Tunable terahertz broadband filter

Abstract

The invention relates to a tunable terahertz broadband filter. The tunable terahertz broadband filter comprises a plurality of unit structures which are periodically arranged, and each filtering unit is structurally characterized in that the lower plane of a first composite layer (1) is tightly attached to the upper plane of a first dielectric layer (2), and the lower plane of the first dielectric layer (2) is tightly attached to the upper plane of a square metal ring (3); the upper plane of the second dielectric layer (4) is tightly attached to the lower plane of the square metal ring (3), and the upper plane of the second composite layer (5) is tightly attached to the lower plane of the second dielectric layer (4). The obtained broadband filter can realize the conversion that the transmissivity is reduced from 95% to below 10%. The invention has the characteristics of simple structure, high transmissivity, tunable bandwidth, polarization insensitivity and the like.

Description

Technical Field

[0001] The present invention belongs to the field of terahertz metamaterial devices, and in particular relates to a tunable terahertz broadband filter, which can be used for the development of terahertz filters. Background Art

[0002] Terahertz waves, also known as terahertz radiation or THz radiation, refer to electromagnetic waves with frequencies between 0.1 and 10 terahertz (THz) and wavelengths between approximately 30 and 300 microns. Terahertz waves lie between microwaves and infrared in the electromagnetic spectrum. This frequency range has long been considered the "terahertz gap" because traditional technologies and materials have struggled to effectively generate, detect, and manipulate these waves. Terahertz wave generation primarily relies on femtosecond laser pulses or synchrotron radiation, while detection typically relies on pyroelectric and photoconductive detectors. Terahertz waves have high penetration through certain non-polar materials, low photon energy, and no ionizing radiation, making them relatively safe for the human body. Their wavelength lies between microwaves and infrared, offering significant potential for application in non-invasive security inspections. With advances in materials science, optoelectronics, and micro-nanofabrication technologies, the research and application of terahertz waves has rapidly expanded. Governments and research institutions worldwide are increasing their investment in terahertz technology, hoping to achieve breakthroughs in areas such as national security, biomedicine, and information technology. Due to its unique physical properties, terahertz waves are considered to be a band with great potential in future technological development.

[0003] Metamaterials are a new type of material with artificially designed microstructures whose physical properties surpass those of traditional natural materials on a macroscopic scale. Metamaterials are designed by designing artificial structures on a subwavelength scale (i.e., smaller than the wavelength of light), which can control the propagation of electromagnetic waves in a specific way. These artificial structures are usually composed of basic materials such as metals, insulators or semiconductors. Through precise geometric arrangement and size design, they can achieve properties that surpass those of natural materials, such as negative refractive index, perfect lens effect, and stealth. Metamaterials have a wide range of applications in the field of optics, including optical imaging, photonic crystals and optical communications. They are not only of profound significance in theoretical physics, but also have great potential in practical applications, greatly promoting the development of related technologies and sciences.

[0004] Traditional terahertz metamaterial filters based on metal patches are limited by their fixed operating frequency, which restricts their practical applications. With the continuous advancement of research, many single-band, multi-band, and broadband tunable terahertz metamaterial filters have been designed and fabricated. However, some challenges remain. Therefore, a large number of researchers are aiming to develop terahertz metamaterial filters with excellent tunability. Specifically, while maintaining excellent filtering performance, they are pursuing terahertz metamaterial filters with advantages such as a wide tuning range, wide tuning frequency, simple tuning methods, and high tuning accuracy.

[0005] Liang Lan Ju et al. (Liang LJ, Yao JQ, Yan X. Ultrabroad terahertz bandpass filter based on a multiple-layered metamaterial with flexible substrates [J]. Chinese physics letters, 2012, 29(9): 094209.) designed a flexible wide-bandpass filter based on multilayer metamaterials for the terahertz band. The filter, consisting of a five-layer structure, exhibits excellent filtering capability, with a 3dB bandwidth of approximately 0.47 THz, a rising slope of 80 dB / THz, and a falling slope of 96 dB / THz. The authors elaborated on the mechanism of each resonant mode in the metamaterial structure, and the proposed filter provides an effective approach for realizing ultrabroadband terahertz devices. However, its narrow passband and low transmittance limit its application.

[0006] Huang et al. (J.Huang, J.Li, Y.Yang, J.Li, J.Li, Y.Zhang, and J.Yao, “Active controllable bandwidth of THz metamaterial bandpass filter based on vanadiumdioxide,” Opt. Commun. 465, 125616 (2020).) studied a terahertz metamaterial bandpass filter based on vanadium dioxide. The structure utilizes the phase change properties of vanadium dioxide and applies a bias voltage to it to change its conductivity, thereby achieving dynamic control of bandwidth and passband frequency.

[0007] Li Jining et al. (Li J, Chang S. Alternative-frequency-selected terahertz bandstop filter [C] 2014 39th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz). IEEE, 2014: 1-2.) proposed a terahertz metamaterial bandstop filter consisting of a split ring (SRR) and a pair of vanadium dioxide rods. Under conditions of optical pumping or temperature fluctuations, the conductivity of the vanadium dioxide changes, thereby altering the geometry of the metal resonant unit, causing the filter's transmission characteristics to switch between two resonant modes. When the vanadium dioxide is in an insulating state, the resonant frequency is approximately 0.44 THz; when the vanadium dioxide is in a metallic state, the filter structure resonates at 0.67 THz. This filter has a simple structure, but its narrow operating bandwidth and lack of tunability limit its application. Summary of the Invention

[0008] In response to the technical problems existing in the prior art, the primary purpose of the present invention is to provide a tunable terahertz broadband filter based on vanadium dioxide. The filter has a simple structure and a wide filtering bandwidth. In addition, the transmittance of the filter can be tuned by adjusting the conductivity of vanadium dioxide.

[0009] To achieve the above objectives, the present invention proposes a tunable terahertz broadband filter based on vanadium dioxide, providing the following solutions:

[0010] It should be noted that since the terahertz metamaterial filter is composed of unit structures arranged in a periodic manner, its size can be arranged according to one's own needs, so the following steps only introduce the design of the unit structure.

[0011] like Figure 2 As shown, the structure of the terahertz broadband filter unit consists of a first composite layer (1), a first dielectric layer (2), a square metal ring (3), a second dielectric layer (4), and a second composite layer (5); the lower plane of the first composite layer (1) is in close contact with the upper plane of the first dielectric layer (2), and the lower plane of the first dielectric layer (2) is in close contact with the upper plane of the square metal ring (3); the upper plane of the second dielectric layer (4) is in close contact with the lower plane of the square metal ring (3), and the upper plane of the second composite layer (5) is in close contact with the lower plane of the second dielectric layer (4).

[0012] The square metal ring (3) is located at the middle position in the height direction of the terahertz filter unit; the first composite layer (1) and the second composite layer (5) are symmetrically arranged; the first dielectric layer (2) and the second dielectric layer (4) are symmetrically arranged; and the centroids of the first composite layer (1), the first dielectric layer (2), the square metal ring (3), the second dielectric layer (4) and the second composite layer (5) coincide with the center line of the terahertz filter unit.

[0013] like Figure 3 As shown, the first composite layer (1) is based on a VO2 square ring with a side length of L = 10 μm and a ring width of W1 = 3 μm as the central body. The outer center of the VO2 square ring is connected to four VO2 semicircular rings with an outer ring radius of R3 = 5 μm and a ring width of W2 = 1, and the outer ring radius of each adjacent VO2 semicircular ring is tangent. Each VO2 semicircular ring is nested inside a metal semicircular ring with an outer ring radius of R1 = 2 μm and a ring width of W3 = 0.5. The four vertices of the VO2 square ring serve as the center of the VO2 semicircular ring and the metal semicircular ring structure, respectively, forming a central symmetrical structure as a whole. The second composite layer (5) is the same as the first composite layer (1);

[0014] like Figure 4 As shown, the square metal ring (3) is a square ring structure, and the ring width W of the square metal ring (3) is 0.5

[0015] The first dielectric layer (2) and the second dielectric layer (4) both use polyimide as a filling medium, and the dielectric constants of the first dielectric layer (2) and the second dielectric layer (4) are 3.5.

[0016] The material of the metal ring is gold.

[0017] The dielectric constant ε of the vanadium dioxide is expressed by the Drude model in the THz band:

[0018]

[0019] ε ∞ =12 is the relative dielectric constant at infinite frequency;

[0020] γ represents the collision frequency, γ=5.75×10 13 rad / s;

[0021] ω p (σ) is the plasma frequency, ω p The relationship between (σ) and σ is:

[0022]

[0023] Where: σ0 represents the temperature-related conductivity, σ0 = 3 × 10 5 S / m;

[0024] σ represents the electrical conductivity of vanadium dioxide, σ=200S / m in insulating state; σ=200000S / m in metallic state;

[0025] ω p (σ) represents the plasma frequency which depends on the conductivity; ω p (σ)=1.4×10 15 rad / s.

[0026] The material of the square metal ring (3) is the same as that of the first composite layer (1) and the second composite layer (5); the thickness of the square metal ring (3) is the same as the thickness t2 of the first composite layer (1) and the second composite layer (5). t2 = 0.5 μm.

[0027] The length of the four sides of the first dielectric layer (2), the square metal ring (3) and the second dielectric layer (4) of the terahertz filter is p=20 μm.

[0028] The first dielectric layer (2) and the second dielectric layer (4) have the same thickness t1, t1 = 3 μm.

[0029] The vanadium dioxide is set to an insulating state and a metallic state in the simulation, wherein the temperature of the insulating state is ≤300K and the temperature of the metallic state is ≥340K;

[0030] The beneficial effects of the present invention are:

[0031] (1) The present invention proposes a tunable terahertz broadband filter based on vanadium dioxide. By leveraging the phase change properties of vanadium dioxide, the filter's filtering performance can be adjusted without changing structural parameters. Specifically, when the conductivity of vanadium dioxide is 200 S / m, the filter operates in transmission mode; when the conductivity of vanadium dioxide reaches 200,000 S / m, the filter switches from transmission mode to reflection mode.

[0032] (2) The tunable terahertz broadband filter based on vanadium dioxide proposed in the present invention exhibits good polarization insensitivity to both TE and TM polarized waves.

[0033] (3) The present invention proposes a tunable terahertz broadband filter based on vanadium dioxide. When VO2 is in the insulating state, this filter can achieve ultra-wideband bandpass filtering with a transmission coefficient exceeding 90% in the range of 3.15 to 8.81 THz, and a relative bandwidth of up to 116%. When VO2 is in the metallic state, it transforms into a band-stop filter with a transmission coefficient of less than 15% in the range of 0.1 to 8.78 THz. The maximum modulation depth of the transmission coefficient can reach an impressive 91.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0035] Figure 1 This is a schematic diagram of the three-dimensional structure of a 3×3 unit array of a tunable terahertz broadband filter based on vanadium dioxide according to the present invention;

[0036] Figure 2 This is a schematic structural diagram of a single unit of a tunable terahertz broadband filter based on vanadium dioxide according to the present invention;

[0037] Figure 3 for Figure 1 A schematic top view of the first composite layer (1) in the tunable terahertz broadband filter unit based on vanadium dioxide;

[0038] Figure 4 for Figure 1 A schematic top view of a square metal ring (3) in a tunable terahertz broadband filter unit based on vanadium dioxide;

[0039] Figure 5 for Figure 1 Transmittance simulation results of the tunable terahertz broadband filter based on vanadium dioxide at vertical incidence when the conductivity of vanadium dioxide is 200 S / m and 200,000 S / m respectively;

[0040] Figure 6 for Figure 1 Simulation results of the transmittance of the tunable terahertz broadband filter based on vanadium dioxide as the conductivity of vanadium dioxide changes;

[0041] Figure 7 for Figure 1 Simulation results of the transmittance of the tunable terahertz broadband filter based on vanadium dioxide that varies with polarization angle when the vanadium dioxide is in an insulating state;

[0042] Figure 8 for Figure 1 Simulation results of the transmittance of the tunable terahertz broadband filter based on vanadium dioxide as it changes with polarization angle when the vanadium dioxide is in a metallic state; DETAILED DESCRIPTION

[0043] The following, with reference to the accompanying drawings of the present invention, provides a complete and clear explanation of the technical methods in the present invention examples, enabling those skilled in the art to easily and smoothly understand the various advantages and details of the present invention. Obviously, the example described below is merely a specific example of the present invention, and the present invention can also be implemented through different examples. Details such as the dimensions and positions described in the present invention can also be slightly modified for different examples without violating the spirit of the present invention. Therefore, other examples obtained without inventive changes also fall within the scope of the present invention.

[0044] like Figure 2 As shown, the structure of the terahertz broadband filter unit consists of a first composite layer (1), a first dielectric layer (2), a square metal ring (3), a second dielectric layer (4), and a second composite layer (5); the lower plane of the first composite layer (1) is in close contact with the upper plane of the first dielectric layer (2), and the lower plane of the first dielectric layer (2) is in close contact with the upper plane of the square metal ring (3); the upper plane of the second dielectric layer (4) is in close contact with the lower plane of the square metal ring (3), and the upper plane of the second composite layer (5) is in close contact with the lower plane of the second dielectric layer (4).

[0045] The square metal ring (3) is located at the middle position in the height direction of the terahertz filter unit; the first composite layer (1) and the second composite layer (5) are symmetrically arranged; the first dielectric layer (2) and the second dielectric layer (4) are symmetrically arranged; and the centroids of the first composite layer (1), the first dielectric layer (2), the square metal ring (3), the second dielectric layer (4) and the second composite layer (5) coincide with the center line of the terahertz filter unit.

[0046] like Figure 3 As shown, the first composite layer (1) is based on a VO2 square ring with a side length of L = 10 μm and a ring width of W1 = 3 μm as the central body. The outer center of the VO2 square ring is connected to four VO2 semicircular rings with an outer ring radius of R3 = 5 μm and a ring width of W2 = 1, and the outer ring radius of each adjacent VO2 semicircular ring is tangent. Each VO2 semicircular ring is nested inside a metal semicircular ring with an outer ring radius of R1 = 2 μm and a ring width of W3 = 0.5. The four vertices of the VO2 square ring serve as the center of the VO2 semicircular ring and the metal semicircular ring structure, respectively, forming a central symmetrical structure as a whole. The second composite layer (5) is the same as the first composite layer (1);

[0047] like Figure 4 As shown, the square metal ring (3) is a square ring structure, and the ring width W of the square metal ring (3) is 0.5

[0048] The first dielectric layer (2) and the second dielectric layer (4) both use polyimide as a filling medium, and the dielectric constants of the first dielectric layer (2) and the second dielectric layer (4) are 3.5.

[0049] The material of the metal ring is one of aluminum, gold, silver and copper.

[0050] The dielectric constant ε(ω) of the vanadium dioxide is expressed by the Drude model in the THz band:

[0051]

[0052] ε ∞ =12 is the relative dielectric constant at infinite frequency;

[0053] γ represents the collision frequency, γ=5.75×10 13 rad / s;

[0054] ω p (σ) is the plasma frequency, ω p The relationship between (σ) and σ is:

[0055]

[0056] Where: σ0 represents the temperature-related conductivity, σ0 = 3 × 10 5 S / m;

[0057] σ represents the electrical conductivity of vanadium dioxide, σ=200S / m in insulating state; σ=200000S / m in metallic state;

[0058] ω p (σ) represents the plasma frequency which depends on the conductivity; ω p (σ)=1.4×10 15 rad / s.

[0059] The material of the square metal ring (3) is the same as that of the first composite layer (1) and the second composite layer (5); the thickness of the square metal ring (3) is the same as the thickness t2 of the first composite layer (1) and the second composite layer (5). t2 = 0.5 μm.

[0060] The length of the four sides of the first dielectric layer (2), the square metal ring (3) and the second dielectric layer (4) of the terahertz filter is p=20 μm.

[0061] The first dielectric layer (2) and the second dielectric layer (4) have the same thickness t1, t1 = 3 μm.

[0062] The vanadium dioxide is set to an insulating state and a metallic state in the simulation, wherein the temperature of the insulating state is ≤300K and the temperature of the metallic state is ≥340K;

[0063] When VO2 is in the insulating state, the filter achieves ultra-wideband bandpass filtering with a transmission coefficient exceeding 90% in the range of 3.15 to 8.81 THz, with a relative bandwidth of up to 116%. When VO2 is in the metallic state, it transforms into a band-stop filter, with a transmission coefficient below 15% in the range of 0.1 to 8.78 THz. The maximum modulation depth of the transmission coefficient can reach an impressive 91.5%.

[0064] The beneficial effects of the present invention are:

[0065] (1) The present invention proposes a tunable terahertz broadband filter based on vanadium dioxide. Through the phase change characteristics of vanadium dioxide, the filtering performance can be adjusted without changing the structural parameters. Figure 5 It can be seen that when the conductivity of vanadium dioxide is 200S / m, the working mode of the filter is transmission mode; when the conductivity of vanadium dioxide is 200000S / m, the working mode of the filter switches from transmission mode to reflection mode.

[0066] (2) The tunable terahertz broadband filter based on vanadium dioxide proposed in the present invention exhibits good polarization insensitivity to both TE and TM polarization waves. Figure 7 and Figure 8 The transmittance simulation results of the terahertz ultra-wideband filter with polarization angle changes when vanadium dioxide is in insulating state and metallic state are analyzed. Figure 7 and Figure 8 It can be seen that the transmittance of the filter hardly changes with the change of angle.

[0067] (3) The present invention proposes a tunable terahertz broadband filter based on vanadium dioxide. When VO2 is in the insulating state, this filter can achieve ultra-wideband bandpass filtering with a transmission coefficient exceeding 90% in the range of 3.15 to 8.81 THz, and a relative bandwidth of up to 116%. When VO2 is in the metallic state, it switches from transmission mode to reflection mode in the range of 0.1 to 8.78 THz, with a transmission coefficient of less than 15%. The maximum modulation depth of the transmission coefficient can reach an impressive 91.5%. Figure 6 is a simulation result diagram of the transmittance of the terahertz ultra-wideband filter as the conductivity of vanadium dioxide changes; Figure 6 It can be seen that with the increase of the conductivity of vanadium dioxide, the transmission peak of the filter can be reduced from 95% to below 10%.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to specific implementation methods, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A tunable terahertz broadband filter, characterized in that: The structure of the terahertz broadband filter unit is as follows: the lower plane of the first composite layer (1) is in close contact with the upper plane of the first dielectric layer (2), and the lower plane of the first dielectric layer (2) is in close contact with the upper plane of the square metal ring (3); the upper plane of the second dielectric layer (4) is in close contact with the lower plane of the square metal ring (3), and the upper plane of the second composite layer (5) is in close contact with the lower plane of the second dielectric layer (4). The first composite layer (1) and the second composite layer (5) are symmetrically arranged, and the first dielectric layer (2) and the second dielectric layer (4) are symmetrically arranged; the centroids of the first composite layer (1), the first dielectric layer (2), the square metal ring (3), the second dielectric layer (4), and the second composite layer (5) coincide with the center line of the terahertz ultra-wideband filter unit; the four sides of the first dielectric layer (2), the square metal ring (3), and the second dielectric layer (4) are flush, and the lengths p×p of the four sides are equal.

2. The tunable terahertz broadband filter according to claim 1, characterized in that: The length of the four sides of the first dielectric layer (2), the square metal ring (3) and the second dielectric layer (4) of the terahertz filter is p=20 μm.

3. The tunable terahertz broadband filter according to claim 1, characterized in that: The first composite layer (1) is based on a VO2 square ring with a side length of L = 10 μm and a ring width of W1 = 3 μm. The center of the outer edge of the VO2 square ring is connected to four VO2 semicircular rings with an outer ring radius of R2 = 5 μm and a ring width of W2 = 1, and the outer ring radius of each adjacent VO2 semicircular ring is tangent. Each VO2 semicircular ring is nested inside a metal semicircular ring with an outer ring radius of R1 = 2 μm and a ring width of W3 = 0.

5. The four vertices of the VO2 square ring serve as the centers of the VO2 semicircular ring and the metal semicircular ring structure, respectively, forming a centrally symmetrical structure as a whole. The second composite layer (5) is the same as the first composite layer (1).

4. The tunable terahertz broadband filter according to claim 1, wherein: The material of the first dielectric layer (2) and the second dielectric layer (4) is polyimide, and the dielectric constant is 3.

5.

5. The tunable terahertz broadband filter according to claim 1, characterized in that: The first dielectric layer (2) and the second dielectric layer (4) have the same thickness t1, which is 3 μm.

6. The tunable terahertz broadband filter according to claim 1, characterized in that: The first composite layer (1) and the second composite layer (5) have the same thickness t2, which is 0.5 μm.

7. The tunable terahertz broadband filter according to claim 1, characterized in that: The square metal ring (3) is located at the middle position of the terahertz filter unit in the height direction; the square metal ring (3) is a square ring structure, and the ring width W of the square metal ring (3) is 0.

5.

8. The tunable terahertz broadband filter according to claim 1, characterized in that: The material of the metal ring is gold.

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