CN110794596A

CN110794596A - Reflection type broadband polarization controller based on graphene-medium composite super surface

Info

Publication number: CN110794596A
Application number: CN201911111543.XA
Authority: CN
Inventors: 关胜男; 程洁嵘; 陈铁红; 常胜江
Original assignee: Nankai University
Current assignee: Nankai University
Priority date: 2019-06-18
Filing date: 2019-11-14
Publication date: 2020-02-14
Anticipated expiration: 2039-11-14
Also published as: CN110794596B

Abstract

The invention relates to a graphene-medium composite super-surface-based reflective broadband dynamic polarization controller, and belongs to the technical field of novel artificial electromagnetic materials and terahertz science. The polarization controller comprises four layers of structures, wherein the lowermost layer is a metal substrate (1), the middle layer is a low-refractive-index medium (2), the uppermost layer is a high-refractive-index medium super-surface array (3) embedded in the low-refractive-index medium (2), the upper surface of the super-surface array is exposed in the air, and uniform single-layer graphene (4) is arranged between the low-refractive-index medium layer and the high-refractive-index medium super-surface. A bias voltage control device (5) is arranged between the graphene layer (4) and the metal substrate (1), and linearly polarized light is converted into orthogonal linearly polarized light or circularly polarized light within a certain frequency range by changing the conductivity of graphene. The invention integrates the double functions of the half-wave plate and the quarter-wave plate, and has the advantages of high efficiency, simple structure, easy integration and the like.

Description

Reflection type broadband polarization controller based on graphene-medium composite super surface

Technical Field

The invention relates to a graphene-dielectric composite super-surface-based reflective broadband polarization controller, and belongs to the technical field of novel artificial electromagnetic materials and terahertz science.

Background

A metasurface is a periodic or quasi-periodic two-dimensional array of sub-wavelength scale artificial building units. The amplitude, phase and polarization of scattered light are accurately controlled by changing the size, shape, direction and the like of the unit, and key functions such as beam steering, focusing, holographic imaging, polarization control, optical stealth and the like are realized in the ultrathin structure. Compared with the traditional curved surface element depending on phase accumulation, the super-surface device has flexible design, high integration level and easy processing. In order to focus the beam to sub-wavelength dimensions and control the resonance frequency by structural parameters, metal surface plasmonic super-surfaces and high refractive index dielectric super-surfaces become two main types of super-surfaces. However, these super-surfaces are often single in function and even impossible to dynamically adjust.

The conductivity of the graphene can be controlled by an external bias voltage, and a wave beam is localized in a scale range far smaller than the wavelength through surface plasma resonance in a terahertz to infrared light wave band, so that a new opportunity is provided for the development of the adjustable super-surface device. By patterning the graphene layer or attaching the graphene layer to the metal super surface, the electromagnetic response of the artificial unit can be dynamically controlled by using voltage, so that a modulator, a dynamic phase shifter, a polarization controller and the like are realized. The common problems of low efficiency and large loss of the metal super surface are still accompanied with the dynamic response process.

The graphene and the dielectric super-surface are combined, the tunable characteristic is endowed while the advantages of high dielectric super-surface efficiency, rich electromagnetic resonance modes and the like are continued, and a novel dynamic super-surface device is expected to be developed by utilizing the interaction between the graphene and the dielectric resonance modes, so that a highly integrated multifunctional dynamic regulation device is provided for terahertz and infrared optical systems.

Disclosure of Invention

The purpose of the invention is as follows: the invention provides a graphene-medium composite super-surface polarization controller, which changes the conductivity of graphene through bias voltage and realizes the dynamic switching of functions of a reflection type half-wave plate and a quarter-wave plate in a certain frequency range.

The technical scheme is as follows: the invention relates to a reflection type broadband polarization controller based on a graphene-medium composite super surface, which is characterized by comprising a four-layer structure, wherein the lowest layer is a metal substrate (1), the middle layer is a low-refractive-index medium layer (2), the uppermost layer is a high-refractive-index medium super surface array (3) embedded in the low-refractive-index medium (2), the upper surface of the super surface array is exposed in the air, and uniform single-layer graphene (4) is arranged between the low-refractive-index medium layer (2) and the high-refractive-index medium super surface (3). A bias voltage control device (5) is arranged between the graphene layer (4) and the metal substrate (1).

Further, the super surface of the medium is a two-dimensional medium column antenna array periodically arranged in an x-y plane, the arrangement period is smaller than the working wavelength, and the period refers to the distance between the geometric centers of two adjacent medium columns on a two-dimensional axis.

Furthermore, the material selection of the dielectric column in the invention needs to be high in refractive index and small in absorption coefficient in the working frequency band, for example, the dielectric column is suitable for a high-resistance silicon material of a terahertz wave band.

Furthermore, the shape of the medium column has anisotropy, and the medium column comprises a rectangular column, an elliptical column, an asymmetric cross-shaped column, an open split ring column and the like, and has different electromagnetic responses in two orthogonal polarization directions of a long axis and a short axis, and equivalent birefringence characteristics. Further, the long and short axes of the dielectric column form 45 degrees with the x and y axes^oThe angle, the polarization direction of the incident linearly polarized light is along the x or y axis.

Furthermore, the low-refractive-index dielectric material is PMMA or SiO₂A dielectric material having an iso-refractive index of about 1.5.

Further, the metal substrate material in the invention is gold, silver or copper.

The bias voltage applied to the graphene is used for adjusting the carrier concentration of the graphene, so that the Fermi level of the graphene is continuously changed within the range of 0-0.5eV, and the conductivity of the graphene is further controlled.

The graphene is almost transparent to terahertz waves when the Fermi level is 0 eV, and gradually approaches to perfect metal along with the increase of the Fermi level.

By optimizing the thickness of the low-refractive-index dielectric layer and the length, width and height of the high-refractive-index dielectric antenna, the Fermi level of the graphene is E_F1If the difference is not less than 0 eV, the amplitudes of the reflected lights polarized along the major and minor axes of the dielectric cylinder antenna are made equal to each other, and the phase difference is pi, thereby realizing a reflective half-wave plate in a frequency range as wide as possible.

Gradually increasing the Fermi level of graphene to E_F2The amplitudes of the reflected light polarized along the long axis and the short axis of the dielectric column antenna are equal, the phase difference is 1.5 pi or 0.5 pi, the function of a reflective quarter-wave plate is realized, and the reflected wave is converted into a circular polarization state from a linear polarization state.

Controlling the bias voltage to enable the Fermi level of the graphene to be E_F1Or E_F2The dynamic switching of the half-wave plate and the quarter-wave plate can be realized on the premise of not changing the structure of the device.

The half-wave plate and the quarter-wave plate have a common working frequency band.

The half-wave plate and the quarter-wave plate can keep the polarization conversion efficiency of more than 70% in a certain frequency range, wherein the polarization conversion efficiency is defined as the ratio of the power of a target polarization state to the incident light power.

The invention has the beneficial effects that: the graphene layer has no pattern, and the whole structure is easy to design and process; the electromagnetic response of the dielectric resonance unit is regulated and controlled by the graphene, the technical problem that the existing polarization control super-surface is single in function and cannot be tuned is solved, and a multifunctional polarization control device which is efficient, wide in bandwidth, high in integration degree and adjustable and controllable is provided for terahertz and infrared optical systems.

Drawings

Fig. 1 is a schematic structural and functional diagram of a graphene-dielectric composite super surface provided by an embodiment of the present invention;

FIG. 2 is a schematic diagram of the structural elements of an embodiment of the present invention;

FIG. 3 is the reflection coefficient and phase difference of the embodiment of the invention when the Fermi level of the graphene is 0 eV, and the polarization is along the long and short axes;

FIG. 4 is a rotation angle for linearly polarized incident light in the y-direction at a graphene Fermi level of 0 eV according to an embodiment of the present invention;

FIG. 5 is the reflection coefficient and phase difference of the embodiment of the invention when the Fermi level of the graphene is 0.2 eV, and the polarization is along the long and short axes;

FIG. 6 is the reflected beam ellipticity for a linearly polarized incident light in the y-direction for embodiments of the present invention at a graphene Fermi level of 0.2 eV;

FIG. 7 is the ellipsometry of the reflected beam as a function of graphene Fermi level at a frequency of 1.08 THz for an embodiment of the present invention;

FIG. 8 shows the variation of the resonant frequency of the long and short axis modes of the dielectric column with the Fermi level of graphene in the embodiment of the present invention.

The figure shows that: the device comprises a metal substrate 1, a low-refractive-index dielectric layer 2, a high-refractive-index dielectric super-surface array 3 embedded in the low-refractive-index dielectric, a single-layer graphene 4 and a bias voltage control device 5.

Detailed Description

The following further describes embodiments of the present invention with reference to the drawings. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without any creative work belong to the protection scope of the present invention.

The reflection type broadband polarization controller based on the graphene-medium composite super surface comprises four layers of structures, in the specific embodiment shown in fig. 1, the lowest layer is a gold substrate (1), the middle layer is a low-refractive-index medium PMMA (2), the uppermost layer is a silicon rectangular column super surface array (3) embedded in the PMMA (2), and uniform single-layer graphene (4) is arranged between the PMMA layer and the silicon medium column super surface. And a bias voltage control device (5) is arranged between the graphene layer and the gold substrate.

As shown in the schematic diagram of the structural unit in fig. 2, the Fermi level of the graphene is E_F1When the thickness is not less than 0 eV, the unit period Lambda is determined to be 130 mu m and the gold substrate thickness h is determined by parameter scanning₃0.2 μm, thickness h of PMMA layer₂33 μm, siliconColumn thickness h ₁70 μm, a silicon column length L of 124 μm, and a silicon column width S of 50 μm, in which case broadband orthogonal linear polarization conversion can be achieved around 1.08 THz.

More specifically, the performance of the structure is simulated by using a three-dimensional finite-difference time domain method, and fig. 3 shows that the fermi level of the graphene is E_F1The variation curve of the amplitude and the phase difference of the reflected wave polarized along the long axis and the short axis along with the frequency when the frequency is not less than 0 eV, the reflection coefficient of the light wave polarized along the long axis and the short axis is more than 80 percent, and the phase difference is about pi.

Accordingly, FIG. 4 shows the angle of rotation of the reflected light relative to the y-polarized incident light, which is between 80 and 90 over the frequency range of 0.93-1.15 THz.

FIG. 5 shows that the Fermi level of graphene is E_F2The variation curve of the amplitude and the phase difference of the reflected wave polarized along the long axis and the short axis along with the frequency when the frequency is not less than 0.2 eV, the reflectivity of the polarized light of the long axis and the short axis is still kept similar near 1.08 THz, and the phase difference is about 1.5 pi.

Accordingly, FIG. 6 shows the ellipsometry of the reflected light relative to the incident y-polarized light, in the range of 1.0-1.1 THz, between-0.85 and-1, with the reflected light maintaining a good circular polarization.

Fig. 7 is a plot of the ellipsometry of the reflected beam as a function of graphene fermi level at a frequency of 1.08 THz for an embodiment of the present invention. Maintaining good circular polarization over a wide range of 0.2 eV and 0.3-0.5 eV, while E is shown in FIG. 7_FThe polarization states of reflected light at 0 eV, 0.07 eV, 0.13 eV, and 0.2 eV are changed from linearly polarized light in the x direction to left circularly polarized light with increasing fermi level.

FIG. 8 is a graph of the effect of varying the graphene Fermi level on the resonant frequency of the silicon column for the long and short axis orthogonal polarization modes. With the increase of the graphene Fermi level, the resonant frequencies of the two modes move towards high frequency, but the moving speed of the long-axis resonant frequency is higher, and the polarization state of reflected light can dynamically change along with the bias voltage due to the fact that the two modes are regulated and controlled by the change of the graphene Fermi level.

By varying the size of the structural elements shown in fig. 2, the same dynamic polarization control can be achieved in other frequency ranges.

Claims

1. The utility model provides a reflection-type broadband polarization controller based on graphite alkene-medium composite super surface, characterized in that, this polarization controller includes four layers of structures, and the lower floor is metal substrate (1), and the centre is low refracting index medium (2), and the superiors are high refracting index medium super surface array (3) of embedding in low refracting index medium (2), makes the upper surface of super surface array expose in the air simultaneously, is even monolayer graphite alkene (4) between low refracting index medium layer and the high refracting index medium super surface, is equipped with bias voltage controlling means (5) between graphite alkene layer (4) and metal substrate (1).

2. The reflective broadband polarization controller based on the graphene-dielectric composite super surface as claimed in claim 1, wherein the dielectric super surface is a two-dimensional dielectric cylinder antenna array with sub-wavelength periodic arrangement.

3. The reflective broadband polarization controller based on the graphene-dielectric composite super surface as claimed in claim 1, wherein the material of the dielectric column is selected to have a high refractive index and a small absorption coefficient in an operating frequency band, such as a high-resistance silicon material suitable for terahertz band, and the low-refractive-index dielectric material is PMMA or SiO₂A dielectric material having an iso-refractive index of about 1.5.

4. The reflective broadband polarization controller based on the graphene-dielectric composite super-surface as claimed in claim 1, wherein the dielectric cylinders have anisotropic shapes, including rectangular cylinders, elliptical cylinders, asymmetric cross-shaped cylinders, split ring split cylinders, and the like, have different electromagnetic responses in two orthogonal polarization directions of long and short axes, and have equivalent birefringence characteristics.

5. The broadband reflective polarization controller based on graphene-dielectric composite super-surface as claimed in claim 1, wherein the polarization direction of incident linearly polarized light is in accordance with claim 4The length and the short axis of the dielectric antenna are 45^oAnd (4) an included angle.

6. The reflective broadband polarization controller based on the graphene-dielectric composite super surface as claimed in claim 1, wherein the metal substrate material is gold, silver or copper.

7. The reflective broadband polarization controller based on the graphene-dielectric composite super surface as claimed in claim 1, wherein the bias voltage adjusts the carrier concentration of the graphene layer, so that the fermi level of the graphene is changed within the range of 0-0.5eV, and the conductivity of the graphene is further controlled.

8. The broadband reflective polarization controller based on the graphene-dielectric composite super-surface as claimed in claim 1, wherein the reflected light polarized along the long axis and the short axis of the antenna has equal amplitude and phase difference pi by selecting a proper antenna size and a proper thickness of the low refractive index dielectric layer.

9. On the basis of claim 8, the phase difference of the long and short axis polarized reflected light is changed to 3 pi/2 or pi/2 by adjusting the conductivity of the graphene through voltage, and the dynamic switching from the half-wave plate to the quarter-wave plate is realized.

10. The reflective broadband polarization controller based on the graphene-dielectric composite super surface as claimed in claim 1, wherein the half-wave plate and the quarter-wave plate have the same operating frequency band.