A Design Method of Terahertz Time Delay Device Based on Anderson Localization Principle

Abstract

The present invention proposes a design method for a terahertz time delay device based on the Anderson localization principle, including the following steps: Step S1, use electromagnetic field simulation software to find a photonic crystal structure model as the initial structure; Step S2, divide it into 10×10 small pixel blocks according to the arrangement of the crystal lattice, and use the particle swarm optimization algorithm to control the presence or absence of the dielectric columns in the pixel blocks by assigning "0" and "1" values to the pixel blocks; Step S3, randomly generate the lattice arrangement of the dielectric columns and set the boundary conditions to the perfectly matched layer boundary; Step S4, perform electromagnetic field simulation to obtain the calculated transmittance spectrum line, return it and calculate the initial FOM value; Step S5, generate a new lattice arrangement through the automatic search of the particle swarm and perform simulation, and compare it with the previous FOM value; Step S6, repeat Step S5 and make comparisons, and output the finally optimized structure. The present invention solves the problems of difficult design and long design cycle of the existing terahertz time delay device.

Description

Technical Field

[0001] The present invention relates to the technical fields of terahertz devices and numerical calculation technologies, and in particular to a design method for a terahertz time delay device based on the Anderson localization principle. Background Art

[0002] The application of terahertz technology not only requires efficient wave sources and sensitive detectors, but also high-performance functional components, such as filters, modulators, switches, distributors, wavelength division multiplexers, and time delay devices, etc., to effectively manipulate terahertz waves.

[0003] There are many different types of terahertz functional devices, and the corresponding theories of these devices are also independent of each other. Therefore, when a terahertz functional device with a specific function is required, first, a suitable structure is found in the matching template library according to the specific function as the basic model. If no suitable structure is found, the device model needs to be designed according to the mature theory. Subsequently, by adjusting the characteristic structure parameters in the model, the target device with the required performance is finally generated. Obviously, this direct design method not only takes a lot of time to find the corresponding template in the template library, but also requires a lot of experience in analysis modeling and theoretical analysis, which poses high requirements for scientific workers and engineering technicians to complete their design tasks in a timely and effective manner. Therefore, using a reverse design method that can directly go from a specific function to the device structure can omit the adjustment stage of model parameters, thereby shortening the design cycle of functional devices and achieving the goal of efficiently designing terahertz functional devices. Summary of the Invention

[0004] The present invention proposes a design method for a terahertz time delay device based on the Anderson localization principle, which solves the problems of difficult design and long design cycle of existing terahertz time delay devices.

[0005] The present invention adopts the following technical solutions.

[0006] A design method for a terahertz time delay device based on the Anderson localization principle, the delay device includes a substrate and dielectric columns fixed on the substrate, and microcavities are formed between some of the dielectric columns. By adjusting the refractive index of the dielectric columns around the microcavities, the residence time of photons in the microcavities is controlled to achieve controllable time delay. The design method includes the following steps;

[0007] Step S1, use an electromagnetic field simulation software to find a two-dimensional periodic dielectric column type photonic crystal structure model with a wide bandgap as the initial structure, and the total number of dielectric columns in the initial structure is 10×10;

[0008] Step S2: Divide it into 10×10 small pixel blocks according to the arrangement of the two-dimensional photonic crystal lattice at the substrate. Use the particle swarm algorithm to control the presence or absence of the dielectric columns in the pixel blocks by assigning "0" and "1" values. "0" represents no insertion of the dielectric column, and "1" represents insertion of the dielectric column.

[0009] Step S3: Randomly generate a 10×10 arrangement of dielectric column lattices composed of "0" and "1" pixel points, and set the boundary condition to the perfectly matched layer boundary.

[0010] Step S4: Select the incident wave frequency as the center frequency of the forbidden band, and set the boundary condition in the electromagnetic field simulation software to the perfectly matched layer boundary. Set the figure of merit evaluation function FOM representing the device performance. FOM is set as the sum of the root mean square errors between the transmittances at multiple frequency points with equal frequency intervals on both sides of the center frequency of the forbidden band and the set transmittance. Set the transmittance spectrum line as a transmittance peak with a fixed bandwidth and peak value whose center frequency is equal to the incident wave frequency. Import the initial structure into the electromagnetic field simulation software for electromagnetic field simulation, obtain the calculated transmittance spectrum line, and return it to calculate the initial FOM value.

[0011] Step S5: Generate a new lattice arrangement through the automatic search of the particle swarm, import the changed new structure into the electromagnetic field simulation software for simulation, calculate its FOM value, and compare it with the previous FOM value to decide whether to retain the current lattice arrangement.

[0012] Step S6: Repeat Step S5 and compare the FOM values of the previous and current iterations. If the FOM value is improved, continue to repeat; otherwise, the optimization ends, and the finally optimized structure is output.

[0013] The dielectric column is a structure with low loss to terahertz, and its material includes high-resistivity silicon.

[0014] In Steps S1 and S2, the frequency range and the position of the transmittance peak are defined in the objective function representing the device performance in the electromagnetic field simulation software. In the assignment of the pixel blocks in Step S2, "0" represents no insertion of the dielectric column at this pixel block, and "1" represents insertion of the dielectric column at this pixel block.

[0015] In the iterative optimization in Steps S5 and S6, control the presence or absence of the dielectric columns on the substrate lattice through the algorithm. By the method of adding dielectric columns, transform the photonic crystal with a perfect lattice into a defective photonic crystal, and make the defective photonic crystal generate a transmittance peak in the forbidden band, which is also called a defect mode.

[0016] When terahertz light corresponding to the frequency of the defect mode is injected into the defect structure formed by the defect mode, at this time, the frequency supported by the defect mode is consistent with the frequency of the injected photon, so that the photon is localized in the defect structure, generating the time delay effect required for the photon delay device.

[0017] In the time delay effect, photons oscillate repeatedly between adjacent defect cavities A and B, thereby forming a delay effect. The photons in the two defect cavities have an attenuation effect and an interactive transfer effect.

[0018] Microcavities are formed between some of the dielectric pillars. By adjusting the refractive index of the dielectric pillars around the microcavities, the residence time of photons in the microcavities can be controlled to achieve controllable time delay. The specific method is as follows: Inject femtosecond laser pulses into the defect cavity where photons are localized, change the refractive index of the dielectric pillars around the microcavity, so that the local frequency of the defect cavity shifts due to the laser pulses, creating a difference from the frequency of the incident terahertz light, thereby releasing the localized photons and achieving the purpose of releasing photons at a specific time.

[0019] The terahertz time delay device controls the residence time of localized photons in the defect cavity by adjusting the injection start time of femtosecond laser pulses to achieve the function of the time delay device.

[0020] The electromagnetic field simulation software is FDTD Solutions; the perfectly matched layer is Perfectly Matched Layer, abbreviated as PML; the binary particle swarm optimization algorithm is Binary Particle Swarm Optimization, abbreviated as BPSO.

[0021] The design method rapidly designs the terahertz time delay device through the BPSO algorithm. Its FOM evaluation function is set as the root mean square error between the transmittance corresponding to multiple frequencies and the ideal value. The particle swarm algorithm outputs the finally optimized structure through multiple iterations.

[0022] The terahertz time delay device based on the Anderson localization principle in the present invention is a device that utilizes the multiple scattering of light in a disordered medium to form photon localization, thereby confining the light in a certain region or path to change the optical path. It has wide applications in optical detection fields such as terahertz time-domain spectroscopy (THz-TDS), optical coherence tomography (OCT), and ultrafast time-resolution spectroscopy. By changing the lattice arrangement of the two-dimensional dielectric pillar-type photonic crystal to form a random medium with a certain degree of disorder, photon localization is used to confine photons in a certain region or path to achieve the function of time delay.

[0023] The method of the present invention can design a terahertz time delay device based on a two-dimensional photonic crystal using an intelligent algorithm and achieve the required performance.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The design method disclosed by the present invention has a higher design efficiency. First, based on the two-dimensional dielectric pillar photonic crystal model, the presence or absence of the dielectric pillar at the lattice center is controlled according to its lattice arrangement, and a group of initialized scattering dielectric pillar arrangements are randomly generated. The initial structure is imported into FDTD Solutions for electromagnetic field simulation, and the simulation results are recorded and returned to calculate the FOM value (Figure of Merit, FOM). Using this method, iterative optimization design can be carried out. After the optimization is completed, the required scattering dielectric pillar type time delay structure can be generated. The entire optimization direction is judged based on the gradient information of the change in the quality factor FOM, which can more purposefully find the direction of structure adjustment. This reverse design method avoids searching for corresponding templates in the template library during the device design process and can greatly save the time and energy consumption for modeling analysis and theoretical analysis.

[0026] 2. The terahertz time delay device disclosed by the present invention is small in volume, with a side length of 4.2 mm and a height of 2 mm, and the overall size can be further optimized according to actual needs.

[0027] 3. The material used for the dielectric pillar in the design method disclosed by the present invention is high-resistivity silicon or other materials with less terahertz loss, which is convenient for integration with on-chip silicon-based terahertz systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described in detail below with reference to the drawings and specific embodiments:

[0029] FIG Figure 1 is a schematic diagram of each stage of the terahertz time delay device design method in the method embodiment of the present invention;

[0030] FIG Figure 2 is the final structure (left) of the terahertz time delay device in the method embodiment of the present invention and its corresponding transmittance spectrum (right);

[0031] FIG Figure 3 is a graph of the electric field change with time at positions A and B of the terahertz time delay device structure in the method embodiment of the present invention when photons matching the local frequency are injected with and without dielectric pillars. The left figure is for cavity A, the right figure is for cavity B, and the inset corresponds to the case without dielectric pillars;

[0032] FIG Figure 4 is a dynamic change graph of the internal optical field with time when photons with a completely matched local frequency are injected into the terahertz time delay device structure in the method embodiment of the present invention;

[0033] FIG Figure 5 is a schematic diagram of a dynamic release of local photons by the terahertz time delay device in the method embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] As shown in the figure, a design method of a terahertz time delay device based on the Anderson localization principle. The delay device includes a substrate and dielectric columns fixed on the substrate. Microcavities are formed between some of the dielectric columns. By adjusting the refractive index of the dielectric columns around the microcavities, the residence time of photons in the microcavities is controlled to achieve controllable time delay. The design method includes the following steps;

[0035] Step S1: Use electromagnetic field simulation software to find a two-dimensional periodic dielectric column type photonic crystal structure model with a wide bandgap as the initial structure. The total number of dielectric columns in the initial structure is 10×10;

[0036] Step S2: Divide the two-dimensional photonic crystal lattice arrangement at the substrate into 10×10 small pixel blocks. The presence or absence of the dielectric column in the middle of the pixel block is controlled by assigning "0" and "1" values to the pixel blocks through the particle swarm algorithm. "0" represents no insertion of the dielectric column, and "1" represents insertion of the dielectric column;

[0037] Step S3: Randomly generate a 10×10 dielectric column lattice arrangement composed of "0" and "1" pixel points, and set the boundary condition to a perfectly matched layer boundary;

[0038] Step S4: Select the incident wave frequency as the center frequency of the bandgap. Set the boundary condition in the electromagnetic field simulation software to a perfectly matched layer boundary; Set the figure of merit evaluation function FOM representing the device performance. FOM is set as the sum of the root mean square errors of the transmittances at multiple frequency points with equal frequency intervals on both sides of the center frequency of the bandgap and the set transmittance. Set the transmittance spectrum line as a transmittance peak with a fixed bandwidth and peak value whose center frequency is equal to the incident wave frequency. Import the initial structure into the electromagnetic field simulation software for electromagnetic field simulation to obtain the calculated transmittance spectrum line, and return it and calculate the initial FOM value;

[0039] Step S5: Generate a new lattice arrangement through the automatic search of the particle swarm. Import the changed new structure into the electromagnetic field simulation software for simulation, calculate its FOM value and compare it with the previous FOM value to decide whether to retain the current lattice arrangement;

[0040] Step S6: Repeat Step S5 and compare the FOM values of the previous and current iterations. If the FOM value is improved, continue to repeat; otherwise, the optimization ends and the finally optimized structure is output.

[0041] The dielectric column is a structure with low loss to terahertz, and its material includes high-resistivity silicon.

[0042] In the above-mentioned step S1 and step S2, the frequency range and the position of the transmission peak are defined in the objective function representing the device performance in the electromagnetic field simulation software. In the assignment of the pixel blocks in step S2, "0" represents not inserting a dielectric column at this pixel block, and "1" represents inserting a dielectric column at this pixel block.

[0043] In the iterative optimization in step S5 and step S6, the presence or absence of dielectric columns on the substrate lattice is controlled by an algorithm. By adding dielectric columns, the photonic crystal of the perfect lattice is transformed into a defective photonic crystal, and a transmission peak, also known as a defect mode, is generated in the bandgap of the defective photonic crystal.

[0044] When terahertz light corresponding to the frequency of the defect mode is injected into the defect structure formed by the defect mode, at this time, the frequency supported by the defect mode is consistent with the frequency of the injected photon, so that the photon is localized in the defect structure, generating the time delay effect required for a photon delay device.

[0045] In the above-mentioned time delay effect, the photon oscillates repeatedly between adjacent defect cavities A and B, thus forming a delay effect. The photons in the two defect cavities have an attenuation effect and an interactive transfer effect.

[0046] Microcavities are formed between some of the dielectric columns. By adjusting the refractive index of the dielectric columns around the microcavity, the residence time of photons in the microcavity is controlled to achieve controllable time delay. The specific method is as follows: Inject femtosecond laser pulses into the defect cavity where photons are localized, change the refractive index of the dielectric columns around the microcavity, so that the local frequency of the defect cavity shifts due to the laser pulse, generating a difference from the frequency of the incident terahertz light, thereby releasing the localized photons and achieving the purpose of releasing photons at a specific time.

[0047] The terahertz time delay device controls the residence time of localized photons in the defect cavity by adjusting the injection start time of the femtosecond laser pulse to achieve the function of the time delay device.

[0048] The electromagnetic field simulation software is FDTD Solutions; the perfectly matched layer is Perfectly Matched Layer, abbreviated as PML; the binary particle swarm optimization algorithm is Binary Particle Swarm Optimization, abbreviated as BPSO.

[0049] The above-mentioned design method quickly designs the terahertz time delay device through the BPSO algorithm. Its FOM evaluation function is set as the root mean square error between the transmittance corresponding to multiple frequencies and the ideal value. The particle swarm algorithm outputs the finally optimized structure through multiple iterations.

[0050] Example 1:

[0051] Figure 1Schematic diagrams of each stage of the design method of the terahertz time delay device in the method embodiment of the present invention. S1 is the initialization stage, randomly generate a 10×10 arrangement of dielectric column lattices composed of "0" and "1" pixel points, where "0" represents inserting a silicon dielectric column in the middle of the lattice and "1" represents not inserting a dielectric column, and set the boundary condition to the PML boundary. At this time, import the initialized dielectric column lattice arrangement into the FDTD Solution simulation software to calculate the corresponding optical response curve, and then return it and calculate the figure of merit FOM; S2 is the stage of searching for a certain particle in a certain iteration. The number of particles in the particle swarm for each iteration has been set during initialization, usually set to 5 particles here. In this iteration, five corresponding lattice arrangements will be globally searched. Similarly, import the corresponding new structures into FDTD Solution for 5 electromagnetic field simulations, calculate their FOM values respectively, and set the particle search result corresponding to the structure with the best FOM as the global optimum; S3 is the stage of multiple iterations. The particle swarm searches continuously. By repeatedly comparing the FOM values corresponding to the global optimum of this time and the global optimum of the previous time, if the former is better than the latter, keep the result of the former. If the FOM value of the former is not as good as the latter, keep the global optimum value of the previous iteration; S4 is the stage of exporting the final structure. After the set number of iterations, the optimization ends, and the finally optimized structure is output.

[0052] Example 2:

[0053] Figure 2 The final structure (left) of the terahertz time delay device in the method embodiment of the present invention and its corresponding transmittance spectrum (right). The frequency range and the position of the transmission peak can be defined in the objective function representing the device performance. The essence of iterative optimization is to transform the photonic crystal of the perfect lattice into a defective photonic crystal by controlling the presence or absence of dielectric columns on the lattice through an algorithm. The defective photonic crystal will generate a transmission peak in the bandgap, also called a defect mode. When terahertz light with a frequency corresponding to the defect mode is injected into the defect structure, at this time, the frequency supported by the defect mode is the same as the frequency of the injected photon, and the photon will be localized in the defect, thus producing a certain time delay effect on the photon. As shown in the left figure, two defect cavities A and B are formed, and photons will oscillate repeatedly between these two cavities, thus forming a delay effect.

[0054] Example 3:

[0055] Figure 3This is a graph showing the variation of the electric field with time at positions A and B of the terahertz time delay structure in the method embodiments of the present invention when photons matching the local frequency are injected, with and without dielectric columns. The left figure shows cavity A, and the right figure shows cavity B. The inset corresponds to the case without dielectric columns. At this time, the central frequency of the injected photons is 0.27 THz. It can be seen from the figure that in the case without dielectric columns (i.e., light passes through the blank area corresponding to the size of the device), the duration of the electric field component at positions A and B is only about 0.07 ns (inset). Therefore, it can be ignored when calculating the time delay of a photon by this device. In the presence of dielectric columns, the time delay of a photon by cavities A and B is about 0.7 ns, and in addition to a certain attenuation of the photons in the two cavities, there is also a certain interaction transfer effect.

[0056] Example 4:

[0057] Figure 4 This is a graph showing the dynamic variation of the internal optical field with time when photons with a local frequency completely matching are injected into the terahertz time delay structure in the method embodiments of the present invention. It can be seen from the figure that at positions A and B of the cavities, the photons are perfectly localized in the defective microcavities. At the moment of 0.14 ns, the vast majority of the photons are localized in cavity B; as time goes by, at the moment of 0.23 ns, in addition to a certain attenuation, the photons gradually couple into cavity A; at the moment of 0.33 ns, the photons gradually couple into cavity B again. After multiple rounds of cycles, at 0.7 ns, the photons are finally completely absorbed by the perfect matching layer around the structure, including the part of the photons that attenuate during the dynamic transfer process.

[0058] Example 5:

[0059] Figure 5 This is a schematic diagram of the dynamic release of local photons by the terahertz time delay in the method embodiments of the present invention. In the figure, by injecting femtosecond laser pulses into a defective cavity where a certain number of photons are localized, the refractive index of the dielectric column around the microcavity will change with the irradiation of the laser pulses, resulting in a shift in the local frequency of the defective cavity, causing the mode supported by the defective mode to be inconsistent with the incident light frequency, and the localized photons will be released. Therefore, by adjusting the injection start time of the femtosecond pulse, the terahertz time delay can control the residence time of local photons in the defect and realize the function of the time delay.

[0060] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. A design method for a terahertz time delay device based on the Anderson localization principle, characterized in that: The delay device includes a substrate and dielectric columns fixed on the substrate. Microcavities are formed between some of the dielectric columns. By adjusting the refractive index of the dielectric columns around the microcavities, the residence time of photons in the microcavities is controlled to achieve controllable time delay. The design method includes the following steps; Step S1: Use an electromagnetic field simulation software to find a two-dimensional periodic dielectric column type photonic crystal structure model with a wide bandgap as the initial structure. The total number of dielectric columns in the initial structure is 10×10; Step S2: Divide the two-dimensional photonic crystal lattice at the substrate into 10×10 small pixel blocks. The presence or absence of the dielectric column in the middle of the pixel block is controlled by assigning "0" and "1" values to the pixel blocks through a particle swarm algorithm; Step S3: Randomly generate a 10×10 dielectric column lattice arrangement composed of "0" and "1" pixel points, and set the boundary condition to a perfectly matched layer boundary; Step S4: Select the incident wave frequency as the center frequency of the bandgap, and set the boundary condition in the electromagnetic field simulation software to a perfectly matched layer boundary; Set the figure of merit evaluation function FOM representing the device performance. FOM is set as the sum of the root mean square errors of the transmittances at the center frequency of the bandgap and multiple frequency points with equal frequency intervals on both sides and the set transmittance. Set the transmittance spectrum line as a transmittance peak with a fixed bandwidth and peak value whose center frequency is equal to the incident wave frequency. Import the initial structure into the electromagnetic field simulation software for electromagnetic field simulation to obtain the calculated transmittance spectrum line, and return it to calculate the initial FOM value; Step S5: Generate a new lattice arrangement through the automatic search of the particle swarm, import the changed new structure into the electromagnetic field simulation software for simulation, calculate its FOM value and compare it with the previous FOM value to decide whether to retain the current lattice arrangement; Step S6: Repeat Step S5 and compare the FOM values of the previous and current iterations. If the FOM value is improved, continue to repeat; otherwise, the optimization ends and the finally optimized structure is output; The dielectric column is a structure with low loss to terahertz, and its material includes high-resistivity silicon; In the iterative optimization in Step S5 and Step S6, the presence or absence of the dielectric column on the substrate lattice is controlled by an algorithm. By adding dielectric columns, the photonic crystal with a perfect lattice is transformed into a defective photonic crystal, and a transmittance peak, also called a defect mode, is generated in the bandgap of the defective photonic crystal; When terahertz light corresponding to the frequency of the defect mode is injected into the defect structure formed by the defect mode, at this time, the frequency supported by the defect mode is consistent with the frequency of the injected photons, so that the photons are localized in the defect structure, and the time delay effect required by the delay device for the photons is generated.

2. The design method of a terahertz time delay device based on the Anderson localization principle according to claim 1, wherein: In Step S1 and Step S2, the frequency range and the transmittance peak position are defined in the objective function representing the device performance in the electromagnetic field simulation software. In the assignment of the pixel blocks in Step S2, "0" represents not inserting a dielectric column at this pixel block, and "1" represents inserting a dielectric column at this pixel block.

3. A design method of a terahertz time delay device based on the Anderson localization principle according to claim 1, characterized in that: In the time delay effect, photons oscillate repeatedly between adjacent defect cavities A and defect cavities B, thereby forming a delay effect. The photons in the two defect cavities have an attenuation effect and an interactive transfer effect.

4. A design method of a terahertz time delay device based on the Anderson localization principle according to claim 1, characterized in that: Microcavities are formed between the partial dielectric columns. By adjusting the refractive index of the dielectric columns around the microcavities, the residence time of photons in the microcavities can be controlled to achieve controllable time delay. The specific method is as follows: Inject femtosecond laser pulses into the defect cavity where photons are localized, change the refractive index of the dielectric columns around the microcavity, so that the local frequency of the defect cavity is shifted due to the laser pulses, resulting in a difference from the frequency of the incident terahertz light, thereby releasing the localized photons and achieving the purpose of releasing photons at a specific time.

5. A design method of a terahertz time delay device based on the Anderson localization principle according to claim 4, characterized in that: The terahertz time delay device controls the residence time of localized photons in the defect cavity by adjusting the injection start time of the femtosecond laser pulses to achieve the function of the delay device.

6. The design method of a terahertz time delay device based on the Anderson localization principle according to claim 4, characterized in that: The electromagnetic field simulation software is FDTD Solutions; the perfectly matched layer is Perfectly Matched Layer, abbreviated as PML; the particle swarm algorithm is the binary particle swarm optimization algorithm, namely Binary Particle Swarm Optimization, abbreviated as BPSO.

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