A read-write optical memory based on high-order topology

Abstract

The present invention provides a readable and writable optical memory based on a high-order topological structure, belonging to the field of all-optical communication technology. The present invention implements the read and write functions of the optical memory through the L-shaped boundary configuration of the PC. Because the angle configuration is connected to the boundary, the angle state can be transitioned to the boundary state by applying an external bias magnetic field. Ultimately, the angle state is used to store the optical signal, and the conversion from the angle state to the boundary state is used to extract or read the optical signal. Through the above design, a readable and writable optical memory is realized. The optical memory designed by the present invention can solve the problems of the short storage time and complex storage and reading processes of existing optical storage devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of all-optical communication, and in particular relates to a readable and writable optical memory based on a high-order topological structure. Background Art

[0002] Optical memory is a key component in optical communication systems and quantum computing applications. Due to the extremely high speed of light, storing photons is much more difficult than storing electrons. Consequently, most current optical storage efforts rely on slow-light technologies, such as electromagnetically induced transparency (EIT). However, slow-light technologies also have numerous drawbacks. For example, EIT is mostly based on atomic systems and requires complex experimental conditions. Furthermore, in practical applications, optical signal storage methods typically rely on low-temperature, high-refractive-index media, but the storage time is limited by the length of the medium.

[0003] Given these realities, designing efficient and easily implementable optical signal storage is an urgent task. Currently, topological insulators and topological photonic crystals (PCs) are attracting widespread research interest. Their ability to support topological edge states (TESs) that are perturbation- and backscatter-immune offers potential for designing functional topological devices. It is generally believed that d-1-dimensional gapless edge states should exist at the boundaries of one-dimensional nontrivial materials. However, with the discovery of higher-order topological insulators (HOTIs), two-dimensional (2D) second-order topological insulators can accommodate nontrivial zero-dimensional (0D) corner states. In particular, HOTIs have been extended to photonic crystals that can confine light to specific regions. By exploiting the ability of topological corner states (TCSs) to capture photons, optical signals can be stored. However, as optical signal storage, in addition to signal storage, it must also have the ability to output signals. While TCSs and general defect states in photonic crystals can also be used to store optical signals, the lack of a mechanism to achieve this output makes it difficult to release the signal. Consequently, the design and development of read-write memories based on conventional PCs remains challenging. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention provides a readable and writable optical memory based on a high-order topological structure to solve the problem of storing and extracting optical signals.

[0005] The present invention achieves the above technical objectives through the following technical means.

[0006] A readable and writable optical memory based on a high-order topological structure, characterized by comprising: a two-dimensional photonic lattice plate, wherein the two-dimensional photonic lattice plate is divided into two regions, an L-shaped right-angle boundary is formed between the two regions, and two ends of the L-shaped right-angle boundary serve as an input end and an output end respectively;

[0007] The two-dimensional photonic lattice in the concave side region of the L-shaped right-angle boundary is composed of a unit cell A, which is a square structure. A dielectric film is provided at the center of the unit cell A, which is a square structure and is made of a gyromagnetic material;

[0008] The two-dimensional photonic lattice in the convex side area of ​​the L-shaped right-angle boundary is composed of unit cells B. The unit cells B are square structures with side lengths equal to those of the unit cells A. 1 / 4 of the dielectric film is provided at each of the four corners of the unit cells B.

[0009] Furthermore, the reading and writing method of the optical storage is as follows: when there is no external magnetic field, the optical signal is input from the input end at the angular state frequency to realize optical signal storage; when a bias magnetic field is applied to the two-dimensional photonic lattice, the optical signal is output from the output end at the angular state frequency to realize optical signal reading.

[0010] Furthermore, the side lengths of the unit cell A and the unit cell B are 0.029 m, and the side length of the dielectric film is 0.0087 m.

[0011] Furthermore, the gyromagnetic material is yttrium iron garnet ferrite.

[0012] Furthermore, the dielectric film is arranged in an air background with a refractive index of 1.

[0013] Furthermore, the intensity of the bias magnetic field is 1600 Gauss.

[0014] Furthermore, the two-dimensional photonic lattice in the concave side region of the L-shaped right-angled boundary is composed of 8×8 unit cells A, and the two-dimensional photonic lattice in the convex side region of the L-shaped right-angled boundary is composed of three groups of 8×8 unit cells B.

[0015] Furthermore, the angular state frequency is 0.2813 2πc / a.

[0016] The beneficial effects of the present invention are:

[0017] (1) The present invention provides a readable and writable optical memory based on a high-order topological structure and proposes a new optical memory structure design, in which the read and write functions of the optical memory are realized by configuring the L-shaped boundary of the PC. Since the configuration of the angle is connected to the boundary, the angle state can be transitioned to the boundary state by applying an external bias magnetic field. Ultimately, the angle state is used to store the optical signal, and the conversion from the angle state to the boundary state is used to export or read the optical signal. Through the above design, a readable and writable optical memory is realized.

[0018] (2) The optical storage device designed by the present invention can solve the problems of short storage time and complicated storage and reading process of existing optical storage devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A square lattice designed for the present invention;

[0020] Figure 2(a) shows the band structure of UC A under TM polarization;

[0021] Figure 2(b) shows the band structure of UC B under TM polarization;

[0022] Figure 2(c) shows the E of the two unit cells at the high symmetry point X in the first and second bands. z Mode field diagram;

[0023] Figure 3(a) shows the band structure of UC A in a magnetic field under TM polarization;

[0024] Figure 3(b) shows the band structure of UC B in a magnetic field under TM polarization;

[0025] Figure 3(c) shows the E of the two unit cells at the high symmetry point X in the first and second bands when there is a magnetic field. z Mode field diagram;

[0026] Figure 4 This is the structural diagram of the supercell made by the present invention to verify the boundary state;

[0027] Figure 5(a) shows the case when there is no magnetic field Figure 4 Band diagram of the supercell;

[0028] Figure 5(b) shows the magnetic field Figure 4 Band diagram of the supercell;

[0029] Figure 6 This is a diagram of the supercell structure made by the present invention to verify the angular state;

[0030] Figure 7(a) shows the case when there is no magnetic field Figure 6 Eigenfrequency plot of supercell;

[0031] Figure 7(b) shows the magnetic field Figure 6 Eigenfrequency plot of supercell;

[0032] Figure 8(a) shows the E of the four corner states in Figure 7(a) z Mode field diagram;

[0033] Figure 8(b) shows the E in Figure 7(b) at the same frequency as the corner state in Figure 7(a). z Mode field diagram;

[0034] Figure 9 This is a structural diagram of the optical storage device of the present invention;

[0035] FIG10( a ) shows the E of the optical storage device of the present invention when the incident light is incident at an angular frequency without applying a magnetic field.z Field map;

[0036] FIG10( b ) shows the E of the optical storage device of the present invention when a magnetic field is applied. z Field map;

[0037] FIG10( c ) is a graph showing the change in field value with position along the dotted lines in FIG10( a ) and FIG10( b );

[0038] Figure 11 is a graph showing the change of field values ​​of corner points and source points over time;

[0039] Figure 12 This is a curve fitting graph for the corner point field value data. DETAILED DESCRIPTION

[0040] The following describes embodiments of the present invention in detail. Examples of the illustrated embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0041] 1. Two-dimensional photonic lattice

[0042] like Figure 1 The two-dimensional photonic square lattice designed by the present invention is shown. It consists of several dielectric thin films (the dark rectangles in the figure) arranged in a square array against an air background. The dielectric films are square and made of gyromagnetic material, and the refractive index of the air background is 1. As a specific example, in this embodiment, the dielectric films are made of yttrium iron garnet (YIG); the side length of the dielectric films is 0.0087m, and the center-to-center distance between adjacent dielectric films is 0.029m.

[0043] Figure 1 In the two-dimensional photonic square lattice shown, the unit cell can adopt two different symmetry center forms, namely the portion outlined by the dashed line on the left and the portion outlined by the dashed line on the right. For convenience, the unit cell outlined by the dashed line on the right is referred to as UC A (unit cell A), and the unit cell outlined by the dashed line on the left is referred to as UC B (unit cell B). For UC A, a dielectric film is placed at the center of the unit cell; for UC B, a quarter of a dielectric film is placed at each of the four corners of the unit cell. Both unit cells have the same first Brillouin zone.

[0044] Under the action of the external bias magnetic field H0, the magnetic permeability μ of the YIG material is a tensor, which is expressed as satisfy:

[0045]

[0046] Where μr is the relative magnetic permeability, μ k is the magnetization permeability caused by the magnetic field, i is the imaginary unit, μ r and μ k They are:

[0047]

[0048]

[0049] Where μ0 is the vacuum permeability, ω is the operating frequency of the optical memory (i.e. the normalized value of the incident light frequency f0), ω0 = μ0γH0 is the Larmor frequency, ω m =μ0γM s is the characteristic frequency, where γ is the gyromagnetic ratio, H0 is the external bias magnetic field, and M s The saturation magnetization intensity caused by the steady magnetic field, the saturation magnetization intensity M of YIG s =1780Gauss. For example, when the bias magnetic field H0 is 1600Gauss and the incident light frequency f0 is 4.28GHz, μ in the YIG material tensor is r and μ k 14μ0 and 12.4μ0 respectively.

[0050] 2. Characteristics of Two-Dimensional Photonic Lattice

[0051] To illustrate why the above-mentioned two-dimensional photonic lattice can be applied to optical memory, the following explanation and verification of the characteristics of the above-mentioned two-dimensional photonic lattice are given: 1)

[0053] First, consider the case where H0 = 0 (i.e., no bias magnetic field is applied), the dielectric constant ε = 15.26ε0 and the magnetic permeability μ = μ0 of the YIG dielectric film. Based on the finite element method, the eigenfrequency can be calculated using Comsol software. By scanning the wave vector k along the Γ-MX-Γ direction in the first Brillouin zone, the frequency band diagrams shown in Figures 2(a) and 2(b) are obtained for UC A and UC B, respectively. In the figure, the "+" and "-" signs represent even parity and odd parity, respectively. Although the band structures of the two unit cells UC A and UC B are the same, they have different topological phases.

[0054] The topological properties of topological photonic crystals (PCs) can be described by the vector Zak phase, which is the integral of the Berry connection in momentum space:

[0055]

[0056] Where 1stBZ represents the first Brillouin zone, which represents the range of integration in the formula, and dk xrepresents the component of the wave vector k in the momentum space in the x direction, dk y represents the component of the wave vector k in the y direction in the momentum space, Tr() represents the trace of the matrix, |u m >and|u n > are the Bloch functions of the mth and nth frequency bands respectively (Note: the formula is to find the inner product of the two Bloch wave function derivatives, so the writing method in the formula is slightly different from here), A is the integral of the two Bloch function derivatives, is the partial differential symbol, which is used here for derivation; where m and n are the occupied frequency bands. 4V The symmetry of the point group, the Zak phase can be rewritten as:

[0057]

[0058] Where q is an integer calculated from the second half of the formula, mod represents the remainder, mod2 is the remainder obtained by dividing by 2, Γ is the high symmetry point of the first Brillouin zone, i = x, y represents the direction of the components on the x-axis and y-axis in momentum space, i is the imaginary unit used to represent complex numbers, X i represents the X point in the first Brillouin zone, η n is the parity check of the high symmetry point of the nth frequency band in the first Brillouin zone.

[0059] For points P1, P2, P3, and P4 in Figure 2(a) and Figure 2(b), the mode field E from the first to the second band is drawn using the COMSO simulation software. z The distribution is shown in Figure 2(c). As can be seen from Figure 2(c), for UC A, the first band gap is topologically trivial; this can also be calculated using the above Zak phase formula, that is, the calculated result is P x =0, where P x is the component in the x direction calculated when i = x in the previous formula; for UC B, the first band gap is topologically nontrivial, and the corresponding calculation result is P x =π.

[0060] When H0 = 1600 Gauss (1600 Gauss is only for this demonstration, and other bias magnetic field strengths can also be selected), the frequency band is calculated using UC A and UC B respectively. In this case, μ r and μ kIt is related to the operating frequency, so the characteristic frequency of the YIG lattice must be obtained by the nonlinear solver in the Comsol software. Similarly to when H0=0, the frequency band diagrams of UCA and UC B are shown in Figures 3(a) and 3(b) respectively; the mode field E from the first to the second band is plotted for points P1, P2, P3, and P4 in Figures 3(a) and 3(b). z The distribution is shown in Figure 3(c). Through the mode field symmetry analysis, it can be concluded that for UC A, the first band gap is topologically trivial, and the corresponding calculation result P x = 0; for UC B, the first band gap is topologically nontrivial, and the corresponding calculation result P x =π.

[0061] From the comparative tests of the above two unit cells with and without a bias magnetic field, it can be seen that the frequency position of the first band gap is different when there is an external bias magnetic field and when there is no external bias magnetic field. 2)

[0063] The different topological phases between UC A and UC B will result in the generation of unprotected boundary states at the boundary of the two unit cell structures. To further obtain the frequency range of the boundary states, the present invention constructs a super cell. Its structure is as follows Figure 4 As shown, it is specifically the minimum period combination of UC A and UC B along the FK direction.

[0064] For the supercell, energy band diagrams of the TM mode were obtained without and with an applied bias magnetic field (also at 1600 Gauss). The results are shown in Figures 5(a) and 5(b). The red curve in the figure represents the topological edge state. In Figure 5(a), without an applied bias magnetic field, the frequency range of the edge state is 0.2903 to 0.3228 (2πc / a); in Figure 5(b), with an applied bias magnetic field, the frequency range of the edge state is 0.2729 to 0.2976 (2πc / a). 3)

[0066] In order to obtain the angular state, the present invention establishes a super unit cell model with the structure as follows Figure 6 The constructed supercell consists of a 14×14 UC B surrounding a 6×6 UC A. Figure 7(a) and Figure 7(b) show the characteristic frequencies of the supercell with and without an external bias magnetic field, respectively.

[0067] As can be seen from Figure 7(a), there are four separate points C1, C2, C3, and C4 concentrated at the frequency 0.2813 (2πc / a). These four points are in the angular state, and the frequency 0.2813 2πc / a is the angular state frequency ω that can excite the angular state. c. Draw the eigenmode field E for C1, C2, C3, and C4 respectively z , the results are shown in Figure 8(a). As can be seen from Figure 8(a), for states C1 and C4, the energy is concentrated on the four corners; for states C2 and C3, the energy is concentrated on the two diagonal corners.

[0068] In Figure 7(b), no corner state is excited, but an edge state appears at the frequency position of the original corner state (red dashed line). The eigenmode field E is also plotted based on the original corner state frequency. z ,The results are shown in Figure 8(b).,As can be seen from Figure 8(b), all the energy is on the four boundaries.

[0069] In summary, it is concluded that under the condition of an external bias magnetic field, the constructed two-dimensional unit cell model will undergo mode conversion from corner states to edge states. Based on this characteristic, the read and write functions of optical storage can be realized.

[0070] 3. Optical Storage

[0071] like Figure 9 The optical memory device designed according to the present invention comprises a two-dimensional photonic lattice slab divided into two regions with an L-shaped right-angled boundary between them. The two-dimensional photonic lattice on the concave side of the L-shaped boundary is composed of UC A, while the two-dimensional photonic lattice on the convex side of the L-shaped boundary is composed of UC B. The two ends of the L-shaped boundary serve as the input and output of the optical memory, respectively.

[0072] Comsol software was used to simulate and test the above optical memory. When no bias magnetic field was applied to the two-dimensional photonic lattice, the angular state frequency ω c As the incident light frequency, light is input from the input port. The results are shown in Figure 10(a). It can be seen that under the excitation of the incident light, strong optical wave resonance occurs at the corner of the L-shaped boundary. Because the corner state is within the band gap, energy transfer from the source to the corner occurs solely through coupling of the evanescent field, resulting in no waveform being found along the path from source to corner. The capture of light at the corner precisely enables the storage of the optical signal.

[0073] Then, a bias magnetic field of H0 = 1600 Gauss is applied to the above two-dimensional photonic lattice. The results are shown in Figure 10(b). It can be found that there is clear wave propagation along the L-shaped boundary. In order to compare the capture in the corner state and the release in the boundary state, the E in Figure 10(a) and Figure 10(b) are plotted. zFigure 10(c) shows the one-dimensional amplitude distribution of the field along the boundary. As can be seen from Figure 10(c), when capturing light in the corner state, the field is focused at the corner; while when releasing light in the boundary state, the field is evenly distributed along the boundary. In summary, under the influence of an external magnetic field, the optical memory of the present invention transitions from capturing light to releasing it, and the release of light is equivalent to the extraction of optical signals.

[0074] Storage time is a very important performance indicator for optical storage. In order to verify the storage time of the present invention, a time domain simulation is performed based on Comsol software. A value of I=sin(ω c t) μA line current source, the harmonics are excited as a signal from 0 to 74ns, after 74ns I = 0. Record E at the corner point z Field changes over time. Figure 11 The figure shows the evolution of the source field to the angular field over time, indicating that the angular field increases monotonically from 0 to 74 ns, reaches a maximum at 74 ns, and decreases monotonically after 74 ns. The data fitting of the attenuated field amplitude is further performed, and the results are shown in Fig. Figure 12 As shown. Figure 12 It was found that the decay was similar to The exponential function of the loss rate is γ = 4.86 × 10 6 s -1 , E0=20.28V / m. Based on this, the time it takes for the maximum value E0 to decay to E0 / e is 232ns, that is, the storage time of the above optical memory is 232ns.

[0075] In the description of the present invention, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0076] The present invention is not limited to the above-mentioned embodiments. Any obvious improvement, replacement or modification that can be made by those skilled in the art without departing from the essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A readable and writable optical storage device based on a high-order topological structure, characterized by: The two-dimensional photonic lattice plate is divided into two regions, an L-shaped right-angle boundary is formed between the two regions, and two ends of the L-shaped right-angle boundary serve as an input end and an output end respectively; The two-dimensional photonic lattice in the concave side region of the L-shaped right-angle boundary is composed of a unit cell A, which is a square structure. A dielectric film is provided at the center of the unit cell A, which is a square structure and is made of a gyromagnetic material; The two-dimensional photonic lattice in the convex side area of ​​the L-shaped right-angle boundary is composed of unit cells B. The unit cells B are square structures with side lengths equal to those of the unit cells A. 1 / 4 of the dielectric film is provided at each of the four corners of the unit cells B.

2. The optical storage device according to claim 1, wherein: The optical storage device has a reading and writing method as follows: when there is no external magnetic field, the optical signal is input from the input end at the angular state frequency to realize optical signal storage; when a bias magnetic field is applied to the two-dimensional photonic lattice, the optical signal is output from the output end at the angular state frequency to realize optical signal reading.

3. The optical storage device according to claim 1, wherein: The side lengths of the unit cell A and the unit cell B are 0.029 m, and the side length of the dielectric film is 0.0087 m.

4. The optical storage device according to claim 1, wherein: The gyromagnetic material is yttrium iron garnet ferrite.

5. The optical storage device according to claim 1, wherein: The dielectric film is placed in an air background with a refractive index of 1.

6. The optical storage device according to claim 2, wherein: The intensity of the bias magnetic field is 1600 Gauss.

7. The optical storage device according to claim 1, wherein: The two-dimensional photonic lattice in the concave side region of the L-shaped right-angle boundary is composed of 8×8 unit cells A, and the two-dimensional photonic lattice in the convex side region of the L-shaped right-angle boundary is composed of three groups of 8×8 unit cells B.

8. The optical storage device according to claim 2, wherein: The angular state frequency is 0.2813 2πc / a.

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