CN113219572A

CN113219572A - Distributed feedback chirped grating based on graphene array

Info

Publication number: CN113219572A
Application number: CN202110516813.6A
Authority: CN
Inventors: 张亚平
Original assignee: Hubei University of Science and Technology
Current assignee: Hubei University of Science and Technology
Priority date: 2021-05-12
Filing date: 2021-05-12
Publication date: 2021-08-06

Abstract

The invention provides a distributed feedback chirped grating based on a graphene array, and belongs to the technical field of filters. The chirped grating comprises a defect layer and two chirped gratings symmetrically distributed on two sides of the defect layer, wherein the chirped gratings comprise a first dielectric layer and a plurality of second dielectric layers, and each second dielectric layer is positioned between the first dielectric layer and the defect layer; the thickness of each second dielectric layer is gradually increased from the first dielectric layer to the defect layer, and the thickness difference of the adjacent second dielectric layers is an isoquantitative constant; a graphene layer is arranged between the adjacent second dielectric layers, between the first dielectric layer and the adjacent second dielectric layer, and between the defect layer and the adjacent second dielectric layer. The invention has the functions of wave selection and wave filtering.

Description

Distributed feedback chirped grating based on graphene array

Technical Field

The invention belongs to the technical field of filters, and relates to a distributed feedback chirped grating based on a graphene array.

Background

The simplest optical filter and wavelength selector is a fabry-perot cavity, and the wave meeting the standing wave condition can be output in a resonant mode, but the monochromaticity of the transmitted wavelength of the structure is not good enough. To enhance wavelength selectivity, i.e., monochromaticity, two bragg gratings may be used instead of two mirrors in the fabry-perot cavity to form a distributed feedback bragg grating. The Bragg gratings are spatially distributed with a periodicity that results in a single number of transmission modes for a wavelength selector based on a distributed feedback Bragg grating. To increase the number of transmission modes, a chirped grating may be used instead of a bragg grating.

The spatial period of the chirped grating is no longer constant but is a function of position, e.g. two adjacent grids differ from each other in spatial period by a fixed length ad. Generally, a chirped grating refers to a grating structure whose period varies spatially. Alternatively, the chirped grating may be formed by spatially varying the refractive index. The varied spatial periodic structure can support multi-wavelength resonance output, so that the chirped grating is widely applied to multi-wavelength optical filters. In addition, the chirped grating can also be applied to fiber lasers, sensors and the like. The conventional chirped grating is generally formed by changing the refractive index of a dielectric medium in a space alternating manner, so that once the chirped grating is formed, the structure of the chirped grating is basically fixed, and the performance parameters of the chirped grating are difficult to regulate through external physical quantities.

Graphene is used as an ultrathin two-dimensional material and has good adjustable conductivity. Single layer graphene is embedded in a dielectric, non-periodically arranged. When the space periods of adjacent graphene are different by a constant, the chirped grating is formed. And arranging the two chirped gratings at two ends of a section of dielectric medium to form symmetrical distribution, thus forming the distributed feedback chirped grating based on the graphene array.

The surface conductivity of the graphene can be flexibly regulated and controlled through the chemical potential, and the surface conductivity of the graphene can also be represented by the equivalent refractive index, so that the equivalent refractive index of the graphene can also be controlled through the chemical potential of the graphene. When the distributed feedback chirped grating based on the graphene array is applied to a multi-wavelength selector and an optical filter, the number and the wavelength of transmission wavelengths, the cut-off wavelength and the bandwidth of the filter can be flexibly regulated and controlled through the chemical potential of graphene.

Disclosure of Invention

The present invention is directed to provide a distributed feedback chirped grating based on a graphene array, which solves the above problems in the prior art, and provides a multilayer structure with optical wave selection and filtering functions.

The purpose of the invention can be realized by the following technical scheme: a distributed feedback chirped grating based on a graphene array is characterized by comprising a defect layer and two chirped gratings symmetrically distributed on two sides of the defect layer, wherein the chirped grating comprises a first dielectric layer and a plurality of second dielectric layers, and each second dielectric layer is positioned between the first dielectric layer and the defect layer; the thickness of each second dielectric layer is gradually increased from the first dielectric layer to the defect layer, and the thickness difference of the adjacent second dielectric layers is an isoquantitative constant; a graphene layer is arranged between the adjacent second dielectric layers, between the first dielectric layer and the adjacent second dielectric layer, and between the defect layer and the adjacent second dielectric layer.

Further, the first dielectric layer, the second dielectric layer and the defect layer are all silicon dioxide.

Single layer graphene is embedded in a silica matrix material to form two chirped grating structures. The two chirped gratings are positioned at two ends of a section of dielectric medium and are symmetrically distributed, so that the distributed feedback chirped grating based on the graphene array is formed. The chirped grating is a non-periodic array formed by alternately arranging single-layer graphene and silicon dioxide dielectric medium; the spatial period difference between two adjacent graphenes is constant.

The structure is a resonant cavity, the two chirped graphene gratings are reflecting surfaces of the resonant cavity, and a dielectric medium sandwiched between the two chirped graphene gratings is a cavity of the resonant cavity. When the wavelength of the incident light wave just satisfies the resonance condition, resonance is formed. A formant appears on the transmittance curve, with a peak transmittance close to 1, while light waves of other wavelengths are suppressed. The more the number of periods of the graphene grating, the more the resonant mode. The structure is applicable to a multi-wavelength selector and an optical filter. The transmission wavelength of the selector and the bandwidth of the filter can be regulated and controlled by the periodicity of the chirped grating, the length of the cavity and the chemical potential of the graphene.

Drawings

Fig. 1 is a schematic diagram of a distributed feedback chirped grating structure based on a graphene array.

Figure 2 is a graph of transmittance and reflectance in a distributed feedback chirped grating based on graphene arrays.

Fig. 3 is transmittance for different chemical potentials.

Fig. 4 shows the transmission spectrum and the reflection spectrum corresponding to N-40 (N is the number of spatial periods of the chirped grating).

In the figure, a first dielectric layer; B. a second dielectric layer; C. a defect layer; sigma, graphene layer.

Detailed Description

The following are specific embodiments of the present invention and are further described with reference to the drawings, but the present invention is not limited to these embodiments.

Distributed feedback chirped grating based on graphene array as shown in fig. 1, 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 respectively located at two ends of a defect layer C and are symmetric about a central point of the defect layer C, and a dielectric a is located at an incident side and an exit side. The chirped grating is a non-periodic grating, the symbol σ represents graphene, and the spatial period between adjacent graphene differs by a constant value. In the chirped grating, the B-type dielectrics between two adjacent graphene arrays are sequentially numbered as B from the middle to two sides₁,B₂,L,B_NWhere N is the number of spatial periods of the chirped grating.

The materials used for dielectric A, B and C are silicon dioxide, and have a refractive index n of 1.449. The thickness of the dielectric A is d_AThe thickness of the dielectric C is d_CI-th layer of B-type dielectric B_iHas a thickness of d_iI.e. the space period of the chirped grating, and the difference between two adjacent space periods is a constant d_i-d_i-1Δ d. The whole structure can also be expressed as A σ B_NσLσB₂σB₁σCσB₁σB₂σLσB_Nσ A. The surface conductivity of graphene is related to temperature, chemical potential and wavelength of input light, taking the room temperature as T ═ 23 ℃ (deg.c in degrees celsius).

When the number of space periods of the chirped grating is N-20, the transmittance and the reflectance of the distributed feedback chirped grating based on the graphene array are shown in fig. 2. The ordinate T represents the transmittance, R the reflectance, the abscissa λ the wavelength, and the wavelength unit μm the micrometer. Here take d_A＝2μm，d_C＝16μm，d₁＝300nm，d₂＝290nm，…，d_N100nm (nm represents nm), that is, the difference in thickness between two adjacent layers of B-type dielectrics is Δ d 10 nm. Let the input wave be transverse electric wave and vertically incident. Non-linear effects are ignored. The transmittance T is shown by a solid line and the reflectance R is shown by a broken line, taking the chemical potential of graphene as μ ═ 0.2eV (eV represents electron volts). It can be seen that many resonance peaks, i.e., multi-mode resonances, appear on the transmittance curve as the wavelength is varied. When λ is 3 μm, an upward transition occurs in the transmittance curve, and the transmittance transitions from 0.69 to about 0.87. The distance between two adjacent formants is increased from left to right in sequence, and the peak value of the formant is gradually reduced. Continuing to increase the wavelength of the incident light, the transmittance is cut off.

As the wavelength increases, the wavelength interval between adjacent formants increases. As the wavelength of the incident light continues to increase, the transmittance decreases rapidly and the transmittance curve is cut off. Because there are many resonant cavities in the middle of the graphene array, resonant modes exist. The electrons of graphene undergo an in-band transition to an inter-band transition at an incident light wavelength λ of 3 μm, and thus undergo a step in transmittance. When the incident light wave is increased, the resonance condition is not satisfied, and the surface current effect of the graphene causes that the transmissivity is sharply reduced to cut off, so that the distributed feedback chirped grating can be applied to a low-pass multi-wavelength selector or a filter.

The reflectivity curve and the transmissivity curve form a pair of complementary curves, namely, the reflectivity is low at a place with high transmissivity; where the transmittance is convex, the reflectance is concave; as the wavelength changes, the reflectance increases as the transmittance decreases; when the transmitted light is completely cut off, the reflected light intensity is maximum, and the maximum reflectance is 1 regardless of the loss.

Keeping the number N of the periods of the chirped grating constant at 20, changing the chemical potential of the graphene, and keeping the other parameters constant, and fig. 3 shows transmittance curves corresponding to different chemical potentials of the graphene. It can be seen that: each curve has a jump around λ ═ 3 μm, and as the chemical potential increases, a certain blue shift occurs in the incident wavelength when the jump occurs. Therefore, when the device is used for a low-pass multi-wavelength selector or a filter, the wavelength corresponding to each transmission mode of the filter can be flexibly adjusted through the chemical potential of graphene.

When the number of spatial periods of the chirped grating is increased to N-40, the transmission spectrum and the reflection spectrum of the chirped grating based on the graphene array are shown in fig. 4. Accordingly, the spacing between adjacent graphene becomes d₁＝500nm，d₂＝490nm，…，d_NThe difference in thickness between two adjacent layers of B-type dielectric is still Δ d equal to 10nm at 100nm, and the other parameters are kept constant. It can be seen that: the transmittance of the resonance peak gradually decreases with increasing wavelength and is quite obvious; the greater the chemical potential, the smaller the transmittance of the formants; the larger the chemical potential, the lower the cut-off wavelength of the low-pass filter. The transmittance curve becomes steeper as the speed of decrease of the cutoff wavelength corresponding to N40 is higher than that of the case of N20. This is because as the number of chirped grating cycles increases, on the one hand, the resonance increases, and on the other hand, the loss of graphene also increases; in addition, as the chemical potential of graphene increases, the loss also increases.

Keeping the space period number N of the chirped grating constant at 20, changing the space period of the chirped grating, and taking d₁＝500nm，d₂＝480nm，…，d_NThe difference in thickness between two adjacent layers of B-type dielectric becomes Δ d 20nm at 100nm, and the other parameters remain unchanged. Compared with the case where the small spatial period Δ d is 10nm, the number of resonance peaks is increased, that is, the wavelength satisfying the resonance condition is increased; the bandwidth of the low-pass filter is also increased. Thus, when the device is applied to a wavelength selector, the number of transmission modes increases. And the bandwidth of the filter can be regulated and controlled by the chemical potential of the graphene.

In summary, the distributed feedback chirped grating based on the graphene array can be used for an optical low-pass multi-wavelength selector and a filter, and parameters such as the transmission wavelength of the wavelength selector and the transmissivity, the cut-off wavelength, the bandwidth and the like of the filter can be regulated and controlled through the space period number, the space period and the graphene chemical potential of the chirped grating.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.

Claims

1. A distributed feedback chirped grating based on a graphene array is characterized by comprising a defect layer (C) and two chirped gratings symmetrically distributed on two sides of the defect layer, wherein the chirped grating comprises a first dielectric layer (A) and a plurality of second dielectric layers (B), and each second dielectric layer is positioned 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 of the adjacent second dielectric layers (B) is an isoquantity constant; a graphene layer (sigma) is arranged between the 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 a graphene array according to claim 1, wherein the first dielectric layer (a), the second dielectric layer (B) and the defect layer (C) are all silicon dioxide.