CN113991313A - Design method of pixel type terahertz band-pass filter - Google Patents
Design method of pixel type terahertz band-pass filter Download PDFInfo
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Abstract
The invention relates to a design method of a pixel type terahertz band-pass filter, which comprises the following steps: dividing the frequency selection surface structure unit into M multiplied by N pixel points, wherein the pixel points are metal squares and are represented by '1', and the air squares are represented by '0'; initializing, and randomly generating a group of M multiplied by N pixel blocks consisting of '0' and '1' pixel points; setting a figure of merit FOM, performing electromagnetic field simulation on the initial structure, and calculating an initial FOM value; randomly selecting a pixel point, changing the state of the pixel point, calculating the FOM value of the pixel point and comparing the FOM value with the FOM value of the previous time, and if the FOM value is improved, keeping the changed state of the pixel point; traversing all pixel points for one iteration, and reserving the optimal FOM value in the iteration; and repeating the steps and comparing FOM values of the previous iteration and the current iteration, finishing optimization after meeting the set cycle termination condition, and outputting the optimized structure. The method is beneficial to quickly and efficiently designing the terahertz band-pass filter with excellent performance.
Description
Technical Field
The invention belongs to the technical field of terahertz, and particularly relates to a design method of a pixel type terahertz band-pass filter.
Background
The application of terahertz technology requires not only an efficient wave source and a sensitive detector, but also high-performance functional elements such as filters, modulators, switches, distributors, wavelength division demultiplexers, polarizers, and the like to effectively manipulate terahertz waves.
The terahertz functional device has a plurality of different types, and the corresponding theories of the devices are independent from each other, so when the terahertz functional device with a certain specific function is needed, firstly, a proper structure is searched in a template library matched with the terahertz functional device according to the specific function to be used as a basic model, if the proper structure cannot be found, a device model is designed according to a mature theory and the theory, and then, a target device with the required performance is finally generated by adjusting characteristic structure parameters in the model.
Obviously, the direct design method not only needs to spend a lot of time to search the corresponding template in the template library, but also needs a lot of experience of analysis modeling and theoretical analysis, which puts high requirements on scientists and engineering technicians to timely and effectively complete their design tasks. Therefore, by utilizing a reverse design method which can directly realize the design from a specific function to a device structure, the adjustment stage of model parameters can be omitted, so that the design period of a functional device is shortened, and the aim of efficiently designing the terahertz functional device is fulfilled.
The terahertz filter can separate noise from signals and is an important functional device in terahertz communication and terahertz imaging systems. The implementation of terahertz bandpass filtering with a frequency selective surface is one of the most common methods. The frequency selection surface can realize different filtering characteristics through different unit shapes, has the advantages of higher tolerance to processing errors, simple shape and the like, receives more attention, can realize wider frequency band range, more frequency bands and steeper frequency cut-off characteristics through multi-layer combination, and has a more complete template library.
Disclosure of Invention
The invention aims to provide a design method of a pixel type terahertz band-pass filter, which is beneficial to quickly and efficiently designing the terahertz band-pass filter with excellent performance.
In order to achieve the purpose, the invention adopts the technical scheme that: a design method of a pixel type terahertz band-pass filter comprises the following steps:
s1, dividing a structural unit of the frequency selection surface into M multiplied by N small blocks, namely pixel points, wherein the M multiplied by N pixel points are composed of a metal square or an air square, the air square is a processing part, if the pixel points are the metal square, the pixel points are represented by '1', and the air square is represented by '0'; initializing, and randomly generating a group of M multiplied by N pixel blocks consisting of '0' and '1' pixel points;
s2, setting an objective function capable of expressing device performance, namely a figure of merit FOM, wherein the sum of squares of differences between the transmittance corresponding to multipoint frequencies and an ideal value is set on the terahertz frequency selection surface, then introducing the initial structure into electromagnetic field simulation software for electromagnetic field simulation, returning the simulation result and calculating the initial FOM value;
s3, randomly selecting one pixel point of the M multiplied by N pixel points, changing the state of the pixel point, changing the pixel point to be 0 if the pixel point is 1, changing the pixel point to be 1 if the pixel point is 0, performing electromagnetic field simulation on the new structure after the change, calculating the FOM value of the new structure, comparing the FOM value with the FOM value of the last time, if the FOM value is improved, keeping the state of the changed pixel point, and otherwise, restoring the changed value; carrying out M multiplied by N changes in total, namely traversing all pixel points, namely one iteration, and reserving the optimal FOM value in the iteration;
and S4, repeating the step S3, comparing the FOM value of the previous iteration with the FOM value of the current iteration, finishing the optimization when the FOM value is smaller than the set minimum value or the cycle number reaches the set iteration number, and outputting the finally optimized structure.
Further, the distribution of the pixel points in the frequency selective surface structure unit is set to 1/8 symmetrical structure, that is, the device is symmetrical not only about the x-axis and the y-axis, but also about two diagonal lines, so as to structurally and directly cancel the influence caused by the electric field polarization without simulating the electromagnetic field of each polarization angle multiple times on the program to obtain the polarization stability.
Further, the formula for calculating the FOM value is as follows:
wherein,T 1representing center frequency within pass bandfThe transmittance of the light source (c) is,T 2andT 3is composed off ±B FWHMThe transmittance at the point of/2 is,B FWHMin order to set the bandwidth of the communication channel,T 4、T 5、T 6、T 7the transmission of the frequency points evenly distributed outside the pass band.
Furthermore, the method is not only suitable for designing the terahertz band-pass filter, but also suitable for designing two-dimensional structures of other terahertz functional devices, including terahertz cross connectors and terahertz grating couplers.
Compared with the prior art, the invention has the following beneficial effects:
1. the method has more efficient design efficiency. The method can perform iterative optimization design only by randomly generating a group of initialized pixel points and taking 1/8 in the symmetrical structure, a required band-pass filter structure can be generated after optimization is completed, the whole optimization direction is judged by gradient information of objective function change, the structure adjustment direction can be found more purposefully, and the reverse design method avoids that a corresponding template is searched in a template library in the device design process and a large amount of time and energy are consumed due to the need of analyzing modeling and theoretical analysis experience.
2. The method can design the band-pass filter on the basis of a completely unknown structure, so that all possibilities in a design area can be explored, the structure with the corresponding function can be realized, the side length of the pixel point of the designed terahertz wave band-pass filter is only 15, the whole size is determined according to actual requirements, and the terahertz wave band-pass filter can be integrated into a terahertz system.
3. The 1/8 symmetrical structure of the method can directly counteract the influence caused by electric field polarization without simulating the electromagnetic field of each polarization angle for many times on a program to obtain polarization stability, and compared with the traditional cross-shaped band-pass filter, the pixel type terahertz band-pass filter generated by the reverse design method has better out-of-band suppression effect.
Drawings
FIG. 1 is a schematic diagram of various stages of a terahertz band-pass filter design method according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of 1/8 symmetrical structure of a terahertz band-pass filter according to an embodiment of the present invention;
fig. 3 is a graph of two optimal structures and normalized transmittance of the terahertz band-pass filter according to the embodiment of the invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
Fig. 1 is a schematic diagram of each stage of the design method of the terahertz band-pass filter in this embodiment. In the figure, S1: initializing a pixel point structure; s2: setting a target function; s3, randomly selecting pixel points to optimize and traverse all the pixel points; s4, iteration is carried out until the FOM value is not optimized.
As shown in fig. 1, the present embodiment provides a method for designing a pixel-type terahertz band-pass filter, including the following steps:
s1, dividing the structural unit of the frequency selective surface into M × N small blocks (pixel points). Only the upper left 4 x 4 part of the M x N matrix is shown in fig. 1. The M multiplied by N pixel points are composed of two states of a metal square or an air square (a processing part), if the pixel points are the metal square, the pixel points are represented by '1', and if the pixel points are the air square, the pixel points are represented by '0'. Initializing and randomly generating a group of M multiplied by N pixel blocks consisting of '0' and '1' pixel points.
S2, setting an objective function capable of representing the device performance, which may also be called figure of merit (FOM), generally being the sum of squares of the actual value of normalized power and the target value, and the sum of squares of the differences between the transmittance corresponding to the multi-point frequency and the ideal value in the terahertz frequency selective surface, then introducing the initial structure into electromagnetic field simulation software (FDTD Solutions) to perform electromagnetic field simulation, returning the simulation result and calculating the initial FOM value.
S3, randomly selecting one pixel point of the M multiplied by N pixel points, changing the state of the pixel point, changing the pixel point to be 0 if the pixel point is 1, changing the pixel point to be 1 if the pixel point is 0, performing electromagnetic field simulation on the new structure after the change, calculating the FOM value of the new structure, comparing the FOM value with the FOM value of the last time, if the FOM value is improved, keeping the state of the changed pixel point, and if the FOM value is not improved, restoring the changed value. And totally carrying out M multiplied by N changes, namely traversing all pixel points, called one iteration, and keeping the optimal FOM value in the iteration.
And S4, repeating the step S3, comparing the FOM value of the previous iteration with the FOM value of the current iteration, finishing the optimization when the FOM value is smaller than the set minimum value or the cycle number reaches the set iteration number, and outputting the finally optimized structure.
Fig. 2 is a schematic diagram of 1/8 symmetrical structure of the thz band-pass filter in the embodiment of the present invention. In the present embodiment, the distribution of pixel points in the frequency selective surface structure unit is set to 1/8 symmetrical structure, that is, the device is not only with respect toxShaft andythe axial symmetry and the symmetry about two diagonal lines can directly cancel the influence caused by the electric field polarization structurally without simulating the electromagnetic field of each polarization angle multiple times on the program to obtain the polarization stability.
Fig. 3 is a graph showing two optimal structures and normalized transmittance curves of the terahertz band-pass filter in this embodiment. Taking the design of a band-pass filter with a center frequency of 0.5THz as an example, the period of the structural unit at this time is 300μmThe unit period is divided into 20 × 20 small blocks, and the side length of each small block is 15μmThickness of foil 10μmThe metal material is aluminum, and the substrate is 10 thickμmThe material is polyimide. The FOM may be defined as:
whereinT 1Representing center frequency within pass bandfThe transmittance of the light source (c) is,
andT 3is composed off ±B FWHMThe transmittance at the point of/2 is,B FWHMin order to set the bandwidth of the communication channel,T 4 ，T 5，T 6，T 7the transmission of the frequency points evenly distributed outside the pass band. The more frequency points are set and the more uniform the distribution is, the more ideal the obtained terahertz spectrogram is.
It can be seen from fig. 3 that the pixel-type terahertz band-pass filter generated by the reverse design method has a band-pass filtering characteristic, and the out-of-band rejection effect outside the center frequency is good, that is, the normalized transmittance amplitude of the frequency outside the pass band is low.
In the method of the present invention, the distribution of the pixel points in the frequency selective surface is set to 1/8 symmetrical structure, i.e. the device is symmetrical not only about the x-axis and the y-axis, but also about two diagonal lines, so as to structurally and directly cancel the influence caused by the electric field polarization without simulating the electromagnetic field of each polarization angle multiple times in the procedure to obtain the polarization stability.
The method is not only suitable for designing the terahertz band-pass filter, freely controlling the degree of freedom in a design area and designing the terahertz band-pass filter meeting the requirements, but also suitable for designing two-dimensional structures of other terahertz functional devices, including terahertz cross connectors, terahertz grating couplers and the like.
The foregoing is directed to preferred embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. However, any simple modification, equivalent change and modification of the above embodiments according to the technical essence of the present invention are within the protection scope of the technical solution of the present invention.
Claims (4)
1. A design method of a pixel type terahertz band-pass filter is characterized by comprising the following steps:
s1, dividing a structural unit of the frequency selection surface into M multiplied by N small blocks, namely pixel points, wherein the M multiplied by N pixel points are composed of a metal square or an air square, the air square is a processing part, if the pixel points are the metal square, the pixel points are represented by '1', and the air square is represented by '0'; initializing, and randomly generating a group of M multiplied by N pixel blocks consisting of '0' and '1' pixel points;
s2, setting an objective function capable of expressing device performance, namely a figure of merit FOM, wherein the sum of squares of differences between the transmittance corresponding to multipoint frequencies and an ideal value is set on the terahertz frequency selection surface, then introducing the initial structure into electromagnetic field simulation software for electromagnetic field simulation, returning the simulation result and calculating the initial FOM value;
s3, randomly selecting one pixel point of the M multiplied by N pixel points, changing the state of the pixel point, changing the pixel point to be 0 if the pixel point is 1, changing the pixel point to be 1 if the pixel point is 0, performing electromagnetic field simulation on the new structure after the change, calculating the FOM value of the new structure, comparing the FOM value with the FOM value of the last time, if the FOM value is improved, keeping the state of the changed pixel point, and otherwise, restoring the changed value; carrying out M multiplied by N changes in total, namely traversing all pixel points, namely one iteration, and reserving the optimal FOM value in the iteration;
and S4, repeating the step S3, comparing the FOM value of the previous iteration with the FOM value of the current iteration, finishing the optimization when the FOM value is smaller than the set minimum value or the cycle number reaches the set iteration number, and outputting the finally optimized structure.
2. The design method of a pixel-type terahertz band-pass filter according to claim 1, wherein the distribution of pixel points in the frequency selective surface structure unit is set to 1/8 symmetrical structure, that is, the device is symmetrical not only about the x-axis and the y-axis, but also about two diagonal lines, so as to directly cancel the influence caused by electric field polarization structurally, without simulating the electromagnetic field of each polarization angle multiple times in procedure to obtain polarization stability.
3. The design method of the pixel type terahertz band-pass filter according to claim 1, wherein the formula for calculating the FOM value is as follows:
wherein,T 1representing center frequency within pass bandfThe transmittance of the light source (c) is,T 2andT 3is composed off ±B FWHMThe transmittance at the point of/2 is,B FWHMin order to set the bandwidth of the communication channel,T 4、T 5、T 6、T 7the transmission of the frequency points evenly distributed outside the pass band.
4. The design method of the pixel-type terahertz band-pass filter as claimed in claim 1, wherein the method is not only suitable for designing terahertz band-pass filters, but also suitable for designing two-dimensional structures of other terahertz functional devices, including terahertz cross connectors and terahertz grating couplers.
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Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20020067480A1 (en) * | 1999-06-21 | 2002-06-06 | Hamamatsu Photonics K. K. | Terahertz wave spectrometer |
| JP2008008842A (en) * | 2006-06-30 | 2008-01-17 | Matsushita Electric Ind Co Ltd | Electromagnetic wave measuring instrument |
| US20200167897A1 (en) * | 2016-09-30 | 2020-05-28 | Kiarash Ahi | Method and System for Enhancing Resolution of Terahertz Imaging |
| CN112287567A (en) * | 2020-11-28 | 2021-01-29 | 福州大学 | Rapid design method of subminiature terahertz wavelength division multiplexer |
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Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20020067480A1 (en) * | 1999-06-21 | 2002-06-06 | Hamamatsu Photonics K. K. | Terahertz wave spectrometer |
| JP2008008842A (en) * | 2006-06-30 | 2008-01-17 | Matsushita Electric Ind Co Ltd | Electromagnetic wave measuring instrument |
| US20200167897A1 (en) * | 2016-09-30 | 2020-05-28 | Kiarash Ahi | Method and System for Enhancing Resolution of Terahertz Imaging |
| CN112287567A (en) * | 2020-11-28 | 2021-01-29 | 福州大学 | Rapid design method of subminiature terahertz wavelength division multiplexer |
Non-Patent Citations (2)
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| 夏治川: "CD552R3-Ⅲ模拟锁相放大及太赫兹脉冲信号快速采集系统研究", 中国优秀硕士学位论文全文数据库, 15 February 2018 (2018-02-15) * |
| 陈燕青: "表面等离子体共振耦合导致的太赫兹滤波器多带通效应", 激光与光电子学进展, vol. 56, no. 20, 20 November 2019 (2019-11-20) * |
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