CN112162421A - Reflective broadband adjustable polarization converter based on multilayer graphene-medium composite super surface - Google Patents

Abstract

The invention discloses a reflective broadband adjustable polarization converter based on a multi-layer graphene-medium composite metasurface, which belongs to the fields of novel artificial electromagnetic materials and terahertz science and technology. The polarization converter comprises an upper high-refractive-index medium metasurface array (1), a middle graphene-electrode layer (2), and a lower low-refractive-index base layer (3). The 5-layer graphene films (4) and (6) are respectively placed on both sides of the uniform dielectric spacer thin layer (5), and a controllable bias voltage device (7) is arranged between the upper and lower graphene layers, which together constitute an intermediate layer ( 2). By changing the electrical conductivity of the graphene layer, multiple discrete frequency bands of orthogonal polarization conversion can be moved simultaneously, and an extremely wide dynamic frequency range can be realized. Simple and flexible handling.

Description

A Reflective Broadband Tunable Polarization Converter Based on Multilayer Graphene-Dielectric Composite Metasurface

technical field

The invention relates to a reflection type broadband tunable polarization converter based on a multilayer graphene-medium composite metasurface, and belongs to the fields of novel artificial electromagnetic materials and terahertz science and technology.

Background technique

Metasurfaces are usually two-dimensional planar arrays of periodic or quasi-periodic subwavelength metal/dielectric structured antennas. Different from traditional optical elements relying on the phase accumulation on the propagation path for beam control, the metasurface array changes the resonant response of the antenna by adjusting the shape, parameters and arrangement of the antenna unit, and then adjusts the amplitude, polarization, phase and other properties of the incident light. Precise manipulation can realize many new properties that natural materials do not have, such as high artificial birefringence, so that polarization control devices such as wave plates, polarizers, and complex vector beam generators can be realized by using ultra-thin structures.

The resonant response of the antenna determines that the bandwidth of such devices is often narrow, which is difficult to meet the needs of practical applications. Therefore, it is of great significance to develop broadband and frequency-tunable metasurface polarization devices. By introducing functional materials into metasurfaces that can respond to external excitations such as light, electricity, and heat, a series of bandwidth-tunable metasurface polarizers have been developed in recent years. Graphene has become the main functional material of terahertz tunable metasurface devices because its conductivity can be controlled by an external bias voltage, its response speed is fast, and it supports surface plasmon resonance in the terahertz band.

By patterning graphene or combining it with metal metasurfaces, the electrical conductivity of graphene is controlled by voltage to achieve dynamic control of surface plasmon resonance, resulting in tunable half-wave plates, quarter-wave plates, Polarizers, etc. However, there are three problems: first, the electrical regulation of graphene often requires a high bias voltage; second, the design with static broadband response has a small dynamic frequency range of electrical regulation, while the design with static narrowband response, The dynamic frequency range of electrical regulation is wide, and it cannot take into account the large static bandwidth and the large dynamic bandwidth. Third, due to the loss of metal and graphene itself, the work efficiency is very low. Therefore, the present invention proposes to combine graphene and dielectric metasurface to realize the function of orthogonal polarization transformation, which not only reduces the material loss, but also can adjust the dynamic bandwidth in a wide range by using a lower bias voltage, which is suitable for terahertz spectrum, Systems such as imaging provide practical polarization conversion devices.

SUMMARY OF THE INVENTION

Purpose of the invention: The present invention proposes a solution method for a reflective broadband tunable polarization conversion based on a multi-layer graphene-dielectric composite metasurface with a compact structure.

The present invention is a reflective broadband tunable polarization converter based on a multilayer graphene-dielectric composite metasurface, which is characterized in that the device comprises a three-layer structure, the upper layer is a high-refractive-index dielectric metasurface array (1), and the middle is a graphene- The electrode layer (2), the lower layer is a low refractive index substrate (3), and the 5-layer graphene films (4) and (6) are respectively placed on both sides of the dielectric spacer thin layer (5) for applying a bias voltage structure (7), (4) (5) (6) (7) together constitute the intermediate layer (2).

The object of the present invention is achieved in this way:

Selecting the material, structure, and geometric size of the metasurface unit, when the Fermi energy level of graphene is close to 0 eV, the normally incident linearly polarized light is converted into orthogonal polarization states at multiple frequency points to realize static multi-band polarization transform.

Further, the selection of the material of the dielectric unit described in the present invention requires high refractive index and low absorption coefficient in the working frequency band, for example, a high-resistance silicon material suitable for the terahertz band.

Further, the medium unit structure described in the present invention has anisotropic structures with different electromagnetic responses in the orthogonal polarization directions of the long and short axes, such as rectangular columns, asymmetric cross-shaped columns and split-split ring columns. The long and short axes of the column form an included angle of 45 ^° with the x and y axes, and the polarization direction of the incident ray polarized light is along the x or y axis.

Further, the geometric dimensions of the dielectric unit described in the present invention include the length L, width S, height h ₁ of the rectangular column and the period Λ of the xy plane where it is located. By optimizing the above structural parameters, the reflected light polarized along the long and short axes of the dielectric antenna is made. The phase difference is 180° at multiple frequency points, and the orthogonal polarization conversion of multiple bands is realized.

A bias voltage is applied to the upper and lower graphene layers (4) and (6) through a bias device (7), and the phase of the reflected wave is controlled by adjusting the electrical conductivity, so as to realize the frequency shift of the orthogonal polarization conversion without changing the device structure .

Continue to increase the bias voltage until the frequency shift completely covers the frequency gap of the multi-band, and realize the extremely broadband dynamic tuning of orthogonal polarization conversion.

Further, the uniform dielectric thin layer and the low-refractive-index base material in the present invention are dielectric materials such as PMMA or SiO ₂ that are transparent in the terahertz band.

Further, in the present invention, the bias voltage applied to graphene is used to adjust its carrier concentration, so that the Fermi level of graphene is continuously changed in the range of 0-0.25 eV, and the Fermi level of graphene layer is 0. It is nearly transparent to terahertz waves at eV, and gradually transitions to near-perfect metal at 0.25 eV.

The orthogonal polarization conversion realized by the invention can be dynamically tuned in the broadband range of 0.88 THz-1.81 THz, and the polarization conversion efficiency of more than 85% is maintained, wherein the polarization conversion efficiency is defined as the power of the orthogonal linear polarization state and the total reflected light. power ratio.

Beneficial effects of the present invention: the present invention makes full use of the structural birefringence of the medium metasurface and the abundant electromagnetic resonance modes, and on the basis of the multi-band orthogonal polarization transformation, the phase of the reflected wave is changed by adjusting the electrical conductivity of graphene, In this way, multiple discrete working frequency bands are dynamically moved, and continuous tuning in a very wide frequency range is finally realized, which overcomes the problems of narrow bandwidth and limited tuning range of previous polarizer devices, and the graphene layer has no pattern, and the overall structure is easy to design and process. High efficiency, providing efficient, broadband, and highly integrated polarization conversion devices for terahertz optical systems.

Description of drawings

Fig. 1 is the structure and function schematic diagram of the graphene-dielectric composite metasurface provided in the embodiment of the present invention;

Fig. 2 is the structural unit schematic diagram of the embodiment of the present invention;

Fig. 3 is the variation of the phase difference of the reflected beam polarized along the x-axis in the long and short axis directions with frequency and graphene Fermi level in the embodiment of the present invention;

Fig. 4 is the polarization conversion efficiency (left-axis coordinate system) and the corresponding graphene Fermi level (right-axis coordinate system) at a series of frequency points selected in Fig. 3 .

FIG. 5 is the corresponding reflected wave intensity at a series of frequency points selected in FIG. 3 .

Fig. 6 is the variation of the reflection phase of different Fermi level graphene layers with frequency in the embodiment of the present invention;

In the picture: high refractive index metasurface array (1), graphene-electrode layer (2) (including 5-layer graphene (4), dielectric thin layer (5), 5-layer graphene (6), polarized Set up the voltage control device (7)) and the low refractive index substrate (3).

Detailed ways

The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

A reflective broadband tunable polarization converter based on a multilayer graphene-dielectric composite metasurface, as shown in Figure 1, includes a three-layer structure, an upper rectangular silicon pillar metasurface array layer (1), and a middle graphene-electrode layer (2), the lower glass substrate layer (3). The middle layer (2) is composed of the upper 5-layer graphene (4), the uniform PMMA spaced thin layer (5), and the lower 5-layer graphene (6), and a controllable bias voltage device is arranged between the upper and lower graphene layers (7).

In the structural unit shown in Figure 2, when the Fermi level of graphene is close to 0 eV, the unit period Λ is 130 μm, the thickness h ₁ of the silicon pillar is 37 μm, and the length L of the silicon pillar is 78 μm through parameter scanning. The silicon pillar width S is 35 μm.

As shown in Figure 3, when the graphene Fermi level EF = _0.02eV , this structure makes the reflected light polarized along the long and short axes of the silicon column have a 180° phase difference at three frequency points of 0.92 THz, 1.05 THz and 1.32 THz .

，石墨烯费米能级在0 eV-0.25 eV的调控范 围内需要最高31V的偏置电压。其中，ɛ ₀ 和ɛ _r 为真空中的介电常数和PMMA的相对介电常数，e 和ν _f 为电荷量和费米速率（1.1×10⁶ m/s）。 The thickness t _s of the uniform PMMA spacer thin layer between the upper and lower graphene is 0.1 μm, according to the approximate relationship between the _Fermi level EF and the bias voltage V _g

, the graphene Fermi level requires a bias voltage of up to 31 V in the regulated range of 0 eV-0.25 eV. Among them, ɛ ₀ and ɛ _r are the permittivity in vacuum and the relative permittivity of PMMA, e and ν _f are the charge amount and Fermi rate (1.1×10 ⁶ m/s).

Figure 3 is a graph showing the phase difference of reflected waves polarized along the long and short axes as a function of frequency when the Fermi level of graphene is from 0 to 0.25 eV. The three curves in the figure correspond to a phase difference of about 180°, and the white dots on the curves correspond to A series of selected operating frequency points, these frequency points are respectively located on the three curves, and evenly cover the frequency range of 0.88-1.81THz.

Correspondingly, FIG. 4 shows the polarization conversion rate PCR of x-polarized reflected light relative to y-polarized incident light at a series of frequency points selected in FIG. 3 = |R _xy | ² /|R _xy | ² +|R _yy | ² , where R _xy and R _yy are the reflection coefficients of the x- and y-polarized reflected light, respectively. Through the adjustment of the corresponding Fermi level in the figure, the polarization conversion efficiency is greater than 85% in the frequency range of 0.88-1.81 THz.

Correspondingly, with the adjustment of the graphene Fermi level, the intensity of the reflected light also changes. Figure 5 shows the intensity of the reflected light relative to the incident light at a series of frequency points selected in Figure 3, which fluctuates between 0.3 and 0.92 .

Figure 6 is a separate study of the reflection phase of the graphene layer. When the Fermi level of graphene is 0 eV, the reflection phase is -157° at 0.8 THz, and increases with the increase of frequency. As the Fermi level increases, graphene tends to be a perfect metal, and the reflection phase gradually approaches -180°. Due to the different reflection phases of graphene at different Fermi levels, the three frequency points that undergo orthogonal polarization transformation move to high frequencies at the same time, finally covering a broadband frequency range of 0.88-1.81 THz.

Claims (8)

1. A reflection-type broadband adjustable polarization converter based on a multi-layer graphene-medium composite super surface is characterized in that a polarization controller comprises a three-layer structure, wherein the upper layer is a high-refractive-index medium super surface array (1), the middle layer is a graphene-electrode layer (2), the lower layer is a low-refractive-index substrate (3), 5-layer graphene films (4) and (6) are respectively arranged on two sides of a uniform medium interval thin layer (5) to form a bias voltage structure (7), and the middle layer (2) is formed together.

2. The reflective broadband adjustable polarization converter based on the multilayer graphene-dielectric composite super surface according to claim 1, wherein the dielectric super surface is a two-dimensional dielectric cylinder antenna array with sub-wavelength periodic arrangement, and the selected material has a high refractive index and a small absorption coefficient in an operating frequency band, for example, a high-resistance silicon material suitable for terahertz waveband.

3. The reflective broadband tunable polarization converter based on the multilayer graphene-dielectric composite super-surface according to claim 1, wherein the dielectric units have anisotropic structures with different electromagnetic responses in the orthogonal directions of the long axis and the short axis, such as rectangular columns, asymmetric cross-shaped columns, split ring columns, and the like.

4. The reflective broadband tunable polarization converter based on the multilayer graphene-dielectric composite super surface as claimed in claim 1, wherein the reflected light polarized along the long and short axes of the antenna is 180 ° out of phase at a plurality of discrete frequency points by selecting a suitable antenna size under the condition that no voltage is applied to the graphene.

5. The reflective broadband tunable polarization converter based on the multilayer graphene-dielectric composite super surface as claimed in claim 1, wherein the polarization direction of the incident linearly polarized light forms 45 degrees with the long and short axes of the dielectric antenna as claimed in claim 3^oAnd (4) an included angle.

6. The reflective broadband tunable polarization converter based on the multi-layer graphene-dielectric composite super surface of claim 1, wherein the uniform dielectric thin layer and the substrate material between the upper graphene layer and the lower graphene layer are PMMA or SiO₂Is transparent in terahertz wavebandThe low refractive index dielectric material of (1).

7. The reflective broadband tunable polarization converter based on the multilayer graphene-dielectric composite super-surface as claimed in claim 1, wherein the bias voltage adjusts the carrier concentration of the graphene, so that the fermi level of the graphene is changed within a range of 0-0.25 eV.

8. The reflective broadband-adjustable polarization converter based on the multilayer graphene-dielectric composite super-surface according to claim 1, wherein a reflected wave propagation phase is dynamically compensated through voltage adjustment of graphene conductivity, so that discrete frequency points of orthogonal polarization conversion move in the same direction, and finally, an extremely wide dynamic frequency range is completely covered, and broadband-adjustable polarization conversion is realized.

Images