阅读说明：本技术 一种实现类电磁诱导透明效应的超材料及方法 (Metamaterial and method for realizing electromagnetic-like induced transparent effect ) 是由 韦中超 孙昊瀛 许宁 于 2019-07-24 设计创作，主要内容包括：本发明涉及一种实现类电磁诱导透明效应的超材料及方法,衬底以及在所述衬底上的周期性的结构子单元,所述结构子单元包括作为暗模谐振器的平行切割线对和作为亮膜谐振器的垂直切割线对,所述平行切割线对和所述垂直切割线对由金属材料制成；在结构子单元和衬底之间设置有两条不同宽度的墨烯带,一条石墨烯带置于亮膜谐振器下实现透射窗的展宽,另一条石墨烯带用于连接所有亮膜谐振器。本发明应用前景广泛。(The invention relates to a metamaterial and a method for realizing an electromagnetic induction-like transparent effect, and further relates to a substrate and a periodic structure subunit on the substrate, wherein the structure subunit comprises a parallel cutting line pair serving as a dark mode resonator and a vertical cutting line pair serving as a bright film resonator, and the parallel cutting line pair and the vertical cutting line pair are made of metal materials; two graphene strips with different widths are arranged between the structural subunit and the substrate, one graphene strip is arranged under the bright film resonator to realize widening of the transmission window, and the other graphene strip is used for connecting all the bright film resonators. The invention has wide application prospect.)

1. A metamaterial for realizing the electromagnetic induction-like transparent effect, which is characterized by comprising a substrate and periodic structural subunits on the substrate, wherein the structural subunits comprise a parallel cutting line pair serving as a dark-mode resonator and a vertical cutting line pair serving as a bright-film resonator, and the parallel cutting line pair and the vertical cutting line pair are made of metal materials; two graphene strips with different widths are arranged between the structural subunit and the substrate, one graphene strip is arranged under the bright film resonator to realize widening of the transmission window, and the other graphene strip is used for connecting all the bright film resonators.

2. Metamaterial for realizing electromagnetic-like induced transparency effects as in claim 1, characterized in that the graphene strips have a surface conductivity σ_gThe calculation formula of (a) is as follows:

wherein e is the electronic charge, K_BIs the boltzmann constant, T is the ambient temperature,is a simplified Planck constant, ω is the angular frequency of the incident light, Γ is the carrier relaxation time, E_FIs the fermi level of graphene.

3. The metamaterial for realizing the electromagnetic-like induced transparent effect as claimed in claim 2, wherein in the terahertz waveband, the Fermi level E of graphene is satisfied according to the Pauli incompatibility principle_F＞＞K_BT andin the case of (2), the surface conductivity σ of the graphene tape_gThe calculation formula of (c) can be simplified as follows:

wherein e is the charge of an electron,is a simplified Planck constant, ω is the angular frequency of the incident light, Γ is the carrier relaxation time, E_FIs the fermi level of graphene.

4. A metamaterial for achieving an electromagnetically-induced-transparency-like effect as claimed in claim 1, wherein the metallic material is aluminum.

5. A metamaterial for achieving an electromagnetically induced transparent effect as claimed in claim 4, wherein the optical properties of the aluminum are represented as follows:

where γ is the damping constant, ω is the angular frequency of the incident light, ω_PIs the plasma frequency.

6. A method for realizing the electromagnetic induction-like transparent effect is characterized in that voltages are applied to the upper pole and the lower pole of the metamaterial according to claim 1, so that the Fermi level of a graphene band is changed, and a destructive interference phenomenon is generated between the parallel cutting line pair and the vertical cutting line pair in the process of changing the Fermi level of the graphene band, so that the electromagnetic induction-like transparent phenomenon is generated.

7. The method for achieving the electromagnetic-like induced transparency effect as claimed in claim 6, wherein the resonance frequency of destructive interference phenomenon generated between the parallel cutting line pair and the perpendicular cutting line pair is 2.14 THz.

8. The method for realizing the electromagnetic-like induced transparency effect according to claim 7, wherein the width of the graphene band which is placed under the bright film resonator to realize the broadening of the transmission window is controlled to change, so that the bandwidth of the transmission window of the transmission spectrum can be continuously changed.

9. The method of achieving an electromagnetically induced transparency effect as claimed in claim 8 wherein the transmission window bandwidth is continuously modulated from 0.4-1.29 THz.

10. The method for realizing the electromagnetic induction-like transparent effect according to claim 7, wherein the fermi level of the graphene is controlled to be increased from 0.4 to 1.2ev, and the spectral extinction ratio can be dynamically adjusted from 32% to 94%.

Technical Field

The invention relates to the technical field of photoelectricity, in particular to a metamaterial and a method for achieving an electromagnetic-like induced transparent effect.

Background

Electromagnetically Induced Transparency (EIT) describes an experimental phenomenon of a sharp transmission window generated in a broad absorption curve, which is accompanied by a change in the dispersion characteristics of light, and thus has many potential applications such as slow light, biochemical sensing, filtering, etc. This phenomenon was originally discovered in experiments with three-level atomic systems and is caused by quantum destructive interference between two different excitation paths. However, since the experiment of the atomic system requires extremely harsh conditions, such as optical pumping and extreme low temperature, the application of EIT is greatly limited. Recently, an analogue of electromagnetically induced transparency (EIT-like) has been proposed, i.e. based on metamaterials to achieve an electromagnetically induced transparency-like effect. The metamaterial based on the EIT-like structure has the advantages of flexible design, easiness in implementation and the like, and can be widely applied to terahertz wave bands.

Although many researches on meta-materials of EIT are carried out at present, the inventor finds that the defects of the prior art in the researches: the transmission window bandwidth of the EIT-like metamaterials is narrow, the application range of the EIT-like metamaterials is severely limited, and the prior art can only adjust the geometric dimension of the metamaterial structure to solve the problem of narrow bandwidth, which is very inconvenient for the practical application of the metamaterial with a fixed structure. Therefore, further research on a simpler and more practical method for realizing the broadening of the transmission window is urgently needed.

Disclosure of Invention

In view of the above, the present invention provides a metamaterial and a method for achieving the electromagnetic-induced transparency-like effect.

A metamaterial for realizing an electromagnetic induction-like transparent effect comprises a substrate and periodic structural subunits on the substrate, wherein the structural subunits comprise a parallel cutting line pair serving as a dark-mode resonator and a vertical cutting line pair serving as a bright-film resonator, and the parallel cutting line pair and the vertical cutting line pair are made of metal materials; two graphene strips with different widths are arranged between the structural subunit and the substrate, one graphene strip is arranged under the bright film resonator to realize widening of the transmission window, and the other graphene strip is used for connecting all the bright film resonators.

Surface conductivity σ of the graphene tape_gThe calculation formula of (a) is as follows:

wherein e is the electronic charge, K_BIs the boltzmann constant, T is the ambient temperature,is a simplified Planck constant, ω is the angular frequency of the incident light, Γ is the carrier relaxation time, E_FIs the fermi level of graphene.

In a terahertz waveband, the Fermi level E of graphene is satisfied according to the Pauli incompatibility principle_F＞＞K_BT andin the case of (2), the surface conductivity σ of the graphene tape_gThe calculation formula of (c) can be simplified as follows:

wherein e is the charge of an electron,is a simplified Planck constant, ω is the angular frequency of the incident light, Γ is the carrier relaxation time, E_FIs the fermi level of graphene.

The metal material is aluminum.

The optical properties of the aluminum are expressed as follows:

where γ is the damping constant, ω is the angular frequency of the incident light, ω_PIs the plasma frequency.

A method for realizing the electromagnetic induction-like transparent effect is characterized in that voltages are applied to the upper pole and the lower pole of the metamaterial, so that the Fermi level of a graphene band is changed, and in the process of changing the Fermi level of the graphene band, a destructive interference phenomenon is generated between the parallel cutting line pair and the vertical cutting line pair, so that the electromagnetic induction-like transparent phenomenon is generated.

The resonance frequency at which destructive interference occurs between the pair of parallel cutting lines and the pair of perpendicular cutting lines is 2.14 THz.

The width change of the graphene band which is placed under the bright film resonator to realize the broadening of the transmission window is controlled, and the continuous change of the bandwidth of the transmission window of the transmission spectrum can be realized.

The transmission window bandwidth is continuously modulated from 0.4-1.29 THz.

The spectral extinction ratio can be dynamically adjusted from 32% to 94% by controlling the Fermi level of the graphene to be increased from 0.4 to 1.2 ev.

The invention has the beneficial effects that:

the metamaterial for realizing the electromagnetic-induction-like transparent effect consists of parallel cutting line pairs, vertical cutting line pairs and two graphene bands with different widths, and realizes continuous modulation of the transmission window bandwidth from 0.4 to 1.29 thz. On the other hand, the adjustable range of the spectral extinction ratio can be realized by adjusting the Fermi level of the graphene. The EIT-like metamaterial provided by the invention has wide application prospect in broadband slow light devices, filters and modulators in terahertz communication.

Drawings

FIG. 1 is a schematic diagram of a metamaterial bright-dark mode structure and a metamaterial overall structure;

FIG. 2 is a graph of graphene conductivity versus Fermi level;

FIG. 3 is a transmission line graph of PCWP, VCWP and overall structure EIT in a metamaterial device;

FIG. 4 is a graph of metamaterial transmission spectra versus graphene bandwidth read w;

FIG. 5 is a schematic representation of the transmission spectra of graphene at different Fermi levels;

FIG. 6 is an electric field distribution of the metamaterial at 0.82THz with graphene Fermi levels at 0.4eV and 1.2 eV;

fig. 7 is a transmission phase shift and group delay for a metamaterial without graphene.

Detailed Description

The invention provides a metamaterial for realizing an electromagnetic-like induced transparency effect, in particular to a mixed metal graphene metamaterial for realizing a transmission window for expanding an EIT (enhanced information technology) under a terahertz spectrum. Compared with the prior metamaterial, the metamaterial has superior performance in the widest bandwidth and adjustable bandwidth range. The metamaterial provides a new way for actively controlling an EIT transmission window, and has wide application prospect in broadband slow-light devices, filters and modulators in terahertz communication.

Fig. 1(a) shows a schematic perspective view of a metamaterial realizing an electromagnetic-like induced transparency effect. His subunit structure consists of a parallel cut line pair (PCWP) as a dark mode resonator and a vertical cut line pair (VCWP) as a bright mode resonator. Metallic aluminum is used as the material of the bright-and-dark mode resonator. Two graphene strips with different widths are laid on the substrate, one graphene strip is placed under the bright-mode resonator to realize widening of the transmission window, and the other graphene strip is used for connecting all subunit structures. All metamaterial structures were laid on a substrate with a relative dielectric constant of 11.7. The thickness of the substrate and the aluminum were 400nm and 200nm, respectively. FIG. 1(b) depicts parameters of an EIT-like metamaterial single substructure. Specifically, in this embodiment, px is 80 microns, py is 120 microns, L is 60 microns, m is 5 microns, w is 4.5 microns, n is 42 microns, s is 4 microns, r is 1 micron, and d is 10 microns.

Aluminum (Al) is selected as a material of the bright-dark mode resonator, and a de-rod model can be described by the following formula in the terahertz waveband:

where γ is the damping constant and ω is the plasma frequency, values of 1.22X 1014rad/s and 2.24X 1016rad/s, respectively. The conductivity of graphene is a major factor affecting its performance. According to the Kubo equation, the conductivity of graphene consists of the in-band conductivity and the inter-band conductivity:

where kb is boltzmann's constant, e is the electronic charge, t is the ambient temperature (t 300K),for the simplified planck constant, ω is the incident plane wave angular frequency. In the formula, the Fermi energy level of graphene is represented by ef, and the average relaxation time is represented by Γ. According to the Pauli incompatibility principle, the in-band partial conductivity of the graphene can be ignored in the terahertz wave band. When graphene satisfies E_F＞＞K_BT andthe conditions can be expressed from time to time by simplified de-rod equations:

the carrier relaxation time is defined asWherein V_FRepresenting the fermi velocity, and μ is the carrier mobility. The conductivity of graphene at different fermi levels is shown in figure 2.

To study the functionality of the proposed EIT metamaterial, we used Finite Difference Time Domain (FDTD) software. In the simulation calculation, the basic settings are as follows: the background index is set to 1.0, the simulated temperature is 300T, periodic boundary conditions are applied in the x and y directions, and a perfectly matched layer is applied in the z direction. FIG. 3(a) shows the transmission spectra of three metamaterials, including PCWP, VCWP and a combination of the two. As can be seen from the figure, the spectral characteristics of the VCWP metamaterial show that the material has a symmetrical Lorentzian type resonance mode, and the center frequency of the material is 2.14 terahertz. In contrast, the PCWP metamaterial has no optical response to incident light, and almost completely transmits and transmits in the terahertz wave band. When plane waves are polarized in the X direction, an EIT-like transmission window with a bandwidth of 0.33Thz can be obtained under the combination of VCWP and PCWP.

To demonstrate the physical mechanism of transmission window formation in EIT metamaterials, we plot the transmission electric field distributions of VCWP, PCWP, and the combined metamaterials at the 2.14thz resonance frequency, as shown in fig. 3 (b-d). FIG. 3(b) shows that strong coupling of VCWP to the incident plane wave can excite part of the surface plasmon (PSP). Fig. 3(c) is an electric field distribution diagram of PCWP, which shows that it is not excited by incident light at a corresponding frequency, since the polarization direction of incident plane wave is symmetric along the X-direction of the structure, resulting in a very weak electric field distribution thereof. From this point we can prove that VCWP and PCWP are light and dark modes, respectively. The dark mode can be excited by the local field induced by the bright mode by near field coupling. Fig. 3(d) depicts the electric field distribution of the EIT metamaterial under 2.14 terahertz. When the PCWP and the VCWP are integrated into the subunit, there is a strong electric field concentration across the PCWP, which indicates that the PCWP becomes unipolar mode upon excitation of the VCWP. It is the destructive interference between monopole and dipole modes that causes the EIT-like effect to occur.

To obtain a wider EIT transmission window we add graphene under VCWP. First we investigated the effect of graphene structure size on the transmission window. The partial structure is shown in fig. 4 (a). Fig. 4(b) shows the transmission spectra of graphene at different w (fermi level fixed at 1.2 ev). As the length of W increases from 0.5 micron to 4.5 microns, the left transmission valley gradually moves towards the blue spectrum, while the right transmission valley does not change substantially. We plot the bandwidth variation of the transmission lines of different lengths w in fig. 4 (c). The bandwidth of the transmission window may be increased from 0.4 to 1.29 thz. It is noted that the amplitude of the transmission window does not change during the modulation.

Next, we investigated the variation of the transmission window of graphene at different fermi levels. FIG. 5 shows a transmission spectrum at a Fermi level of from 0.4 to 1.2 eV. With the addition of graphene and adjustment of its fermi level to 0.4eV, the frequency of the left transmission trough is blue shifted to 0.71thz and the transmission amplitude is 0.64, while the right transmission trough resonance frequency is almost unchanged, compared to the case without graphene. The amplitude of the left and right transmission valleys gradually decreases as the fermi level increases from 0.4ev to 1.2 ev. When the fermi level of the graphene reaches 1.2eV, the maximum modulation depths of the left and right transmission valleys are 85% and 92%, respectively. We evaluated the performance of EIT metamaterial devices with spectral extinction ratios, which can be described as:

S_con＝(T_peak-T_dip)/(T_peak+T_dip)×100％

wherein, T_peakIs the intensity at the transmission peak, T_dipIs the intensity of the transport trough. The spectral extinction ratio can be dynamically adjusted from 32% to 94% by increasing the Fermi level of the graphene during the modulation process. Therefore, the EIT metamaterial provided by the embodiment has wide application prospects in optical filters and modulators.

To explain the physical mechanism behind this new phenomenon, we plot the corresponding transmitted electric field distributions of graphene at 0.4eV and 1.2eV at the 0.82thz resonance frequency. See in particular fig. 6.

As shown in fig. 6(a), when the fermi level of graphene is 0.4eV, energy transfer of current between the VCWP resonator and the PCWP resonator is blocked due to low conductivity of graphene, thereby suppressing formation of EIT, resulting in occurrence of high transmission of left and right transmission valleys. When the fermi level of graphene is increased to 1.2ev, the electric field of VCWP therein is significantly decreased, and the electric field of PCWP is significantly increased. The energy on the bright mode resonator is almost entirely transferred to the dark mode resonator, with a high intensity electric field concentrated at both ends of the dark mode resonator. By adjusting the fermi level of the graphene, active modulation of the spectral extinction ratio can be achieved.

Slow light is produced due to strong dispersion in the transmission window and is one of the most important applications of EIT. The group delay tg, which is used to represent the ability of slow light, can be calculated by the following equation:

here, Φ is the phase change introduced by EIT. Fig. 6 shows the phase change and group delay of EIT metamaterials without graphene. There is a group delay of 3.48ps in the transmission window, indicating that the device we designed performs well in slow light.

In summary, we propose a metamaterial. The graphene band is composed of a parallel cutting line pair (PCWP), a vertical cutting line pair (VCWP) and two graphene bands with different widths. Through simulation calculation, we prove that the EIT metamaterial designed by us realizes continuous modulation of transmission window bandwidth from 0.4 to 1.29 thz. On the other hand, the adjustable range of the spectral extinction ratio can be realized by adjusting the Fermi level of the graphene. Furthermore, we calculate the group delay of the slow light effect. The EIT-like metamaterial designed by us is believed to have wide application prospects in broadband slow-light devices, filters and modulators in terahertz communication.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.