Magnetic adjustable multi-band terahertz wave absorber

Abstract

The invention relates to the technical field of terahertz wave absorbers, in particular to a magnetic adjustable multi-band terahertz wave absorber. The magnetic adjustable multi-band terahertz wave absorber sequentially comprises a graphene layer, a spacing layer and a Fibonacci quasi-periodic photonic crystal unit from top to bottom, as the Fibonacci quasi-periodic photonic crystal unit is formed by alternately stacking the first dielectric layers and the second dielectric layers according to the Fibonacci sequence, a periodic structure which is optimized and complicated or a random structure which is lacked regularly in the traditional design is replaced, so that the design of the multi-band terahertz wave absorber does not need to repeatedly try the position of a defect layer; the number of absorption bands can be systematically controlled only by adjusting a single order, and meanwhile, due to the fact that the Fibonacci quasi-periodic photonic crystal unit can generate a plurality of photonic band gaps and the graphene layer and the spacer layer have strong locality on incident light, multi-band terahertz perfect wave absorption is achieved.

Description

Technical Field

[0001] This invention relates to the field of terahertz absorber technology, and more particularly to a magnetically tunable multi-band terahertz absorber. Background Technology

[0002] Terahertz absorbers are key components in terahertz communication, imaging, and sensing systems. Graphene, due to its excellent properties such as tunable conductivity and strong surface plasmon resonance in the terahertz band, is widely used in absorber design. Meanwhile, photonic crystals are artificial materials composed of dielectric materials arranged in a periodic structure. Depending on their spatial dimension, photonic crystals can be classified as one-dimensional, two-dimensional, and three-dimensional. Among them, one-dimensional photonic crystals are suitable for combination with graphene to enhance the absorption performance of terahertz absorbers due to their simple structure, easy fabrication, controllable light transmission, and low loss.

[0003] Currently, terahertz absorbers based on graphene and photonic crystals mostly employ external electric fields for performance tuning. However, this method requires the introduction of electrodes, increasing the complexity of device fabrication and operation. In contrast, magnetic field tuning eliminates the need for electrodes, simplifying the device structure. Therefore, some magnetically tunable terahertz absorbers based on graphene and one-dimensional photonic crystal structures have been proposed in the prior art, achieving narrowband, broadband, and multi-band terahertz absorption through external magnetic field tuning.

[0004] However, the photonic crystals in existing magnetically tunable multi-band terahertz absorbers based on graphene and one-dimensional photonic crystal structures are mostly periodic or random photonic crystals. Periodic photonic crystals usually require a lot of design to determine the insertion positions of multiple defect layers, while random photonic crystals lack predictive models and it is difficult to form deterministic engineering optimization rules.

[0005] In summary, existing magnetically tunable multiband absorbers based on graphene and one-dimensional photonic crystals suffer from complex design optimization or a lack of deterministic rules due to the periodic or random structure of the photonic crystals. Summary of the Invention

[0006] The purpose of this invention is to provide a magnetically tunable multi-band terahertz absorber, which solves the problem that existing magnetically tunable multi-band absorbers based on graphene and one-dimensional photonic crystals have problems such as complex design optimization or lack of deterministic rules due to the periodic or random structure of the photonic crystal.

[0007] To achieve the above objectives, the present invention provides a magnetically tunable multi-band terahertz absorber, which comprises, from top to bottom, a graphene layer, a spacer layer, and a Fibonacci quasi-periodic photonic crystal unit, wherein the Fibonacci quasi-periodic photonic crystal unit is formed by alternating stacking of a first dielectric layer and a second dielectric layer in a Fibonacci sequence.

[0008] The first dielectric layer is magnesium fluoride, the refractive index of the first dielectric layer is 1.38, and the thickness of the first dielectric layer is 18.12 μm.

[0009] The second dielectric layer is silicon dioxide, the refractive index of the second dielectric layer is 2.25, and the thickness of the second dielectric layer is 11.11 μm.

[0010] The spacer layer is made of silicon carbide, has a refractive index of 3.59, and a thickness of 6.96 μm.

[0011] The graphene layer is a single-layer graphene, and the Fermi level E of the graphene layer is... F = -0.34 eV, the scattering rate Γ of the graphene layer is 10 meV / The Fermi rate v of the graphene layer F = 10 6 The speed is m / s, and the thickness of the graphene layer is 0.335 nm.

[0012] The electrical conductivity of the graphene layer must follow the Drud model.

[0013] The layered structure of the magnetically tunable multi-band terahertz absorber requires the use of the transfer matrix method to calculate its absorption characteristics.

[0014] The number of frequency bands of the magnetically tunable multi-band terahertz absorber increases with the order of the Fibonacci quasi-periodic photonic crystal unit. The absorption amplitude of the multi-band can be adjusted by an external magnetic field, the center frequency of the multi-band can be adjusted by the incident angle, and wide-angle absorption can be achieved.

[0015] The present invention discloses a magnetically tunable multi-band terahertz absorber, comprising, from top to bottom, a graphene layer, a spacer layer, and a Fibonacci quasi-periodic photonic crystal unit. Since the Fibonacci quasi-periodic photonic crystal unit is formed by alternating stacking of the first and second dielectric layers in a Fibonacci sequence, replacing the complex periodic structure or random structure lacking regularity in traditional designs, the design of the multi-band terahertz absorber does not require repeated attempts at defect layer locations. The number of absorption bands can be systematically controlled simply by adjusting a single order. Furthermore, because the Fibonacci quasi-periodic photonic crystal unit can generate multiple photonic band gaps, and the graphene layer and the spacer layer have strong localization of incident light, perfect multi-band terahertz absorption is achieved. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the structure of the magnetically adjustable multi-band terahertz absorber provided by the present invention.

[0018] Figure 2 The magnetically tunable multi-band terahertz absorber GC(B) provided by this invention n-1 A) n In B, n = 6, that is, GC(B) 5 A) 6 When B is applied and a magnetic field of 7 T is applied (i.e., B = 7 T), the absorption (a), transmittance (b), and reflectance (c) of the structure for right-hand circularly polarized waves ("+") and left-hand circularly polarized waves ("-").

[0019] Figure 3 The magnetically tunable multi-band terahertz absorber GC(B) provided by this invention n-1 A) n In B, n = 6, that is, GC(B) 5 A) 6 B, and the absorption rate of the structure for left-handed circularly polarized waves when different magnetic fields B = 0, 4 and 7 T are applied.

[0020] Figure 4 The magnetically tunable multi-band terahertz absorber GC(B) provided by this invention n-1 A) n In B, n = 6, that is, GC(B) 5 A) 6 When B is applied and a magnetic field of 7 T is applied (i.e., B = 7 T), the absorption rate of the structure for left-handed circularly polarized waves varies with the incident angle (i.e., θ = 0°, 30° and 50°).

[0021] Figure 5 The magnetically tunable multi-band terahertz absorber GC(B) provided by this invention n-1 A) n In B, n = 6, that is, GC(B) 5 A) 6 When B is applied and a magnetic field of 7 T is applied (i.e., B = 7 T), the absorption rate of the structure for left-handed circularly polarized waves varies with the order of the quasi-periodic photonic crystal (i.e., n = 4, 6, and 8).

[0022] G - Graphene layer, C - Spacer layer, A - First dielectric layer, B - Second dielectric layer. Detailed Implementation

[0023] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0024] Please see Figures 1 to 5 The present invention provides a magnetically tunable multi-band terahertz absorber, which includes, from top to bottom, a graphene layer G, a spacer layer C, and a Fibonacci quasi-periodic photonic crystal unit. The Fibonacci quasi-periodic photonic crystal unit is formed by alternating stacking of a first dielectric layer A and a second dielectric layer B in a Fibonacci sequence.

[0025] The arrangement of the first dielectric layer A and the second dielectric layer B follows the following pattern:

[0026]

[0027] Where n and j represent the order and generation number of the Fibonacci quasi-periodic photonic crystal unit, respectively. In this technical solution, j = 3. Therefore, the magnetically tunable multi-band terahertz absorber can be represented as GC(B n-1 A) n B.

[0028] In this embodiment, since the Fibonacci quasi-periodic photonic crystal unit is formed by alternating stacking of the first dielectric layer A and the second dielectric layer B in a Fibonacci sequence, replacing the complex periodic structure or random structure lacking rules in traditional design, the design of multi-band terahertz absorbers does not require repeated attempts at defect layer positions. The number of absorption bands can be systematically controlled simply by adjusting a single order. At the same time, since the Fibonacci quasi-periodic photonic crystal unit can generate multiple photonic band gaps and the graphene layer G and the spacer layer C have strong localization of incident light, perfect multi-band terahertz absorption is achieved.

[0029] Furthermore, the electrical conductivity of the graphene layer G must follow the Drood model.

[0030] In this embodiment, when a magnetic field is incident perpendicularly onto the graphene layer G, the electrical conductivity of the graphene layer G includes its lateral electrical conductivity. Hall conductivity Within the terahertz frequency range, the electrical conductivity of the graphene layer G follows the Drude model, with the following equation:

[0031] (1)

[0032] and

[0033] (2)

[0034] in, It is Drud's weight. The cyclotron frequency, , , , and These represent the electron charge, Fermi level, reduced Planck constant, Fermi velocity, and scattering rate, respectively. Meanwhile, the dielectric constant of graphene is as follows:

[0035] (3)

[0036] in, , and ,in , and These are the operating angular frequency, the vacuum dielectric constant, and the thickness of the graphene layer G, respectively.

[0037] Furthermore, the layered structure of the magnetically tunable multi-band terahertz absorber needs to be calculated using the transfer matrix method to determine its absorption characteristics.

[0038] In this embodiment, since linearly polarized waves can be decomposed into left-handed and right-handed circularly polarized waves with equal amplitude but opposite directions of rotation, and right-handed and left-handed circularly polarized waves exhibit different absorption characteristics when passing through the graphene layer G under the influence of a magnetic field, and the 4 × 4 transfer matrix method can simultaneously calculate the absorption characteristics of both right-handed and left-handed circularly polarized waves, this technical solution will use the 4 × 4 transfer matrix method to calculate the absorption characteristics of the proposed structure. The specific 4 × 4 transfer matrix method is as follows:

[0039] S1: Assuming the designed structure is placed in air, the refractive index of both the incident wave space (0th layer of medium) and the outgoing wave space (N+1th layer of medium) is 1, that is... Furthermore, the incident terahertz wave is parallel to the xz plane and the incident angle is... .exist Incident at point, and at The point of origin is the point of origin. Let this be the thickness of the proposed model. Therefore, the wave vector... The tangential components are respectively and ,in (Right now ( ) represents the free space wavenumber. For graphene under a magnetic field, the wave vector is... The longitudinal component is , where j = 1, 2, 3 and 4. These are the four distinct z-components of the wave vector.

[0040] For one side of monolayer anisotropic graphene, its electric field and magnetic field It can be represented as follows:

[0041] (4)

[0042] in,

[0043] (5)

[0044] in, and , For free space impedance. Furthermore, in equation (4)... It can be represented as:

[0045] (6)

[0046] When an external magnetic field is incident perpendicularly on graphene, the dielectric constant tensor of graphene (i.e., equation (3)) , and They are respectively: , and In equation (3) , , and If all are zero, then equation (6) can be simplified to:

[0047] (7)

[0048] S2: The tangential components of the electric and magnetic fields on the other side of monolayer anisotropic graphene can be expressed as:

[0049] (8)

[0050] in, It is a 4 × 4 transition matrix. It can be represented as:

[0051] (9)

[0052] in, The diagonal propagation matrix has the following elements: .matrix For matrix In elements The matrix composed of the corresponding eigenvectors. and Both can be achieved by analyzing the matrix. Obtained by diagonalization.

[0053] S3: Because the first dielectric layer A, the second dielectric layer B, and the spacer layer C are all isotropic media, their transition matrix , and The same method can be used. The first medium will be used as an example for explanation. For a thickness of d... A isotropic medium, torque matrix It can be represented as:

[0054] (10)

[0055] in, The diagonal propagation matrix has the following elements: . for The corresponding feature vector, and Similarly, this can be achieved by manipulating the matrix. Obtained by diagonalization. It is worth noting that in isotropic media… and ,then It can be represented as:

[0056] (11).

[0057] S4: For Figure 1 The structures shown, z = 0 and z = L e The electric or magnetic field at a point should satisfy the following equation:

[0058] (12)

[0059] Since a linearly polarized wave can be equivalently represented as two circularly polarized waves with equal amplitude and opposite directions—namely, a right-handed circularly polarized wave and a left-handed circularly polarized wave—therefore... and It can be represented as:

[0060] (13)

[0061] in,

[0062] (14)

[0063] in, , and Let represent the incident, reflected, and transmitted amplitudes of a right-hand circularly polarized wave (or a left-hand circularly polarized wave), respectively. Therefore, equation (12) can be rewritten as:

[0064] (15)

[0065] in, This is the total transition matrix, used for connecting... and The field. Therefore, the transmission and reflection coefficients of co-polarized (same subscript) and cross-polarized (different subscript) fields are respectively:

[0066] (16).

[0067] Furthermore, in order to achieve multi-band absorption with tunable magnetic field in the terahertz frequency range, after many attempts, the final result was determined. Figure 1 The structural parameters are as follows:

[0068] The first dielectric layer A is magnesium fluoride (MgF2), and the refractive index of the first dielectric layer A is n. A = 1.38, the thickness of the first dielectric layer A is d A = 18.12 μm;

[0069] The second dielectric layer B is silicon dioxide (SiO2), and the refractive index of the second dielectric layer B is n. B = 2.25, the thickness of the second dielectric layer B is d B = 11.11 μm;

[0070] The spacer layer C is silicon carbide (SiC), and the refractive index of the spacer layer C is n. C = 3.59, the thickness of the spacer layer C is d C = 6.96 μm;

[0071] The graphene layer G is a single layer of graphene, and the Fermi level E of the graphene layer G is... F = -0.34 eV, the scattering rate Γ of the graphene layer G is 10 meV / The Fermi rate v of the graphene layer G F = 10 6 m / s, the thickness d of the graphene layer G g = 0.335 nm.

[0072] The incident wave is a linearly polarized wave with a center frequency of 3 THz, and the magnetic field is perpendicular to the xz plane.

[0073] In this embodiment, from Figure 2 (a) It can be seen that when the magnetic field B = 7 T, the proposed model achieves perfect absorption of left-handed circularly polarized waves at three frequency points (i.e., f1 = 1.99 THz, f2 = 2.99 THz, and f3 = 4 THz) in the frequency range of 1.5 – 4.5 THz, with absorption rates of 95.93%, 99.27%, and 97.59%, respectively. The perfect absorption of the three bands is primarily attributed to the Fibonacci quasi-periodic photonic crystal unit (B n-1 A) n B can generate 3 photonic band gaps, such as Figure 2 As shown in (b). Secondly, this is attributed to the fact that the graphene layer G is the only one in the model with terahertz wave absorption properties, and that it, along with the spacer layer C, acts as a defect layer capable of strong localization of terahertz waves, thus resulting in no reflected waves, as... Figure 2 As shown in (c). Furthermore, from Figure 2 (a) It is also evident that at the corresponding frequencies of the three perfect absorption peaks, the absorption rate of the proposed model for right-hand circularly polarized waves is less than 50%. This stems from magnetic circular dichroism; under the same magnetic field, left-hand and right-hand circularly polarized waves exhibit different absorption characteristics. Since the proposed structure has a much higher absorption rate for left-hand circularly polarized waves than for right-hand circularly polarized waves, the following discussion will primarily focus on the absorption rate characteristics of the proposed structure for left-hand circularly polarized waves under the influence of a magnetic field.

[0074] from Figure 3 It can be seen that with the increase of magnetic field B, the absorption rates at f2 = 2.99 THz and f3 = 4 THz increase accordingly, while the absorption at f1 = 1.99 THz initially increases and then slightly decreases. This is because the light energy at this frequency does not match the energy required for the intraband Landau level transitions of graphene under the influence of the magnetic field. That is, as the magnetic field increases, the energy required for the intraband transitions of graphene becomes closer to the light energy at that frequency, thus increasing the absorption rate. However, as the magnetic field further increases, the energies gradually diverge, resulting in a decrease in absorption rate. Although the absorption rate at f1 decreases slightly when the magnetic field B = 7 T, the absorption rates at all three frequencies are greater than 95%, achieving perfect absorption across all three bands.

[0075] from Figure 4 It can be seen that as the incident angle θ increases, the amplitudes of all three absorption peaks decrease, while the center frequencies all exhibit a blue shift. This is because the propagation angle in the dielectric layer... The relationship with the response frequency f satisfies Therefore, as the incident angle θ increases, according to Snell's law, As the angle of incidence decreases, the response frequency f increases, resulting in a blue shift in the absorption peak frequency. Furthermore, the absorption rate of the three absorption peaks remains greater than 90% even at an incident angle of 50°, demonstrating that this structure can be applied to wide-angle microwave absorption.

[0076] from Figure 5 It can be seen that as the order n increases, the number of frequency bands with perfect absorption increases. When n = 8, the number of frequency bands with an absorption rate greater than 90% has reached 5. Therefore, compared with designing multi-band perfect absorbers based on periodic photonic crystals, using quasi-periodic photonic crystals only requires adjusting the order n to obtain more perfect absorption bands, while the design based on periodic photonic crystals requires repeatedly trying different defect layer positions to achieve multi-band absorption effects.

[0077] Therefore, the number of frequency bands of the magnetically tunable multi-band terahertz absorber increases with the order of the Fibonacci quasi-periodic photonic crystal unit. The absorption amplitude of the multi-band can be adjusted by an external magnetic field, the center frequency of the multi-band can be adjusted by the incident angle, and wide-angle absorption can be achieved.

[0078] In summary, this technical solution employs numerical methods to investigate the influence of graphene and a one-dimensional Fibonacci quasi-periodic photonic crystal structure on the absorption characteristics of terahertz waves under magnetic field modulation. This structure achieves perfect absorption across multiple frequency bands and wide angles in the terahertz band. This structure eliminates the need for electrodes on micro / nano devices, allowing for tuning of absorption characteristics through non-contact magnetic field manipulation. Furthermore, this structure eliminates the need for repeatedly designing multiple defect layer locations; the number of frequency bands can be increased simply by changing the order of the quasi-periodic photonic crystal. This design provides a new approach for the development of multi-channel terahertz detectors, sensors, and modulators.

[0079] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A magnetically tunable multi-band terahertz absorber, characterized in that, The magnetically tunable multi-band terahertz absorber comprises, from top to bottom, a graphene layer, a spacer layer, and a Fibonacci quasi-periodic photonic crystal unit. The Fibonacci quasi-periodic photonic crystal unit is formed by alternating stacking of a first dielectric layer and a second dielectric layer in a Fibonacci sequence.

2. The magnetically tunable multi-band terahertz absorber as described in claim 1, characterized in that, The first dielectric layer is magnesium fluoride, the refractive index of the first dielectric layer is 1.38, and the thickness of the first dielectric layer is 18.12 μm.

3. The magnetically tunable multi-band terahertz absorber as described in claim 2, characterized in that, The second dielectric layer is silicon dioxide, the refractive index of the second dielectric layer is 2.25, and the thickness of the second dielectric layer is 11.11 μm.

4. The magnetically tunable multi-band terahertz absorber as described in claim 3, characterized in that, The spacer layer is made of silicon carbide, has a refractive index of 3.59, and a thickness of 6.96 μm.

5. The magnetically tunable multi-band terahertz absorber as described in claim 4, characterized in that, The graphene layer is a single layer of graphene, and the Fermi level E of the graphene layer is... F = -0.34 eV, the scattering rate Γ of the graphene layer is 10 meV / The Fermi rate v of the graphene layer F = 10 6 The speed is m / s, and the thickness of the graphene layer is 0.335 nm.

6. The magnetically tunable multi-band terahertz absorber as described in claim 1, characterized in that, The electrical conductivity of the graphene layer must follow the Drood model.

7. The magnetically tunable multi-band terahertz absorber as described in claim 1, characterized in that, The layered structure of the magnetically tunable multi-band terahertz absorber requires the use of the transfer matrix method to calculate its absorption characteristics.

8. The magnetically tunable multi-band terahertz absorber as described in claim 1, characterized in that, The number of frequency bands of the magnetically tunable multi-band terahertz absorber increases as the order of the Fibonacci quasi-periodic photonic crystal unit increases.

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