An optical logic device based on Rudin-Shapino photonic crystal

Abstract

The present invention provides an optical logic device based on Rudin-Shapino photonic crystals, belonging to the field of all-optical communication technology. It includes two symmetrically distributed binary RS photonic crystals and two graphene monolayers, the binary RS photonic crystals include a plurality of first dielectric layers and a plurality of second dielectric layers, the first dielectric layer is denoted as H, the second dielectric layer is denoted as L, the graphene monolayer is denoted as G, the binary RS photonic crystal is denoted as HHHLHHLH, and the optical logic device is denoted as HHHLHHLH 1 GH 2 H 2 GH 1 LHHLHHH, where H 1 GH 2 and H 2 GH 1 Both represent a three-layer structure formed by embedding a single layer of graphene in a first dielectric layer, the thickness of the first dielectric layer and the second dielectric layer are 1 / 4 of their respective optical wavelengths, and the first dielectric layer and the second dielectric layer are two uniform dielectric sheets with high and low refractive indices, respectively. The present invention has the advantages of low judgment threshold and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of all-optical communication and relates to an optical logic device based on Rudin-Shapino photonic crystal. Background Art

[0002] In all-optical communication, information needs to be stored, transmitted, relayed, judged, timed, amplified and shaped in the optical domain. This requires the vigorous development of all-optical devices that control light, and optical logic devices based on optical bistable states are an important type of them. Optical bistable states are a nonlinear optical effect based on the optical Kerr effect of materials. When the incident light is strong enough, one input light intensity value can correspond to two different output light intensity values, that is, one incident light intensity value can induce two stable resonant output states.

[0003] When optical bistable is applied to optical logic, the upper and lower thresholds of the bistable state correspond to the decision thresholds of logic 1 and logic 0 of the optical logic respectively; the larger the decision threshold, the stronger the light intensity required to trigger the decision of the optical logic. However, as the power of the device increases, the stability of the device will deteriorate during operation, and the requirements for heat dissipation conditions will also increase. In addition, the smaller the interval between the upper and lower thresholds of the bistable state, the smaller the distinction between logic 1 and logic 0 of the corresponding optical logic, which will lead to an increase in the misjudgment rate. Therefore, current research on optical bistable devices mainly focuses on how to reduce the threshold of optical bistable through new materials and new structures, and increase the interval between the upper and lower thresholds.

[0004] In order to achieve a low-threshold optical bistability effect, on the one hand, it is necessary to seek materials with larger third-order nonlinear optical coefficients; on the other hand, the local electric field should be enhanced by optimizing the system structure. The optical Kerr effect is proportional to the local electric field, so a strong local electric field can enhance the third-order nonlinear optical effect of the material, thereby reducing the threshold of optical bistability.

[0005] Graphene is an emerging two-dimensional material with ultra-thinness and excellent electrical conductivity. The surface conductivity of graphene can be flexibly controlled by its chemical potential. Importantly, graphene has a considerable third-order optical nonlinear optical coefficient, which makes graphene a hot material in the study of optical bistability. In addition, in order to further reduce the threshold of bistability, the surface plasmon of graphene can be used to enhance the local electric field of graphene; graphene can also be embedded in defect photonic crystals to enhance its nonlinear effect. In defect photonic crystals, the energy of the defect mode is mainly distributed in the defect layer. Therefore, embedding graphene in the defect layer can greatly enhance the nonlinear effect of graphene, thereby achieving low-threshold optical bistability.

[0006] Two dielectric sheets with different refractive indices are arranged alternately in space to form a photonic crystal with a periodic structure. In wave vector space, photonic crystals have a photonic band structure similar to the electronic band in semiconductors. Light waves in the band gap will be completely reflected. If defects are introduced into the photonic crystal, a transmission mode will appear in the transmission spectrum. The transmission mode is a defect mode that has a strong local effect on the electric field and is often used to enhance the third-order nonlinear effect of the material.

[0007] There are natural defect layers in quasi-photonic crystals or non-periodic photonic crystals, and the number of defect modes increases geometrically with the increase of sequence numbers. Therefore, quasi-photonic crystals or non-periodic photonic crystals are often used to enhance the localization of the electric field.

[0008] The Thue-Morse (TM) sequence is a quasi-periodic sequence in mathematics, and its corresponding photonic crystal is a quasi-periodic photonic crystal. By embedding graphene into TM photonic crystals, optical bistability can be achieved, and the threshold of optical bistability is about 100GW / cm 2 (Gigawatts per square centimeter). TM photonic crystals have multiple defect cavities, and in the same defect cavity there are multiple defect modes, namely resonant transmission modes. As the serial number increases, the number of dielectric layers in the TM photonic crystal increases accordingly, and the transmission modes in the transmission spectrum split geometrically, so these resonant modes are also called optical fractals. Optical fractals are localized to the electric field and are often used to enhance the third-order nonlinear effects of materials, thereby achieving low-threshold optical bistability. Whether it is possible to find another quasi-periodic photonic crystal with a stronger localized electric field, and then composite it with graphene to further enhance the nonlinear effect of graphene and lower the threshold of optical bistability is a research focus in this field. Summary of the invention

[0009] The purpose of the present invention is to provide an optical logic device based on Rudin-Shapino photonic crystals 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 reduce the decision threshold of the optical logic device.

[0010] The objective of the present invention can be achieved through the following technical solutions: An optical logic device based on Rudin-Shapino photonic crystals, characterized in that it includes two symmetrically distributed binary RS photonic crystals and two graphene monolayers, the binary RS photonic crystals include a plurality of first dielectric layers and a plurality of second dielectric layers, the first dielectric layer is denoted as H, the second dielectric layer is denoted as L, the graphene monolayer is denoted as G, the binary RS photonic crystals are denoted as HHHLHHLH, and the optical logic device is denoted as HHHLHHLH 1 GH 2 H 2 GH 1 LHHLHHH, where H 1 GH2 and H 2 GH 1 Both represent a three-layer structure formed by a single layer of graphene embedded in a first dielectric layer, the thickness of the first dielectric layer and the second dielectric layer are respectively 1 / 4 of their respective optical wavelengths, and the first dielectric layer and the second dielectric layer are respectively two uniform dielectric sheets with high and low refractive indices; the optical logic device based on Rudin-Shapino photonic crystals can realize low-threshold optical bistable states, and the upper and lower thresholds of the bistable states correspond to the decision thresholds of logic 1 and logic 0 of the optical memory, respectively.

[0011] Furthermore, the first dielectric layer is made of lead telluride, a material with a high refractive index, and the second dielectric layer is made of cryolite, a material with a low refractive index.

[0012] Furthermore, the logic 1 decision threshold, the logic 0 decision threshold, and the interval between the logic 1 decision threshold and the logic 0 decision threshold of the optical logic device are regulated by the chemical potential of the graphene monolayer.

[0013] Furthermore, the logic 1 decision threshold, the logic 0 decision threshold, and the interval between the logic 1 decision threshold and the logic 0 decision threshold of the optical logic device are regulated by the incident wavelength.

[0014] Two dielectric sheets A and B with different refractive indices, high and low, are arranged in sequence according to the Rudin-Shapiro (RS) sequence with sequence number N=3 to form an RS photonic crystal pair that is symmetrical about the origin; two graphene monolayers are then embedded in the RS photonic crystal pair to form a composite structure; there is an optical fractal form in the RS photonic crystal pair, and the optical fractal form has a local effect on the electric field; the two graphene monolayers are located exactly at the position where the local electric field corresponding to one of the optical fractal forms is the strongest, so the third-order nonlinear effect of graphene is greatly enhanced, thereby realizing a low-threshold optical bistability; the threshold of the optical bistability in this structure can be as low as 100MW / cm 2 , which is 3 orders of magnitude lower than the threshold of optical bistability in the composite structure of Thue-Morse photonic crystal and graphene.

[0015] The upper and lower thresholds of the optical bistability in the Rudin-Shapino photonic crystal structure, as well as the interval between the upper and lower thresholds, increase with the increase of the chemical potential of graphene and the incident wavelength. Therefore, when the optical bistability effect is applied to an optical logic device, the logic 1 and logic 0 decision thresholds of the optical logic device, as well as the interval between the logic 1 and logic 0 decision thresholds, can be flexibly controlled by the chemical potential of graphene and the incident wavelength. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1This is a schematic diagram of the composite structure of RS photonic crystal and graphene with sequence number N=3.

[0017] Figure 2 It is the linear transmission spectrum of light waves in the RS photonic crystal with sequence number N=3.

[0018] Figure 3 is the wavelength λ 3 =The normalized electric field distribution of the optical spectrum corresponding to 1.4058μm.

[0019] Figure 4 This is the schematic diagram of an optical logic device based on optical bistable state.

[0020] Figure 5 The middle figure (a) shows the input-output light intensity relationship corresponding to different graphene chemical potentials; Figure 5 Figure (b) shows the relationship between the upper and lower thresholds of the bistable state and the change of the chemical potential of graphene.

[0021] Figure 6 Figure (a) shows the input-output intensity relationship for different incident wavelengths; Figure 6 The middle figure (b) shows the relationship between the upper and lower thresholds of the bistable state and the incident wavelength.

[0022] In the figure, H, the first dielectric layer; L, the second dielectric layer; G, the graphene monolayer. DETAILED DESCRIPTION

[0023] 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.

[0024] Mathematically, the iteration rule of the Rudin-Shapiro (RS) sequence is: 0 =H, S 1 =HH,S 2 =HHHL, S 3 =HHHLHHLH,……,S N =S N-1 (HH→HHHL, HL→HHLH, LH→LLHL, LL→LLLH), ..., where N (N=0, 1, 2, 3, ...) represents the sequence number, S N represents the Nth item of the sequence, HH→HHHL represents changing S N-1 Replace HH in with HHHL.

[0025] Figure 1The following is a schematic diagram of the composite structure of a binary RS photonic crystal and graphene with sequence number N=3. The two binary RS photonic crystals are symmetrically distributed about the origin, which can be expressed as: HHHLHHLHHLHHLHHH, where the letters H and L represent two uniform dielectric sheets with high and low refractive indices respectively; there is an optical fractal effect in the photonic crystal, and two graphene monolayers are respectively embedded in the position where the local electric field corresponding to one of the fractal forms is the strongest, forming a composite structure, which can be expressed as: HHHLHHLH 1 GH 2 H 2 GH 1 LHHLHHH, where G represents a graphene monolayer. The entire composite structure is symmetrically distributed about the origin, similar to a distributed feedback Bragg grating.

[0026] In the composite structure of the RS photonic crystal pair and graphene, H is a high refractive index material lead telluride, whose refractive index is n H =4.1; L, L 1 and L 2 The low refractive index material cryolite has a refractive index of n L =1.35. The thickness of H and L is 1 / 4 optical wavelength, that is, the thickness of H is d H =λ 0 / 4 / n H = 0.0945 μm (μm means micrometer), where λ 0 =1.55μm is the central wavelength, and the thickness of L is d L =λ 0 / 4 / n L =0.287μm. 1 The thickness is d L1 =0.0088μm, L 2 The thickness is d L2 =0.0857μm, meeting the condition d L1 +d L2 =d L The incident light is a transverse magnetic wave, incident vertically from the left. The horizontal right is the positive direction of the coordinate axis Z.

[0027] The thickness of a single layer of graphene is about 0.33nm (nm stands for nanometer), which is equivalent to the size of an atom. The thickness of graphene can be ignored relative to the thickness of the dielectric sheets H and L. Here, the ambient temperature is set to 300K (K stands for Kelvin), and the relaxation time of electrons in graphene is τ = 0.5ps (ps stands for picoseconds).

[0028] Changing the frequency of incident light, when the effect of graphene is not considered, Figure 2The linear transmission spectrum of light in the RS photonic crystal with sequence number N=3 is given. The ordinate T represents the transmittance of the light wave; the abscissa (ω-ω 0 ) / ω gap represents the normalized angular frequency, where ω=2πc / λ, ω 0 =2πc / λ 0 and ω gap =4ω 0 arcsin│(n H -n L ) / (n H +n L )| 2 / π represent the incident light angular frequency, the incident light center angular frequency and the angular frequency band gap, c is the speed of light in a vacuum, and arcsin is the inverse sine function. In the normalized frequency range [-0.25, 0.25], there are three transmittance resonance peaks, corresponding to three resonant optical fractal forms. They are independent of each other and are separated by appropriate distances. These three transmittance peaks are all 1, and the corresponding medium wavelengths are: λ 1 =1.7271μm,λ 2 =1.55μm and λ 3 =1.4058μm. These three optical sub-states all have a local effect on the electric field. Here, only the third resonance state (marked with an asterisk ☆) is selected to obtain its corresponding mode field distribution. Then, the graphene monolayer is embedded in the position where the electric field intensity is the strongest in the structure, thereby enhancing the nonlinear effect of graphene and realizing a low-threshold optical bistability. In addition, to achieve a low-threshold optical bistability near the third resonance state, the incident wavelength must be relative to the wavelength λ of the third resonance state. 3 =1.4058μm appropriately red detuned.

[0029] Figure 3 The given is Figure 2 The electric field distribution of the third resonant optical component in the composite structure, and the corresponding resonant wavelength λ 3 =1.4058μm. The dotted line indicates the interface between two adjacent dielectric layers. Two graphene monolayers G are respectively embedded in the two locations with the strongest local electric field in the structure. The ordinate indicates the normalized Z component electric field intensity. It can be seen that the distribution of electric field energy in the structure is uneven and localized. The two graphene monolayers are located exactly at the two locations with the strongest local electric field. The optical third-order nonlinear effect of graphene is proportional to the local electric field intensity. Therefore, the nonlinear effect of graphite is greatly enhanced.

[0030] Fixed incident wavelength λ = 1.423μm, relative to the optical fractal resonance wavelength λ 3=1.4058μm, there is a certain red detuning. Increasing the incident light intensity, considering the nonlinear effect of graphene, the chemical potential of graphene is set to μ=0.4eV, and other parameters remain unchanged. Figure 4 The optical bistability phenomenon in the input-output light intensity relationship is given and applied to optical logic devices. i Indicates the input light intensity, the ordinate I o Indicates the output light intensity; unit: MW / cm 2 Indicates megawatts per square centimeter. An S-shaped curve segment appears in the contour of the input-output light intensity relationship, which is a typical feature of optical bistability. When the input light intensity gradually increases from a low value, at the right turning point of the S-shaped curve segment, the output light intensity will jump upward. The input light intensity I i =I u This process is called the upper threshold of optical bistability, which corresponds to the logic 1 of the optical logic device. i =I u It is called the decision threshold of the optical logic device logic 1; when the input light intensity gradually decreases from a higher value, at the left turning point of the S-shaped curve segment, the output light intensity will jump downward, and the input light intensity I i =I d This process is called the lower threshold of optical bistability, which corresponds to the logic 0 of the optical logic device. i =I d It is called the decision threshold of logic 0 of the optical logic device. At this time, the decision threshold of logic 1 is I u =275.7236MW / cm 2 The logic 0 judgment threshold is I d =232.2537MW / cm 2 , the interval between the logic 1 and logic 0 decision thresholds is I u -I d =43.4699MW / cm 2 .

[0031] Keep the incident wavelength λ = 1.423μm and other parameters unchanged, Figure 5 (a) shows the input-output light intensity relationship corresponding to different graphene chemical potentials μ. It can be seen that when μ = 0.3eV, 0.4eV and 0.5eV, there is an S-shaped curve in the input-output light intensity relationship curve, that is, a bistable relationship.

[0032] As the μ value increases, different chemical potentials correspond to different bistable curves, and the upper and lower thresholds and threshold intervals of the bistable state are also different. When 0.2eV≤μ<0.42eV and 0.44eV≤μ, the upper and lower thresholds of the bistable state, as well as the threshold interval of the bistable state increase with the increase of the graphene chemical potential, such as Figure 5(b) When 0.42eV≤μ<0.44eV, the upper and lower thresholds of bistable and the threshold interval of bistable decrease with the increase of graphene chemical potential. This phenomenon occurs because when the wavelength of incident light is λ=1.423μm, the internal electrons of graphite will transition from intra-band transition to inter-band transition near the chemical potential of 0.43eV. th Indicates the threshold of bistable state; symbol I u and I d Represent the upper and lower thresholds of the bistable state, respectively. Therefore, the upper and lower thresholds and threshold interval of the bistable state can be regulated by the chemical potential of graphene.

[0033] Optical bistability threshold of 100 GW / cm in a Thue-Morse photonic crystal and graphene composite system 2 In the composite structure of RS photonic crystal and graphene, the threshold of optical bistability is reduced to 100MW / cm 2 Magnitude.

[0034] In addition, different incident wavelengths correspond to different bistable curves and thresholds.

[0035] When the graphene chemical potential μ is fixed at 0.4 eV, other parameters remain unchanged. Figure 6 (a) shows the input-output light intensity relationship for different incident wavelengths. It can be seen that: when λ = 1.746μm ~ 1.749μm, the input-output optics are bistable; the bistable curves for different incident wavelengths are different, that is, the upper and lower thresholds and the width of the threshold interval of the bistable state are different; as the incident wavelength increases, that is, the wavelength detuning increases, the upper and lower thresholds of the bistable state increase, and the threshold interval of the bistable state increases, such as Figure 6 (b) As shown. The greater the wavelength detuning, the more nonlinear effects are needed to bridge this difference in order to achieve optical resonance, and the stronger the incident light energy required to achieve resonance. Therefore, the upper and lower thresholds and threshold interval of the bistable state can be controlled by the incident wavelength.

[0036] In summary, in the composite of RS photonic crystal and graphene, there is a resonant optical morphology, which has a strong local effect on the electric field. The two monolayer graphenes are located at the position where the local electric field corresponding to one of the optical morphologies is the strongest. Therefore, the nonlinear effect of graphene is greatly enhanced, thus realizing a low-threshold optical bistability. The threshold of the optical bistability is as low as 100MW / cm 2The optical bistability is 3 orders of magnitude smaller than that in the composite of Thue-Morse photonic crystal and graphene. The optical bistability can be applied to optical logic devices, and the decision thresholds of logic 1 and logic 0 of the optical logic device, as well as the interval between the decision thresholds of logic 1 and logic 0, can be flexibly controlled by the chemical potential of graphene and the incident wavelength.

[0037] 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. An optical logic device based on Rudin-Shapino photonic crystal, characterized in that: The invention comprises two symmetrically distributed binary RS photonic crystals and two graphene monolayers, wherein the binary RS photonic crystal comprises a plurality of first dielectric layers and a plurality of second dielectric layers, wherein the first dielectric layer is denoted as H, the second dielectric layer is denoted as L, and the graphene monolayer is denoted as G. The binary RS photonic crystal is represented as HHHLHHLH, and the optical logic device is represented as HHHLHHLH1GH2H2GH1LHHLHHH, wherein H1GH2 and H2GH1 both represent a three-layer structure formed by graphene monolayers embedded in the first dielectric layer, the thickness of the first dielectric layer and the second dielectric layer are respectively 1 / 4 of their respective optical wavelengths, and the first dielectric layer and the second dielectric layer are respectively two uniform dielectric sheets with high and low refractive indices; the optical logic device based on the Rudin-Shapino photonic crystal can realize a low-threshold optical bistable state, and the upper and lower thresholds of the bistable state correspond to the decision thresholds of logic 1 and logic 0 of the optical memory, respectively.

2. The optical logic device based on Rudin-Shapino photonic crystal according to claim 1, characterized in that: The first dielectric layer is lead telluride, a material with a high refractive index, and the second dielectric layer is cryolite, a material with a low refractive index.

3. An optical logic device based on Rudin-Shapino photonic crystal according to claim 1 or 2, characterized in that: The logic 1 decision threshold, the logic 0 decision threshold, and the interval between the logic 1 decision threshold and the logic 0 decision threshold of the optical logic device are regulated by the chemical potential of the graphene monolayer.

4. An optical logic device based on Rudin-Shapino photonic crystal according to claim 1 or 2, characterized in that: The logic 1 decision threshold, the logic 0 decision threshold, and the interval between the logic 1 decision threshold and the logic 0 decision threshold of the optical logic device are adjusted by the incident wavelength.

Images