CN113687560B - An optical logic device based on Rudin-Shapino photonic crystal - Google Patents

Abstract

The invention provides an optical logic device based on Lu Ding-Xia Pinuo photonic crystals, and belongs to the technical field of all-optical communication. The two-dimensional RS photonic crystal comprises two symmetrical 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, the first dielectric layers are marked as H, the second dielectric layers are marked as L, the graphene monolayers are marked as G, the binary RS photonic crystal is marked as HHHLHHLH, the optical logic is marked as HHHLHHLH ₁GH₂H₂GH₁ LHHLHHH, both H ₁GH₂ and H ₂GH₁ represent three-layer structures formed by embedding the graphene monolayers into the first dielectric layers, the thicknesses of the first dielectric layers and the second dielectric layers are respectively 1/4 of the respective optical wavelengths, and the first dielectric layers and the second dielectric layers are respectively two uniform dielectric sheets with high refractive indexes and low refractive indexes. The invention has the advantages of low decision threshold value and the like.

Description

Lu Ding-Xia Pinuo photonic crystal-based optical logic device

Technical Field

The invention belongs to the technical field of all-optical communication, and relates to an optical logic device based on Lu Ding-Xia Pinuo photonic crystals.

Background

In all-optical communication, information needs to be stored, transmitted, relayed, decided, timed, amplified, shaped and the like in an optical domain, which is an all-optical device for controlling light, and an optical logic based on optical bistable state is an important category. Optical bistable is a nonlinear optical effect based on the optical kerr effect of the material. When the incident light is sufficiently strong, one input intensity value may correspond to two different output intensity values, i.e., one incident intensity value may induce two stable resonant output states.

When the optical bistable state is applied to the optical logic device, the upper and lower threshold values of the bistable state correspond to the decision threshold values of logic 1 and logic 0 of the optical logic device respectively, and the larger the decision threshold value is, the stronger the light intensity required for triggering the decision of the optical logic device is. However, as the power of the device increases, the stability of the device during operation becomes poor, and the requirements for heat dissipation conditions are also increased. In addition, the smaller the upper and lower threshold intervals of bistable state, the smaller the discrimination of the logic 1 and logic 0 of the corresponding optical logic, which leads to an increase in the false positive rate. Thus, current research on optical bistable devices is focused mainly on how to lower the threshold value of optical bistable by new materials and new structures, and to increase the interval between upper and lower threshold values.

In order to achieve a low threshold optical bistable effect, materials with a large third order nonlinear optical coefficient are sought on the one hand, and local electric fields are enhanced by optimizing the system structure on the other hand. The optical kerr effect is proportional to the local electric field, so that a strong local electric field can increase the third order nonlinear optical effect of the material, thereby lowering the threshold of optical bistable state.

Graphene is an emerging two-dimensional material with ultra-thin properties and excellent electrical conductivity. The surface conductivity of graphene can be flexibly regulated and controlled by the chemical potential of graphene. Importantly, graphene has a considerable third-order optical nonlinear optical coefficient, which makes graphene a hotspot material in optical bistable research. In addition, in order to further reduce the threshold value of bistable state, the surface plasmon of the graphene can be utilized to enhance the local electric field of the graphene, and the graphene can be embedded into the defect photonic crystal to enhance the nonlinear effect. In the defect photonic crystal, the energy of the defect mode is mainly distributed in the defect layer, so that graphene is embedded in the defect layer, the nonlinear effect of the graphene can be greatly enhanced, and the low-threshold optical bistable state is realized.

The two dielectric thin plates with different refractive indexes are alternately arranged in space to form the photonic crystal with a periodic structure. In the wave vector space, the photonic crystal has a photonic band structure similar to the electron band in semiconductors. Light waves within the band gap will be totally reflected. If defects are introduced into the photonic crystal, a transmission mode appears in the transmission spectrum, and the transmission mode is a defect mode and has strong local action on an electric field, and is often used for enhancing the third-order nonlinear effect of a material.

The quasi-photonic crystal or the non-periodic photonic crystal has a natural defect layer, and the number of the defect modes increases with the increase of the sequence number in geometric progression, so the quasi-photonic crystal or the non-periodic photonic crystal is often used for enhancing the locality of an electric field.

The Thue-Morse (TM) sequence is mathematically a quasi-periodic sequence whose corresponding photonic crystal is a quasi-periodic photonic crystal. The graphene is embedded in a TM photonic crystal to achieve optical bistable, with a threshold of approximately 100GW/cm ² (gigawatts per square centimeter). The TM photonic crystal has a plurality of defect cavities, and a plurality of defect modes, namely resonant transmission modes, exist in the same defect cavity. As the serial number increases, the number of dielectric layers in TM photonic crystals increases accordingly, and transmission modes in the transmission spectrum are geometrically split, so these resonance modes are also called optical morphologies. The optical morphology has locality to the electric field and is commonly used for enhancing the third-order nonlinear effect of materials, so that the low-threshold optical bistable state can be realized. Whether a quasi-periodic photonic crystal with stronger electric field locality can be found and then the composite structure of the quasi-periodic photonic crystal and graphene is adopted, so that the nonlinear effect of the graphene is further enhanced, the threshold value of optical bistable state is reduced, and the research focus in the field is achieved.

Disclosure of Invention

The invention aims to solve the problems in the prior art and provide an optical logic device based on Lu Ding-Xia Pinuo photonic crystals, and the technical problem to be solved by the invention is how to reduce the decision threshold of the optical logic device.

The optical logic device based on the Lu Ding-Xia Pinuo photonic crystal is characterized by comprising two symmetrically distributed binary RS photonic crystals and two graphene monolayers, wherein the binary RS photonic crystals comprise a plurality of first dielectric layers and a plurality of second dielectric layers, the first dielectric layers are marked as H, the second dielectric layers are marked as L, the graphene monolayers are marked as G, the binary RS photonic crystals are marked as HHHLHHLH, the optical logic device is marked as HHHLHHLH ₁GH₂H₂GH₁ LHHLHHH, both H ₁GH₂ and H ₂GH₁ are respectively marked as three-layer structures formed by embedding the graphene monolayers into the first dielectric layers, the thicknesses of the first dielectric layers and the second dielectric layers are respectively 1/4 of the respective optical wavelengths, the first dielectric layers and the second dielectric layers are respectively two uniform dielectric sheets with high refractive indexes and low refractive indexes, the optical logic device based on the Lu Ding-Xia Pinuo can realize low-threshold optical decision, and the bistable photonic crystal device can respectively realize the upper and lower optical threshold value corresponding to the threshold value of 1 and the logic 0 of the bistable state memory.

Further, the first dielectric layer is lead telluride with high refractive index material, and the second dielectric layer is cryolite with low refractive index material.

Further, 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 and controlled by the chemical potential of the graphene monolayer.

Further, 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 are regulated by the incident wavelength.

The method comprises the steps of sequentially arranging two dielectric thin sheets A and B with different refractive indexes according to a Lu Ding-Xia Pinuo (Rudin-shape: RS) sequence with sequence number of N=3 to form an RS photon crystal pair symmetrical about an origin, embedding two graphene monolayers into the RS photon crystal pair to form a composite structure, wherein the RS photon crystal pair is in an optical fractional form, the optical fractional form has a local effect on an electric field, the two graphene monolayers are exactly positioned at the position with the strongest local electric field corresponding to one optical fractional form, so that the third-order nonlinear effect of graphene is greatly enhanced, further, the low-threshold optical bistable state is realized, and the threshold of the optical bistable state in the structure can be as low as 100MW/cm ², which is 3 orders of magnitude lower than that of the optical bistable state in the composite structure of Thue-Morse photon crystal and graphene.

The upper and lower thresholds of the optical bistable state in the Lu Ding-Xia Pinuo photonic crystal structure and the interval between the upper and lower thresholds increase with the chemical potential and the incident wavelength of the graphene. Therefore, when the optical bistable effect is applied to the optical logic device, the logic 1, the logic 0 judgment threshold value and the interval between the logic 1 and the logic 0 judgment threshold value of the optical logic device can be flexibly regulated and controlled through the chemical potential and the incident wavelength of the graphene.

Drawings

Fig. 1 is a schematic diagram of an RS photonic crystal and graphene composite structure with sequence number n=3.

Fig. 2 is a linear transmission spectrum of light waves in an RS photonic crystal with sequence number n=3.

Fig. 3 is a normalized electric field distribution for an optical fractal corresponding to wavelength λ ₃ = 1.4058 μm.

Fig. 4 is a schematic diagram of an optical logic based on optical bistable states.

The graph (a) in fig. 5 shows the input-output light intensity relationship corresponding to different graphene chemical potentials, and the graph (b) in fig. 5 shows the bistable upper and lower threshold values in the relationship with the change of the graphene chemical potentials.

Fig. 6 (a) shows the input-output light intensity relationship corresponding to different incident wavelengths, and fig. 6 (b) shows the bistable upper and lower thresholds as a function of incident wavelengths.

In the figure, H, a first dielectric layer, L, a second dielectric layer and G, a graphene monolayer.

Detailed Description

The following are specific embodiments of the present invention and the technical solutions of the present invention will be further described with reference to the accompanying drawings, but the present invention is not limited to these embodiments.

Mathematically, lu Ding-Xia Pinuo (Rudin-shape: 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 of the sequence, S _N represents the nth item of the sequence, hh→ HHHL represents the replacement of HH in S _N-1 with HHHL.

Fig. 1 shows a schematic diagram of a composite structure of a binary RS photonic crystal with sequence number n=3 and graphene. The two binary RS photon crystals are symmetrically distributed about an origin and can be expressed as HHHLHHLHHLHHLHHH, wherein letters H, L respectively represent two uniform dielectric thin plates with different refractive indexes, an optical fractal effect exists in the photon crystals, and then two graphene monolayers are respectively embedded into the position with the strongest local electric field corresponding to one of the fractal forms to form a composite structure, and the composite structure can be expressed as HHHLHHLH ₁GH₂H₂GH₁ LHHLHHH, wherein G represents the graphene monolayers. The whole composite structure is symmetrically distributed about the origin, similar to a distributed feedback Bragg grating.

In the composite structure of the RS photonic crystal pair and graphene, H is a high refractive index material of lead telluride, the refractive index of which is n _H＝4.1;L、L₁ and L ₂ is a low refractive index material of cryolite, and the refractive index of which is n _L =1.35. Both H and L have a thickness of 1/4 optical wavelength, i.e. H has a thickness d _H＝λ₀/4/n_H = 0.0945 μm (μm represents micrometers), where λ ₀ =1.55 μm is the center wavelength, L has a thickness d _L＝λ₀/4/n_L＝0.287μm.L₁ and d _L1＝0.0088μm,L₂ has a thickness d _L2 = 0.0857 μm, satisfying the condition d _L1+d_L2＝d_L. The incident light is transverse magnetic wave and vertically enters from the left. The horizontal right direction is the positive direction of the coordinate axis Z direction.

The thickness of the single-layer graphene is about 0.33nm (nm represents nanometers), which corresponds to the size of one atom. The thickness of the graphene is negligible relative to the thickness of the dielectric sheets H and L. Here, the ambient temperature is set to 300K (K represents kelvin), and the relaxation time τ=0.5 ps (ps represents picoseconds) of electrons in graphene.

Changing the frequency of the incident light, when the influence of graphene is not considered, fig. 2 shows a linear transmission spectrum of light in the RS photonic crystal of sequence number n=3. The ordinate T represents the transmittance of the light wave; the abscissa (ω - ω ₀)/ω_gap represents the normalized angular frequency, where ω=2ρc/λ, ω ₀＝2πc/λ₀ and ω _gap＝4ω₀arcsin│(n_H-n_L)/(n_H+n_L)|²/pi represent the incident light angular frequency, the incident light center angular frequency and the angular frequency band gap, respectively, c is the light velocity in vacuum, arcsin is an arcsin function, within the normalized frequency interval of [ -0.25,0.25], there are 3 formants of transmittance corresponding to 3 resonant optical fraction forms, which are independent of each other and at a suitable distance, these 3 transmittance peaks are all 1, the corresponding mid-wavelengths are respectively: the 3 optical sub-forms of lambda ₁＝1.7271μm、λ₂ =1.55 mu m and lambda ₃ = 1.4058 mu m have local effect on the electric field, only the 3 rd resonance state (marked by star) is selected to obtain the corresponding mode field distribution, and then the graphene single layer is inlaid at the position with the strongest electric field intensity in the structure, so that the nonlinear effect of the graphene is enhanced, and the low-threshold optical bistable state is realized.

Fig. 3 shows the electric field distribution of the 3 rd resonant optical fraction in the composite structure of fig. 2, corresponding to a resonant wavelength λ ₃ = 1.4058 μm. The dashed lines represent the interfaces of two adjacent dielectric layers, and two graphene monolayers G are respectively embedded at two positions in the structure where the local electric field is strongest. The ordinate represents the normalized Z-component electric field strength. It can be seen that the distribution of electric field energy in the structure is non-uniform, with localized properties. The two graphene monolayers are located exactly at the two locations where the local electric field is strongest. The optical third-order nonlinear effect of graphene is proportional to the local electric field strength, and thus, the nonlinear effect of graphite is greatly enhanced.

There is some red detuning of the fixed incident wavelength λ= 1.423 μm, relative to the optical fractal resonance wavelength λ ₃ = 1.4058 μm. Increasing the incident light intensity, considering the nonlinear effect of graphene, the chemical potential of graphene is set to μ=0.4ev, other parameters remain unchanged, and fig. 4 shows that an optical bistable phenomenon occurs in the input-output light intensity relationship, and applies it to an optical logic device. The abscissa I _i represents the input light intensity, the ordinate I _o represents the output light intensity, and the unit MW/cm ² represents megawatts per square centimeter. An S-shaped curve segment appears in the contour of the input-output intensity relationship, which is a typical feature of optical bistable states. When the input light intensity gradually increases from a lower value, the output light intensity at the right inflection point of the S-shaped curve section is in an upward jump, the input light intensity I _i＝I_u is called an upper threshold value of optical bistable state, the process corresponds to logic 1 of an optical logic device, I _i＝I_u is called a judgment threshold value of logic 1 of the optical logic device, when the input light intensity gradually decreases from a higher value, the output light intensity at the left inflection point of the S-shaped curve section is in a downward jump, the input light intensity I _i＝I_d is called a lower threshold value of optical bistable state, the process corresponds to logic 0 of the optical logic device, and I _i＝I_d is called a judgment threshold value of logic 0 of the optical logic device. At this time, the decision threshold of logic 1 is I _u＝275.7236MW/cm², the decision threshold of logic 0 is I _d＝232.2537MW/cm², and the interval between the decision thresholds of logic 1 and logic 0 is I _u-I_d＝43.4699MW/cm².

Keeping the incident wavelength λ= 1.423 μm and other parameters unchanged, fig. 5 (a) shows the input-output light intensity relationship corresponding to different graphene chemical potentials μ. It can be seen that the input-output intensity curves each have a sigmoid curve, i.e. a bistable relationship, when μ=0.3 eV, 0.4eV and 0.5 eV.

The value of mu is increased, bistable curves corresponding to different chemical potentials are different, and the upper threshold value, the lower threshold value and the threshold interval of the bistable state are also different. When 0.2 eV.ltoreq.mu.0.42 eV and 0.44 eV.ltoreq.mu, the bistable upper and lower thresholds, and the bistable threshold interval increase with increasing graphene chemical potential, as shown in FIG. 5 (b). And when 0.42eV is less than or equal to mu <0.44eV, the bistable upper and lower thresholds and bistable threshold intervals decrease with the increase of the chemical potential of the graphene. This occurs because at incident light wavelengths λ= 1.423 μm, the internal electrons of graphite transition from intraband to interband transitions around the chemical potential 0.43 eV. The ordinate I _th represents the bistable threshold value, and the symbols I _u and I _d represent the bistable upper and lower threshold values, respectively. Thus, the upper and lower thresholds and threshold intervals of bistable states can be regulated by the chemical potential of graphene.

In Thue-Morse photonic crystal and graphene composite system, the optical bistable threshold is of the order of 100GW/cm ², whereas in RS photonic crystal and graphene composite structure, the optical bistable threshold is reduced to the order of 100MW/cm ².

In addition, the corresponding bistable curves and thresholds are different for different incident wavelengths.

Other parameters remained unchanged when graphene chemical potential μ=0.4 eV was immobilized, and fig. 6 (a) shows the input-output light intensity relationship corresponding to different incident wavelengths. It can be seen that when λ= 1.746 μm to 1.749 μm, the input-output optics are bistable, the bistable curves corresponding to different incident wavelengths are different, i.e. the upper and lower thresholds and the threshold widths of the bistable states are different, and as the incident wavelength increases, i.e. the wavelength detuning amount increases, the upper and lower thresholds of the bistable states increase, and the threshold interval of the bistable states increases, as shown in fig. 6 (b). The greater the amount of wavelength detuning, the more nonlinear effects are needed to bridge this difference to achieve optical resonance, and the more intense the incident light energy is needed to meet resonance. Thus, the bistable upper and lower thresholds and threshold intervals can be modulated by the incident wavelength.

In summary, in the combination of the RS photonic crystal pair and graphene, there is a resonant optical sub-form, the optical sub-form has a strong local action on the electric field, and the two single-layer graphene layers are just located at the position where the local electric field corresponding to one of the optical sub-forms is the strongest, so that the nonlinear effect of the graphene is greatly enhanced, thereby realizing low-threshold optical bistable state, the threshold of which is as low as 100MW/cm ², and is 3 orders of magnitude smaller than that in the combination of the Thue-Morse photonic crystal and graphene. The optical bistable state can be applied to an optical logic device, the decision threshold values of logic 1 and logic 0 of the optical logic device, and the interval between the decision threshold values of logic 1 and logic 0 can be flexibly regulated and controlled through the chemical potential and the incident wavelength of graphene.

The specific embodiments described herein are offered by way of example only to illustrate the spirit of the invention. Those skilled in the art may make various modifications or additions to the described embodiments or substitutions thereof without departing from the spirit of the invention or exceeding the scope of the invention as defined in the accompanying claims.

Claims (4)

1. An optical logic device based on Rudin-Shapino photonic crystals, characterized in that it comprises two symmetrically distributed binary RS photonic crystals and two graphene monolayers, wherein the binary RS photonic crystals comprise a plurality of first dielectric layers and a plurality of second dielectric layers, wherein the first dielectric layers are denoted as H, the second dielectric layers are denoted as L, the graphene monolayers are 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, wherein H ₁ GH ₂ and H ₂ GH ₁ represents a three-layer structure formed by a single graphene layer 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. 2. According to claim 1, an optical logic device based on Rudin-Shapino photonic crystal is characterized in that the first dielectric layer is a high refractive index material lead telluride, and the second dielectric layer is a low refractive index material cryolite. 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 regulated by the incident wavelength.