CN112612072B - Chirp grating based on graphene array - Google Patents

Abstract

The present invention provides a chirped grating based on a graphene array, belonging to the field of optical technology. It comprises two dielectric layers one, a non-periodic grating is sandwiched between the two dielectric layers one, the non-periodic grating comprises a plurality of dielectric layers two, a single layer of graphene is sandwiched between adjacent dielectric layers two and between dielectric layers one and two; from the incident direction to the exit direction, the thickness of each dielectric layer two gradually decreases, and the thickness difference between adjacent dielectric layers two is a fixed constant. The present invention has the advantages of optical low-pass filtering function and the like.

Description

A chirped grating based on graphene array

Technical Field

The invention belongs to the field of optical technology and relates to a chirped grating based on a graphene array.

Background technique

When the grating changes periodically in space, it is called a Bragg grating. When the spatial period of the grating is no longer a constant but a function of position, for example, two adjacent gratings differ from each other by a fixed length in spatial period, it is called a chirped grating. Chirping originally referred to the sound of birds, that is, the change in the pitch of birdsong; in physics, chirping refers to the change of physical quantity over time or space. The chirped grating here refers to a grating structure whose period varies with space. In addition, the refractive index can also be changed with space to form a chirped grating. Chirped gratings can be widely used in optical filters, fiber lasers and sensors. Once the traditional chirped grating is formed, its structural parameters are basically fixed. Therefore, it is difficult to adjust the grating performance parameters through external parameters.

Summary of the invention

The purpose of the present invention is to provide a chirped grating based on a graphene array in view of the above-mentioned problems existing in the prior art. The technical problem to be solved by the present invention is how to make the multilayer structure have the function of optical low-pass filtering and the cut-off wavelength and bandwidth have flexible adjustability.

The object of the present invention can be achieved by the following technical solutions: A chirped grating based on a graphene array, characterized in that it includes two dielectric layers one, a non-periodic grating is sandwiched between the two dielectric layers one, the non-periodic grating includes a plurality of dielectric layers two, a single layer of graphene is sandwiched between adjacent dielectric layers two and between dielectric layers one and two;

From the incident direction to the exit direction, the thickness of each dielectric layer 2 gradually decreases, and the thickness difference between adjacent dielectric layers 2 is a fixed constant.

Furthermore, the dielectric layer 1 and the dielectric layer 2 are made of silicon dioxide.

As an ultra-thin two-dimensional material, graphene has excellent electrical conductivity. A single layer of graphene can be embedded in a dielectric and arranged non-periodically. When the spatial period of adjacent graphene differs by a constant, a chirped grating is formed. The surface conductivity of graphene can be flexibly adjusted by chemical potential, and its equivalent refractive index can also be controlled by the chemical potential of graphene.

When the chirped grating based on graphene array is applied to the optical low-pass filter, its cut-off wavelength and bandwidth can be controlled by the chemical potential of graphene. The chemical potential of graphene is related to the impurity doping concentration of graphene and the external gate voltage applied to graphene. Therefore, the cut-off wavelength and bandwidth of the optical low-pass filter can be flexibly controlled by the external gate voltage.

A single layer of graphene is embedded in a silicon dioxide matrix material to form a chirped grating structure. The chirped grating is composed of a non-periodic graphene array and a silicon dioxide dielectric; the spatial period difference between two adjacent graphene layers is constant.

The graphene array forms many resonant cavities. When the wavelength of the incident light wave just meets the resonance condition, resonance will be formed, and a resonance peak will appear on the transmittance curve, while the long wavelength will be suppressed to form a cutoff wavelength. Therefore, this structure can be applied to optical low-pass filters. The transmittance, cutoff wavelength and bandwidth of the optical low-pass filter can be controlled by the number of periods of the chirped grating, the spatial period and the chemical potential of graphene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of a chirped grating based on a graphene array.

Figure 2 shows the transmittance and reflectance in a chirped grating based on a graphene array.

Figure 3 shows the transmittance corresponding to different chemical potentials.

FIG4 shows the transmittance of a chirped grating with a spatial period of 40.

FIG5 shows the transmission in a chirped grating with a large spatial period.

In the figure, δ, single-layer graphene; A, dielectric layer one; B, dielectric layer two.

Detailed ways

The following are specific embodiments of the present invention and the accompanying drawings to further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

As shown in FIG. 1 , the chirped grating is a non-periodic grating, the symbol δ represents graphene, and the periods of adjacent graphene differ by a constant value.

There are two types of dielectrics, dielectric layer 1 A and dielectric layer 2 B, where dielectric layer 1 A is located at the incident side and the exit side, and dielectric layer 2 B is located between the graphene arrays of the chirped grating. Each dielectric layer 2 B is numbered from left to right as B ₁ , B ₂ ,…, B _N . Dielectric layer 1 A and dielectric layer 2 B can be made of the same material or different materials. The thickness of dielectric layer 1 A is d _A , the thickness of the i-th dielectric layer 2 B (i.e., _Bi ) is d _i , N is the spatial period of the chirped grating, i.e., the number of dielectric layers 2 B, and the difference between two adjacent spatial periods is a constant d _i -d _i-1 = Δd. The entire structure can also be expressed as AσB ₁ σB ₂ …σB _i σB _i+1 …σB _N σA. Here, the materials of dielectrics A and B we choose are both silicon dioxide, and their refractive index is n = 1.449. The surface conductivity of graphene is related to temperature, chemical potential and input light wavelength, with room temperature being T = 23°C.

When the number of spatial periods of the chirped grating is N=20, Figure 2 shows the transmittance and reflectance of the chirped grating based on the graphene array. The ordinate T represents the transmittance, R represents the reflectance, the abscissa λ represents the wavelength of the incident wave, and the wavelength unit μm represents micrometers. Here, d _A = 2μm, d ₁ = 300nm, d ₂ = 290nm, ..., d _N = 100nm (nm represents nanometers), that is, the thickness difference between two adjacent layers of B-type dielectric is Δd = 10nm. Assume that the input wave is a transverse electric wave, vertically incident, and ignore the nonlinear effect. Take the graphene chemical potential as μ = 0.2eV (eV represents electron volts), the solid line represents the transmittance T, and the dotted line represents the reflectance R. It can be seen that with the change of wavelength, many resonance peaks appear on the transmittance curve. When λ = 3μm, there is an upward jump on the transmittance curve, and the transmittance jumps from 0.69 to about 0.87. As the wavelength increases, the wavelength interval between adjacent resonance peaks increases. If the wavelength of the incident light continues to increase, the transmittance will decrease rapidly, and the transmittance curve will show a cutoff phenomenon. Because there are many resonant cavities in the middle of the graphene array, there are resonant modes. When the wavelength of the incident light is λ = 3μm, the electrons in the graphene will transition from intra-band transition to inter-band transition, so the transmittance will have a step phenomenon. When the wavelength of the incident light increases and cannot meet the resonance condition, coupled with the surface current of the graphene, the transmittance decreases sharply, and even cuts off. Therefore, the chirped grating can be used in optical low-pass filters. The reflectivity curve and the transmittance curve form a pair of complementary curves, that is, where the transmittance is large, the reflectivity is small; where the transmittance is convex, the reflectivity is concave; when the wavelength changes, the transmittance decreases and the reflectivity increases; when the transmitted light is completely cut off, the reflected light intensity is the largest. If the loss is not considered, the maximum reflectivity is 1.

Keeping the number of periods of the chirped grating N=20 unchanged and changing the chemical potential of graphene, Figure 3 shows the transmittance curves corresponding to different graphene chemical potentials. It can be seen that each curve has a jump near λ=3μm, and as the chemical potential increases, the corresponding wavelength will blueshift (move to a shorter wavelength) when the jump occurs. When the device light is used for a low-pass filter, the cutoff wavelength of the filter decreases as the chemical potential of graphene increases, that is, the bandwidth becomes narrower.

When the number of spatial periods of the chirped grating increases to N = 40, Figure 4 shows the transmittance at this time. Here, d ₁ = 500nm, d ₂ = 490nm, ..., d _N = 100nm, the thickness difference between two adjacent layers of B-type dielectric is still Δd = 10nm, and other parameters remain unchanged. It can be seen that: with the increase of wavelength, the transmittance of the resonance peak gradually decreases more obviously; the greater the chemical potential, the smaller the transmittance of the resonance peak; the greater the chemical potential, the lower the cutoff wavelength of the low-pass filter, that is, the bandwidth becomes narrower. Compared with the case of N = 20, the cutoff wavelength corresponding to N = 40 decreases faster, and the transmittance curve becomes steeper. This is because, on the one hand, the increase in the number of periods of the chirped grating enhances the resonance, and on the other hand, it also increases the loss of graphene; secondly, as the chemical potential of graphene increases, the loss will also increase.

Keeping the number of spatial periods N of the chirped grating unchanged at 20, and changing the spatial period of the chirped grating, Figure 5 shows the transmittance of the chirped grating. Here, d ₁ = 500nm, d ₂ = 480nm, ..., d _N = 100nm, the thickness difference between two adjacent layers of B-type dielectric becomes Δd = 20nm, and other parameters remain unchanged. Compared with the case of a smaller spatial period and Δd = 10nm, the number of resonance peaks increases, that is, the number of wavelengths that meet the resonance condition increases; the bandwidth of the low-pass filter also increases, and the bandwidth can be controlled by the chemical potential of graphene.

In summary, chirped gratings based on graphene arrays can be used for optical low-pass filters, and the filter parameters such as transmittance, cutoff wavelength, and bandwidth can be controlled by the number of spatial periods of the chirped grating, the spatial period, and the chemical potential of graphene.

The specific embodiments described herein are merely examples of the spirit of the present invention. A person skilled in the art of the present invention may make various modifications or additions to the specific embodiments described or replace them in a similar manner, but this will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims (2)

1. A chirped grating based on a graphene array, characterized in that it comprises two dielectric layers (A), a non-periodic grating is sandwiched between the two dielectric layers (A), the non-periodic grating comprises a plurality of dielectric layers (B), a single layer of graphene (δ) is sandwiched between adjacent dielectric layers (B) and between dielectric layer (A) and dielectric layer (B); From the incident direction to the exit direction, the thickness of each dielectric layer 2 (B) gradually decreases, and the thickness difference between adjacent dielectric layers 2 (B) is a fixed constant; When the number of spatial periods of the chirped grating is N=20, the thickness of the dielectric layer 1 (A) is d _A =2μm, the thickness of the adjacent dielectric layer 2 (B) is Δd=10nm, the thickness of the first dielectric layer 2 (B) close to the incident direction is d ₁ =300nm, the input wave is assumed to be a transverse electric wave, and the incidence is vertical, the nonlinear effect is ignored, and the chemical potential of graphene is taken as μ=0.2eV. With the change of wavelength, multiple resonance peaks appear on the transmittance curve. When the wavelength of the incident light is λ=3μm, the transmittance jumps from 0.69 to 0.87. With the increase of wavelength, the wavelength interval between adjacent resonance peaks increases. If the wavelength of the incident light continues to increase, the transmittance will decrease rapidly, and the transmittance curve will show a cutoff phenomenon. The electrons in the graphene will appear in the band when the wavelength of the incident light is λ=3μm. The transition from transition to inter-band transition, when the wavelength of the incident light continues to increase and cannot meet the resonance condition, coupled with the surface current of graphene, causes the transmittance to drop sharply, and even cutoff occurs. Therefore, the chirped grating can be applied to optical low-pass filters; the reflectivity curve and the transmittance curve form a pair of complementary curves, that is, where the transmittance is large, the reflectivity is small; where the transmittance increases suddenly, the reflectivity decreases suddenly; when the transmittance decreases, the reflectivity increases; when the transmitted light is completely cut off, the reflected light intensity is the maximum. If the loss is not considered, the maximum reflectivity is 1. 2. A chirped grating based on a graphene array according to claim 1, characterized in that the dielectric layer 1 (A) and the dielectric layer 2 (B) are silicon dioxide.