Dynamically Tunable Triple-Band Terahertz Perfect Absorber Based on Graphene Metamaterials

Abstract

The present invention discloses a dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials, which is formed by periodically arranging M×N absorption units with a multi-layer structure in an array. The absorption units are distributed in a two-dimensional M×N pattern, where both M and N are positive integers. The absorption unit includes a metal substrate, a first dielectric layer, a graphene square ring, a second dielectric layer, and a graphene disk, which are arranged in sequence from bottom to top. The metal substrate, the first dielectric layer, and the second dielectric layer are all cuboid-shaped structures with a square cross-section in the horizontal direction, and the side lengths of the square cross-sections of the three are equal. The graphene square ring is a square ring structure, and its outer side length is smaller than the side length of the metal substrate, the first dielectric layer, or the second dielectric layer. The diameter of the graphene disk is smaller than the inner side length of the graphene square ring. The terahertz perfect absorber of the present invention can achieve high absorption at three frequency points simultaneously, and has the characteristics of dynamic tunability, polarization insensitivity, and wide-angle absorption, with a simple structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz wave absorbers, and relates to a dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials. Background Art

[0002] Terahertz (THz) waves generally refer to electromagnetic waves with frequencies in the range of 0.1 - 10 THz. Due to their low energy, strong penetrability, and unique frequency and wavelength, the applications of terahertz waves and related technologies have become research hotspots in various fields. As a basic functional device for terahertz applications, terahertz absorbers are widely used in detectors, spectral imaging, stealth, and other fields.

[0003] The terahertz absorber based on metamaterials is a device that can efficiently absorb incident terahertz waves. The main principle is to utilize different loss mechanisms to convert terahertz waves into heat energy or other forms of energy, ultimately achieving the purpose of absorbing terahertz waves. Among various absorbers, absorbers based on metamaterial structures have received extensive attention from domestic and foreign scholars in recent years. Compared with traditional absorbers, metamaterial absorbers have the characteristics of small volume, high absorption rate, and easy integration. Existing metamaterial absorbers use a stacked multi-layer structure or multiple resonant structures with different sizes in the same plane to achieve multi-frequency absorption. The structure is complex, the manufacturing and processing are difficult, and once the device is determined, its absorption function is not adjustable, which restricts the application of the absorber. Therefore, there is an urgent need to develop a terahertz multi-frequency narrowband absorber with a simple structure, convenient manufacturing, and flexible tunability to meet the actual needs of terahertz applications. Summary of the Invention

[0004] The purpose of the present invention is to provide a dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials, which can simultaneously achieve high absorption, dynamic tunability, polarization insensitivity, wide-angle absorption, and a simple structure at three frequency points.

[0005] The technical solution adopted by the present invention is that the dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials is formed by periodically arranging M×N absorbing units with multi-layer structures in an array, and the absorbing units are distributed in a two-dimensional M×N pattern, where both M and N are positive integers;

[0006] The absorbing unit includes a metal substrate, a first dielectric layer, a graphene square ring, a second dielectric layer, and a graphene disk, which are arranged in sequence from bottom to top;

[0007] The metal substrate, the first dielectric layer, and the second dielectric layer are all cuboid-shaped structures with a square cross-section in the horizontal direction, and the side lengths of the square cross-sections of the three are equal; the graphene square ring is a square-ring structure, and its outer side length is smaller than the side length of the metal substrate, the first dielectric layer, or the second dielectric layer; the diameter of the graphene disk is smaller than the inner side length of the graphene square ring.

[0008] The features of the present invention also lie in:

[0009] The material of the metal substrate is gold, with a conductivity of 4.56×10 7 S / m, the side length is 8 - 10 μm, and the thickness is 0.2 - 0.5 μm.

[0010] The material of the first dielectric layer is silicon dioxide, with a relative dielectric constant of 3.9, the side length is 8 - 10 μm, and the thickness is 8.5 - 9 μm.

[0011] The material of the second dielectric layer is silicon dioxide, with a relative dielectric constant of 3.9, the side length is 8 - 10 μm, and the thickness is 3 - 3.4 μm.

[0012] The graphene square ring is a single-layer graphene, with a thickness of 0.34 nm, an outer side length of 7.2 - 7.8 μm, and a width of 0.8 - 1.4 μm.

[0013] The graphene disk is a single-layer graphene, with a thickness of 0.34 nm and a radius of 1.6 - 2.2 μm.

[0014] The beneficial effects of the present invention are:

[0015] (1) The absorption unit of the absorber of the present invention is composed of a metal substrate, a first dielectric layer, a graphene square ring, a second dielectric layer, and a graphene disk. By using two layers of graphene patterning structures, the impedance matching is improved, the frequency selectivity is enhanced, and the band absorption rate is increased. There are three perfect absorption peaks at 1.23 THz, 4.2 THz, and 6.38 THz for this absorber, and the absorption rates reach 99.9%, 99.8%, and 98.3% respectively;

[0016] (2) The absorber of the present invention can change the chemical potential of graphene by changing the external bias voltage of the graphene square ring and the graphene disk, thereby changing the conductivity of graphene, and further adjusting the impedance characteristics of the entire terahertz absorber, so that the center frequencies and absorption rates of the three absorption peaks can be dynamically tuned;

[0017] (3) The absorber of the present invention has polarization insensitivity and can maintain excellent and stable absorption performance for both TE waves (transverse electric waves) and TM waves (transverse magnetic waves) at a wide range of incident angles. Description of the Drawings

[0018] Figure 1It is a schematic diagram of the periodic structure of the absorber of the present invention;

[0019] Figure 2 It is a schematic diagram of the unit structure of the absorber of the present invention;

[0020] Figure 3 It is a schematic diagram of the structure of the graphene square ring in the absorber of the present invention;

[0021] Figure 4 It is a schematic diagram of the structure of the graphene disk in the absorber of the present invention;

[0022] Figure 5 It is the chemical potential μ of the graphene square ring provided in Embodiment 1 of the present invention c1 is 0.7 eV, and the chemical potential μ of the graphene disk c2 is 0.6 eV, and it is the absorption spectrum diagram of the terahertz absorber;

[0023] Figure 6 It is the electric field distribution of the absorber provided in Embodiment 1 of the present invention at three central frequencies. Among them, Figures a-c are the top view of the graphene disk, the top view of the graphene square ring, and the side view at 1.23 THz, respectively; Figures d-f are the top view of the graphene disk, the top view of the graphene square ring, and the side view at 4.2 THz, respectively; Figures g-i are the top view of the graphene disk, the top view of the graphene square ring, and the side view at 6.38 THz, respectively;

[0024] Figure 7 It is the influence diagram of the chemical potential μ of the graphene disk in the absorber provided in Embodiment 1 of the present invention c2 on the absorption spectrum;

[0025] Figure 8 It is the influence diagram of the chemical potential μ of the graphene square ring in the absorber provided in Embodiment 1 of the present invention c1 on the absorption spectrum;

[0026] Figure 9 It is the influence diagram of the change of the incident wave polarization angle on the absorption spectrum in Embodiment 1 of the present invention;

[0027] Figure 10 It is the influence diagram of the change of the incident angle in the TE mode on the absorption spectrum in Embodiment 1 of the present invention;

[0028] Figure 11 It is the influence diagram of the change of the incident angle in the TM mode on the absorption spectrum in Embodiment 1 of the present invention. Detailed implementation manners

[0029] The present invention will be described in detail below with reference to the accompanying drawings and specific implementation manners.

[0030] The present invention is a dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials, such asFigure 1 As shown, it is composed of a periodic array of M×N tunable triple-band narrowband terahertz absorption units with a multi-layer structure. The absorption units are distributed in a two-dimensional M×N pattern, where both M and N are positive integers. As Figure 1 and Figure 2 shown, each absorption unit includes a metal substrate 1, a first dielectric layer 2, a graphene square ring 3, a second dielectric layer 4, and a graphene disk 5 arranged in sequence from bottom to top.

[0031] The metal substrate 1 and the two dielectric layers are all cuboid structures with a square cross-section in the horizontal direction, and the side lengths of the square cross-sections of the three are equal, and they serve as the period of the absorber unit. Among them, the material of the metal substrate 1 is gold, and the conductivity is 4.56×10 7 S / m, the side length is 8 - 10 μm, and the thickness t is 0.2 - 0.5 μm. The materials of the two dielectric layers are both silicon dioxide, the relative dielectric constant is 3.9, the side lengths are both 8 - 10 μm, the thickness H1 of the first dielectric layer 2 is 8.5 - 9 μm, and the thickness H2 of the second dielectric layer 4 is 3 - 3.4 μm.

[0032] Both the graphene square ring 3 and the graphene disk 5 are single-layer graphene, and their thicknesses are both 0.34 nm;

[0033] As Figure 3 shown, the graphene square ring 3 is a square ring structure, and its outer side length a is less than the unit period (P x or P y ), which is 7.2 - 7.8 μm, and the width d is 0.8 - 1.4 μm; as Figure 4 shown, the diameter of the graphene disk 5 is less than the inner side length a - 2d of the graphene square ring 3, and its radius r is 1.6 - 2.2 μm. The chemical potential of the graphene square ring 3 is μ c1 , the chemical potential of the graphene disk 5 is μ c2 , and the chemical potential of graphene can be adjusted by an external bias voltage, which is the basis for the dynamic tuning ability of the proposed absorber.

[0034] Example 1:

[0035] As Figures 1 - 4 shown, the terahertz absorber in this example includes 3×3 absorption units. The number of units does not affect the overall absorption performance. Each absorption unit includes a metal substrate 1, a first dielectric layer 2, a graphene square ring 3, a second dielectric layer 4, and a graphene disk 5 arranged in sequence from bottom to top.

[0036] Both the graphene square ring 3 and the graphene disk 5 are single-layer graphene, and their thicknesses are both 0.34 nm; the outer side length a of the graphene square ring 3 is 7.4 μm, the width d of the square ring is 1 μm; the radius r of the graphene disk 5 is 1.8 μm. The chemical potential μ of the graphene square ring 3 c1is 0.7 eV, and the chemical potential μ of the graphene disk 5 c2 is 0.6 eV.

[0037] The materials of the two dielectric layers are both silicon dioxide, with a relative dielectric constant of 3.9. The side lengths of both are 8 μm. The thickness H1 of the first dielectric layer 2 is 8.9 μm, and the thickness H2 of the second dielectric layer 4 is 3.2 μm.

[0038] The material of the metal substrate 1 is gold, with a conductivity of 4.56×10 7 S / m. The side length of the metal substrate 1 is 8 μm, and the thickness t is 0.2 μm, which is much larger than the skin depth of the metal in the terahertz frequency band, ensuring that the transmittance T of the absorber is 0.

[0039] Figure 5 This is the absorption spectrum of the terahertz absorber in this embodiment. It can be seen that under normal incidence, the absorption rates of the absorber at three central frequencies of 1.23 THz, 4.2 THz, and 6.38 THz are 99.9%, 99.8%, and 98.3% respectively, approaching perfect absorption.

[0040] In order to further study the mechanism of each absorption peak of this triple-frequency terahertz absorber, the electric field distribution of the absorber at each central frequency was tested, and the results are as Figure 6 shown. It can be seen from Figures a and b that at the central frequency of 1.23 THz, the electric field of the graphene disk layer is concentrated at the upper and lower ends of the graphene disk, but the intensity is very low; the graphene square ring layer undergoes dipole resonance, and the electric field is concentrated at the upper and lower ends of the graphene square ring, and the intensity is much greater than that of the graphene disk; therefore, the first absorption peak of the absorber is mainly caused by the dipole resonance of the graphene square ring. It can be seen from Figures d and e that at the central frequency of 4.2 THz, the electric field intensity of the graphene disk layer approaches 0, and the electric field is concentrated in the graphene square ring layer, and the graphene square ring layer undergoes high-order resonance; therefore, the second absorption peak of the absorber is caused by the high-order resonance of the graphene square ring. It can be seen from Figures g and h that at the central frequency of 6.38 THz, the graphene disk layer undergoes dipole resonance, and the electric field is mainly distributed at the upper and lower ends of the graphene disk, while the graphene square ring layer undergoes high-order resonance, and the two effects are superimposed to jointly form the third absorption peak. In addition, it can be seen from the side views c, f, and i that the graphene material excites surface plasmon polaritons (SPPs) at the central frequency, improving the absorption rate and making the absorber achieve almost perfect absorption at the central frequency.

[0041] In order to study the dynamic tuning ability of the absorber, Figure 7 、 Figure 8 shows the influence of the graphene chemical potential on the absorption spectrum of the absorber. It can be seen from Figure 7 that the chemical potential μ of the graphene disk c2When varying between 0.4 - 0.7 eV, both the first absorption peak and the second absorption peak have relatively fixed absorption frequencies and extremely high absorption rates, and are hardly affected by μ c2 . For the third absorption peak, except that the third absorption peak can achieve a perfect absorption of 99.9% at 0.6 eV, the absorption rate of the third absorption peak at other chemical potentials is at most no more than 70%, and there are secondary peaks. It can also be seen from this that μ c2 mainly affects the third absorption peak, which is consistent with the previous analysis of the electric field distribution and surface current distribution. In order to make the absorber exhibit the characteristics of triple - frequency perfect absorption, the chemical potential μ c2 of the graphene disk is fixed at 0.6 eV. Figure 8 shows the influence of the chemical potential μ c1 of the graphene square ring on the absorption spectrum. μ c1 starts from 0.6 eV and increases to 0.9 eV in steps of 0.1 eV. The absorption rates of each absorption peak are as indicated in Figure 8 . All three absorption peaks have the highest absorption rate when μ c1 = 0.7 eV, and then gradually decrease. Among them, the difference in absorption rate between the first absorption peak and the third absorption peak is more obvious between each chemical potential, while the difference in absorption rate of the second absorption peak between each chemical potential is not large. From 0.6 eV to 0.9 eV, the central frequencies of the first absorption peak are 1.25 THz, 1.32 THz, 1.38 THz, 1.42 THz in sequence; the central frequencies of the second absorption peak are 4.02 THz, 4.34 THz, 4.64 THz, 4.9 THz in sequence; the central frequencies of the third absorption peak are 6.33 THz, 6.34 THz, 6.37 THz, 6.39 THz in sequence. All three absorption peaks have a blue - shift in the central frequency as μ c1 increases. Among them, the most obvious frequency - shift amplitude between each chemical potential is the second absorption peak, followed by the first absorption peak, and the smallest frequency - shift amplitude is the third absorption peak. In summary, on the basis of fixing the chemical potential μ c2 of the graphene disk at 0.6 eV to make the absorber exhibit triple - frequency absorption, by changing the chemical potential of the graphene square ring, the central frequencies and absorption rates of the three absorption peaks can be adjusted. Therefore, this absorber has the characteristic of dynamic tunability.

[0042] As Figure 9 shown, the absorption spectrum of the terahertz perfect absorber in this embodiment is independent of the incident polarization angle ψ under normal incidence of electromagnetic waves, showing the characteristic of polarization insensitivity. This polarization insensitivity is attributed to the fact that the metasurface structure is rotationally symmetric.

[0043] Figure 10 、 Figure 11The figures respectively show the influence of the incident angle change on the absorption spectrum in the TE (transverse electric wave) and TM (transverse magnetic wave) modes. It can be seen that for TE polarization, within the range of 0 - 35° of the incident angle, the absorption spectrum does not change with the change of the incident angle; when the incident angle further increases, the absorption rate of the third absorption peak decreases rapidly; after the incident angle exceeds 60°, the absorption rate of the first absorption peak decreases; the second absorption peak maintains a high absorption rate within 0 - 80°, but the central frequency shows a blue shift. For TM polarization, within the range of 0 - 40° of the incident angle, the three absorption peaks are not affected by the change of the incident angle; when the incident angle continues to increase, the central frequencies of the first absorption peak and the third absorption peak show a slight blue shift phenomenon, and the central frequency of the second absorption peak remains unchanged; when the incident angle further increases, the absorption rates of the second absorption peak and the third absorption peak decrease, and the absorption rate of the first absorption peak can maintain a high absorption rate within 0 - 80°. In summary, the absorber of the present invention has the characteristics of polarization insensitivity and supporting large-angle incidence.

[0044] Example 2:

[0045] The terahertz absorber in this example includes 3×3 absorption units. The number of units does not affect the overall absorption performance. Each absorption unit includes a metal substrate 1, a first dielectric layer 2, a graphene square ring 3, a second dielectric layer 4, and a graphene disk 5 arranged in sequence from bottom to top.

[0046] Both the graphene square ring 3 and the graphene disk 5 are single-layer graphene, and their thicknesses are both 0.34 nm; the outer side length a of the graphene square ring 3 is 7.2 μm, and the ring width d is 0.8 μm; the radius r of the graphene disk 5 is 1.6 μm. The chemical potential μ of the graphene square ring 3 c1 is 0.7 eV, and the chemical potential μ of the graphene disk 5 c2 is 0.6 eV.

[0047] The materials of the two dielectric layers are both silicon dioxide, and the relative dielectric constant is 3.9; the side lengths are both 8 μm, the thickness H1 of the first dielectric layer 2 is 8.5 μm, and the thickness H2 of the second dielectric layer 4 is 3 μm.

[0048] The material of the metal substrate 1 is gold, and the conductivity is 4.56×10 7 S / m. The side length of the metal substrate 1 is 8 μm, and the thickness t is 0.2 μm, which is much larger than the skin depth of the metal in the terahertz frequency band, ensuring that the transmittance T of the absorber is 0.

[0049] Example 3:

[0050] The terahertz absorber in this example includes 3×3 absorption units. The number of units does not affect the overall absorption performance. Each absorption unit includes a metal substrate 1, a first dielectric layer 2, a graphene square ring 3, a second dielectric layer 4, and a graphene disk 5 arranged in sequence from bottom to top.

[0051] Both the graphene square ring 3 and the graphene disk 5 are single-layer graphene, and their thicknesses are both 0.34 nm; the outer side length a of the graphene square ring 3 is 7.8 μm, and the ring width d is 1.4 μm; the radius r of the graphene disk 5 is 2.2 μm.

[0052] The materials of the two dielectric layers are both silicon dioxide, and the relative dielectric constant is 3.9; the side lengths are both 10 μm, the thickness H1 of the first dielectric layer 2 is 9 μm, and the thickness H2 of the second dielectric layer 4 is 3.4 μm.

[0053] The material of the metal substrate 1 is gold, and the conductivity is 4.56×10 7 S / m, the side length of the metal substrate 1 is 10 μm, and the thickness t is 0.5 μm, which is much larger than the skin depth of the metal in the terahertz frequency band, ensuring that the transmittance T of the absorber is 0.

[0054] Example 4:

[0055] The terahertz absorber in this example includes 3×3 absorption units. The number of units does not affect the overall absorption performance. Each absorption unit includes a metal substrate 1, a first dielectric layer 2, a graphene square ring 3, a second dielectric layer 4, and a graphene disk 5 arranged in sequence from bottom to top.

[0056] Both the graphene square ring 3 and the graphene disk 5 are single-layer graphene, and their thicknesses are both 0.34 nm; the outer side length a of the graphene square ring 3 is 7.2 μm, and the ring width d is 0.8 μm; the radius r of the graphene disk 5 is 1.6 μm.

[0057] The materials of the two dielectric layers are both silicon dioxide, and the relative dielectric constant is 3.9; the side lengths are both 10 μm, the thickness H1 of the first dielectric layer 2 is 9 μm, and the thickness H2 of the second dielectric layer 4 is 3.4 μm.

[0058] The material of the metal substrate 1 is gold, and the conductivity is 4.56×10 7 S / m, the side length of the metal substrate 1 is 10 μm, and the thickness t is 0.5 μm, which is much larger than the skin depth of the metal in the terahertz frequency band, ensuring that the transmittance T of the absorber is 0.

[0059] Example 5:

[0060] The terahertz absorber in this example includes 3×3 absorption units. The number of units does not affect the overall absorption performance. Each absorption unit includes a metal substrate 1, a first dielectric layer 2, a graphene square ring 3, a second dielectric layer 4, and a graphene disk 5 arranged in sequence from bottom to top.

[0061] Both the graphene square loop 3 and the graphene disk 5 are single-layer graphene, and their thicknesses are both 0.34 nm; the outer side length a of the graphene square loop 3 is 7.8 μm, and the loop width d is 1.4 μm; the radius r of the graphene disk 5 is 2.2 μm.

[0062] The materials of the two dielectric layers are both silicon dioxide, and the relative dielectric constant is 3.9; the side lengths are both 8 μm, the thickness H1 of the first dielectric layer 2 is 8.5 μm, and the thickness H2 of the second dielectric layer 4 is 3 μm.

[0063] The material of the metal substrate 1 is gold, and the conductivity is 4.56×10 7 S / m. The side length of the metal substrate 1 is 8 μm, and the thickness t is 0.2 μm, which is much larger than the skin depth of the metal in the terahertz frequency band, ensuring that the transmittance T of the absorber is 0.

[0064] The present invention is a dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials. By optimizing the structural parameters, good absorption performance is obtained at the three central frequencies of 1.23 THz, 4.2 THz, and 6.38 THz respectively, and the absorption spectrum can be dynamically tuned by changing the chemical potential of graphene, with polarization-insensitive and wide-angle absorption performance. The absorber of the present invention has good absorption performance, can be flexibly tuned, and has potential application value.

Claims

1. A dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterials, characterized in that, The absorber is formed by periodically arranging M×N absorbing units with a multi-layer structure in an array, and the absorbing units are distributed two-dimensionally in M×N, where both M and N are positive integers; The absorbing unit includes a metal substrate (1), a first dielectric layer (2), a graphene square ring (3), a second dielectric layer (4), and a graphene disk (5) arranged in sequence from bottom to top; The metal substrate (1), the first dielectric layer (2), and the second dielectric layer (4) are all cuboid structures with a square cross-section in the horizontal direction, and the side lengths of the square cross-sections of the three are equal; the graphene square ring (3) is a square ring structure, and its outer side length is smaller than the side length of the metal substrate (1), the first dielectric layer (2), or the second dielectric layer (4); the diameter of the graphene disk (5) is smaller than the inner side length of the graphene square ring (3); The material of the first dielectric layer (2) is silicon dioxide, with a relative dielectric constant of 3.9, a side length of 8 - 10 µm, and a thickness of 8.5 - 9 µm; The material of the second dielectric layer (4) is silicon dioxide, with a relative dielectric constant of 3.9, a side length of 8 - 10 µm, and a thickness of 3 - 3.4 µm; The graphene square ring (3) is a single-layer graphene with a thickness of 0.34 nm, an outer side length of 7.2 - 7.8 µm, and a width of 0.8 - 1.4 µm; The graphene disk (5) is a single-layer graphene with a thickness of 0.34 nm and a radius of 1.6 - 2.2 µm.

2. The dynamically tunable triple-band terahertz perfect absorber based on graphene metamaterial according to claim 1, characterized in that, The material of the metal substrate (1) is gold, and its conductivity is 4.56×10 7 S / m. The side length is 8 - 10 µm, and the thickness is 0.2 - 0.5 µm.

Images