CN111678887A

CN111678887A - Novel polarization insensitive sensor based on T-shaped graphene coupling

Info

Publication number: CN111678887A
Application number: CN202010297887.0A
Authority: CN
Inventors: 童胜君; 肖丙刚
Original assignee: China Jiliang University
Current assignee: China Jiliang University
Priority date: 2020-04-15
Filing date: 2020-04-15
Publication date: 2020-09-18

Abstract

The invention discloses a novel polarization insensitive sensor based on T-shaped graphene coupling, which structurally comprises two orthogonal T-shaped graphene resonators, wherein each T-shaped resonator actually comprises a dipole harmonic oscillator and a conductive band. A metal grid is integrated on the graphene, and the conductivity of the graphene can be regulated and controlled by modifying the voltage applied between the grid and the substrate silicon. When the polarization of the electromagnetic wave is in the horizontal or vertical direction, the dipole resonator horizontal to the polarization direction is directly excited as a bright state, and the dipole resonator vertical to the polarization direction is used as a dark state. Destructive interference between the two harmonic oscillators causes the generation of an electromagnetically induced transparent window, and the working frequency band of the transparent window can be electrically tuned with the graphene. Since the two resonators are orthogonal to each other and have substantially the same size, they are insensitive to electromagnetic waves polarized in the horizontal and vertical directions. The sensor can be used for detection of nano-substances, and achieves a sensitivity of 4960nm per unit refractive index and a quality factor higher than 11.4 in two directions.

Description

Novel polarization insensitive sensor based on T-shaped graphene coupling

The technical field is as follows:

the invention relates to a design method of a novel tunable polarization insensitive refractive index sensor, belongs to the field of artificial electromagnetic metamaterials, relates to an electromagnetic induction transparency phenomenon with tunable detection frequency band, and has the capability of detecting nano substances in horizontal and vertical polarization directions in practical application.

Background art:

the electromagnetic induction transparent metamaterial has good application prospect in a refractive index sensor due to high transmissivity and sensitivity to the spatial refractive index on the structure. However, most of the electromagnetic induced transparency is polarization sensitive due to the structural particularity, and the application scene is limited. And because the electromagnetic metamaterial is difficult to be actually processed, the structure of the electromagnetic induction transparent metamaterial is difficult to be adjusted after being fixed, and therefore the working frequency band of the transparent window is difficult to be tuned.

Graphene is a band gap semi-metallic material, which indicates that the conductivity can be adjusted by electrical doping or chemical doping, and the manipulation of the conductivity can be achieved by changing the external gate voltage of graphene. Therefore, the special graphene structure is an effective means for solving the two problems, and a new design method is provided for realizing the refractive index sensor which has tunable working frequency band and is insensitive to polarization.

The invention content is as follows:

the purpose of the invention is as follows: in order to provide a novel efficient nano-material detection method, the invention provides a novel polarization insensitive sensor based on T-shaped graphene coupling, the working frequency range of detection can be adjusted by changing the Fermi level of graphene, and the sensor is insensitive in the horizontal and vertical polarization directions.

The technical scheme is as follows: a novel polarization insensitive sensor based on T-shaped graphene coupling structurally comprises two orthogonal and equal-width T-shaped graphene resonators. The T-shaped resonator is actually composed of a dipole harmonic oscillator and a conductive strip, and the two dipole harmonic oscillators are also perpendicular to each other. The width w of the two dipole harmonic oscillators is 50nm, the length is L1-158 nm, and L2-160 nm. The two conductive strips were 400nm long and 20nm wide. Graphene is integrated on top of a dielectric calcium fluoride, which has a relative dielectric constant of 2.05 and a thickness of 300 nm. The substrate under the medium is doped silicon, the relative dielectric constant is 11.9, and the thickness is 3 um. The metal grid is integrated on the graphene, and the Fermi level of the graphene can be modified by modifying the voltage applied between the grid and the doped silicon, so that the surface conductivity of the graphene can be regulated. The electromagnetically induced transparency effect of this structure results from destructive interference between the two resonators. When the polarization of the electromagnetic wave is in the horizontal direction or the vertical direction, the dipole resonator horizontal to the polarization direction is directly excited as a bright state. The other dipole resonator perpendicular to the polarization direction is excited as a dark state by near-field coupling with the previous dipole. Destructive interference between the two harmonic oscillators results in high-transmittance electromagnetically-induced transparency, and the working frequency band of the transparent window can be electrically tuned with the graphene. Since the two T-resonators are orthogonal to each other and have substantially the same dimensions, they are insensitive to electromagnetic waves polarized in the horizontal as well as in the vertical direction.

Has the advantages that: compared with the prior art, the invention has the following advantages:

1. the working frequency band is tunable: unlike common electromagnetically induced transparent metamaterials, the frequency response of the surface layer cannot be changed after the metal pattern is fixed, the semi-metallic replacement metal of graphene is used as a key resonator, and the electric adjustability of the graphene is used for tuning the electromagnetically induced transparent window, so that the working frequency band of the electromagnetically induced transparent window is controlled.

2. The operation is simple: only the voltage applied between the gate and the doped silicon needs to be modified.

3. And (4) multiple functions: the sensor not only can realize the tuning of the detection frequency band, but also has consistent detection effect on the electromagnetic waves on vertical polarization and horizontal polarization, thereby greatly increasing the application scenes of the sensor.

Description of the drawings:

FIG. 1 is a top view of a metamaterial unit structure and a schematic of a completed device: wherein (a) the period of the display cell structure is Px and Py in the x and y directions, the graphene resonator width W and length L1, L2, the coupling distance S between the two resonators, (b) the display light is incident from the top, the graphene pattern structure and CaF₂the/Si layers are integrated together, with metal electrodes fabricated on single layer graphene.

Fig. 2 is a transmission spectrum of a metamaterial: the solid line is a transmission spectrum under excitation of x-polarized waves, and the dashed line is a transmission spectrum under excitation of y-polarized waves.

FIG. 3 shows the transmission spectrum of the metamaterial under x-polarization excitation: the dotted line is the transmission curve of only the right-side T-shaped resonator at the top of the metamaterial, the dashed line is the transmission curve of only the upper-side T-shaped resonator, and the solid line is the transmission curve of the two resonators when coupled.

Fig. 4 shows the electric field distribution of the transparent window of the proposed metamaterial under x-polarization excitation: wherein (a) is the electric field distribution at the first resonance point 20.33THz, (b) is the electric field distribution at the transparent window peak value 20.66THz, and (c) is the electric field distribution at the second resonance point 21.00 THz.

Fig. 5 shows the transmission spectrum and the frequency shift at the peak of the transparency window of the graphene metamaterial at different graphene fermi levels when an x-polarized wave is incident: (a) the variation of the transmission spectrum with respect to the fermi level, (b) the variation of the frequency shift at the peak of the transparency window with respect to the fermi level.

FIG. 6 shows the transmission spectrum and sensing performance at different refractive indices for x-polarized wave incidence: (a) is the transmission spectrum of the graphene metamaterial as a function of refractive index, (b) is the wavelength at the peak of the transparency window versus refractive index (straight line); the quality factor versus the change in refractive index (polyline).

The specific implementation method comprises the following steps:

the technical solution of the present invention is explained in detail by specific embodiments with reference to the accompanying drawings.

In order to study the electromagnetic induced transparent response of the proposed graphene metamaterial, as shown in fig. 1, electromagnetic simulation was performed using commercial frequency domain finite element method software CST Microwave Studio. Using periodic boundary conditions in the x and y directions, the excitation source is selected to be incident on a plane wave propagating in the-z direction. As shown in FIG. 2, the transmission spectrums of the designed metamaterial under the excitation of x-polarized waves (solid lines) and y-polarized waves (dashed lines), it can be clearly seen that the frequencies of the two spectrums at key points are basically consistent, the transmission rate is not greatly different, and the metamaterial is considered to be insensitive to polarization.

The dotted line is the transmission curve of only the right-side T-shaped resonator on the top of the metamaterial under the excitation of the x-polarized wave, and the dashed line is the transmission curve of only the upper-side T-shaped resonator, as shown in FIG. 3. The solid line is the transmission spectrum when the two resonators are coupled, which shows that the electromagnetically induced transparency phenomenon occurs due to the coupling between the bright and dark states. This transparent window can be used for sensing applications. The peak value of the transparent window and the electric field intensity at two resonance points are shown in fig. 4, and the mutually perpendicular near ends of the dipoles can be seen at the first resonance point 20.33THz to show opposite electric fields, which is called as an antisymmetric mode. At the second resonance point 21.00THz, the near ends of the dipoles exhibit the same electric field, called the symmetric mode. Whereas both resonators are weakly excited at the peak 20.66THz of the transparent window, very small field strengths are indicative of high transmission due to destructive interference between the two resonators.

The transmission spectrum and the frequency shift at the peak of the transparency window of the graphene metamaterial at different graphene fermi levels are simulated as in fig. 5. It can be seen that the operating band of the transparent window shows a frequency shift of about 4.5THz as the Fermi level changes from 0.5eV to 0.8 eV. And the magnitude of the frequency shift has a quadratic function with the fermi level. As fig. 6 simulates the transmission spectrum of the graphene metamaterial with the change of the refractive index under x polarization; the change in wavelength at the peak of the transparency window with respect to the refractive index (solid line); the quality factor versus the change in refractive index (polyline). The sensor can be used for detection of nano-substances, and achieves a sensitivity of 4960nm per unit refractive index and a quality factor higher than 11.4 in two directions.

Claims

1. A novel polarization insensitive sensor based on T-shaped graphene coupling is provided, and the method combines the electrical adjustable property of graphene and the high transmissivity of an electromagnetic induction transparent window to design a novel refractive index sensor with a tunable working frequency band. The sensor is of a unit cell structure and is composed of two orthogonal T-shaped graphene, and calcium fluoride and substrate silicon are integrated below the graphene. The structure can tune the working frequency band by modifying the grid voltage applied to the surface of the graphene, and can detect nano molecules through the refractive index change on the upper side of the structure. The structure is insensitive to electromagnetic waves incident with horizontal and vertical polarization.

2. The novel polarization insensitive sensor based on T-type graphene coupling of claim 1, wherein the unit cell structure has a period Px-Py-400 nm in x and y directions. Both T-shaped structures actually consist of a dipole resonator with a width w of 50nm, a length L1-158 nm, and a length L2-160 nm, and a conductive strip. The two conductive strips are the same in size, 400nm in length and 20nm in width. The thickness of the medium calcium fluoride is 300nm, and the thickness of the substrate silicon is 3 um.

3. The novel polarization insensitive sensor based on T-shaped graphene coupling as claimed in claim 1, wherein two orthogonal T-shaped resonators cause two dipole resonators to be orthogonal, and when the polarization of the electromagnetic wave is in the horizontal direction or the vertical direction, the dipole resonator horizontal to the polarization direction is directly excited as a bright state. The other dipole resonator perpendicular to the polarization direction acts as a dark state. Destructive interference between the two harmonic oscillators results in high-transmittance electromagnetically-induced transparency, and the working frequency band of the transparent window can be electrically tuned with the graphene.

4. The novel polarization insensitive sensor based on T-shaped graphene coupling as claimed in claim 3, wherein the T-shaped graphene of the resonant units of the sensor are orthogonal to each other, thereby realizing the polarization insensitivity phenomenon of incident electromagnetic waves in horizontal and vertical polarization directions.

5. The design method of the novel polarization insensitive sensor based on T-shaped graphene coupling as claimed in claim 1, wherein the working frequency of the electromagnetically induced transparent window is linearly related to the refractive index of the upper side of the structure, so as to realize the detection of biochemical molecules. The sensitivity of the sensing performance is 4960nm/RIU (reflexiveindextunit) in two polarization directions, and the quality factor is more than 11.4.