A trigger threshold adjustable light logic device based on graphene

Abstract

The present invention provides a trigger threshold adjustable optical logic device based on graphene, which belongs to the field of all-optical communication technology. The optical logic device includes a plurality of first dielectric layers A, a plurality of second dielectric layers B and four graphene monolayers G, represented as ABABBBABABB 1 GB 2 BB 3 GB 4 BB 4 GB 3 BB 2 GB 1 BABABBBABA, where B 1 GB 2 , B 2 GB 1 , B 3 GB 4 and B 4 GB 3 Both represent a sandwich structure formed by embedding a graphene monolayer in a second dielectric layer; the first dielectric layer and the second dielectric layer are two uniform dielectric sheets with different refractive indices; the optical logic device has four optical sub-forms, and the electric field corresponding to the optical sub-form has a local effect. The four graphene monolayers are respectively embedded in the strongest position of the local electric field corresponding to one of the sub-forms, so that the third-order nonlinear effect of the graphene monolayer is enhanced, thereby realizing a low-threshold optical bistability; the thickness of the first dielectric layer and the second dielectric layer is 1 / 4 of their respective optical wavelengths. The present invention has the advantages of low optical bistability threshold and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of all-optical communication and relates to a trigger threshold adjustable optical logic device based on graphene. Background Art

[0002] In all-optical communications, signals need to be stored, relayed, timed, judged, amplified and shaped in the optical domain, which requires the use of optical logic devices, and optical logic devices based on optical bistable are an important type of them.

[0003] Optical bistability is a nonlinear optical effect based on the optical Kerr effect of materials. When the incident light intensity is large 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. When the optical bistability effect is applied to an optical logic device, the upper and lower thresholds of the bistable state correspond to the trigger thresholds of logic 1 and logic 0 of the optical logic device, respectively. The larger the trigger threshold, the greater the light intensity required to trigger the optical logic device. When the power of the device increases, its stability will deteriorate, which puts higher requirements on heat dissipation. In addition, when the interval between the upper and lower thresholds of the optical bistability is small, the distinction between logic 1 and logic 0 in the optical logic device becomes weaker, and the misjudgment rate increases. Therefore, the current research on optical bistability mainly focuses on how to reduce the threshold of optical bistability through new materials and new structures, and increase the interval between the upper and lower thresholds.

[0004] To achieve low-threshold optical bistability, on the one hand, it is necessary to seek materials with larger third-order nonlinear coefficients, and on the other hand, to enhance the local electric field by optimizing the system structure. Because the optical Kerr effect is proportional to the local electric field, a strong local electric field can enhance the third-order nonlinear effect of the material.

[0005] Graphene is an ultra-thin two-dimensional material with excellent electrical conductivity, and its surface conductivity can be flexibly controlled by the chemical potential of graphene. Importantly, graphene has a considerable third-order optical nonlinear coefficient. This makes graphene a preferred material for achieving low-threshold optical bistability. In addition, to further reduce the threshold of bistability, the surface plasmons of graphene can be used to enhance the local electric field of graphene, thereby improving the nonlinear effect of graphene; graphene can also be embedded in the defect layer of a photonic crystal, and the locality of the electric field due to defects can be used to improve the nonlinear effect of graphene. In addition, the third-order nonlinear coefficient of graphene is a function of the chemical potential of graphene, so the threshold of the graphene-based bistability can be flexibly controlled by the chemical potential of graphene. When optical bistability is used in optical logic, the triggering threshold of the optical logic can also be controlled by the chemical potential of graphene.

[0006] Two dielectrics 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 without transmission. If defects are introduced into the photonic crystal, defect modes, namely transmission modes, will appear in the transmission spectrum. The transmission mode has a strong local effect on the electric field and is often used to enhance the third-order nonlinearity of the material. In quasi-photonic crystals or non-periodic photonic crystals, there are natural defects, and the number of defect modes increases geometrically with the increase of the sequence number. This is the optical fractal effect. The optical fractal form is a type of defect mode, which has a strong local effect on the electric field. Therefore, the optical fractal effect in quasi-photonic crystals or non-periodic photonic crystals can be used to enhance the locality of the electric field.

[0007] The Thue-Morse (TM) sequence is a quasi-periodic sequence in mathematics, and the corresponding TM photonic crystal is a quasi-periodic photonic crystal. By combining graphene and TM photonic crystals, a strong local electric field can be obtained, and a low-threshold optical bistability can be achieved. The threshold of the optical bistability is about GW / cm 2 (Gigawatts per square centimeter).

[0008] To further reduce the threshold of optical bistability, graphene is combined with Octonacci photonic crystals. Octonacci photonic crystals are also a quasi-periodic photonic crystal with optical fractal properties, and these optical fractals are more localized to the electric field. In the composite structure of graphene and Octonacci photonic crystals, the threshold of optical bistability can be as low as 100MW / cm 2 (MW / cm 2 Represents megawatts per square centimeter). Whether it is possible to obtain other composite structures of quasi-periodic photonic crystals and graphene to further reduce the threshold of optical bistability, and to achieve flexible and adjustable threshold of optical bistability to obtain optical logic devices with adjustable low trigger threshold has become the research focus in this field. Summary of the invention

[0009] The purpose of the present invention is to provide a graphene-based optical logic device with adjustable trigger threshold in response to the above-mentioned problems existing in the prior art. The technical problem to be solved by the present invention is how to lower the threshold of the optical bistable state and to achieve flexible and adjustable threshold so that it can be applied to optical logic devices.

[0010] The objective of the present invention can be achieved through the following technical solutions: a trigger threshold adjustable optical logic device based on graphene, characterized in that the optical logic device comprises a plurality of first dielectric layers A, a plurality of second dielectric layers B and four graphene monolayers G, the multilayer structure of the all-optical digital encoder is expressed as ABABBBABABB1GB2BB3GB4BB4GB3BB2GB1BABABBBABA, wherein B1GB2, B2GB1, B3GB4 and B4GB3 all represent a sandwich structure formed by a graphene monolayer embedded in a second dielectric layer; the first dielectric layer and the second dielectric layer are two uniform dielectric sheets with different refractive indices, the optical logic device has four optical sub-morphologies, the electric field corresponding to the optical sub-morphology has a local effect, the four graphene monolayers are respectively embedded in the strongest position of the local electric field corresponding to one of the sub-morphologies, so that the third-order nonlinear effect of the graphene monolayer is enhanced, thereby realizing a low-threshold optical bistability; the thickness of the first dielectric layer and the second dielectric layer are both 1 / 4 of their respective optical wavelengths.

[0011] The composite structure includes four graphene monolayers and a Cantor photonic crystal with a sequence number N=3, wherein the Cantor photonic crystal is ABABBBABABBBBBBBBBABABBBABA, and the letters A and B represent two uniform dielectric sheets with different refractive indices respectively; the Cantor photonic crystal has multiple independent optical fractal forms, and the electric field corresponding to the fractal forms is localized. The four graphene monolayers are embedded in the position where the local electric field corresponding to one of the fractal forms is the strongest. The composite structure as a whole can be expressed as: ABABBBABABB1GB2BB3GB4BB4GB3BB2GB1BABABBBABA, wherein G represents a single layer of graphene; the local electric field at the position where the graphene is located is the strongest, so the third-order nonlinear effect of the graphene is greatly enhanced, thereby realizing a low-threshold optical bistability; the thicknesses of the dielectric sheet A and the dielectric sheet B are both 1 / 4 of their respective optical wavelengths; the threshold of the optical bistability in this structure can be as low as MW / cm 2 , which is 2 orders of magnitude lower than the threshold of optical bistability in the composite structure of Octonacci photonic crystal and graphene.

[0012] The upper and lower thresholds of the optical bistability in the composite structure of Cantor photonic crystal and graphene, as well as the interval between the upper and lower thresholds, decrease with the decrease of the chemical potential of graphene and the incident wavelength. Therefore, when the optical bistability in this structure is applied to optical logic devices, the logic trigger threshold of the optical logic device, as well as the interval between the trigger thresholds of logic 1 and logic 0, can be flexibly controlled by the chemical potential of graphene and the incident wavelength.

[0013] Furthermore, the matrix material of the first dielectric layer is lead telluride, and the matrix material of the second dielectric layer is cryolite.

[0014] Furthermore, the logic 1 trigger threshold, logic 0 trigger threshold, and threshold interval of the optical logic device are regulated by the incident wavelength.

[0015] Furthermore, the logic 1 trigger threshold, logic 0 trigger threshold, and threshold interval of the optical logic device are regulated by the chemical potential of the graphene monolayer. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0018] Figure 3 It is the normalized electric field distribution of the optical spectrum corresponding to the wavelength λ=1.7209μm.

[0019] Figure 4 The relationship between the outgoing light intensity and the incident light intensity.

[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] Figure 7 Schematic diagram of a binary optical logic device based on optical bistable state.

[0023] In the figure, A, first dielectric layer; B, second dielectric layer; G, graphene single layer. DETAILED DESCRIPTION

[0024] 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.

[0025] In mathematics, the iteration rule of the Cantor sequence is: S0 = A, S1 = ABA, S2 = ABA (BBB) ​​ABA, S3 = S2 (BBB) 2 S2, ..., S N =SN-1 (BBB) N-1 S N-1 , ..., where N (N = 0, 1, 2, 3, ...) is the sequence number, S N Indicates the Nth item of a sequence; (BBB) N-1 Representation 3 N-1 The letters A and B in the corresponding Cantor photonic crystal represent two uniform dielectric sheets with different refractive indices. Figure 1 The composite structure of Cantor photonic crystal and graphene with sequence number N=3 is given. Symbol I1 represents the incident light, and symbol I2 represents the outgoing light. The dielectric sheet and the single-layer graphene are arranged in sequence along the Z axis, and the center position of the structure is point 0. This structure can also be expressed as ABABBBABABB1GB2BB3GB4BB4GB3BB2GB1BABABBBABA, where the letter G represents a single-layer graphene, B1GB2 and B2GB1 represent a sandwich structure formed by a single-layer graphene embedded in the dielectric layer B; the matrix material of A is lead telluride with a refractive index of n. a =4.1; the matrix materials of B, B1, B2, B3 and B4 are all cryolite, with a refractive index of n b =1.35. The incident light is a transverse magnetic wave, incident vertically from the left. The thickness of dielectric sheets A and B is 1 / 4 of the optical wavelength, that is, the thickness of A is d a =λ0 / 4 / n a =0.0945μm (μm means micrometer), where λ0=1.55μm is the central wavelength, and the thickness of B is d b =λ0 / 4 / n b = 0.287 μm, the thickness of B1 is d b1 = 0.0482 μm, the thickness of B2 is d b2 =0.2388μm, satisfying condition d b1 +d b2 =d b, , the thickness of B3 is d b3 =0.1116μm, the thickness of B4 is d b4 =0.1754μm, satisfying condition d b3 +d b4 =d b .

[0026] The thickness of a single layer of graphene is about 0.33 nm (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 dielectric sheets A, B, B1, B2, B3 and B4. 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).

[0027] Changing the frequency of incident light, when the effect of graphene is not considered, Figure 2 The linear transmission spectrum of light in the Cantor photonic crystal with sequence number N = 3 is given. The vertical axis T represents the transmittance of the light wave; the horizontal axis (ω-ω0) / ω gap represents the normalized angular 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 angular frequency, the incident light center angular frequency and the angular frequency band gap, c is the speed of light in vacuum, and arcsin is the inverse sine function. In the normalized frequency interval [-1,1], there are 8 transmittance resonance peaks, corresponding to 8 resonant optical forms, all of which have a local effect on the electric field. These 8 transmittance peaks are all 1. Here, only the fourth resonance state (marked with an asterisk) is selected to achieve optical bistability. The resonance wavelength corresponding to this peak is λ=1.7209μm. The next step is to calculate the corresponding electric field distribution, and then embed graphene in the position where the local electric field is the strongest, thereby enhancing the nonlinear effect of graphene and achieving low-threshold optical bistability; in addition, to achieve low-threshold optical bistability, the incident wavelength must be appropriately detuned relative to the fourth resonance state wavelength λ=1.7209μm.

[0028] The dielectric and graphene are arranged horizontally from left to right in a regular manner. Figure 3 The electric field distribution of the fourth resonant optical fractal in the composite structure is given. The dotted line represents the interface between two adjacent dielectric layers, and the four graphene monolayers G are respectively embedded in the positions with the strongest electric field intensity in the structure. The ordinate represents the normalized electric field intensity of the Z component. It can be seen that the distribution of the electric field energy in the structure is uneven and localized. The four graphene monolayers are located at the four strongest positions of the local electric field. The third-order nonlinear optical effect of graphene is proportional to the local electric field intensity, so the nonlinear effect of graphite is greatly enhanced.

[0029] Set the chemical potential to μ = 0.45 eV (eV stands for electron volts), and keep other parameters unchanged. Figure 4 The relationship between the output light intensity and the input light intensity is given. The incident light wavelength is λ = 1.724μm, and there is a certain red detuning relative to the fourth resonance wavelength λ = 1.7209μm. i Indicates the input light intensity, the ordinate I o Indicates the output light intensity. Unit: MW / cm 2It represents megawatts per square centimeter. When the light intensity increases to a certain value, an S-shaped curve will appear in the input-output light intensity relationship curve, representing the bistable relationship between input and output. In the composite system of Octonacci photonic crystal and graphene, the optical bistable threshold is 100MW / cm 2 In the composite of TM photonic crystal and graphene, the optical bistability is GW / cm 2 In the structure described herein, the threshold of the optical bistability is reduced to MW / cm 2 It can be seen that in the composite structure of Cantor photonic crystal with N=3 and graphene, the threshold of optical bistability is greatly reduced.

[0030] When the input light intensity gradually increases from a relatively small value, at the right inflection point of the S-curve segment, the corresponding input light intensity I i =I u , the output light intensity jumps upward, so I u It is called the upper threshold of optical bistability; when the input light intensity gradually decreases from a relatively large value, at the left inflection point of the S-curve segment, the corresponding input light intensity I i =I d , the output light intensity changes downward, so I d It is called the lower threshold of optical bistability. The difference between the upper and lower thresholds is I u -I d It is called the threshold interval.

[0031] When the input light intensity is between the upper and lower thresholds, that is, I u i d When the input light intensity value corresponds to two output light intensity values, this is the so-called optical bistable effect. In the contour line of the input-output light intensity relationship, the optical bistable effect appears as an S-shaped curve, which can be used in binary optical logic devices.

[0032] Of course, different incident wavelengths or different chemical potentials of graphene will result in different corresponding bistable curves and thresholds.

[0033] The incident wavelength λ is fixed at 1.724 μm, and other parameters remain 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 = 0.425eV, 0.45eV and 0.475eV, the input-output light intensity relationship is hyperbolic; increasing the μ value, the upper and lower thresholds of the bistable state, as well as the threshold interval of the bistable state, increase. Figure 5 ​​(b) is shown. The horizontal axis μ represents the chemical potential of graphene, and the vertical axis Thresh. represents the threshold of the bistable state. Upper Threshold and Lower Threshold represent the upper and lower thresholds of the bistable state, respectively. The upper and lower thresholds and threshold interval of the bistable state increase with the increase of the chemical potential of graphene. Therefore, the upper and lower thresholds and threshold interval of the bistable state can be flexibly controlled by the chemical potential of graphene.

[0034] When the graphene chemical potential μ is fixed at 0.475 eV, other parameters remain unchanged. Figure 6 (a) shows the input-output light intensity relationship corresponding to different incident wavelengths. It can be seen that: when the three wavelength values ​​λ = 1.724μm, 1.725μm and 1.726μm are given, the input-output light intensity is a bistable relationship; the bistable curves corresponding to different incident wavelengths are different, that is, the upper and lower thresholds and threshold intervals of the bistable state are different; as the incident wavelength increases, that is, the detuning amount increases, the upper and lower thresholds of the bistable state increase, and the threshold interval of the bistable state also increases, such as Figure 6 (b) As shown. It can be seen that in the entire given wavelength range of 1.724μm≤λ≤1.726μm, the bistable state will appear; the upper and lower thresholds and threshold intervals of the bistable state increase with the increase of wavelength. Because the larger the detuning amount of the wavelength, the stronger the incident light energy required to bridge the detuning amount to achieve resonance. Therefore, the upper and lower thresholds and threshold intervals of the bistable state can be controlled by the incident wavelength.

[0035] In summary, there are multiple resonant optical fractal forms in the composite structure of Cantor photonic crystal and graphene with sequence number N=3. The optical fractal form has a strong local effect on the electric field, and the four graphene monolayers are located at the four strongest electric field positions corresponding to one of the fractal forms, so the nonlinear effect of graphene is greatly enhanced, thereby realizing a low-threshold optical bistability; the threshold of the optical bistability is as low as MW / cm 2 The optical bistability is 6 orders of magnitude smaller than that in the composite of TM photonic crystal and graphene, and 2 orders of magnitude smaller than that in the composite of Octonacci photonic crystal and graphene. The upper and lower thresholds and threshold intervals of the optical bistability can be flexibly controlled by the chemical potential of graphene and the incident wavelength. This effect can be applied to binary optical logic devices, and the triggering threshold of the optical logic device can be flexibly controlled by the chemical potential of graphene and the incident wavelength.

[0036] The incident light wavelength is set to λ = 1.724μm, the chemical potential is μ = 0.45eV, and an optical bistability phenomenon appears in the input-output light intensity relationship. It is applied to binary optical logic devices. The principle is as follows Figure 7As shown. When the bistable effect can be applied to optical logic devices. When the input light intensity gradually increases from a low value, at the right turning point, the output light intensity will jump upward, so the input light intensity I i =I u This process corresponds to the logic 1 of the optical logic device. i =I u It is called the logic 1 trigger threshold of the optical logic device; when the input light intensity gradually decreases from a higher value, at the lower left turning point, the output light intensity will jump downward. i =I d It is called the lower threshold. This process corresponds to the logic 0 trigger threshold of the optical logic device. i =I d It is called the logic 0 trigger threshold of the optical logic device. u =2.1162MW / cm 2 , lower threshold I d =0.3424MW / cm 2 , threshold interval I u -I d =1.7738MW / cm 2 .

[0037] Figure 5 (b) shows that the input-output curve of the optical bistable state is affected by the chemical potential of graphene. The positions of the two inflection points of the corresponding S-curve segment change with different chemical potentials. The two inflection points of the S-curve segment correspond to the upper and lower thresholds of the optical bistable state, i.e., the trigger thresholds of logic 1 and logic 0 of the optical logic device. Keeping the incident wavelength λ = 1.724 unchanged, the chemical potential increases to μ = 0.475 eV. At this time, the upper threshold I u =13.6316MW / cm 2 , lower threshold I d =0.0367MW / cm 2， , threshold interval I u -I d =13.6028MW / cm 2 It can be seen that when the chemical potential increases, the trigger thresholds of logic 1 and logic 0 of the optical logic device, as well as the threshold interval increase.

[0038] Figure 6 (b) shows the input-output curve of the optical bistable state affected by the incident wavelength. When the chemical potential is μ = 0.45 eV and the incident wavelength is reduced to λ = 1.723, the corresponding upper threshold I u =1.5487MW / cm 2 , lower threshold I d =0.0187MW / cm 2,, threshold interval I u -I d =1.53MW / cm 2 It can be seen that when the incident wavelength decreases, the wavelength detuning amount decreases, the trigger thresholds of logic 1 and logic 0 of the optical logic device, and the threshold interval decreases.

[0039] The larger the trigger threshold interval, the greater the distinction between logic 1 and logic 0, and the lower the misjudgment rate. To reduce the misjudgment rate of the optical logic device, it is necessary to increase the chemical potential of graphene or the wavelength detuning amount. At the same time, the trigger threshold increases. Therefore, reducing the misjudgment rate of the optical logic device comes at the expense of increasing the trigger threshold.

[0040] 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 trigger threshold adjustable light logic device based on graphene, characterized in that: The optical logic device includes a plurality of first dielectric layers A, a plurality of second dielectric layers B and four graphene monolayers G. The multilayer structure of the optical logic device is expressed as ABABBBABABB1GB2BB3GB4BB4GB3BB2GB1BABABBBABA, wherein B1GB2, B2GB1, B3GB4 and B4GB3 all represent sandwich structures formed by graphene monolayers embedded in the second dielectric layer; the first dielectric layer and the second dielectric layer are two uniform dielectric sheets with different refractive indices. The optical logic device has four optical sub-forms, and the electric field corresponding to the optical sub-forms has a local effect. The four graphene monolayers are respectively embedded in the strongest position of the local electric field corresponding to one of the sub-forms, so that the third-order nonlinear effect of the graphene monolayer is enhanced, thereby realizing a low-threshold optical bistability; the optical bistability can be applied to a binary optical logic device with an adjustable trigger threshold; the thickness of the first dielectric layer and the second dielectric layer are both 1 / 4 of their respective optical wavelengths.

2. The trigger threshold adjustable light logic device based on graphene according to claim 1, characterized in that: The matrix material of the first dielectric layer is lead telluride, and the matrix material of the second dielectric layer is cryolite.

3. A graphene-based trigger threshold adjustable light logic device according to claim 1 or 2, characterized in that: The logic 1 trigger threshold, logic 0 trigger threshold and threshold interval of the optical logic device are regulated by the chemical potential of the graphene monolayer.

4. A graphene-based trigger threshold adjustable light logic device according to claim 1 or 2, characterized in that: The logic 1 trigger threshold, logic 0 trigger threshold and threshold interval of the optical logic device are adjusted by the incident wavelength.

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