An optical memory based on Rudin-Shapino photonic crystal

Abstract

The present invention provides an optical storage device based on Rudin-Shapino photonic crystals, belonging to the field of all-optical communication technology. The optical storage device comprises 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 storage device is represented as: HHHLHHL1GL2HHL2GL1HHLHHH, wherein L1GL2 and L2GL1 both represent a three-layer structure formed by embedding a graphene monolayer in a second 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 storage device based on Rudin-Shapino photonic crystals can realize a low-threshold optical bistable state, and the upper and lower thresholds of the bistable state correspond to the write and read decision thresholds of the optical storage device, respectively. The present invention has the advantages of being applicable to optical storage devices, etc.

Description

Technical Field

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

[0002] In all-optical communication, information needs to be stored, transmitted, relayed, timed, amplified and shaped in the optical domain. This requires the development of all-optical devices that control light, and optical storage based on optical bistability is an important type of optical storage. Optical bistability is 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 output resonant states.

[0003] When optical bistable is applied to optical storage, the upper and lower thresholds of the bistable state correspond to the decision thresholds for writing and reading the optical storage respectively; the larger the decision threshold, the stronger the light intensity required to trigger the writing and reading of the optical storage. However, as the power of the device increases, the stability of the device during operation will deteriorate, and the requirements for heat dissipation conditions will also become higher. In addition, the smaller the interval between the upper and lower thresholds of the bistable state, the smaller the distinction between writing and reading the corresponding optical storage, which will lead to an increased probability of misoperation. Therefore, current research on optical bistable devices mainly focuses on how to reduce the threshold of optical bistable states through new materials and new structures, and how to 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 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 threshold of the 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 nonlinear optical 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 strength 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 third-order nonlinear optical 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. 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 is localized to 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 transmission modes in the TM photonic crystal split geometrically, so this effect is also called optical fractal. The localization of the electric field in the optical fractal form can achieve low-threshold optical bistability. Whether it is possible to find quasi-periodic photonic crystals with stronger electric field localization, and then composite them with graphene to further enhance the nonlinear effect of graphene and lower the threshold of optical bistability; and then apply optical bistability to optical storage to obtain optical logic devices with adjustable thresholds for writing and reading decisions, is the research focus in this field. Summary of the invention

[0009] The purpose of the present invention is to provide an optical storage 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 enhance the nonlinear effect of graphene and reduce the threshold of optical bistability so that it can be applied to optical storage devices.

[0010] The objective of the present invention can be achieved through the following technical solutions: an optical memory based on Rudin-Shapino photonic crystals, characterized in that it comprises 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, and the layered structure of the optical memory is represented as: HHHLHHL1GL2HHL2GL1HHLHHH, wherein L1GL2 and L2GL1 both represent a three-layer structure formed by embedding a graphene monolayer in a second 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 memory based on Rudin-Shapino photonic crystals can realize a low-threshold optical bistable state, and the upper and lower thresholds of the bistable state correspond to the write and read decision thresholds 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 write decision threshold, the read decision threshold, and the interval between the write decision threshold and the read decision threshold of the optical memory are regulated by the chemical potential of the graphene monolayer.

[0013] Furthermore, the write decision threshold, the read decision threshold, and the interval between the write decision threshold and the read decision threshold of the optical storage 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 optical 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 10MW / cm 2 , which is 4 orders of magnitude lower than the optical bistability threshold in the Thue-Morse photonic crystal and graphene composite structure.

[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 optical storage, the write and read decision thresholds of the optical storage, as well as the interval between the write and read decision thresholds, 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 two RS photonic crystals and graphene with sequence number N=3.

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

[0018] Figure 3 It is the normalized electric field distribution of the optical spectrum corresponding to the wavelength λ1 = 1.7271 μm.

[0019] Figure 4 The middle figure (a) shows the input-output light intensity relationship corresponding to different graphene chemical potentials; Figure 4 Figure (b) shows the relationship between the upper and lower thresholds of the bistable state and the change of the chemical potential of graphene.

[0020] Figure 5 Figure (a) shows the input-output intensity relationship for different incident wavelengths; Figure 5 The middle figure (b) shows the relationship between the upper and lower thresholds of the bistable state and the incident wavelength.

[0021] Figure 6 This is the schematic diagram of the optical memory based on optical bistable state.

[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: S0 = H, S1 = HH, S2 = HHHL, S3 = 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 SN-1 Replace HH in with HHHL.

[0025] Figure 1 The schematic diagram of the composite structure of two binary RS photonic crystals and graphene with sequence number N=3 is given. The two binary RS photonic crystals are symmetrically distributed about the origin, which can be expressed as: HHHLHHLHHLHHGHHH, where the letters H and L represent two uniform dielectric sheets with high and low refractive indices respectively; then two graphene monolayers are 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: HHHLHHL1GL2HHL2GL1HHLHHH, where G represents a graphene monolayer. The composite structure is centrally symmetrically distributed about the origin, similar to a distributed feedback Bragg grating. The horizontal right is the positive direction of the coordinate axis Z direction.

[0026] In the RS photonic crystal pair, H is a high refractive index material lead telluride, whose refractive index is n H =4.1; L, L1 and L2 are low refractive index materials 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 =0.287μm. The thickness of L1 is d L1 = 0.1933 μm, the thickness of L2 is d L2 =0.0937μm, meeting the condition d L1 +d L2 =d L The incident light is a transverse magnetic wave, incident vertically from the left.

[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, L, L1 and L2. 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 spectra of light waves in two RS photonic crystal composite structures with sequence number N = 3 are given. The vertical axis T represents the transmittance of light waves; the horizontal axis (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 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 [-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 an appropriate distance. These three transmittance peaks are all 1, and the corresponding medium wavelengths are: λ1 = 1.7271μm, λ2 = 1.55μm and λ3 = 1.4058μm. These three optical fractal forms all have a local effect on the electric field. Here, only the first resonance state (marked with an asterisk ☆) is selected to obtain its corresponding mode field distribution. Then, a graphene monolayer is embedded in the position where the local 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 realize low-threshold optical bistability near the first resonance peak, the incident wavelength must be appropriately detuned relative to the first resonance wavelength λ1 = 1.7271 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 λ1 = 1.7271μm. 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 intensity 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 optical third-order nonlinear 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 1.748μm, and there is a certain red detuning relative to the first optical fractal resonance wavelength λ1 = 1.7271μm. When the input light intensity is strong enough, considering the nonlinear effect of graphene, other parameters remain unchanged. Figure 4 (a) shows the input-output light intensity relationship corresponding to different graphene chemical potential μ. i Indicates the input light intensity, the ordinate I o Indicates the output light intensity; unit: MW / cm 2 It represents megawatts per square centimeter. It can be seen that when μ = 0.4eV and 0.5eV, there is an S-shaped curve segment in the input-output light intensity relationship curve, corresponding to the bistable relationship; while when μ = 0.3eV, the input-output light intensity relationship is non-bistable.

[0031] When the input light intensity increases, the output light intensity jumps upward at the right turning point of the S-curve segment. The corresponding input light intensity value at this time is called the upper threshold of the optical bistable state. When the input light intensity gradually decreases from a relatively large value, the output light intensity jumps downward at the left turning point of the S-curve segment. The corresponding input light intensity value at this time is called the lower threshold of the optical bistable state. The difference between the upper and lower thresholds is called the threshold interval.

[0032] When the input light intensity is between the upper and lower thresholds, one input light intensity value corresponds to two output light intensity values, which is called optical bistability. The S-shaped curve segment in the input-output light intensity relationship contour is a typical feature of optical bistability, and this effect can be used in optical storage.

[0033] As the μ value increases, different graphene chemical potentials correspond to different bistable curves, and the upper and lower thresholds and threshold intervals of the bistable state are also different; as the graphene chemical potential increases, the upper and lower thresholds of the bistable state increase, and the interval between the bistable thresholds increases, such as Figure 4 (b) is shown. th Indicates the threshold of bistable state; symbol I u and I d Respectively represent the upper and lower thresholds of the bistable state. When μ ≥ 0.36 eV, the input-output light intensity relationship is bistable; 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 regulated by the chemical potential of graphene.

[0034] Optical bistability threshold of 100 GW / cm in a composite system of Thue-Morse photonic crystal and graphene 2 In the composite structure of two RS photonic crystals and graphene, the optical bistability threshold is greatly reduced to 10 MW / cm 2 Magnitude.

[0035] In addition, different incident wavelengths correspond to different bistable curves and thresholds.

[0036] The graphene chemical potential μ is fixed at 0.4 eV, and other parameters remain unchanged. Figure 5 (a) shows the input-output light intensity relationship for different incident wavelengths. It can be seen that: when λ = 1.57μm ~ 1.58μm, the input-output optics are bistable; the bistable curves for different incident wavelengths are different, that is, the upper and lower thresholds and threshold intervals of the bistable are different; as the incident wavelength increases, that is, the wavelength detuning increases, the upper and lower thresholds of the bistable increase, and the upper and lower threshold intervals of the bistable also increase, such as Figure 5(b) As shown. The greater the detuning of the wavelength, the more nonlinear effects are needed to bridge this difference in order to achieve 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.

[0037] In summary, two graphene monolayers are embedded in the RS photonic crystal pair to form a composite structure, in which there is a resonant optical fractional form, which has a strong local effect on the electric field; the two graphene monolayers are located exactly at the strongest position of the local electric field corresponding to one of the optical fractional 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 10MW / cm 2 The optical bistability is 4 orders of magnitude smaller than that in the composite of Thue-Morse photonic crystal and graphene. The optical bistability can be applied to optical memory, and the write and read decision thresholds of the optical memory, as well as the interval between the write and read decision thresholds, can be flexibly controlled by the chemical potential of graphene and the incident wavelength.

[0038] The incident light wavelength is set to λ = 1.748μm, the chemical potential is μ = 0.4eV, and an optical bistability phenomenon appears in the input-output light intensity relationship. It is applied to optical storage. The principle is as follows Figure 6 As shown in the figure, when the input light intensity gradually increases from a low value, at the right inflection point of the S-curve segment, the output light intensity will jump upward. i =I u This process is called the upper threshold of optical bistability, which corresponds to the writing process of optical storage. i =I u It is called the write decision threshold of the optical memory. When the input light intensity gradually decreases from a higher value, at the left turning point of the S-curve segment, the output light intensity will jump downward. i =I d It is called the lower threshold of optical bistability. This process corresponds to the reading process of optical storage. i =I d It is called the read decision threshold of the optical storage. At this time, the corresponding write decision threshold is I u =44.4985MW / cm 2 , the reading decision threshold is I d =31.9573MW / cm 2 , the interval between the write and read decision thresholds is I u -I d =12.5412MW / cm 2 .

[0039] Figure 4 (b) and Figure 5 (b) shows that the input-output curve of the optical bistability is affected by the chemical potential of graphene and the incident wavelength. The positions of the two inflection points in the corresponding S-curve segment change with different graphene chemical potential and input wavelength. The two inflection points of the S-curve segment correspond to the upper and lower thresholds of the optical bistability, that is, the read and write decision thresholds of the optical memory. Therefore, the read and write decision thresholds of the optical memory, as well as the interval between the read and write decision thresholds, can be regulated by the chemical potential of graphene and the input wavelength. It can be seen that when the chemical potential and the input wavelength increase, the read and write decision thresholds of the optical memory, as well as the interval between the read and write decision thresholds, all increase.

[0040] The larger the decision threshold interval of the optical memory, the greater the distinction between write and read operations, and the smaller the probability of misoperation. To reduce the misoperation rate of the optical memory, it is necessary to increase the chemical potential or wavelength detuning of graphene, while at the same time the write and read decision thresholds of the optical memory will increase. Therefore, reducing the misoperation rate of the optical memory comes at the expense of increasing the decision threshold of the optical memory.

[0041] 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 memory based on Rudin-Shapino photonic crystal, characterized in that: The optical memory device comprises 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 memory device is expressed as: HHHLHHL1GL2HHL2GL1HHLHHH, wherein L1GL2 and L2GL1 both represent a three-layer structure formed by embedding a graphene monolayer in a second 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 memory device based on Rudin-Shapino photonic crystals can realize low-threshold optical bistable states, and the upper and lower thresholds of the bistable states respectively correspond to the write and read decision thresholds of the optical memory device.

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

4. The optical memory based on Rudin-Shapino photonic crystal according to claim 1 or 2, characterized in that: The write decision threshold, the read decision threshold, and the interval between the write decision threshold and the read decision threshold of the optical memory are regulated by the incident wavelength.

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