An optical logic device based on period-doubling photonic crystal

Abstract

The present invention provides an optical logic device based on Period‑Doubling photonic crystals, which belongs to the field of all-optical communication technology. It includes a plurality of first dielectric layers, a plurality of second dielectric layers and two graphene monolayers, the first dielectric layer is denoted as H, the second dielectric layer is denoted as L, the graphene monolayer is denoted as G, and the layered structure of the optical logic device is represented as: HLH1GH2HH2GH1LHL, 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 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 optical logic device based on PD 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 logic device, respectively. The present invention has the advantages of being able to be used as an optical logic device.

Description

Technical Field

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

[0002] In all-optical communication, the storage, transmission, relay, judgment, timing, amplification and shaping of information are all carried out and completed in the optical domain, which requires the vigorous development of all-optical devices that control light, and optical logic devices based on optical bistable are an important type of them. Optical bistable is a nonlinear optical effect that utilizes 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 output resonant 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 input light field energy 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, materials with larger third-order nonlinear optical coefficients are sought; on the other hand, the local electric field is enhanced by optimizing the system structure. The optical Kerr effect is proportional to the local electric field strength, so a strong local electric field can enhance the third-order nonlinear effect of the material, thereby reducing the bistability threshold.

[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 coefficient, which makes graphene a popular 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. The energy of the defect mode is mainly distributed in the defect layer, so embedding graphene in the defect layer can greatly improve the nonlinear effect of graphene.

[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, and 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 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-photonic crystal. By embedding graphene into TM photonic crystals, optical bistability can be achieved, and the threshold of the 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 photonic crystal sequence 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 transmission modes are also called fractal resonance states. The optical fractal state has a strong localization to the electric field and can be used to achieve low-threshold optical bistability. Whether it is possible to find quasi-periodic photonic crystals with stronger electric field localization, and then compound them with graphene to further enhance the nonlinear effect of graphene and lower the threshold of the optical bistability; and then apply the optical bistability to optical logic devices to obtain optical logic devices with low decision thresholds and adjustable thresholds, is the 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 Period-Doubling 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 to design a photonic crystal that can be used for an optical logic device.

[0010] The objectives of the present invention can be achieved through the following technical solutions: an optical logic device based on Period-Doubling photonic crystals, characterized in that it includes a plurality of first dielectric layers, a plurality of second dielectric layers and two graphene monolayers, the first dielectric layer is denoted as H, the second dielectric layer is denoted as L, the graphene monolayer is denoted as G, the layered structure of the optical logic device is expressed as: HLH1GH2HH2GH1LHL, 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 Period-Doubling 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 logic device, 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 decision thresholds and threshold intervals of logic 1 and logic 0 of the optical logic device are regulated by the chemical potential of the graphene monolayer.

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

[0014] Two dielectric sheets H and L with different refractive indices, high and low, are arranged in sequence according to the Period-Doubling (PD) sequence with sequence number N=3; two graphene monolayers are then embedded in the PD photonic crystal to form a composite structure; there is an optical fractal form in the PD 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 strongest electric field position corresponding to one of the optical fractal forms, 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 TM photonic crystal and graphene.

[0015] The upper and lower thresholds of the optical bistable state in the PD 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 bistable effect is applied to an optical logic device, 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. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0018] Figure 3 It is the normalized electric field intensity distribution of the optical spectrum corresponding to the wavelength λ1=2.0855μm.

[0019] Figure 4 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 Period-Doubling (PD) sequence is: S0 = H, S1 = HL, S2 = HLHH, ..., S N =S N-1 S N-2 S N-2 , ..., where N (N = 0, 1, 2, 3, ...) represents the sequence number, S N Indicates the Nth item of the sequence. In the corresponding PD photonic crystal, the letters H and L represent two uniform dielectric sheets with high and low refractive indices, respectively.

[0025] Figure 1The schematic diagram of the composite structure of PD photonic crystal and graphene with sequence number N=3 is given. The PD photonic crystal with sequence number N=3 can be expressed as: HLHHHLHL, 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 of one fractal form is the strongest to form a composite structure, which can be expressed as: HLH1GH2HH2GH1LHL, where G represents a graphene monolayer.

[0026] In the PD photonic crystal, H, H1 and H2 are high refractive index materials such as lead telluride, whose refractive index is n H =4.1; L is the low refractive index material cryolite, whose refractive index is n L =1.35. The thickness of H and L is 1 / 4 optical wavelength, that is, the thickness of H is d H =λ0 / 4 / n H = 0.0945 μm (μm means micrometer), where λ0 = 1.55 μm is the central wavelength, and the thickness of L is d L =λ0 / 4 / n L = 0.287 μm. The thickness of H1 is d H1 = 0.0145 μm, the thickness of H2 is d H2 =0.08μm, satisfying condition d H1 +d L2 =d H The incident light is a transverse magnetic wave, incident 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 0.33 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 the dielectric sheets H, H1, H2 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 2 The linear transmission spectrum of light in the PD photonic crystal with sequence number N = 3 is given. The ordinate T represents the transmittance of the light wave; the abscissa (ω-ω0) / ω gap represents the normalized angular frequency, where ω = 2πc / λ, ω0 = 2πc / λ0 and ω gap =4ω0arcsin│(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, respectively, c is the speed of light in vacuum, and arcsin is the inverse sine function. In the normalized frequency interval [-1,1], there is a photonic band gap, and two transmission peaks appear in the middle of the band gap, corresponding to two resonant optical fractal forms; they are independent of each other and separated by an appropriate distance; the two transmittance peaks are both 0.9687, and the corresponding medium wavelengths are: λ1=2.0855μm and λ2=1.2333μm. Both of these optical fractal forms have a local effect on the electric field. Here, only the first resonant 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 local electric field is the strongest in the composite 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 first resonance wavelength λ1, the incident wavelength must be appropriately red-detuned relative to λ1 = 2.0855 μm.

[0029] Figure 3 The given is Figure 2 The electric field distribution of the first resonant optical fractal in the composite structure, the corresponding resonant wavelength is λ1 = 2.0855μm. The dotted line represents the interface between two adjacent dielectric sheets, and the two graphene monolayers G are respectively embedded in the two locations with the strongest local electric field in the structure. The ordinate represents 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 third-order nonlinear optical effect of graphene is proportional to the local electric field intensity, so the nonlinear effect of graphite is greatly enhanced.

[0030] The incident wavelength λ is fixed at 2.23 μm, and there is a certain red detuning relative to the first resonance wavelength λ1 = 2.0855 μm of the optical spectrum. When the input light is strong enough, considering the nonlinear effect of graphene, the chemical potential of graphene is fixed at μ = 0.4 eV, 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. There is an S-shaped curve segment in the contour line 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 uIt 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 =525.2662MW / cm 2 The logic 0 judgment threshold is I d =314.3101MW / cm 2 The interval between the logic 1 and logic 0 decision thresholds is I u -I d =210.9561MW / cm 2 .

[0031] Keep the incident wavelength λ = 2.23μm 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 segment 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, such as Figure 5 (b) is shown. th Indicates the threshold of the bistable state, symbol I u and I d Respectively represent the upper and lower thresholds of the bistable state; when 0.2eV≤μ<0.27eV and 0.28eV≤μ, the upper and lower thresholds of the bistable state, as well as the upper and lower threshold intervals of the bistable state increase with the increase of the graphene chemical potential; and when 0.27eV≤μ<0.28eV, the upper and lower thresholds of the bistable state, as well as the upper and lower threshold intervals of the bistable state decrease with the increase of the graphene chemical potential. This is because when the wavelength of the incident light is λ=2.23μm, the internal electrons of graphite will transition from intra-band transition to inter-band transition near the chemical potential μ=0.275eV. Therefore, the upper and lower thresholds and threshold intervals of the bistable state can be regulated by the chemical potential of graphene.

[0033] In the composite structure of TM photonic crystal and graphene, the optical bistability threshold is 100GW / cm 2 In the composite structure of PD photonic crystal and graphene, the threshold of optical bistability is reduced to 100MW / cm 2It can be seen that the threshold of optical bistability is reduced by 3 orders of magnitude.

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

[0035] The graphene chemical potential μ is fixed at 0.4 eV, and other parameters remain unchanged. Figure 6 (a) shows the input-output intensity relationship for different incident wavelengths. It can be seen that: when λ = 2.22μm ~ 2.25μm, the input-output intensity relationship is bistable; 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 wavelength detuning increases, the upper and lower thresholds of the bistable state increase, and the upper and lower threshold intervals of the bistable state increase, such as Figure 6 (b) As shown. The greater the wavelength detuning, the stronger the incident light field energy is required to bridge this difference by using nonlinear effects to achieve resonance. Therefore, the upper and lower thresholds of the bistable state, as well as the interval between the upper and lower thresholds, can be controlled by the incident wavelength.

[0036] In summary, in the composite structure of PD photonic crystal and graphene, there is a resonant optical morphology, which has a strong local effect on the electric field. The two graphene monolayers are located at the strongest position of the local electric field corresponding to one of the optical morphologies, so the nonlinear effect of graphene is greatly enhanced, thereby realizing a low-threshold optical bistability, and the threshold of the optical bistability can be as low as 100MW / cm 2 The optical bistability threshold is 3 orders of magnitude smaller than that in the composite structure of TM photonic crystal and graphene. The optical bistability can be applied to an optical logic device, and the decision thresholds of logic 1 and logic 0 of the optical logic device correspond to the upper and lower thresholds of the optical bistability, respectively. 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 Period-Doubling photonic crystal, characterized in that: It includes a plurality of first dielectric layers, a plurality of second dielectric layers and two graphene monolayers, 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 layered structure of the optical logic device is expressed as: HLH1GH2HH2GH1LHL, 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 of the Period-Doubling 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 logic device.

2. According to claim 1, an optical logic device based on Period-Doubling photonic crystal is 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 Period-Doubling photonic crystal according to claim 1 or 2, characterized in that: The decision values ​​and threshold intervals of logic 1 and logic 0 of the optical logic device are regulated by the chemical potential of the graphene monolayer.

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

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