阅读说明：本技术 一种基于Rudin-Shapiro光子晶体对和石墨烯复合结构的光学双稳态 (Optical bistable state based on Rudin-Shapiro photonic crystal pair and graphene composite structure ) 是由 刘芳华 于 2021-09-26 设计创作，主要内容包括：本发明提供了一种基于Rudin-Shapiro光子晶体对和石墨烯复合结构的光学双稳态,属于光学技术领域。包括两个对称分布的二元RS光子晶体和若干石墨烯单层,所述二元RS光子晶体包括若干第一电介质层H和若干第二电介质层L,所述Rudin-Shapiro光子晶体对和石墨烯复合结构表示为HHHLGHHGLHHLGHHGLHHH,所述第一电介质层和第二电介质层分别为两种折射率高、低不同的均匀电介质薄片,所述第一电介质层和第二电介质层的厚度分别为各自光学波长的1/4。本发明具有光学双稳态的阈值低等优点。(The invention provides an optical bistable state based on a Rudin-Shapiro photonic crystal pair and graphene composite structure, and belongs to the technical field of optics. The two-dimensional RS photonic crystal comprises two symmetrically distributed binary RS photonic crystals and a plurality of graphene single layers, the binary RS photonic crystals comprise a plurality of first dielectric layers H and a plurality of second dielectric layers L, the Rudin-Shapiro photonic crystal pair and the graphene composite structure are represented as HHHLGHHGLHHLGHHGLHHH, the first dielectric layers and the second dielectric layers are respectively two uniform dielectric sheets with different refractive indexes of high and low, and the thicknesses of the first dielectric layers and the second dielectric layers are respectively 1/4 of respective optical wavelength. The invention has the advantages of low threshold value of optical bistable state and the like.)

1. An optical bistable state based on a Rudin-Shapiro photonic crystal pair and a graphene composite structure is characterized by comprising two binary RS photonic crystals and a plurality of graphene single layers which are symmetrically distributed, wherein the binary RS photonic crystals comprise a plurality of first dielectric layers H and a plurality of second dielectric layers L, and the Rudin-Shapiro photonic crystal pair and graphene composite structure is represented as HHHLGHHGLHHLGHHGLHHH; the composite structure has optical morphologies and has a local effect on an electric field, and the four graphene single layers are just positioned at the positions with the strongest electric field respectively; the first dielectric layer and the second dielectric layer are respectively two uniform dielectric sheets with different refractive indexes; the first dielectric layer and the second dielectric layer have a thickness of 1/4 a of the respective optical wavelength.

2. The optical bistable state based on the Rudin-Shapiro photonic crystal pair and graphene composite structure as claimed in claim 1, wherein said first dielectric layer is a high refractive index material of lead telluride, and said second dielectric layer is a low refractive index material of cryolite.

3. The optical bistability based on the Rudin-Shapiro photonic crystal pair and graphene composite structure as claimed in claim 1 or 2, wherein the upper threshold, the lower threshold and the interval between the upper and lower thresholds of the optical bistability are regulated by the chemical potential of the graphene monolayer.

4. The optical bistability based on the Rudin-Shapiro photonic crystal pair and graphene composite structure as claimed in claim 1 or 2, wherein the upper threshold, the lower threshold and the interval between the upper and lower thresholds of the optical bistability are regulated by the wavelength of incident light.

Technical Field

The invention belongs to the technical field of optics, and relates to an optical bistable state based on a Rudin-Shapiro photonic crystal pair and graphene composite structure.

Background

Optical bistability is a nonlinear optical effect based on the optical kerr effect of a material. When the incident light is sufficiently strong, one input intensity value may correspond to two different output intensity values, i.e. one input intensity value may induce two stable output resonance states. In all-optical communication, information needs to be transmitted, relayed, timed, amplified, shaped, and the like in the optical domain, which leads to the vigorous development of all-optical devices for optical control of light, and an important class of all-optical switches based on optical bistable state are provided.

When the optical bistable state is applied to the all-optical switch, the upper threshold value and the lower threshold value of the bistable state respectively correspond to the on-off trigger threshold value of the optical switch; the greater the trigger threshold, the greater the intensity of light required to trigger the light switch on or off. However, as the power of the device increases, the stability of the device during operation deteriorates and the requirements for heat dissipation conditions also increase. In addition, the smaller the interval between the bistable upper and lower thresholds is, the smaller the on and off discrimination of the corresponding all-optical switch is, which may result in the increase of the misoperation rate. Therefore, current research on optical bistable devices focuses on reducing the threshold of optical bistable states and increasing the interval between upper and lower thresholds by new materials and new structures.

In order to achieve low threshold optical bistable effects, materials with large third-order nonlinear coefficients are sought on the one hand; on the other hand, the local electric field is enhanced by optimizing the system structure, and the optical Kerr effect is in direct proportion to the local electric field, so that the strong local electric field can improve the third-order nonlinear effect of the material, thereby reducing the bistable threshold value.

Graphene is a new two-dimensional material, has ultrathin property and excellent conductivity, and the surface conductivity of the graphene can be flexibly regulated and controlled through the chemical potential of the graphene. Importantly, graphene has a considerable third-order optical nonlinear coefficient, which makes graphene a popular material in optical bistable studies. In addition, in order to further reduce the threshold value of bistable state, the local electric field of graphene can be enhanced by using the surface plasmon polariton of the graphene; graphene may also be embedded into the defect layer of the photonic crystal to enhance its nonlinear effects. The mode field energy of the defect mode is mainly distributed in the defect layer, and graphene is embedded in the defect layer, so that the nonlinear effect of the graphene can be greatly enhanced.

Two dielectric sheets with different refractive indexes are arranged alternately in space to form the photonic crystal with a periodic structure. In the wave vector space, a photonic crystal has a photonic band structure similar to an electron band in a semiconductor. Light waves within the band gap will be totally reflected without transmission. If a defect layer is introduced into the photonic crystal, a transmission mode appears in the transmission spectrum. The transmission mode is also a defect mode, has strong local property to an electric field, and is often used for enhancing the third-order nonlinear effect of the material.

Quasi-photonic crystals or aperiodic photonic crystals can be used to enhance the local electric field because there is a natural defect layer in the quasi-photonic crystals or aperiodic photonic crystals and the number of defect modes increases geometrically with the increase of the sequence number.

The Thue-Morse (TM) sequence is mathematically a quasi-periodic sequence, and its corresponding photonic crystal is a quasi-periodic photonic crystal. The graphene is embedded into the TM photonic crystal, so that the optical bistable state with low threshold value can be realized, and the threshold value of the optical bistable state is about 100GW/cm²(gigawatts per square centimeter). The TM photonic crystal is provided with a plurality of defect cavities, and a plurality of defect modes, namely resonant transmission modes, exist in the same defect cavity. With the increase of the serial number, the number of dielectric layers in the TM photonic crystal is correspondingly increased, and the transmission modes in the transmission spectrum are split in a geometric series manner, so the resonance modes are called fractal resonance states. The optical fractal has locality to an electric field, and can be used for enhancing the nonlinear effect of graphene, so that low-threshold optical bistable state is realized. Whether another quasi-periodic photonic crystal and graphene composite structure can be found is important in the field of research, so that the nonlinear effect of the graphene is further enhanced, and the threshold of the optical bistable state is reduced.

Disclosure of Invention

The invention aims to solve the problems in the prior art and provides an optical bistable state based on a Rudin-Shapiro (RS: rutin-Charcino) photonic crystal pair and a graphene composite structure, and the technical problem to be solved by the invention is how to enable a multilayer structure to have optical bistable state with lower threshold value.

The purpose of the invention can be realized by the following technical scheme: an optical bistable state based on a Rudin-Shapiro photonic crystal pair and graphene composite structure is characterized by comprising two binary RS photonic crystals and a plurality of graphene single layers which are symmetrically distributed, wherein each binary RS photonic crystal comprises a plurality of first dielectric layers H and a plurality of second dielectric layers L, and the RS photonic crystal pair and graphene composite structure is represented as HHHLGHHGLHHLGHHGLHHH; the composite structure has optical morphologies and has a local effect on an electric field, and the four graphene single layers are just positioned at the positions with the strongest electric field respectively; the first dielectric layer and the second dielectric layer are respectively two uniform dielectric sheets with different refractive indexes; the first dielectric layer and the second dielectric layer have a thickness of 1/4 a of the respective optical wavelength.

Further, the first dielectric layer is made of lead telluride which is a high-refractive-index material, and the second dielectric layer is made of cryolite which is a low-refractive-index material.

Further, the upper threshold, the lower threshold, and the interval between the upper threshold and the lower threshold of the optical bistable state are regulated by the chemical potential of the graphene monolayer.

Further, the upper threshold, the lower threshold and the interval between the upper threshold and the lower threshold of the optical bistable state are regulated by the wavelength of incident light.

Two dielectric sheets A and B with different refractive indexes are sequentially arranged according to a binary RS sequence to form an RS photonic crystal pair; and respectively embedding the four graphene single layers into the interface of two adjacent dielectric sheets to form a composite structure.

The RS photonic crystal pair has an optical fractal shape, an electric field corresponding to the optical fractal shape has locality, and the four graphene single layers are just positioned at the strongest positions of the local electric fields corresponding to the optical fractal shape respectively, so that the third-order nonlinear effect of the graphene is greatly enhanced, and further the optical bistable state with a low threshold value is realized. The threshold value of the optical bistable state in the structure can be as low as 100MW/cm²This is 3 orders of magnitude lower than the threshold for optical bistability in the composite structure of the true-Morse photonic crystal and graphene.

The upper and lower thresholds of the optical bistable state in the RS photonic crystal pair and graphene composite structure and the interval between the upper and lower thresholds are increased along with the increase of the chemical potential and incident wavelength of the graphene. The optical bistable state can be applied to all-optical switches, and the on-off trigger thresholds and the intervals between the on-off trigger thresholds of all-optical switches can be flexibly regulated and controlled through the chemical potential and the incident wavelength of graphene.

Drawings

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

Fig. 2 shows transmission spectra corresponding to RS photonic crystals of different numbers (fig. a, b, and c) in which the numbers of the RS photonic crystal pairs are N2, 3, and 4, respectively).

FIG. 3 shows a wavelength λ with the number N equal to 3₂Normalized electric field distribution of optical fractal 1.55 μm.

In FIG. 4, (a) is a graph showing the input-output intensity relationship corresponding to different chemical potentials of graphene; the graph (b) in fig. 4 shows the variation of the upper and lower bistable thresholds with the chemical potential of graphene.

FIG. 5 is a graph (a) showing the input-output intensity relationship for different incident wavelengths; the graph (b) in fig. 5 shows the variation of the upper and lower thresholds of the bistable state with the incident wavelength.

Fig. 6 is a schematic diagram of a bi-level all-optical switch based on optical bistability.

In the figure, H, a first dielectric layer; l, a second electrolyte layer; G. a graphene monolayer.

Detailed Description

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

Mathematically, the iterative rule for a binary Rudin-Shapiro (RS: rutin-Charino) sequence is: s₀＝H，S₁＝HH，S₂＝HHHL，S₃＝HHHLHHLH，……，S_N＝S_N-1(HH → HHHL, HL → HHLH, LH → LLHL, LL → LLLH), … …, wherein N (N ═ 0, 1, 2, 3, … …) denotes the sequence number, S ═ S_NThe Nth item representing the sequence, HH → HHHL representing S_N-1HH in (1) is replaced by HHHL.

Fig. 1 shows a schematic diagram of a composite structure of a binary RS photonic crystal pair with sequence number N ═ 3 and graphene. The composite structure can be represented as HHHLGHHGLHHLGHHGLHHH, wherein the letters H, L respectively represent two uniform dielectric sheets with different refractive indexes, and four single-layer graphene G is embedded into the strongest position of the local electric field corresponding to the fractal optical state, and is just positioned at the interface of the two adjacent dielectric sheets. The composite structure is symmetrically distributed about the origin, similar to a distributed feedback bragg grating.

In the RS photonic crystal pair, H is a high-refractive-index material lead telluride, and the refractive index of the material is n_H4.1 as the ratio; l is cryolite of low refractive index material with refractive index n_L1.35. Both H and L have a thickness of 1/4 optical wavelengths, i.e., H has a thickness d_H＝λ₀/4/n_H0.0945 μm (μm denotes μm), where λ₀1.55 μm as the center wavelength and L as the thickness d_L＝λ₀/4/n_L0.287 μm. The incident light is transverse magnetic wave and is vertically incident from the left.

Single layer graphene has a thickness of about 0.33nm (nm means 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 denotes kelvin), and the relaxation time τ of the electrons in the graphene is 0.5ps (ps denotes picosecond).

In quasi-photonic crystals, there is an optical fractal effect. The non-linear effect of graphene can be enhanced by utilizing the locality of optical fractal effect on electric field. When the transverse magnetic wave is vertically incident, fig. 2(a) shows the transmission spectrum corresponding to the RS photonic crystal pair with N ═ 2. The ordinate T represents the transmittance, and the abscissa (ω - ω)₀)/ω_gapDenotes a normalized angular frequency, where ω is 2 π c/λ, ω₀＝2πc/λ₀And ω_gap＝4ω₀arcsin│(n_H-n_L)/(n_H+n_L)|²And/pi respectively represents incident light angular frequency, incident light central angular frequency and angular frequency band gap, c is light speed in vacuum, and arcsin is an inverse sine function. It can be seen that between the two dotted lines at the normalized frequency, the number of transmission peaks is 1. Fig. 2(b) shows a transmission spectrum corresponding to an RS photonic crystal pair with N ═ 3, and the number of transmission peaks between two imaginary lines is 3. Fig. 2(c) shows a transmission spectrum corresponding to an RS photonic crystal pair with N-4, and the number of transmission peaks between two dotted lines is 5. It can be seen that the number of transmission peaks rapidly expands with the increase of the sequence number, which is the optical fractal effect.

Here, taking a composite structure of an RS photonic crystal pair of N-3 and graphene as an example, as shown in fig. 2(b), a local effect of an optical fractal state on an electric field and an enhancement effect on a graphene nonlinear effect are explained, so that low-threshold optical bistable is realized. The central wavelengths corresponding to the three transmission peaks in the middle of the transmission spectrum are respectively lambda₁＝1.7271μm、λ₂1.55 μm and λ₃＝1.4058μm。

The dielectric sheets and the graphene are arranged in order from left to right along a horizontal direction, i.e., a Z-axis, according to a rule. FIG. 3 shows the electric field distribution of the central resonance optical fractal of FIG. 2(b) in a composite structure corresponding to an incident wavelength λ₂1.55 μm. The dotted lines represent the interface between two adjacent dielectric sheets, and four single-layer graphene G are respectively embedded at the positions of the structure where the electric field intensity is strongest. The ordinate represents the normalized electric field strength of the Z component. It can be seen that the distribution of the electric field energy in the structure is not uniform and localized. The strongest point of the local electric field is just positioned at the single-layer graphene. The optical third-order nonlinear effect of the graphene is in direct proportion to the local electric field intensity, so that the nonlinear effect of the graphite is greatly enhanced.

Fixed incident wavelength λ 1.57 μm relative to the optical fractal resonance wavelength λ₂There is some red detuning at 1.55 μm. Other parameters are kept unchanged, and the input-output light intensity relationship corresponding to different graphene chemical potentials mu is shown in fig. 4 (a). Abscissa I_iRepresenting input light intensity, ordinate I_oIndicating the output light intensity. Unit MW/cm²Representing megawatts per square centimeter. It can be seen that: when mu is 0.4eV and 0.5eV, the input-output light intensity relation curve has a section of S-shaped curve section, namely, the bistable relation; and when μ ═ 0.3eV, the input-output light intensity relationship is not bistable.

Increasing the output light intensity, wherein the output light intensity jumps upwards at the right corner of the S curve section, and the corresponding input light intensity value is called as an upper threshold value of the optical bistable state; when the input light intensity is gradually reduced from a relatively large value, the output light intensity generates a downward jump at the left turning point of the S curve section, and the corresponding input light intensity value at the moment is called as a lower threshold value of the optical bistable state; the difference between the upper and lower thresholds is called the threshold interval.

When the input intensity is between the upper and lower threshold values, one input intensity value corresponds to two output intensity values, which is called optical bistability. The sigmoid curve segment in the contour line of the input-output light intensity relation is the typical characteristic of optical bistable state, and the effect can be used for all-optical switches of binary light control light.

Increasing the value of mu, wherein 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; as the graphene chemical potential increases, both the upper and lower thresholds of bistability increase, and the threshold interval of bistability increases, as shown in fig. 4 (b). Ordinate I_thA threshold value indicative of bistability; i is_uAnd I_dRepresenting bistable upper and lower thresholds, respectively. When mu is more than or equal to 0.4eV, the input-output light intensity relation is bistable; the bistable upper and lower thresholds, and the threshold interval, increase as the chemical potential of the graphene increases. Therefore, the upper and lower thresholds and the threshold interval of the bistable state can be regulated by the chemical potential of graphene.

In a composite system of the Thue-Morse photonic crystal and graphene, the optical bistable threshold is 100GW/cm²Magnitude, while in the composite structure of RS photonic crystal and graphene, the threshold of optical bistability is lowered to 100MW/cm²Magnitude.

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

When the chemical potential μ of the fixed graphene is 0.5eV, other parameters are kept unchanged, and fig. 5(a) shows the input-output light intensity relationship corresponding to different incident wavelengths. It can be seen that: when λ is 1.56 μm, the input-output light intensity relationship is non-bistable; and when λ ═ 1.57 μm and 1.58 μm, both input-output optics are bistable; the bistable curves corresponding to different incident wavelengths are different, namely the upper threshold value, the lower threshold value and the threshold value interval of the bistable state are different; as the incident wavelength increases, i.e., the amount of detuning increases, the bistable upper and lower thresholds increase, and the bistable threshold interval also increases, as shown in fig. 5 (b). Ordinate I_thIndicating a bistable thresholdA value; i is_uAnd I_dRepresenting bistable upper and lower thresholds, respectively. It can be seen that the input-output intensity relationship is bi-stable when λ ≧ 1.565 μm; the threshold and threshold separation of the bistability increases with increasing wavelength. Since the larger the amount of wavelength detuning, the more the difference needs to be made up by the nonlinear effect, the stronger the incident light energy needed to satisfy the resonance. Thus, the upper and lower thresholds and threshold spacing of the bistable states can be tuned by the incident wavelength.

In a word, the RS photonic crystal has a resonant optical fractal state in the composite structure of the RS photonic crystal and the graphene, and the optical fractal state has a strong local effect on an electric field; the single-layer graphene is just positioned at the strongest position of the electric field corresponding to the optical fractal, so that the nonlinear effect of the graphene is greatly enhanced, and the optical bistable state with the low threshold value is realized, and the threshold value of the optical bistable state is as low as 100MW/cm²The optical bistable state is 3 orders of magnitude smaller than that in the compounding of the Thue-Morse photonic crystal and the graphene. The optical bistable state can be applied to a binary all-optical switch, and the on/off trigger threshold value and the trigger threshold value interval of the switch can be flexibly regulated and controlled through the chemical potential and the incident wavelength of graphene.

The wavelength of incident light is set to 1.57 μm, the chemical potential is set to 0.5eV, and an optical bistable phenomenon occurs in the input-output light intensity relationship. When the optical bistable effect is applied to a binary all-optical switch, the principle is shown in fig. 6. When the input light intensity is gradually increased from a lower value, the output light intensity generates an upward jump at the right turning point of the S curve section, and the input light intensity I_i＝I_uAn upper threshold called bistable, which corresponds to the switching-on process of the all-optical switch, and a lower threshold called bistable_i＝I_uA switching-on trigger threshold value of the called all-optical switch; when the input light intensity is gradually reduced from a higher value, the output light intensity generates a downward jump at the left turning point of the S curve section, and the input light intensity I_i＝I_dA lower threshold called bistable state, which corresponds to the switch-off process of the all-optical switch, and a lower threshold called bistable state_i＝I_dCalled the off trigger threshold of the all-optical switch. At this time, the opening trigger threshold is I_u＝112.0129MW/cm²The turn-off trigger threshold is I_d＝90.0979MW/cm²Trigger threshold interval of I_u-I_d＝21.915MW/cm²。

Fig. 4(b) and 5(b) show that the input-output curve of the optical bistable state is influenced by the chemical potential of graphene, the chemical potential and the input wavelength are different, and the positions of two inflection points of the corresponding S curve segment are also changed. Two inflection points of the S curve segment respectively correspond to an upper threshold and a lower threshold of the optical bistable state, namely an on-off trigger threshold of the all-optical switch. Therefore, the on-off trigger threshold and the trigger threshold interval of the all-optical switch can be regulated and controlled through the chemical potential and the input wavelength of the graphene. It can be seen that as the chemical potential and input wavelength increase, the on, off trigger thresholds of the fully-off switch, and the trigger threshold interval, both increase.

The larger the switch trigger threshold interval is, the larger the degree of distinction between on and off operations is, and the smaller the probability of erroneous operation is. To reduce the mishandling rate of the switch, the detuning amount of the chemical potential or wavelength of the graphene needs to be increased, and at the same time, the switch trigger threshold is increased, so that the reduction of the mishandling rate of the all-optical switch is achieved at the cost of increasing the trigger threshold of the switch.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.