Reverse design super-resolution terahertz line focusing device and method thereof

Abstract

The invention discloses a reverse design super-resolution terahertz line focusing device and a method thereof, and belongs to the technical field of terahertz wave application. The invention aims to provide an efficient and rapid method for designing the device with the super-resolution line focusing function. The reverse design method comprises the following steps: determining target light field intensity distribution (line focusing); a device design area is divided into 200 structural units with the same size, and a transmission function of a device is reversely calculated according to a vector angular spectrum diffraction theory in combination with target light field distribution; and target function fitness values corresponding to different transmission functions are updated by combining a binary particle swarm and a genetic algorithm, and continuous updating and iteration are carried out until an optimal structure meeting constraint conditions is found. Taking a 0.1 THz incident light source as an example, the light spot of a focusing line of the super-resolution line focusing device is 3.5 mm and is far smaller than the diffraction limit 5.856 mm, the side lobe suppression ratio is 21%, the energy utilization rate is 52.71%, and the super-resolution line focusing device has the advantages of uniform line focusing capability, adjustable focal length, high resolution, low manufacturing cost and the like, and has application value in the terahertz imaging field.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz wave applications, and in particular to a reverse-designed super-resolution terahertz line focusing device and a method thereof. Background Art

[0002] Terahertz (THz) waves usually refer to electromagnetic radiation with a frequency range of 0.1 to 10 THz. They have the advantages of low coherence, strong penetration, and low energy, and have broad application prospects in imaging fields such as industrial non-destructive testing, human body security inspection, and medical diagnosis. At present, mainstream terahertz imaging systems mostly use point detectors for imaging, and research methods mainly focus on point focusing of terahertz beams (such as patent application number 202111629641.X, literature E.Arbabi, A.Arbabi, SM Kamali, et al, Nature Communications 9:812, 2018; J.Cheng, Y.Yang, S.Chen, et al, Photonics Research 11(1), 44-54, 2023). Such methods usually require point-by-point scanning of the target sample, which leads to a complex imaging system and limited imaging speed, making it difficult to meet real-time imaging requirements. Compared with the point focusing method, the key to terahertz line focusing is to produce a narrow line width and high uniformity line focusing spot, which appears as an ultra-thin terahertz light sheet when viewed from the side. It can illuminate and detect the entire cross section of the sample at one time, greatly reducing the scanning steps and increasing the imaging speed. Since the waves radiated by the terahertz source are approximately circular spots with Gaussian distribution, a breakthrough in focusing devices is required to obtain the above-mentioned desired terahertz focal line. Therefore, it is urgent to develop terahertz super-resolution line focusing devices with sub-wavelength line focusing control capabilities.

[0003] Metasurface is a planar optical element composed of periodic arrangement of sub-wavelength nano-antennas, which is mostly used for terahertz focusing control. Traditional metasurfaces are based on the forward design method, which uses numerical simulation to screen and arrange structural units that meet the requirements one by one. However, for non-periodic structures or large-radix structural units, it consumes a lot of computing resources and time, which seriously hinders the metasurface design process (Q. Wang, X. Zhang, Y. Xu, et al, Advanced Optical Materials 3 (6), 779-785, 2015). Therefore, a fast and efficient design method is needed, which is different from the parameter scanning or empirical design method of forward design to improve the design efficiency of metasurfaces and provide new design ideas and solutions for terahertz wave application technology. Summary of the invention

[0004] In view of the defects existing in the existing terahertz application technology, the purpose of the present invention is to propose a reverse-designed super-resolution terahertz line focusing device to achieve fast super-resolution terahertz imaging effect. Taking a 0.1THz incident light source as an example, the device line focusing spot reaches 3.5mm, which is much smaller than the diffraction limit of 5.856mm, the sidelobe suppression ratio is 21%, and the energy utilization rate is 52.71%. The generated line focusing spot distribution is narrow and uniform, and the focal length can be adjusted at will. Applying the reverse design method, this device design process does not require parameter scanning. It only needs to constrain the target light field to obtain the optimal structural distribution that meets the requirements. Since the unit spacing is determined by the optimization algorithm, it is different from the equidistant distribution of the traditional grating metasurface structure (as described in patent application numbers 202311364852.4; 202311007849.7), and a super-resolution line focusing effect that breaks through the diffraction limit is achieved. The super-resolution line focusing device of the present invention has uniform line focusing capability, adjustable focal length, is manufactured using common 3D printing technology, has low manufacturing cost, and has application value in the field of terahertz high-resolution imaging.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A reverse design super-resolution terahertz line focusing device and method thereof, wherein the structural distribution of the device is determined by reverse design, including the structural distribution of dielectric column units in an upper layer and a terahertz high-transmittance material substrate in a lower layer.

[0007] Preferably, the inverse design method first determines the key indicators of the super-resolution focal spot according to the system resolution requirements, such as the half-height full width of the focused beam of the target light field line, the resolution sidelobe suppression ratio, the energy utilization rate, etc.; secondly, the transmission function is randomly assigned according to the design indicators, and an optimization model is established in combination with the vector angular spectrum diffraction theory, and an optimization algorithm combining a binary particle swarm and a genetic algorithm is used to find the global optimal design structure through multiple iterative optimizations, which should be no less than 1000 times.

[0008] Preferably, the determined target light field expression is:

[0009]

[0010] Where E x (x,z),E y (y,z) and E z (x,z) are the components of the electric field in the x, y, and z directions respectively, A x,0 (l) is the angular spectrum, T(x) is the transmission function, λ is the wavelength of the incident light, (f x ,f y ) is the spatial frequency, and the focal length can be arbitrarily adjusted by changing the focusing position f of the target light field.

[0011] Preferably, the reverse design area, i.e., the upper unit structure, is composed of 200 dielectric columns with a width of 0.25 mm, and the height of each dielectric column is 0 or meets The total design area of ​​the device is required to be 50mm*50mm.

[0012] Preferably, the entire device is made of a terahertz high-transmittance material with a refractive index of n=1.74, and the substrate thickness is 1 mm.

[0013] Preferably, a phase distribution of (0, π) is randomly assigned to the design region.

[0014] Preferably, by adjusting the initial parameters of the inverse design, the super-resolution terahertz line focusing device can operate in any frequency band.

[0015] Compared with the existing technical solutions, the advantages of the present invention are: different from the traditional terahertz point focusing device affected by the diffraction limit, the device can produce a super-resolution uniform line focusing spot, effectively improving the scanning efficiency; different from the traditional metasurface device application parameter scanning design, the inverse design method can directly obtain the optimal structural distribution by specifying the characteristics of the target light field, thereby improving the design efficiency; it is processed and manufactured using common 3D printing technology, with low manufacturing cost, and can have application value in the field of terahertz high-resolution imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0017] Figure 1 This is a schematic diagram of a super-resolution terahertz line focusing device in Embodiment 1 of the present invention;

[0018] Figure 2 A top view of a super-resolution terahertz line focusing device and a partially enlarged schematic diagram thereof in Embodiment 1 of the present invention;

[0019] Figure 3 This is a working schematic diagram of a super-resolution terahertz line focusing device in Embodiment 1 of the present invention;

[0020] Figure 4 This is a flow chart of a reverse design method for a super-resolution terahertz line focusing device in an embodiment of the present invention;

[0021] Figure 5 This is a line focusing spot distribution diagram of the super-resolution terahertz line focusing device in Example 1 of the present invention on the xy plane at a propagation distance z=80 mm, and the focal length f=80 mm;

[0022] Figure 6 It is a normalized focusing energy distribution diagram of the super-resolution terahertz line focusing device in Example 1 of the present invention at a propagation distance z=80 mm on the x-axis, and the focal length f=80 mm;

[0023] Figure 7 This is a propagation trajectory diagram of the super-resolution terahertz line focusing device on the xz plane in Example 1 of the present invention, with a focal length of f=80 mm;

[0024] Figure 8 This is a propagation trajectory diagram of the super-resolution terahertz line focusing device on the xz plane in the second embodiment of the present invention, with a focal length of f=40 mm;

[0025] Explanation of symbols:

[0026] 1. The upper dielectric column of the device; 2. The device substrate. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The purpose of the present invention is to introduce a reverse-designed super-resolution terahertz line focusing device and a method thereof. The device exhibits line focusing characteristics, and the focused light spot breaks through the diffraction limit to achieve the purpose of super-resolution. It is manufactured using 3D printing technology and has the advantages of high resolution, high focusing efficiency, and low cost.

[0029] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0030] Embodiment 1:

[0031] The present invention produces a terahertz line focusing device with super resolution by a reverse design method, such as Figure 1 As shown, the device is mainly composed of two parts. The upper layer is composed of unevenly distributed terahertz high-transmittance material medium column units determined by inverse design, and its purpose is to form the phase arrangement required for the line focusing spot with super-resolution capability. The lower layer is a terahertz high-transmittance material substrate, and its purpose is to ensure the complete transmission of the terahertz beam.

[0032] The focusing performance of the device can be specifically set according to actual requirements. This embodiment designs a line focusing device by taking the generation of a line focusing spot that breaks through the diffraction limit, a resolution sidelobe suppression ratio of less than 30%, an energy utilization rate of more than 50%, and a focal length of 80 mm as an example. Figure 2 The top view of the device that meets the above requirements and its partial enlarged view are shown. The structural details of the device are marked in detail in the figure, where the period of the unit dielectric column is set to d = 0.25mm. The height of the unit dielectric column is calculated to be 2.027 mm, where λ is the terahertz incident wavelength 3 mm, and n=1.74 is the refractive index of the material constituting the dielectric column. The overall size of the device is 50 mm*50 mm, and the substrate thickness H=1 mm.

[0033] Figure 3 The working principle of this device is demonstrated. When a 0.1THz light source is incident vertically on this device after being collimated and expanded, the wavefront of the light beam is accurately modulated after passing through a carefully designed dielectric column and is focused into a uniform narrow line spot at a pre-designed focal length. By measuring the maximum half-height full width of the focused line spot, it can be determined whether the diffraction limit is reached. The calculation formula for the half-height full width of the diffraction limit is: NA is the numerical aperture of the device. In this example, NA = 0.3125. When the maximum half-height full width of the measured line focus spot is less than It can be proved that this focusing breaks through the diffraction limit and reaches the super-resolution level.

[0034] The key to achieving super-resolution line focusing lies in the arrangement of the dielectric column units on the upper layer of the device. Through the carefully designed arrangement of the dielectric columns, a phase difference of 0 or π can be formed when the light beam is incident on the device surface, thereby generating a complex interference phenomenon and presenting a super-resolution spot size at the focusing position. It is difficult to accurately determine the actual arrangement of the dielectric columns using traditional design methods, but the inverse design method provides a possibility for this. The flowchart of the inverse design is shown below. Figure 4 As shown. First, according to the actual design requirements, the initial structural parameters, resolution requirements, key indicators of the super-resolution focal spot are set, and the objective function weight is given. In this embodiment, MATLAB software programming is used to implement reverse design, and the half-height full width weight of the target light field line focusing beam is set to 0.5, the resolution sidelobe suppression ratio weight is 0.7, and the energy utilization weight is 0.3. The design area is evenly divided into 200 0.25mm*50mm long and thin slits, and each slit is randomly assigned a phase difference of 0 or π, that is, a transmission function T(x) is randomly assigned, and the maximum and minimum inertia weights are set to 0.9 and 0.4 respectively. The particle swarm algorithm is initialized, and the input transmission function is used as the initial reference. The expression of the target light field is determined as follows:

[0035] Where E x(x,z),E y (y,z) and E z (x,z) are the components of the electric field in the x, y, and z directions respectively, A x,0 (l) is the angular spectrum, T(x) is the transmission function, λ = 3 mm is the incident terahertz wavelength, (f x ,f y ) is the spatial frequency. By changing the focus position f of the target light field, the focal length can be arbitrarily adjusted. 0 (·) and J 1 (·) are the zero-order and first-order Bessel functions, respectively.

[0036] For each different randomly assigned transmission function T(x), the half-maximum full width of the focused light field, the resolution sidelobe suppression ratio and the energy utilization rate are calculated, and the fitness value of the objective function is recorded.

[0037] Dynamically update the inertia weight, record the update speed, introduce the mutation operation of the genetic algorithm, set the mutation probability to 0.2, and randomly mutate the particle group to avoid it falling into the local optimum. Through each iterative optimization, compare different fitness values ​​to determine whether the fitness value is the minimum. If not, continue to update and iterate to find the global optimal solution; if so, output the optimal transmission function, which is the required arrangement of unit dielectric columns. Theoretically, the number of iterations should be no less than 1000 times. The purpose is to prevent the program from falling into the local optimal solution during the search process and failing to find the global optimal solution. The device structure can be drawn according to its transmission function in the later stage.

[0038] The numerical finite-difference time-domain method is used to simulate the focusing condition of the designed device and its focusing spot characteristics. The incident light source is a 0.1THz single-frequency wave with Gaussian distribution, and its waist radius is set to 25mm. The beam propagates along the positive direction of the z-axis, and the waist is located at z=-5mm. The device is located at z=0mm. Figure 5 This is the line focusing spot distribution diagram of the super-resolution terahertz line focusing device in this embodiment on the xy plane at a propagation distance of z=80mm. It can be seen that the designed device achieves a good line focusing effect. The focusing spot presents a symmetrical and uniform linear focusing spot. The simulation data shows that the center half-height full width is 3.5mm, which is much smaller than 5.856mm under the diffraction limit. This proves that the device focuses to a spot size that breaks the diffraction limit and achieves super-resolution.

[0039] Figure 6This is the normalized focusing energy distribution diagram of the super-resolution terahertz line focusing device on the x-axis at the propagation distance z=80mm in this embodiment. The ordinate in the figure is the normalized energy intensity. It can be seen from the figure that the focused spots are symmetrically distributed, the energy at the center is the strongest, and there are symmetrical sidelobe distributions on both sides, but the energy proportion of the side lobes is relatively small, only 21%, and the energy utilization rate is 52.71%, which is achieved under the constraints of the resolution sidelobe suppression ratio being less than 30% and the energy utilization rate being higher than 50% set during the inverse optimization.

[0040] Figure 7 This is the propagation trajectory diagram of the super-resolution terahertz line focusing device in this embodiment on the xz plane. It can also be seen from the figure that the device has a very good focusing effect, focusing into a very bright spot at z = 80 mm, which is perfectly consistent with the focal length f = 80 mm pre-set in the inverse design.

[0041] Embodiment 2:

[0042] In order to demonstrate the simplicity and high efficiency of the reverse design in the present invention, in Example 2, only the focus position of the preset device is changed, that is, the focus distance is set to f=40 mm in this example, and the transmission function is optimized without changing other conditions, and the optimized device is simulated by numerical finite-difference time-domain method. Figure 8 This is a propagation trajectory diagram of the super-resolution terahertz line focusing device in the second embodiment of the present invention on the xz plane. It can also be seen from the figure that the device has a very good focusing effect, focusing into a very bright line spot at z = 40mm, which perfectly matches the pre-set focal length f = 40mm.

[0043] From the above two embodiments, it can be seen that the method of the present invention successfully realizes a reverse design of a super-resolution terahertz line focusing device, and can flexibly control its focal length. The reverse design provides a new idea for the design of terahertz devices, which has more flexible and efficient characteristics under the premise of determining the target light field. The designed device produces a super-resolution line focusing spot, which can be applied in the field of terahertz imaging.

[0044] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0045] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A reverse design super-resolution terahertz line focusing device and method thereof, characterized in that: The structural distribution of the device is determined by inverse design, including the dielectric column unit structure distribution of the upper layer and the terahertz high-transmittance material substrate of the lower layer.

2. A reverse design super-resolution terahertz line focusing device and method thereof as claimed in claim 1, characterized in that: The inverse design method first determines the key indicators of the super-resolution focal spot according to the system resolution requirements, such as the half-height full width of the focused beam of the target light field line, the resolution sidelobe suppression ratio, the energy utilization rate, etc.; secondly, the transmission function is randomly assigned according to the design indicators, and the optimization model is established in combination with the vector angular spectrum diffraction theory. The optimization algorithm combining the binary particle swarm and the genetic algorithm is used to find the global optimal design structure through multiple iterative optimizations, which should be no less than 1000 times.

3. A reverse design super-resolution terahertz line focusing device and method thereof as claimed in claim 2, characterized in that: The target light field expression determined by inverse design is: Where E x (x,z),E y (y,z) and E z (x,z) are the components of the electric field in the x, y, and z directions respectively, A x,0 (l) is the angular spectrum, T(x) is the transmission function, λ is the wavelength of the incident light, (f x ,f y ) is the spatial frequency, and the focal length can be arbitrarily adjusted by changing the focusing position f of the target light field.

4. A reverse design super-resolution terahertz line focusing device and method thereof as claimed in claim 1, characterized in that: The reverse design area, that is, the upper unit structure is composed of 200 dielectric columns with a width of 0.25 mm, and the height of each dielectric column is 0 or meets The total design area of ​​the device is required to be 50mm*50mm.

5. A reverse design super-resolution terahertz line focusing device and method thereof as claimed in claim 1, characterized in that: The entire device is made of terahertz high-transmittance material with a refractive index of n=1.74, and the substrate thickness is 1 mm.

6. A reverse design super-resolution terahertz line focusing device and method thereof as claimed in claim 3, characterized in that: The design region of the transmission function is randomly assigned a phase distribution of (0,π).

7. A reverse design super-resolution terahertz line focusing device and method thereof as claimed in claim 3, characterized in that: By adjusting the initial parameters of the inverse design, the super-resolution terahertz line focusing device can operate in any frequency band.

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