An all-optical switch based on period-doubling photonic crystal

Abstract

The present invention provides an all-optical switch based on Period-Doubling photonic crystal, which belongs to the technical field of all-optical communication. 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, and the graphene monolayer is denoted as G. The layered structure of the all-optical switch is represented as: HL1GL2HH1GH1HL2GL1HL, wherein L1GL2 and L2GL1 both represent a three-layer structure formed by a graphene monolayer embedded in the second dielectric layer, H1GH1 represents a three-layer structure formed by a graphene monolayer embedded in the middle of 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 present invention can be applied to all-optical switches.

Description

Technical Field

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

[0002] In all-optical communication, information needs to be stored, transmitted, relayed, judged, timed, amplified and shaped in the optical domain, which requires the use of all-optical devices that control light, and all-optical switches based on optical bistability are an important type of them. 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 resonant output states.

[0003] When optical bistable is applied to an all-optical switch, the upper and lower thresholds of the bistable state correspond to the on and off trigger thresholds of the all-optical switch, respectively; the larger the trigger threshold, the stronger the input light energy required to trigger the on and off of the all-optical switch. However, as the power of the device increases, the working stability of the device will deteriorate, 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 the on and off of the corresponding all-optical switch, which will lead to an increase in the error rate. 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 large third-order nonlinear optical coefficients are sought; on the other hand, the locality of the 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 nonlinear optical coefficient, which makes graphene an important 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 enhance 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 strong local effect on the electric field and is often used to enhance the third-order nonlinear effect of the material.

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

[0008] The Thue-Morse (TM) sequence is a quasi-periodic sequence in mathematics, and its corresponding TM photonic crystal is a quasi-photonic crystal. By embedding graphene into the TM photonic crystal, optical bistability can be achieved, and the threshold of optical bistability is about 100GW / cm 2 (Gigawatts per square centimeter). There are multiple defect cavities in TM photonic crystals, and there are multiple defect modes in the same defect cavity, that is, 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 optical fractals. The optical fractal is localized to the electric field and can achieve low-threshold optical bistability. Whether it is possible to find a quasi-periodic photonic crystal with stronger electric field localization, so as to compound it with graphene, further enhance the nonlinear effect of graphene, and reduce the threshold of the optical bistability; and then apply the optical bistability to all-optical switches to obtain all-optical switches with low trigger 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 all-optical switch 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 how to utilize the localization of the electric field by the optical fractal form in the Period-Doubling photonic crystals to enhance the nonlinear effect of graphene, thereby reducing the threshold of the optical bistability, so that it can be applied to an all-optical switch with a low threshold and adjustable threshold.

[0010] The object of the present invention can be achieved by the following technical solutions: an all-optical switch based on Period-Doubling photonic crystal, characterized in that it includes a plurality of first dielectric layers, a plurality of second dielectric layers and three graphene monolayers, the first dielectric layer is marked as H, the second dielectric layer is marked as L, the graphene monolayer is marked as G, the layered structure of the all-optical switch is represented as: HL1GL2HH1GH1HL2GL1HL, wherein L1GL2 and L2GL1 both represent a three-layer structure formed by a graphene monolayer embedded in the second dielectric layer, the H1GH1 represents a three-layer structure formed by a graphene monolayer embedded in the middle of 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 all-optical switch based on 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 on and off trigger thresholds of the all-optical switch, 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 on trigger threshold, the off trigger threshold and the interval between the on trigger threshold and the off trigger threshold of the all-optical switch are regulated by the chemical potential of the graphene monolayer.

[0013] Furthermore, the on trigger threshold, the off trigger threshold and the interval between the on trigger threshold and the off trigger threshold of the all-optical switch 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; three graphene monolayers are then embedded in the PD photonic crystal to form a composite structure; there are optical fractals in the PD photonic crystal, and the optical fractals have a strong local effect on the electric field; the three graphene monolayers are located exactly at the position where the local electric field corresponding to one of the optical fractals is the strongest, so the third-order nonlinear effect of graphene is greatly enhanced, thereby realizing a low-threshold optical bistability; the threshold of the optical bistability in this structure can be as low as 100MW / cm 2 , which is 3 orders of magnitude lower than the threshold of optical bistability in the composite structure of TM photonic crystal and graphene.

[0015] The upper and lower thresholds of the optical bistability 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 bistability effect is applied to an all-optical switch, the on and off trigger thresholds of the all-optical switch, as well as the interval between the on and off trigger 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 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 distribution of the optical spectrum corresponding to the wavelength λ2 = 1.2333 μm.

[0019] Figure 4 This is the schematic diagram of the all-optical switch based on optical bistability.

[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 light 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 1 The schematic diagram of the composite structure of the PD photonic crystal and graphene with sequence number N = 3 is given. The PD 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 three graphene monolayers are respectively embedded in the strongest position of the electric field intensity distribution corresponding to one of the fractal forms to form a composite structure, which can be expressed as: HL1GL2HH1GH1HL2GL1HL, where G represents a graphene monolayer.

[0026] In the PD photonic crystal pair, H and H1 are high refractive index materials, 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 =0.2084μm, the thickness of L2 is d L2 =0.0786μm, meeting the condition d L1 +d L2 =d L The thickness of H1 is d H1 =0.04725μm, meeting the condition d H1 =d H / 2. 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, 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 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 respectively represent the incident light angular frequency, the incident light center angular frequency and the angular frequency band gap, c is the speed of light in vacuum, and arcsin is the inverse sine function. In the normalized frequency interval [-1,1], there is a photonic band gap, and two transmittance resonance peaks appear in the middle of the band gap, corresponding to two resonant optical fractal forms. They are independent of each other and are 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 second 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 electric field intensity is the strongest in the structure, thereby enhancing the nonlinear effect of graphene and realizing a low-threshold optical bistability. In addition, to achieve a low-threshold optical bistability near the second resonance state, the incident wavelength must be appropriately red-detuned relative to the second resonance state wavelength λ2 = 1.2333 μm.

[0029] Figure 3 The given is Figure 2 The electric field distribution of the second resonant optical fractal in the composite structure, the resonant wavelength is λ2 = 1.2333μm. The dotted line represents the interface between two adjacent dielectric layers, and the three graphene monolayers G are respectively embedded in the three locations with the strongest 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 three graphene monolayers are located exactly at the three 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 1.275μm, and there is a certain red detuning relative to the resonant wavelength λ2 = 1.2333μm of the second optical fraction. When the incident light is strong enough, considering the nonlinear effect of graphene, the chemical potential of graphene is fixed at μ = 0.4eV, and other parameters remain unchanged. Figure 4 The optical bistability phenomenon in the input-output light intensity relationship is given and applied to all-optical switching. i Indicates the input light intensity, the ordinate I o Indicates the output light intensity. Unit: MW / cm 2Indicates 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 inflection point of the S-shaped curve segment, the output light intensity will jump upward. Therefore, the input light intensity I i =I u It is called the upper threshold of optical bistability. This process corresponds to the opening process of the all-optical switch. i =I u It is called the trigger threshold for the all-optical switch to turn on. When the input light intensity gradually decreases from a higher value, the output light intensity will jump downward at the left turning point of the S-shaped curve segment. Therefore, the input light intensity I i =I d It is called the lower threshold of optical bistability. This process corresponds to the turn-off process of the all-optical switch. i =I d It is called the trigger threshold of the all-optical switch off. At this time, the trigger threshold of the opening is I u =1023.5MW / cm 2 , the shutdown trigger threshold is I d =903.3MW / cm 2 , the interval between the turn-on and turn-off trigger thresholds is I u -I d =120.2MW / cm 2 .

[0031] Keep the incident wavelength λ = 1.275μm 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 the width between thresholds 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 dRespectively represent the upper and lower thresholds of the bistable state; when 0.2eV≤μ<0.47eV and 0.49eV≤μ, the upper and lower thresholds of the bistable state, as well as the threshold interval of the bistable state increase with the increase of the chemical potential of graphene; and when 0.47eV≤μ<0.49eV, the upper and lower thresholds of the bistable state, as well as the interval between the upper and lower thresholds of the bistable state decrease with the increase of the chemical potential of graphene. This phenomenon occurs because when the wavelength of the incident light is λ=1.275μm, the internal electrons of graphite will transition from intra-band transition to inter-band transition near the chemical potential μ=0.48eV. Therefore, the upper and lower thresholds of the bistable state and the threshold interval can be regulated by the chemical potential of graphene.

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

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

[0035] When the graphene chemical potential μ is fixed at 0.4 eV, other parameters remain unchanged. Figure 6 (a) shows the input-output light intensity relationship corresponding to different incident wavelengths. It can be seen that: when λ = 1.26μm, the input-output optics is non-bistable; when λ = 1.27μm and 1.28μm, the input-output optics are bistable; the bistable curves corresponding to different incident wavelengths are different, that is, the upper and lower thresholds and the threshold width of the bistable state are different. When λ>1.2675μm, the bistable phenomenon will appear, and as the incident wavelength increases, that is, the detuning amount increases, the upper and lower thresholds of the bistable increase, and the threshold interval of the bistable increases, such as Figure 6 (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.

[0036] In summary, in the composite of PD photonic crystal and graphene, there is a resonant optical morphology, which has a strong local effect on the electric field. The three monolayer graphenes are located at the strongest electric field position corresponding to one of the optical morphologies, so the nonlinear effect of graphene is greatly enhanced, thus realizing a low-threshold optical bistability, and the threshold of the optical bistability is as low as 100MW / cm 2The optical bistability is 3 orders of magnitude smaller than that in the composite of TM photonic crystal and graphene. The optical bistability can be applied to all-optical switches, and the on and off trigger thresholds of the all-optical switch, as well as the interval between the on and off trigger thresholds, 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 all-optical switch based on Period-Doubling photonic crystal, characterized in that: The invention comprises a plurality of first dielectric layers, a plurality of second dielectric layers and three 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 all-optical switch is represented as: HL1GL2HH1GH1HL2GL1HL, wherein L1GL2 and L2GL1 both represent a three-layer structure formed by embedding a graphene monolayer in the second dielectric layer, and H1GH1 represents a three-layer structure formed by embedding a graphene monolayer in the middle of the first dielectric layer. The thicknesses 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 all-optical switch 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 on and off trigger thresholds of the all-optical switch, respectively.

2. The all-optical switch based on Period-Doubling 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 all-optical switch based on Period-Doubling photonic crystal according to claim 1 or 2, characterized in that: The on trigger threshold, the off trigger threshold and the interval between the on trigger threshold and the off trigger threshold of the all-optical switch are regulated by the chemical potential of the graphene monolayer.

4. The all-optical switch based on Period-Doubling photonic crystal according to claim 1 or 2, characterized in that: The on trigger threshold, the off trigger threshold and the interval between the on trigger threshold and the off trigger threshold of the all-optical switch are regulated by the incident wavelength.

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