Transmission and reflection switchable frequency selective surface structure and unit structure thereof

Abstract

The invention relates to a transmission and reflection switchable frequency selective surface structure and a unit structure thereof, and the frequency selective surface unit structure comprises a first metal layer, a dielectric substrate and a second metal layer which are sequentially arranged from top to bottom. The first metal layer comprises a cross fractal structure, a cross connection square ring and four photodiodes, and the cross fractal structure is arranged on the upper surface of the dielectric substrate; the cross connection square ring is located on the outer side of the cross fractal structure. And the four photodiodes are respectively connected between the four end parts of the cross fractal structure and the cross connection square ring. The transmission mode and the reflection mode can be switched through switching on and switching off the photodiode, and the device is not prone to being damaged by heat, can be compatible with various electromagnetic scenes, is high in stability, can be flexibly switched between the transmission mode and the reflection mode at the frequency of 11.1 GHz, and achieves dynamic regulation and control of electromagnetic waves.

Description

Technical Field

[0001] The invention belongs to the technical field of electromagnetic protection, and in particular relates to a frequency selective surface structure with switchable transmission and reflection and a unit structure thereof. Background Art

[0002] Frequency Selective Surface (FSS) is a two-dimensional periodic array structure composed of resonant units, which exhibits the characteristics of electromagnetic wave spatial filter and can be designed to transmit and reflect electromagnetic waves in a specific frequency band. FSS has a wide range of applications in the fields of radomes, absorbers, spatial filters, shields and wireless communication systems.

[0003] With the development of electromagnetic wave technology, the electromagnetic environment is becoming increasingly complex. In modern communications, how to effectively control or suppress the characteristic signals of the target, significantly reduce the radar cross section (RCS) of the antenna system, and achieve radar stealth has always been a difficult problem that needs to be solved. However, this often conflicts with the electrical performance of the antenna system. Many radar stealth technologies will cause the radiation performance of the antenna to decrease, thereby reducing the overall efficiency of the system. Therefore, how to balance radar stealth capability and signal transmission capability has become a hot topic in industry research, and more and more researchers have begun to explore switchable reconfigurable reflectors. It can flexibly switch between transmission bands and reflection bands, thereby taking into account both electromagnetic wave shielding and long-distance communication capabilities within the working frequency band. It can better adapt to complex and changeable electromagnetic environments and enrich the application scenarios of various types of antennas.

[0004] With the rapid development of high-energy technologies such as lasers, microwaves, and particle beams, higher requirements have been placed on the high-energy protection technology of frequency selective surfaces that can switch between transmission and reflection. Existing frequency selective surfaces that can switch between transmission and reflection cannot maintain stable performance when high-energy technologies generate instantaneous high temperatures, and can no longer meet the current application requirements of high-energy protection scenarios. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a frequency selective surface structure with switchable transmission and reflection and its unit structure. The technical problem to be solved by the present invention is achieved by the following technical solution:

[0006] A first aspect of the present invention provides a frequency selective surface unit structure with switchable transmission and reflection, comprising:

[0007] A first metal layer, a dielectric substrate, and a second metal layer are sequentially arranged from top to bottom;

[0008] The first metal layer includes: a cross fractal structure, a cross-connected square ring and four photodiodes, wherein:

[0009] The cross fractal structure is arranged on the upper surface of the dielectric substrate;

[0010] The cross-connected square ring is located outside the cross fractal structure;

[0011] The four photodiodes are respectively connected between the four ends of the cross fractal structure and the cross-connected square ring;

[0012] The centers of the cross fractal structure and the cross-connected square rings coincide with each other, and both are centrosymmetric structures.

[0013] In an achievable manner, the cross fractal structure comprises: a central rectangular portion and four "X" fractal portions, wherein the central rectangular portion is arranged at the center of the upper surface of the dielectric substrate;

[0014] The four ""-shaped fractal parts are respectively connected to four sides of the central rectangular part.

[0015] In an achievable manner, the ""-shaped" part includes: a first rectangular part, a second rectangular part and a gap, wherein:

[0016] The first rectangular portion connects sides of the central rectangular portion;

[0017] The second rectangular portion is arranged perpendicular to the first rectangular portion and connected to one side of the first rectangular portion;

[0018] The slit is arranged perpendicular to the first rectangular portion, and extends from the other side of the first rectangular portion to the inside of the second rectangular portion.

[0019] In an achievable manner, the cross-connected square ring includes four triangular structures, four square ring connecting parts and four connecting lines, wherein:

[0020] The four triangular structures are respectively arranged at the ends of the four "J" fractal parts, and there is a gap between each triangular structure and each "J" fractal part;

[0021] The four square ring connecting parts are respectively connected between every two adjacent triangular structures;

[0022] The four connecting lines are respectively connected to the midpoint of each of the square ring connecting parts.

[0023] In one achievable manner, the four photodiodes are respectively arranged at intervals between the four triangular structures and the four "" fractal parts, and the anode of the photodiode is connected to the "" fractal part, and the cathode of the photodiode is connected to the triangular structure.

[0024] In one achievable manner, the dielectric substrate and the central rectangular portion are concentric squares;

[0025] The width of the first rectangular portion is equal to the side length of the central rectangular portion;

[0026] The long axis of the first rectangular portion is located on the axis of the dielectric substrate.

[0027] In an achievable manner, the triangular structure is an isosceles triangle, and the vertex of the triangular structure is arranged toward the edge of the dielectric substrate, and the midline of the triangular structure is located on the axis of the dielectric substrate;

[0028] The four connecting lines extend from the midpoint of each square ring connecting portion to the four corners of the dielectric substrate respectively.

[0029] In one achievable manner, the second metal layer includes four rectangular structures;

[0030] The four rectangular structures are all located on the lower surface of the dielectric substrate, and the four rectangular structures extend from the midpoints of the four sides of the dielectric substrate to the center of the dielectric substrate respectively.

[0031] In an achievable manner, the material of the first metal layer and the second metal layer both include: one of copper Cu, aluminum Al, and gold Au;

[0032] The material of the dielectric substrate includes: flexible flame-resistant material.

[0033] The second aspect of the present invention provides a frequency selective surface structure with switchable transmission and reflection, comprising M×N periodically arranged unit structures, wherein the unit structure is the frequency selective surface unit structure with switchable transmission and reflection provided by the first aspect of the present invention;

[0034] Wherein, M and N are positive integers.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The unit structure of the transmission-reflection switchable frequency selective surface structure of the present invention realizes the switching between the transmission mode and the reflection mode by turning on and off the photodiode. It is not easily damaged by heat, is compatible with various electromagnetic scenarios, has high stability, and can flexibly switch between the transmission mode and the reflection mode at a frequency of 11.1 GHz, thereby realizing dynamic regulation of electromagnetic waves. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a structural schematic diagram of a unit structure of a transmission-reflection switchable frequency selective surface structure provided by an embodiment of the present invention;

[0038] Figure 2 It is a side view of a unit structure of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention;

[0039] Figure 3 It is a schematic structural diagram of a first metal layer provided by an embodiment of the present invention;

[0040] Figure 4 It is a schematic structural diagram of a character - shaped fractal part provided by an embodiment of the present invention;

[0041] Figure 5 It is a schematic structural diagram of a second metal layer provided by an embodiment of the present invention;

[0042] Figure 6 It is a schematic structural diagram of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention;

[0043] Figure 7 It is a simulation diagram of the filtering performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is turned on in the TE polarization mode;

[0044] Figure 8 It is a simulation diagram of the filtering performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is cut off in the TE polarization mode;

[0045] Fig. 9 It is a simulation diagram of the filtering performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is turned on in the TM polarization mode;

[0046] Fig.10 It is a simulation diagram of the filtering performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is cut off in the TM polarization mode;

[0047] Fig.11 It is a simulation diagram of the angular stability performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is turned on in the TE polarization mode;

[0048] Fig.12 It is a simulation diagram of the angular stability performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is cut off in the TE polarization mode;

[0049] Fig.13 It is a simulation diagram of the angular stability performance of a transmissive - reflective switchable frequency - selective surface structure provided by an embodiment of the present invention when the photodiode is turned on in the TM polarization mode;

[0050] Fig.14 This is a simulation diagram of the angular stability performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is cut off in the TM polarization mode.

[0051] Reference numerals:

[0052] 1: first metal layer; 11: cross fractal structure; 111: central rectangular portion; 112: "" fractal portion; 1121: first rectangular portion; 1122: second rectangular portion; 1123: gap; 12: cross-connected square rings; 121: triangle structure; 122: square ring connection portion; 123: connection line; 13: photodiode; 2: dielectric substrate; 3: second metal layer; 31: rectangular structure. DETAILED DESCRIPTION

[0053] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0054] Embodiment 1

[0055] See also Figure 1 and Figure 2 , Figure 1 is a schematic structural diagram of a unit structure of a transmissive-reflective switchable frequency selective surface structure provided by an embodiment of the present invention, Figure 2 It is a side view of a unit structure of a transmission-reflection switchable frequency selective surface structure provided by an embodiment of the present invention.

[0056] The present embodiment provides a transmission-reflection switchable frequency selective surface unit structure, comprising: a first metal layer 1, a dielectric substrate 2, and a second metal layer 3 arranged in sequence from top to bottom. The first metal layer 1 comprises: a cross fractal structure 11, a cross-connected square ring 12, and four photodiodes 13, wherein the cross fractal structure 11 is arranged on the upper surface of the dielectric substrate 2. The cross-connected square ring 12 is located outside the cross fractal structure 11, and the four photodiodes 13 are respectively connected between the four ends of the cross fractal structure 11 and the cross-connected square ring 12. The centers of the cross fractal structure 11 and the cross-connected square ring 12 coincide, and both are centrosymmetric structures.

[0057] Specifically, the cross fractal structure 11 is disposed at the center of the upper surface of the dielectric substrate 2. When the photodiode 13 is turned off, the cross fractal structure 11 and the cross-connected square ring 12 are in a disconnected state. When the photodiode 13 is turned on, the cross fractal structure 11 and the cross-connected square ring 12 are in a connected state. In the transmission mode, the external optical signal turns off the photodiode 13, and the photocurrent changes the current path on the surface of the first metal layer 1. At this time, the surface current is mainly concentrated on the cross-connected square ring 12, reducing the reflection and enabling the electromagnetic wave to pass through effectively. Conversely, in the reflection mode, by adjusting the optical signal to turn on the photodiode 13, the surface current is mainly concentrated on the cross fractal structure 11 at this time. At this time, the on state of the photodiode 13 increases the equivalent impedance of the first metal layer 1, preventing the transmission of the electromagnetic wave and causing the electromagnetic wave to be reflected, achieving the stealth effect. In this embodiment, by precisely controlling the on and off states of the photodiode 13, the switching between the transmission mode and the reflection mode is realized, and it can flexibly switch between the transmission and reflection modes at a frequency of 11.1 GHz, realizing the dynamic regulation of the electromagnetic wave. Moreover, since both the cross fractal structure 11 and the cross-connected square ring 12 are centrosymmetric structures, that is, the first metal layer 1 is a centrosymmetric structure, the frequency selective surface unit structure provided in this embodiment has good angular stability and polarization stability.

[0058] In this embodiment, please refer to Figure 3 , the cross fractal structure 11 includes: a central rectangular portion 111 and four "J" fractal portions 112. Among them, the central rectangular portion 111 is disposed at the center of the upper surface of the dielectric substrate 2. The four "J" fractal portions 112 are respectively connected to the four sides of the central rectangular portion 111.

[0059] Furthermore, please refer to Figure 4 , the "J" fractal portion 112 includes: a first rectangular portion 1121, a second rectangular portion 1122, and a slit 1123. Among them, the first rectangular portion 1121 is connected to the side of the central rectangular portion 111. The second rectangular portion 1122 is disposed perpendicular to the first rectangular portion 1121 and is connected to one side of the first rectangular portion 1121. The slit 1123 is disposed perpendicular to the first rectangular portion 1121 and extends from the other side of the first rectangular portion 1121 to the inside of the second rectangular portion 1122. Specifically, the first rectangular portion 1121, the second rectangular portion 1122, and the slit 1123 form the "J" shaped "J" fractal portion 112.

[0060] In this embodiment, please refer to Figure 3The cross-connected square ring 12 includes four triangular structures 121, four square ring connecting parts 122 and four connecting lines 123. The four triangular structures 121 are respectively arranged at the ends of the four "J" fractal parts 112, and there is a gap between each triangular structure 121 and each "J" fractal part 112. The four square ring connecting parts 122 are respectively connected between every two adjacent triangular structures 121. The four connecting lines 123 are respectively connected at the midpoint of each square ring connecting part 122.

[0061] In this embodiment, four photodiodes 13 are respectively arranged at intervals between four triangular structures 121 and four "" fractal parts 112 , and the anode of the photodiode 13 is connected to the "" fractal part 112 , and the cathode of the photodiode 13 is connected to the triangular structure 121 .

[0062] In this embodiment, the dielectric substrate 2 and the central rectangular portion 111 are concentric squares. The width of the first rectangular portion 1121 is equal to the side length of the central rectangular portion 111. The long axis of the first rectangular portion 1121 is located on the axis of the dielectric substrate 2. The triangular structure 121 is an isosceles triangle, and the vertex of the triangular structure 121 is set toward the edge of the dielectric substrate 2, and the midline of the triangular structure 121 is located on the axis of the dielectric substrate 2. The four connecting lines 123 extend from the midpoint of each square ring connecting portion 122 to the four corners of the dielectric substrate 2. It should be understood that the long axis of the first rectangular portion 1121 is the axis of the first rectangular portion 1121 along the length direction.

[0063] See also Figure 5 In this embodiment, the second metal layer 3 includes four rectangular structures 31. The four rectangular structures 31 are all located on the lower surface of the dielectric substrate 2, and the four rectangular structures 31 extend from the midpoints of the four sides of the dielectric substrate 2 to the center of the dielectric substrate 2 respectively.

[0064] Specifically, the long axis of the first rectangular portion 1121 is aligned with the midline of the triangular structure 121, the photodiode 13 is arranged between the end of the first rectangular portion 1121 and the bottom side of the triangular structure 121, the positive electrode of the photodiode 13 is connected to the first rectangular portion 1121, and the negative electrode of the photodiode 13 is connected to the triangular structure 121. The square ring connecting portion 122 is connected between the bottom angles of every two adjacent triangular structures 121, and the four square ring connecting portions 122 and the outer edges of the four triangular structures 121 form a square. The axis of the rectangular structure 31 coincides with the axis of the dielectric substrate 2, and there is a gap between the ends of the four rectangular structures 31. It should be understood that the dielectric substrate 2 has two mutually perpendicular axes, the axes of the two rectangular structures 31 coincide with one axis of the dielectric substrate 2, and the axes of the other two rectangular structures 31 coincide with the other axis of the dielectric substrate 2. In this embodiment, the first metal layer 1 and the second metal layer 3 are both centrally symmetrical structures, and are also axisymmetric structures.

[0065] In this embodiment, the materials of the first metal layer 1 and the second metal layer 3 include: one of copper Cu, aluminum Al, and gold Au. The material of the dielectric substrate 2 includes: a flexible flame-resistant material. Furthermore, the relative dielectric constant of the dielectric substrate 2 is 2.65, the electric cutting loss is 0.0001, the thickness is 0.3 mm, and the thickness of the first metal layer 1 and the second metal layer 3 is 0.017 mm to 0.035 mm. The side length of the dielectric substrate 2 is 9 mm. Exemplarily, the material of the dielectric substrate 2 is F4B plate.

[0066] The transmission-reflection switchable frequency selective surface unit structure provided in this embodiment realizes the transmission-reflection switching function through a photodiode, is not easily damaged by heat, is compatible with various electromagnetic scenarios, has high stability, and flexibly realizes the switching between the transmission mode and the reflection mode near 11.1GHz, and can simultaneously take into account the transmission of signals in the X-band and the stealth capability of the antenna system. In addition, the present embodiment uses a flexible material as a dielectric substrate, which has better conformal capability and greater practical application value, and the surface size of the surface unit structure provided in this embodiment is only 9mm*9mm, which meets the trend of miniaturized structures and the requirements of processing technology. In addition, since the first metal layer 1 and the second metal layer 3 are centrally symmetrical structures, the frequency selective surface unit structure has good angular stability and polarization stability, and the resonant frequency and bandwidth change little when irradiated with incident waves at different angles.

[0067] Embodiment 2

[0068] See also Figure 6 , Figure 6 It is a structural schematic diagram of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention.

[0069] The present embodiment provides a transmissive-reflective switchable frequency selective surface structure, comprising M×N periodically arranged unit structures, which are the transmissive-reflective switchable frequency selective surface unit structures provided in the first embodiment of the present invention, wherein M and N are positive integers. Exemplarily, M and N are both 10 or 20 or 40.

[0070] Embodiment 3

[0071] On the basis of the first and second embodiments, this embodiment further defines the geometric parameters of the transmission-reflection switchable frequency selective surface unit structure, and simulates the surface structure composed of the surface unit structure to analyze the performance of the surface structure composed of the surface unit structure.

[0072] Please combine Figure 3 , Figure 4 and Figure 5In this embodiment, the width W of the first rectangular portion 1121 in the transmission-reflection switchable frequency selective surface unit structure is 1 is 1.3 mm, and the length L of the first rectangular portion 1121 is 1 is 1.25 mm, and the width W of the second rectangular portion 1122 is 2 is 0.5 mm, and the length L of the second rectangular portion 1122 is 3 is 0.6 mm, and the width W of the gap 1123 is 3 is 0.1 mm, and the distance L between the second rectangular portion 1122 and the first rectangular portion 1121 is 2 is 1.05 mm, and the width W of the square ring connecting portion 122 is 4 is 0.1 mm, and the height L of the triangular structure 121 is 4 is 1.4 mm, the distance d between the apex angles of each two adjacent triangular structures 121 is 6.2 mm, and the width W of the connecting line 123 is 5 is 0.3 mm, and the width W of the rectangular structure 31 is 6 is 0.4 mm, and the length L of the rectangular structure 31 5 is 2.5 mm, the side length p of the dielectric substrate 2 is 9.0 mm, and the thickness t of the dielectric substrate 2 is 0.3 mm.

[0073] In this embodiment, the transmission-reflection switchable frequency selective surface structure includes 5×5 periodically arranged unit structures.

[0074] Furthermore, in order to verify the performance of the transmission-reflection switchable frequency selective surface structure FSS provided in this embodiment, a number of performance simulation analyses were performed on the FSS structure using commercial simulation software CST.

[0075] See also Figures 7 to 10 , Figure 7 This is a simulation diagram of the filtering performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is turned on in the TE polarization mode. Figure 8 This is a simulation diagram of the filtering performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is cut off in the TE polarization mode. Fig. 9 This is a simulation diagram of the filtering performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is turned on in the TM polarization mode. Fig.10 This is a simulation diagram of the filtering performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is cut off in the TM polarization mode. Figure 7 and Figure 8It can be seen from the return loss S11 and insertion loss S21 that when the photodiode is turned on, the resonant frequency of the FSS structure of this embodiment is 11.1GHz, the insertion loss S21 is -68.9dB, and the bandwidth is 5.1GHz at -10dB. At this time, the FSS structure is in a reflective state; when the photodiode is cut off, the resonant frequency of the FSS structure of this embodiment is 11.1GHz, the return loss S11 is -20.2dB, and the bandwidth is 0.3GHz at -10dB. At this time, the FSS structure is in a transmissive state. In other words, in the TE polarization mode, the FSS structure has the effect of flexibly switching between the transmission mode and the reflection mode for signals with a frequency around 11.1GHz (10.9~11.2GHz).

[0076] Further, from Fig. 9 and Fig.10 It can be seen from the return loss S11 and insertion loss S21 that when the photodiode is turned on, the resonant frequency of the FSS structure of this embodiment is 11.1GHz, the insertion loss S21 is -69.7dB, and the bandwidth at -10dB is 5.1GHz. At this time, the FSS structure is in a reflective state; when the photodiode is cut off, the resonant frequency of the FSS structure of this embodiment is 11.1GHz, the return loss S11 is -20.1dB, and the bandwidth at -10dB is 0.3GHz. At this time, the FSS structure is in a transmissive state. In other words, in the TM polarization mode, the FSS structure has the effect of flexibly switching between the transmission mode and the reflection mode for signals with a frequency around 11.1GHz.

[0077] Furthermore, comprehensive Figures 7 to 10 It can be seen that the FSS structure provided in this embodiment has good transmission-reflection switching performance in the TE mode and the TM mode, and the resonant frequency is always in the X-band, that is, it has excellent polarization stability.

[0078] In addition, in order to study the angular stability of the FSS structure of this embodiment, it is irradiated with incident waves at incident angles of 0°, 15°, 30°, and 45° in the TE mode and TM mode, and the frequency characteristics of the structure can be obtained. Figures 11 to 14 , Fig.11 This is a simulation diagram of the angular stability performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is turned on in the TE polarization mode. Fig.12 This is a simulation diagram of the angular stability performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is cut off in the TE polarization mode. Fig.13 This is a simulation diagram of the angular stability performance of a frequency selective surface structure with switchable transmission and reflection provided by an embodiment of the present invention when the photodiode is turned on in the TM polarization mode. Fig.14This is a simulation diagram of the angular stability performance of a frequency selective surface structure with switchable transmission and reflection when the photodiode is cut off in the TM polarization mode provided by an embodiment of the present invention. The FSS structure has good angular stability and excellent switching characteristics near the 11.1GHz frequency point. Under the irradiation of electromagnetic incident waves at different angles, the frequency deviation is within an acceptable range, and it has excellent signal transmission capability and anti-interference capability. Figure 8 and Fig. 9 It can be seen from the performance simulation diagram that this FSS structure has the effect of switching between transmission mode and reflection mode in the X-band when irradiated with incident waves at incident angles of 0°, 15°, 30° and 45° in TE mode and TM mode, and has excellent filtering and shielding performance, which means that this structure has excellent angular stability.

[0079] The transmissive-reflective switchable frequency selective surface structure based on the photodiode of this embodiment can flexibly switch between the transmission mode and the reflection mode near 11.1GHz, and can simultaneously take into account the X-band signal transmission capability and the electromagnetic protection capability of the corresponding radio frequency system in a variety of electromagnetic scenarios. The surface structure provided by this embodiment has good angular stability and polarization stability, and the resonant frequency and bandwidth change little when the incident wave is irradiated at different angles, and the resonant frequency has no deviation when the incident wave is vertically irradiated (the incident angle is 0°) in the TE and TM polarization modes, and has no effect on the structural performance.

[0080] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A frequency selective surface unit structure with switchable transmission and reflection, characterized in that: include: A first metal layer (1), a dielectric substrate (2) and a second metal layer (3) are arranged in sequence from top to bottom; The first metal layer (1) comprises: a cross fractal structure (11), a cross-connected square ring (12) and four photodiodes (13), wherein: The cross fractal structure (11) is arranged on the upper surface of the dielectric substrate (2); The cross-connected square ring (12) is located outside the cross fractal structure (11); The four photodiodes (13) are respectively connected between the four ends of the cross fractal structure (11) and the cross-connected square ring (12); The centers of the cross fractal structure (11) and the cross-connected square ring (12) coincide with each other, and both are centrosymmetric structures.

2. The transmission-reflection switchable frequency selective surface unit structure according to claim 1, characterized in that: The cross fractal structure (11) comprises: a central rectangular portion (111) and four "X" fractal portions (112), wherein the central rectangular portion (111) is arranged at the center of the upper surface of the dielectric substrate (2); The four "X" fractal parts (112) are respectively connected to the four sides of the central rectangular part (111).

3. The transmission-reflection switchable frequency selective surface unit structure according to claim 2, characterized in that: The ""-shaped" fractal portion (112) comprises: a first rectangular portion (1121), a second rectangular portion (1122) and a gap (1123), wherein: The first rectangular portion (1121) is connected to the sides of the central rectangular portion (111); The second rectangular portion (1122) is arranged perpendicular to the first rectangular portion (1121) and connected to one side of the first rectangular portion (1121); The slit (1123) is arranged perpendicular to the first rectangular portion (1121), and extends from the other side of the first rectangular portion (1121) to the inside of the second rectangular portion (1122).

4. The transmission-reflection switchable frequency selective surface unit structure according to claim 2, characterized in that: The cross-connected square ring (12) comprises four triangular structures (121), four square ring connecting parts (122) and four connecting lines (123), wherein: The four triangular structures (121) are respectively arranged at the ends of the four "J" fractal portions (112), and there is a gap between each triangular structure (121) and each "J" fractal portion (112); The four square ring connecting parts (122) are respectively connected between every two adjacent triangular structures (121); The four connecting lines (123) are respectively connected to the midpoint of each of the square ring connecting parts (122).

5. The transmission-reflection switchable frequency selective surface unit structure according to claim 4, characterized in that: The four photodiodes (13) are respectively arranged at intervals between the four triangular structures (121) and the four "" fractal parts (112), and the positive electrodes of the photodiodes (13) are connected to the "" fractal parts (112), and the negative electrodes of the photodiodes (13) are connected to the triangular structures (121).

6. The transmission-reflection switchable frequency selective surface unit structure according to claim 3, characterized in that: The dielectric substrate (2) and the central rectangular portion (111) are concentric squares; The width of the first rectangular portion (1121) is equal to the side length of the central rectangular portion (111); The long axis of the first rectangular portion (1121) is located on the axis of the dielectric substrate (2).

7. The transmission-reflection switchable frequency selective surface unit structure according to claim 4, characterized in that: The triangular structure (121) is an isosceles triangle, and the vertex of the triangular structure (121) is arranged toward the edge of the dielectric substrate (2), and the midline of the triangular structure (121) is located on the axis of the dielectric substrate (2); The four connecting lines (123) extend respectively from the midpoint of each of the square ring connecting parts (122) to the four corners of the dielectric substrate (2).

8. The transmission-reflection switchable frequency selective surface unit structure according to claim 1, characterized in that: The second metal layer (3) comprises four rectangular structures (31); The four rectangular structures (31) are all located on the lower surface of the dielectric substrate (2), and the four rectangular structures (31) respectively extend from the midpoints of the four sides of the dielectric substrate (2) towards the center of the dielectric substrate (2).

9. The transmission-reflection switchable frequency selective surface unit structure according to claim 1, characterized in that: The materials of the first metal layer (1) and the second metal layer (3) include: one of copper, aluminum and gold; The material of the dielectric substrate (2) includes: flexible flame-resistant material.

10. A frequency selective surface structure with switchable transmission and reflection, characterized in that: Comprising M×N periodically arranged unit structures, wherein the unit structure is a transmission-reflection switchable frequency selective surface unit structure as claimed in any one of claims 1 to 9; Wherein, M and N are positive integers.

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