A distributed feedback chirped grating based on graphene array

Abstract

The present invention provides a distributed feedback chirped grating based on a graphene array, which belongs to the field of filter technology. It includes a defect layer and two chirped gratings symmetrically distributed on both sides of the defect layer, the chirped grating includes a first dielectric layer and a plurality of second dielectric layers, each second dielectric layer is located between the first dielectric layer and the defect layer; from the first dielectric layer to the defect layer, the thickness of each second dielectric layer gradually increases, and the thickness difference between adjacent second dielectric layers is an equal constant; there is a graphene layer between adjacent second dielectric layers, between the first dielectric layer and the second dielectric layer adjacent thereto, and between the defect layer and the second dielectric layer adjacent thereto. The present invention has the functions of wave selection and filtering.

Description

Technical Field

[0001] The invention belongs to the technical field of filters and relates to a distributed feedback chirped grating based on a graphene array. Background Art

[0002] The simplest optical filter and wavelength selector is the Fabry-Perot cavity. Waves that meet the standing wave condition will resonate and output, but the monochromaticity of the wavelength transmitted by this structure is not good enough. In order to enhance the wavelength selectivity, that is, monochromaticity, two Bragg gratings can be used to replace the two reflectors in the Fabry-Perot cavity to form a distributed feedback Bragg grating. The Bragg grating is periodically distributed in space. This periodic distribution results in a single number of transmission modes for the wavelength selector based on the distributed feedback Bragg grating. In order to increase the number of transmission modes, a chirped grating can be used instead of a Bragg grating.

[0003] The spatial period of the chirped grating is no longer a constant, but a function of the position. For example, two adjacent gratings differ from each other in spatial period by a fixed length Δd. Generally speaking, a chirped grating refers to a grating structure whose period varies with space. In addition, a chirped grating can also be formed by allowing the refractive index to vary with space. This changing spatial period structure can support multi-wavelength resonant output, so chirped gratings are widely used in multi-wavelength optical filters. In addition, chirped gratings can also be used in fiber lasers and sensors. Traditional chirped gratings are generally formed by alternating the refractive index of the dielectric in space. Therefore, once formed, its structure is basically fixed, and it is difficult to regulate the performance parameters of the grating through external physical quantities.

[0004] As an ultra-thin two-dimensional material, graphene has good adjustable conductivity. A single layer of graphene is embedded in a dielectric and arranged non-periodically. When the spatial period of adjacent graphene differs by a constant, a chirped grating is formed. Then two chirped gratings are arranged at both ends of a dielectric to form a symmetrical distribution, which constitutes a distributed feedback chirped grating based on a graphene array.

[0005] The surface conductivity of graphene can be flexibly controlled by chemical potential, and its surface conductivity can also be expressed by an equivalent refractive index, so the equivalent refractive index of graphene can also be controlled by the chemical potential of graphene. When the distributed feedback chirped grating based on graphene array is applied to multi-wavelength selectors and optical filters, the number and wavelength of the transmitted wavelengths, the cutoff wavelength and bandwidth of the filter can be flexibly controlled by the chemical potential of graphene. Summary of the invention

[0006] The purpose of the present invention is to provide a distributed feedback 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 to enable the multilayer structure to have light wave selection and filtering functions.

[0007] The objective of the present invention can be achieved through the following technical solutions: a distributed feedback chirped grating based on a graphene array, characterized in that it comprises a defect layer and two chirped gratings symmetrically distributed on both sides of the defect layer, the chirped grating comprises a first dielectric layer and a plurality of second dielectric layers, each second dielectric layer is located between the first dielectric layer and the defect layer; the thickness of each second dielectric layer gradually increases from the first dielectric layer to the defect layer, and the thickness difference between adjacent second dielectric layers is an equal constant; there is a graphene layer between adjacent second dielectric layers, between the first dielectric layer and the second dielectric layer adjacent thereto, and between the defect layer and the second dielectric layer adjacent thereto.

[0008] Furthermore, the first dielectric layer, the second dielectric layer and the defect layer are all made of silicon dioxide.

[0009] A single layer of graphene is embedded in a silicon dioxide matrix material to form two chirped grating structures. The two chirped gratings are located at both ends of a dielectric and are symmetrically distributed, thus forming a distributed feedback chirped grating based on a graphene array. The chirped grating is a non-periodic array formed by alternating single layers of graphene and silicon dioxide dielectrics; the spatial period difference between two adjacent graphene layers is a constant.

[0010] This structure is a resonant cavity. The two chirped graphene gratings are the reflection surfaces of the resonant cavity, and the dielectric sandwiched between the two chirped graphene gratings is the cavity of the resonant cavity. When the wavelength of the incident light wave just meets the resonance condition, resonance will be formed. A resonance peak appears on the transmittance curve, and its peak transmittance is close to 1, while light waves of other wavelengths are suppressed. The more periods of the graphene grating, the more resonant modes there are. This structure can be applied to multi-wavelength selectors and optical filters. The transmission wavelength and filter bandwidth of the selector can be controlled by the period number of the chirped grating, the cavity length and the chemical potential of graphene. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is a schematic diagram of the distributed feedback chirped grating structure based on graphene array.

[0012] Figure 2 Transmittance and reflectivity in distributed feedback chirped gratings based on graphene arrays.

[0013] Figure 3 is the transmittance corresponding to different chemical potentials.

[0014] Figure 4 These are the transmission and reflection spectra corresponding to N=40 (N is the number of spatial periods of the chirped grating).

[0015] In the figure, A, first dielectric layer; B, second dielectric layer; C, defect layer; σ, graphene layer. DETAILED DESCRIPTION

[0016] 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.

[0017] Distributed feedback chirped grating based on graphene array Figure 1 As shown in the figure, a single layer of graphene is embedded in a dielectric material B to form two symmetrically distributed chirped gratings M. The two chirped gratings are located at the two ends of the defect layer C, and are symmetrical about the center point of C. The dielectric A is located on the incident side and the exit side. The chirped grating is a non-periodic grating. The symbol σ represents graphene. The spatial period between adjacent graphene layers differs by a constant value. In the chirped grating, the B-type dielectrics between two adjacent graphene arrays are numbered B1, B2, ..., B from the middle to the sides. N , where N is the number of spatial periods of the chirped grating.

[0018] Here we choose dielectrics A, B and C as silicon dioxide, whose refractive index is n = 1.449. The thickness of dielectric A is d A , the thickness of dielectric C is d C , the i-th layer B-type dielectric B i The thickness is d i , which is the spatial period of the chirped grating, 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 N σ…σB2σB1σCσB1σB2σ…σB N σA. The surface conductivity of graphene is related to temperature, chemical potential and input light wavelength, and the room temperature is taken as T = 23°C (°C represents Celsius).

[0019] When the number of spatial periods of the chirped grating is N = 20, Figure 2 The transmittance and reflectance of the distributed feedback chirped grating based on the graphene array are given. The ordinate T represents the transmittance, R represents the reflectance, and the abscissa λ represents the wavelength. The wavelength unit μm represents micrometers. Here, d A =2μm, d C =16μm, d1=300nm, d2=290nm,…,d N=100nm (nm means nanometer), 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 and is incident vertically. Ignore nonlinear effects. Take the chemical potential of graphene as μ=0.2eV (eV means electron volt), the solid line represents the transmittance T, and the dotted line represents the reflectivity R. It can be seen that with the change of wavelength, many resonance peaks appear on the transmittance curve, that is, multi-mode resonance. When λ=3μm, there is an upward jump on the transmittance curve, and the transmittance jumps from 0.69 to about 0.87. The distance between two adjacent resonance peaks increases from left to right, and the peak value of the resonance peak gradually decreases. Continue to increase the wavelength of the incident light, and the transmittance is cut off.

[0020] 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. Because there are many resonant cavities in the middle of the graphene array, there are resonant modes. When the incident light wavelength is λ=3μm, the electrons in graphene will experience a transition from intra-band transition to inter-band transition, so the transmittance will show a step jump. When the incident light wave increases, the resonance condition is not met, and the surface current of graphene is also affected, causing the transmittance to drop sharply and even cut off. Therefore, the distributed feedback chirped grating can be applied to low-pass multi-wavelength selectors or filters.

[0021] 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.

[0022] Keeping the period number N of the chirped grating unchanged at 20, changing the chemical potential of graphene and keeping other parameters unchanged, Figure 3 The transmittance curves corresponding to different graphene chemical potentials are given. It can be seen that each curve has a jump near λ = 3μm, and as the chemical potential increases, the corresponding incident wavelength will have a certain blue shift when the jump occurs. Therefore, when the device is used in a low-pass multi-wavelength selector or filter, the wavelengths corresponding to each transmission mode of the filter can be flexibly adjusted by the chemical potential of graphene.

[0023] When the number of spatial periods of the chirped grating increases to N = 40, Figure 4 The transmission and reflection spectra of the chirped grating based on graphene array are given. Correspondingly, the spacing between adjacent graphene becomes d1 = 500nm, d2 = 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, and it is quite obvious; 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. 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 with the increase of the number of chirped grating periods, on the one hand, the resonance is enhanced, and on the other hand, the loss of graphene is also increasing; in addition, with the increase of the chemical potential of graphene, the loss will also increase.

[0024] Keep the number of spatial periods of the chirped grating N = 20 unchanged, change the spatial period of the chirped grating, take d1 = 500nm, d2 = 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 small spatial period Δd=10nm, the number of resonance peaks increases, that is, the wavelength that meets the resonance condition increases; the bandwidth of the low-pass filter also increases. In this way, when this device is applied to a wavelength selector, the number of transmission modes increases. And the filter bandwidth can be controlled by the chemical potential of graphene.

[0025] In summary, the distributed feedback chirped grating based on graphene array can be used for optical low-pass multi-wavelength selectors and filters. The transmission wavelength of the wavelength selector and the transmittance, cut-off wavelength, bandwidth and other parameters of the filter can be controlled by the number of spatial periods of the chirped grating, the spatial period and the chemical potential of graphene.

[0026] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A distributed feedback chirped grating based on a graphene array, characterized in that: The invention comprises a defect layer (C) and two chirped gratings symmetrically distributed on both sides of the defect layer. The chirped grating comprises a first dielectric layer (A) and a plurality of second dielectric layers (B), wherein each second dielectric layer is located between the first dielectric layer (A) and the defect layer (C); the thickness of each second dielectric layer (B) gradually increases from the first dielectric layer (A) to the defect layer (C), and the thickness difference between adjacent second dielectric layers (B) is an equal constant; a graphene layer (σ) is provided between adjacent second dielectric layers (B), between the first dielectric layer (A) and the adjacent second dielectric layer (B), and between the defect layer (C) and the adjacent second dielectric layer (B).

2. The distributed feedback chirped grating based on graphene array according to claim 1, characterized in that: The first dielectric layer (A), the second dielectric layer (B) and the defect layer (C) are all made of silicon dioxide.

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