A composite structure of non-periodic photonic crystal and graphene

Abstract

The present invention provides a composite structure of a non-periodic photonic crystal and graphene, belonging to the technical field of all-optical communication systems. It includes a graphene layer and two composite components symmetrically distributed on both sides of the graphene layer, the composite components include multiple first dielectric layers and second dielectric layers, and the arrangement rule of the composite components from outside to inside is: first dielectric layer, second dielectric layer, second dielectric layer, first dielectric layer, second dielectric layer, first dielectric layer, first dielectric layer, second dielectric layer. The present invention can achieve low-threshold optical bistability near multiple wavelengths.

Description

Technical Field

[0001] The invention belongs to the technical field of all-optical communication systems and relates to a composite structure of a non-periodic photonic crystal and graphene. Background Art

[0002] In all-optical communication networks, all-optical switches play a very important role in optical logic devices and optical memories. All-optical switches adjust the output light intensity by adjusting the intensity of the input light. For example, a strong output light intensity corresponds to the on state of the optical switch or optical logic 1, while a weak output light intensity corresponds to the off state of the optical switch or optical logic 0.

[0003] Optical bistability is a nonlinear optical effect. A phenomenon similar to a hysteresis loop will appear between the output light intensity and the input light intensity, that is, one input light intensity can correspond to two stable output light intensities. When the input light intensity increases to a certain level, the output light intensity will jump upward, and the corresponding input light intensity at this time is called the upper threshold of the optical bistability; when the input light intensity decreases to a certain level, the output light intensity will jump downward, and the corresponding input light intensity at this time is called the lower threshold of the optical bistability. Therefore, this jump characteristic of the optical bistability can be applied to all-optical switches, optical logic and optical storage devices. The upper and lower thresholds of the optical bistability correspond to the on and off thresholds of the all-optical switch, respectively.

[0004] Optical bistability is caused by the third-order nonlinear effect of the material. When the input light is strong enough, the local light intensity will cause the refractive index of the material to change, resulting in the multi-valued output light intensity. The stronger the local electric field, or the larger the third-order nonlinear coefficient of the material, the easier it is to achieve the optical bistability effect, and the lower the upper and lower thresholds of the bistable state. The lower the threshold, the lower the light intensity that triggers the switch to turn on and off. In addition, the wider the width between the upper and lower thresholds of the bistable state, the greater the on-off distinction of the fully closed switch, and the lower the misjudgment rate. Therefore, lowering the threshold and increasing the threshold interval are two key research directions for optical bistability.

[0005] In order to enhance the localization of the electric field, the usual approach is to construct a highly resonant feedback cavity or surface plasmon. The simplest resonant cavity is the Fabry-Perot cavity, which consists of two end faces forming two reflectors with a cavity in the middle. In addition, photonic crystals with defects also have a strong localization of the electric field. In particular, photonic crystals with local parity-time symmetry can further enhance the localization of the electric field. However, the construction of photonic crystals is too strict, and it is difficult to achieve parity-time symmetry conditions in the process, which is an important factor restricting the development of low-threshold optical bistable.

[0006] Graphene has a large third-order nonlinear coefficient. In the near-infrared and terahertz bands, the third-order nonlinear coefficient of graphene is one order of magnitude larger than that of silicon and silicon dioxide. In addition, graphene has the characteristics of ultrafast optical response, adjustable conductivity and ultra-thinness. Therefore, graphene is an ideal material for realizing low-threshold optical bistable devices.

[0007] The surface conductivity of graphene is a function of chemical potential (i.e., Fermi level), and the chemical potential of graphene is controlled by factors such as external bias voltage and chemical doping. Therefore, these methods can be used to flexibly regulate the surface conductivity of graphene and then regulate the parameters of optical bistability.

[0008] There are three types of photonic crystals: periodic, quasi-periodic, and non-periodic. In the wave vector space, periodic photonic crystals will have a photonic band structure similar to the electronic band of a semiconductor, with a band gap in the middle of the band. When the wavelength of the incident light is in the band gap, the light wave cannot pass through the photonic crystal and will be completely reflected back. If a layer of defects is inserted into the photonic crystal, a transmission mode, namely the defect mode, will appear in the middle of the band gap. The defect mode has a strong localization to the light field and can be used to achieve low-threshold optical bistability. Because there is only one defect mode in the transmission spectrum, the incident wavelength to achieve optical bistability can only be near the defect mode.

[0009] The Fibonacci sequence is quasi-periodic. The dielectric lattice properties that satisfy the Fibonacci sequence are between random sequences and periodic crystals. Quasi-photonic crystals have continuous Fourier spectra and will have Bragg-like peaks, that is, multiple independent resonance peaks. The localization of the electric field of this structure can also be used to achieve optical bistability, but the localization of the electric field of this quasi-photonic crystal is limited, and the threshold for achieving optical bistability is not low enough.

[0010] Thue-Morse sequence multilayer dielectrics are typical non-periodic photonic crystals. Non-periodic photonic crystals that satisfy the Thue-Morse sequence are a structure between quasi-periodic crystals and disordered systems, with only multiple independent transmission peaks appearing in their Fourier spectrum. Compared with defective photonic crystal structures, there are more transmission modes in Thue-Morse sequence photonic crystals. Compared with Fibonacci sequence photonic crystals, the electric field localization of the transmission mode in Thue-Morse sequence photonic crystals is greater. Therefore, the Thue-Morse sequence photonic crystal can be combined with graphene to achieve multiple wavelengths and low threshold optical bistability. Summary of the invention

[0011] The purpose of the present invention is to provide a composite structure of a non-periodic photonic crystal and graphene in view of the above problems existing in the prior art. The technical problem to be solved by the present invention is how to achieve low-threshold optical bistability near multiple wavelengths.

[0012] The objective of the present invention can be achieved through the following technical scheme: a composite structure of a non-periodic photonic crystal and graphene, characterized in that it includes a graphene layer and two composite components symmetrically distributed on both sides of the graphene layer, the composite components include multiple first dielectric layers and second dielectric layers, and the arrangement pattern of the composite components from outside to inside is: first dielectric layer, second dielectric layer, second dielectric layer, first dielectric layer, second dielectric layer, first dielectric layer, first dielectric layer, second dielectric layer, first dielectric layer, second dielectric layer.

[0013] Furthermore, the first dielectric layer is silicon, and the second dielectric layer is silicon dioxide.

[0014] Defective photonic crystals and quasi-periodic photonic crystals can achieve electric field localization, enhance the local electric field, and thus achieve low-threshold optical bistability. However, there is only a single transmission mode in the transmission spectrum of defective photonic crystals, so low-threshold optical bistability can only be achieved near one wavelength. Multiple transmission peaks can appear in quasi-periodic photonic crystals, but the electric field localization is not strong enough. This structure can achieve optical bistability, but the bistability threshold is not low enough. The transmission spectrum of non-periodic photonic crystals not only has multiple transmission peaks, but also the electric field localization is stronger than that of quasi-periodic photonic crystals. Therefore, by combining non-periodic photonic crystals with graphene, low-threshold optical bistability near multiple wavelengths can be achieved.

[0015] Alternating silicon dioxide and silicon dielectrics according to the Thue-Morse sequence rule can form a non-periodic photonic crystal, and then graphene is embedded in the center of this multilayer structure. Many independent transmission peaks will appear in the transmission spectrum, two of which correspond to the defect modes of the middle defect layer. These two defect modes correspond to two different central input wavelengths. The mode field energy corresponding to these two wavelengths is localized in the central defect layer, and the electric field intensity is the largest at the center of the defect layer. Graphene is embedded in the center of the structure, which is the position with the largest electric field intensity. The strong local electric field can greatly enhance the nonlinear effect of graphene. When the input wavelength is near these two defect modes, low threshold optical bistability can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of the composite structure of non-periodic photonic crystal and graphene.

[0017] Figure 2 is the relationship between transmittance and normalized frequency.

[0018] Figure 3 (a) is the electric field intensity distribution corresponding to transmission mode ①; Figure 3 (b) in the figure is the electric field intensity distribution corresponding to transmission mode ②.

[0019] Figure 4 (a) is the relationship between the output light intensity and the input light intensity near the transmission peak ①; Figure 4 (b) in the figure shows the relationship between the optical bistable threshold and the chemical formula of graphene.

[0020] Figure 5 It is the relationship between the output light intensity and the input light intensity near the transmission peak ②.

[0021] In the figure, A, first dielectric layer; B, second dielectric layer; G, graphene layer. DETAILED DESCRIPTION

[0022] 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.

[0023] like Figure 1 As shown, the permutation iteration rule of the dielectric Thue-Morse sequence is: A—>AB and B—>BA, where the first and second sequences are S1=AB and S2=ABBA, respectively. In this way, the third and fourth sequences can be iterated, which are S3=ABBABAAB and S4=ABBABAABBAABABBA, respectively. Here, we embed graphene in the quasi-photonic crystal of the S4 type Thue-Morse sequence to form a composite structure, namely ABBABAABGBAABABBA, where G is a graphene sheet, and the dielectric A is silicon (Si) with a thickness of d1 and a refractive index of n. A =3.53, dielectric B is silicon dioxide (SiO2), thickness is d2, refractive index is n B =1.46. The surface conductivity of graphene is related to the incident light wavelength λ and the graphene chemical potential μ c , ambient temperature T, relaxation time τ and other factors. Here we take T = 300K (K stands for Kelvin) and τ = 0.5ps (ps stands for picoseconds). Although graphene is ultra-thin, it can also be regarded as a layer with a certain equivalent thickness d g The equivalent thickness of the dielectric sheet can be approximated as d g =1nm (nm stands for nanometer). The input wave is a transverse magnetic wave, which enters the photonic crystal vertically.

[0024] Figure 2The transmission and reflection spectra of the non-periodic photonic lattice composed of a graphene layer embedded in an S4 sequence are given. The vertical coordinate T represents the transmittance. Here, d1 = 110nm is taken as the thickness of dielectric A, d2 = 265nm is taken as the thickness of dielectric B, and the nonlinear effect is ignored. The chemical potential of graphene is set to μ c =0.50eV (eV stands for electron volt). gap represents the normalized frequency, where ω = 2πc / λ, ω0 = 2πc / λ0 and ω gap =4ω0arcsin│(n A -n B ) / (n A +n B )| 2 / π represents the incident light frequency, the incident light center frequency and the bandgap frequency respectively, c is the speed of light in vacuum, λ0=1.55μm (μm represents micrometers) is the center wavelength, and arcsin is the inverse sine function. It can be seen that there are wide bandgaps on both sides of the center frequency of the transmission spectrum, and the transmittance is close to 0. There are seven transmission peaks on the transmittance curve, and their values ​​are close to 1. The transmission peaks correspond to the seven defect modes formed in the non-periodic photonic crystal. When the incident light frequency is equal to the defect mode frequency, resonance is formed. The mode field distribution of the resonant state is concentrated in the defect layer, which is the localization of the electric field. If graphene is embedded in the position where the electric field is the strongest, the third-order nonlinear effect of graphene will be greatly enhanced.

[0025] Different transmission modes correspond to different spatial positions of electric field localization. Among the seven transmission modes, transmission modes ① and ② have the strongest electric field localization, and the electric field is localized in the central defect layer, and the electric field intensity at the location of graphene is the largest. Therefore, this structure can achieve a low-threshold optical bistability effect near these two defect modes.

[0026] Figure 3 (a) shows the electric field distribution corresponding to transmission mode ①. The wavelength of the incident light is λ=λ1=1.9917μm. The horizontal axis Z-axis is the Z axis, which is the direction of dielectric stacking, and the vertical axis |E Z | 2 is the normalized electric field strength. The larger the contour value or the darker the color, the greater the electric field strength at that location. Obviously, the electric field is strongest at the center of the structure, that is, where the graphene is located.

[0027] Figure 3(b) shows the electric field distribution corresponding to transmission mode ②. The wavelength of the incident light at this time is λ=λ2=1.2697μm. The electric field is also localized at the position of the central graphene. Therefore, near the two wavelengths of λ1 or λ2, our structure can greatly enhance the third-order nonlinear effect of graphene, thereby achieving low-threshold optical bistability near the two wavelengths.

[0028] When the wavelength of the incident wave is detuned relative to the transmission peak ① red, for example, λ = 2.01 μm, the relationship between the output light intensity and the output light intensity is as follows Figure 4 (a) is shown. The horizontal axis I i is the input light intensity, the horizontal axis I o is the output light intensity, coordinate unit (GW / cm 2 ) represents gigawatts per square centimeter. It can be seen that when μ c = 0.4eV and 0.5eV, an S curve appears on each curve, which is the bistable phenomenon. The greater the chemical potential, the greater the upper and lower thresholds of the bistable state. c When =0.3eV, there is no S-shaped curve in the input-output relationship curve, so there is no bistability phenomenon in these two cases. It can be seen that near the transmission peak ①, the formation of bistability and the parameters of bistability can be controlled by the chemical potential of graphene.

[0029] Figure 4 (b) shows the relationship between the upper and lower thresholds of the bistable state and the chemical potential of graphene. The vertical axis Thrsh. represents the threshold. It can be seen that when μ c When the value is >0.32eV, the bistable phenomenon will occur. If the chemical potential of graphene is further increased, the width of the upper and lower thresholds of the bistable state will continue to increase. When the optical bistable effect is applied to the all-optical switch, the upper and lower thresholds of the bistable state correspond to the on and off thresholds of the all-optical switch, respectively. Therefore, the larger the switching threshold interval, the smaller the misjudgment rate. The width of the switching threshold of the all-optical switch can also be controlled by the chemical potential of graphene.

[0030] When the wavelength of the incident wave is detuned relative to the transmission peak ② red, for example, λ = 1.276μm, the relationship between the nonlinear transmittance and the output light intensity is as follows Figure 5 As shown. c =0.5eV, an S curve appears on the curve, which is the bistable phenomenon. c = 0.3eV and 0.4eV, there is no S-shaped curve in the input-output relationship curve, so there is no bistability phenomenon in these two cases. It can be seen that near the transmission peak ②, the formation of bistability and the parameters of bistability can also be controlled by the chemical potential of graphene.

[0031] 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 composite structure of a non-periodic photonic crystal and graphene, characterized in that: The invention comprises a graphene layer (G) and two composite components symmetrically distributed on both sides of the graphene layer (G), wherein the composite component comprises a plurality of first dielectric layers (A) and second dielectric layers (B), and the arrangement rule of the composite components from outside to inside is: first dielectric layer (A), second dielectric layer (B), second dielectric layer (B), first dielectric layer (A), second dielectric layer (B), first dielectric layer (A), first dielectric layer (A), second dielectric layer (B); the composite component is a non-periodic photonic crystal formed by alternately stacking the first dielectric layer (A) and the second dielectric layer (B) according to the Thue-Morse sequence rule, and the embedded position of the graphene layer (G) is the center position of the non-periodic photonic crystal.

2. The composite structure of a non-periodic photonic crystal and graphene according to claim 1, characterized in that: The first dielectric layer (A) is silicon, and the second dielectric layer (B) is silicon dioxide.

Images