices based on USMP are more conducive to miniaturization.

[0005] In addition, if USMP is to be applied to photonic devices, it is also necessary to solve lateral constraints and realize three-dimensional waveguide structures. Microwave USMP based on ferrite materials is an E-polarized surface wave, so a three-dimensional waveguide can be constructed by lateral truncation of a pair of metal plates, and this three-dimensional USMP has the same mode properties as the two-dimensional USMP. However, this method is ineffective for terahertz USMP and optical band USMP based on gyroelectric materials. Therefore, how to achieve lateral constraints of high-frequency band USMP and construct three-dimensional waveguides is still an important scientific problem that needs to be solved urgently.

[0006] The Chinese patent application with publication number CN117423996A discloses "A leaky-wave metasurface and method based on unidirectional electromagnetic mode", which is a unidirectional transmission waveguide structure based on magnetic surface plasmon implemented in the microwave band. The USMP implemented by the ferrite material used is an E-polarized surface wave, so a three-dimensional waveguide can be constructed by lateral truncation of a pair of metal plates. However, this method is ineffective for terahertz USMP (H polarization) and optical band USMP (H polarization) based on gyroelectric materials.

[0007] Perfect Magnetic Conductor (PMC) boundary conditions play an important role in electromagnetic design, especially in applications such as waveguide design, surface plasmon waveguides, and electromagnetic shielding. PMC simulates an ideal magnetic surface. When an electromagnetic wave is incident on the PMC surface, the tangential component of the magnetic field is zero. In addition, the reflection of the magnetic field by PMC is different from that of the electric field. The phase of the electric field reflection will be reversed by 180 degrees, while the phase of the magnetic field will change by 0 degrees. Although PMC is an idealized model, artificial materials are usually used in practical applications to approximate the behavior of PMC to meet the needs of electromagnetic design.

[0008] Artificial Magnetic Conductor (AMC), which is usually composed of periodically arranged units that form an artificial electromagnetic material structure at the macroscopic scale. The structural units can be metals, conductive materials, or other dielectric materials, and are usually designed with specific shapes and sizes to generate the desired electromagnetic response within a specific frequency band. By causing local resonance of electromagnetic waves through the periodic structure, this resonance can produce a significant magnetic field effect within a specific frequency range. In the high-frequency case, the behavior of electromagnetic wave transmission is affected by the structure of the material surface. When an electromagnetic wave is incident on the surface of the AMC, the interaction between the surface structure and the incident wave results in reflection or transmission characteristics. These structures simulate the behavior of the PMC through resonance or other electromagnetic effects. In the past, AMC has been mostly used to reduce the back lobe radiation of microstrip antennas, thereby improving the antenna gain and radiation performance. Summary of the Invention

[0009] The present invention aims to overcome the above-mentioned disadvantages of the prior art and provides a terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmons. The present invention can support terahertz three-dimensional USMP, providing a solution for expanding such two-dimensional unidirectional waveguides to three-dimensional unidirectional waveguides, and having great application prospects in the miniaturization of non-reciprocal devices.

[0010] The present invention achieves the above technical objectives through the following technical means, specifically:

[0011] A terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmons, the waveguide is composed of a unidirectional transmission region and two artificial magnetic conductor layers with the same structure and opposite directions spliced on the left and right sides of the unidirectional transmission region. The unidirectional transmission region is a terahertz two-dimensional USMP waveguide structure under the PMC boundary condition, that is, an electrically opaque medium - magnetized semiconductor structure, which can support terahertz two-dimensional USMP; the artificial magnetic conductor is composed of a structure in which square metal rods are periodically extended in a dielectric background and placed on a metal floor, simulating the reflection characteristics of an ideal magnetic conductor.

[0012] Preferably, in the electrically opaque medium - magnetized semiconductor structure, the electrically opaque medium extends infinitely along the positive direction of the axis and the

[0013] positive direction of the

[0014] axis; The relative permittivity of the electrically opaque medium is negative.

[0015] Further, the relative permittivity of the electrically opaque medium is ;

[0015] Preferably, in the electrically opaque medium - magnetized semiconductor structure, the magnetized semiconductor is in a static magnetic field In it, it extends infinitely along the positive direction of the axis and the negative direction of the

[0016] The direction of the static magnetic field is the positive direction of the

[0017] Preferably, the one-way transmission region is obtained by extending an electrically opaque medium-magnetized semiconductor structure; wherein the electrically opaque medium layer extends along the positive direction of the axis and the positive direction of the axis, and the magnetized semiconductor layer extends along the positive direction of the

[0018] axis and the negative direction of the axis. The width of the one-way transmission region is w. Among them , , ; among them is the plasma angular frequency of the magnetized semiconductor, is the electron cyclotron frequency, which is determined by the applied static magnetic field , is the high-frequency relative permittivity of the magnetized semiconductor, and the plasma frequency of the magnetized semiconductor is .

[0019] The artificial magnetic conductor described in the present invention is composed of a structure in which square metal rods are periodically extended in a dielectric background and placed on a metal floor; the parameters of the square metal rods are as follows: a = 7.6 is the period constant, b = 0.8a is the side length of the metal rod, and h = 2a is the height of the square metal rod and the dielectric. The relative permittivity of the dielectric of the structure is 4, and the materials of the metal and the metal floor are copper.

[0020] The one-way transmission region described in the present invention is a terahertz two-dimensional USMP waveguide structure under PMC boundary conditions, that is, an electrically opaque medium-magnetized semiconductor structure, which can achieve two-dimensional one-way transmission, and the transmission direction is the positive direction of the axis, and the polarization type is H polarization

[0021] The artificial magnetic conductor described in the present invention is composed of a structure in which square metal rods are periodically extended in a dielectric background and placed on a metal floor. The artificial magnetic conductor is usually designed with specific shapes and dimensions to generate the required electromagnetic response within a specific frequency band. By causing local resonance of electromagnetic waves through the periodic structure, this resonance can produce a significant magnetic field effect within a specific frequency range, simulating the reflection characteristics of the PMC boundary condition. The artificial magnetic conductor described in the present invention can simulate the reflection characteristics of the PMC near the operating frequency and, at the operating frequency , the magnetic field reflection phase is 0, which is applicable to the above-mentioned terahertz two-dimensional USMP waveguide.

[0022] Innovatively, the present invention applies the characteristics of the artificial magnetic conductor to the terahertz two-dimensional USMP waveguide. By simulating the reflection characteristics of the PMC with an artificial magnetic conductor of a specific structure, terahertz three-dimensional USMP is achieved.

[0023] The advantages of the present invention are as follows: The provided waveguide structure realizes terahertz three-dimensional USMP, providing a solution for expanding such two-dimensional unidirectional waveguides to three-dimensional unidirectional waveguides. The above structure is also applicable to other terahertz two-dimensional USMP waveguide structures and is expected to be extended to the optical frequency, having great application prospects in the miniaturization of non-reciprocal devices. Description of the Drawings

[0024] Figure 1 is a structural diagram of the terahertz three-dimensional unidirectional waveguide of the present invention.

[0025] Figure 2 is Figure 1 the top view.

[0026] Figure 3 is Figure 1 the left view.

[0027] Figure 4 is a structural diagram of the artificial magnetic conductor of the present invention.

[0028] Figure 5 is Figure 4 the top view.

[0029] Figure 6 is Figure 4 the front view.

[0030] Figure 7 is the reflection phase diagram of the artificial magnetic conductor of the present invention.

[0031] Figure 8(a) is a schematic diagram of the transmission of a terahertz two-dimensional unidirectional waveguide with a magnetic field added in the present invention, and Figure 8(b) is a schematic diagram of the transmission of a terahertz two-dimensional unidirectional waveguide without a magnetic field added in the present invention.

[0032] Figure 9(a) is a schematic diagram of unidirectional transmission of terahertz three-dimensional unidirectional waveguide without obstacles in the present invention, and Figure 9(b) is a schematic diagram of unidirectional transmission of terahertz three-dimensional unidirectional waveguide with obstacles in the present invention.

[0033] Figure 10 is the dispersion relation diagram of the terahertz three-dimensional unidirectional waveguide structure of the present invention. Detailed implementation mode

[0034] The following further describes the implementation mode of the present invention in detail.

[0035] Example 1

[0036] First, a terahertz two-dimensional USMP waveguide under PMC boundary conditions is designed to construct a unidirectional transmission region. Secondly, an artificial magnetic conductor is designed to simulate the PMC boundary conditions to achieve 0 reflection of the magnetic field phase, and then support terahertz three-dimensional USMP.

[0037] First, the unidirectional transmission region and the artificial magnetic conductor primitive cell are constructed separately.

[0038] The unidirectional transmission region consists of two layers:

[0039] The first layer is an electrically opaque layer, which is an isotropic homogeneous medium and extends infinitely along the axis and the axis in the positive direction. Its relative dielectric constant is , the relative magnetic permeability is 1, and the relative conductivity is 0, as shown in Figures 1 - 3 .

[0040] The second layer is a magnetized semiconductor layer. Under the action of a static magnetic field (in the positive direction of the axis), the magnetized semiconductor is a gyrotropic anisotropic medium and extends infinitely along the axis and the axis in the negative direction. Its relative dielectric constant tensor is , where , , , the relative magnetic permeability is 1, and the relative conductivity is 0, where is the plasma angular frequency of the magnetized semiconductor, is the electron cyclotron frequency, which is determined by the applied static magnetic field , is the high-frequency relative dielectric constant of the magnetized semiconductor, and the magnetized semiconductor plasma frequency is , as shown in Figures 1 - 3 .

[0041] The above unidirectional transmission region ( axis, lateral) is truncated with a width of .

[0042] The artificial magnetic conductor unit cell has mirror symmetry and consists of two layers:

[0043] The first layer is formed by periodically extending square metal rods in a dielectric background. The relative permittivity of the dielectric is 4, the relative permeability is 1, and the relative conductivity is 0. The height h of the dielectric and the square metal rods is 2a, and the period a = 7.6 , the side length b of the square metal rod is 0.8a, as Figures 4 - 6 shown.

[0044] The second layer is a metal floor made of copper.

[0045] In the present invention, the unidirectional transmission region is a terahertz two-dimensional USMP waveguide, that is, an electrically opaque medium - magnetized semiconductor structure. Under the PMC boundary condition, it can simulate the two-dimensional transmission situation that extends infinitely in the direction. The transmission direction is the positive direction of the axis, and the polarization type is H polarization . By adjusting the w width, unidirectional transmission is achieved in this structure. The working frequency is , excited by a magnetic current source (source). The magnetic field distribution is shown in Fig. 8(a) and is simulated by the commercial simulation software Comsol Multiphysics. When such a two-dimensional waveguide is extended to a three-dimensional waveguide,

[0046] In the direction, it needs to be truncated with real materials, which will lead to the generation of non-unidirectional modes, such as conventional modes, and thus lose the characteristics of unidirectional transmission. Since the PMC boundary condition does not exist in reality, it makes the application of such waveguides difficult and hinders the development of terahertz wave manipulation technology and related photonic devices. Therefore, it is particularly important to find a suitable material to replace the PMC boundary condition under the above structure. Based on the above problems, the present invention designs an artificial magnetic conductor to replace the PMC boundary condition, designs the parameters of the artificial magnetic conductor unit cell, and by adjusting the period constant a, the side length b of the square metal rod, the height h of the square metal rod and the dielectric, and the relative permittivity of the dielectric, makes its performance parameters meet the design. In the present invention, the relative permittivity of the dielectric is 4, the relative permeability is 1, and the relative conductivity is 0. The height h of the dielectric and the square metal rods is 2a, and the period a = 7.6 Figures 5 - 7 , the side length b of the square metal rod is 0.8a, and both the metal rod and the metal floor are made of copper, as shown. This structure can simulate the PMC boundary condition near the working frequency Figure 7 ( Figure 7 gray area), and its reflection phase diagram is as At this time, the reflection phase is 0, and the result is calculated by the CST simulation software. To verify the transmission characteristics of the designed waveguide structure, the unidirectional transmission region and the artificial magnetic conductor are spliced together to form a terahertz three-dimensional USMP waveguide, and the unidirectional transmission is simulated and verified by the commercial simulation software Comsol Multiphysics. The magnetic field distribution is shown in Fig. 9(a). At the same time, the dispersion relation in the neighborhood is calculated to further verify its unidirectionality numerically. The results are as Figure 10 shown.

[0047] Embodiment 2

[0048] The unidirectional transmission region in Embodiment 1 of the present invention is a terahertz two-dimensional USMP waveguide. Under the action of a static magnetic field, the time-reversal symmetry of the system is broken, and unidirectional transmission can be supported. Without the action of an external magnetic field, the unidirectional transmission characteristic is lost, and bidirectional transmission is supported. The magnetic field distribution is shown in Fig. 8(b).

[0049] Embodiment 3

[0050] Embodiment 1 of the present invention is a terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmons, which can be immune to the backscattering caused by defects (or bends). In this embodiment, a cylindrical obstacle is added to the waveguide in Embodiment 1 to further observe the unidirectional transmission of the terahertz three-dimensional USMP. The magnetic field distribution is shown in Fig. 9(b). The diameter of the circular obstacle is 0.67a, and the length is w = 20 μm. In this embodiment, the terahertz three-dimensional USMP can perfectly bypass the obstacle and be immune to the backscattering caused by defects.

[0051] It should be emphasized that the unidirectional transmission region of the above-mentioned invention structure is not limited to the waveguide structure of the embodiment, and can also be other terahertz two-dimensional USMP waveguide structures. The artificial magnetic conductor layer is not limited to the structure of the above embodiment, and can also be other structures with similar phase reflection characteristics.

[0052] The above embodiments are only the preferred embodiments of the present invention, which are more general and do not limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmon, characterized in that: It consists of a unidirectional transmission region and two artificial magnetic conductors with the same structure and opposite directions spliced ​​on the left and right sides of the unidirectional transmission region; the artificial magnetic conductor is composed of square metal rods periodically extended in a dielectric background and placed on a metal floor; the unidirectional transmission region is a terahertz two-dimensional USMP waveguide structure under PMC boundary conditions, that is, an electrically opaque medium-magnetized semiconductor structure, which supports two-dimensional unidirectional transmission based on magnetic surface plasma under ideal magnetic conductor boundary conditions, and the artificial magnetic conductor replaces the ideal magnetic conductor boundary conditions to support three-dimensional unidirectional transmission; The electrically opaque medium-magnetized semiconductor structure, wherein the electrically opaque medium is along Axis positive direction, The positive direction of the axis extends infinitely; The relative dielectric constant of the electrically opaque medium is negative; In the electrically opaque medium-magnetized semiconductor structure, the magnetized semiconductor is in a static magnetic field. In, along Axis positive direction, The negative direction of the axis extends infinitely; The direction of the static magnetic field is Axis positive direction; The magnetized semiconductor is a gyromagnetic optical material. Under the action of a static magnetic field, the relative dielectric constant becomes a tensor with the following form: ,in , , ;in is the plasma angular frequency of the magnetized semiconductor, is the electron cyclotron frequency, determined by the applied static magnetic field Decide, is the high frequency relative permittivity of the magnetized semiconductor, and the plasma frequency of the magnetized semiconductor .

2. The terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmon according to claim 1, characterized in that: The relative dielectric constant of the electrically opaque medium .

3. The terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmon according to claim 1, characterized in that: The artificial magnetic conductor has an operating frequency Simulate the reflection characteristics of PMC nearby, at the working frequency When , the magnetic field reflection phase is 0, which is applicable to the above-mentioned terahertz two-dimensional USMP waveguide.

4. The terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmon according to claim 1, characterized in that: The parameters of the square metal rod are as follows: a=7.6 is the periodic constant, b=0.8a is the side length of the metal rod, h=2a is the height of the square metal rod and the dielectric; the relative dielectric constant of the structural dielectric is 4, and the metal and metal floor materials are copper.

5. The terahertz three-dimensional unidirectional transmission waveguide based on magnetic surface plasmon according to claim 1, characterized in that: The unidirectional transmission region is a terahertz two-dimensional USMP waveguide structure under PMC boundary conditions, which realizes two-dimensional unidirectional transmission. Axis positive direction, polarization type is H polarization ( ).

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