A two-dimensional programmable metasurface controlled by liquid crystal for terahertz frequencies

Abstract

The present invention is a liquid crystal controlled terahertz two-dimensional programmable metasurface, the metasurface is a layered structure, the layered order is a dielectric substrate (1), an array composed of a plurality of metal metasurface units (2), a first polymer film orientation layer (3), a nematic phase liquid crystal layer (4), a second polymer film orientation layer (5), an array composed of a plurality of metal reflector feeding units (6), a dielectric substrate (7), and the designed working frequency is in the terahertz region. The metasurface can independently load a bias voltage to each super subunit composed of 2*2 metal metasurface units (2) to adjust the dielectric constant of the liquid crystal layer in the corresponding area, thereby changing the phase response of the super subunit, and then controlling the phase distribution of different areas of the metasurface to achieve far-field beam manipulation of reflected electromagnetic waves. The liquid crystal controlled terahertz two-dimensional programmable metasurface proposed by the present invention has the advantages of small size, light weight, easy conformality, low cost, etc., and has potential application value in terahertz wireless communication.

Description

Technical Field

[0001] The present invention relates to a working principle, design method and application technology of a liquid crystal controlled terahertz two-dimensional programmable metasurface based on an artificial electromagnetic metasurface and a metal reflector feeding array design, belonging to the technical field of new artificial electromagnetic materials. Background Art

[0002] A coded metasurface is an electromagnetic metasurface that is designed and regulated by digital coding. Its unit structure is described by binary values ​​"0" or "1" (1-bit case), and the phase difference between the two digital states at the corresponding operating frequency is 180°. For a 1-bit metasurface, although each unit has only two states, when these digitally represented unit structures are arranged on a two-dimensional plane, there are countless permutations and combinations. Each arrangement corresponds to a coding pattern. When electromagnetic waves are incident on different coding patterns, different far-field patterns can be generated. Furthermore, by introducing semiconductor elements (such as switching diodes), dielectric tunable materials (such as liquid crystals) and phase change materials (such as vanadium dioxide) into the metasurface to regulate the phase response of the unit, the real-time control of the coding pattern and far-field pattern can be achieved by combining a field programmable gate array (FPGA).

[0003] The terahertz frequency band is between microwaves and far infrared in the electromagnetic spectrum and is usually defined as 0.1 to 10 THz (1 THz = 10 12 Hz). Since the size of the metasurface unit is generally sub-wavelength, for terahertz metasurfaces, the unit size is usually only tens to hundreds of microns. Traditional semiconductor devices such as diodes and varactors are difficult to apply to terahertz field programmable metasurfaces due to size limitations and parasitic effects. Real-time control of terahertz beams is mainly achieved through methods such as vanadium dioxide, liquid crystal, graphene and micromechanical structures.

[0004] Liquid crystal materials have good optical anisotropy and electro-optic modulation properties, and have the advantages of low cost, stable performance, wide operating frequency band, etc., and have also been applied to a certain extent in the terahertz frequency band. The present invention proposes a terahertz two-dimensional programmable reflective metasurface based on liquid crystal regulation, which forms an independent feeding unit by arraying a metal reflective backplane, fills liquid crystal material between the metal metasurface unit and the metal reflector feeding unit, and uses an electric control method to control the orientation of liquid crystal molecules to change the dielectric constant of the liquid crystal layer, thereby changing the unit resonance characteristics and phase response to achieve "0" and "1" coding. The metasurface can realize dynamic scanning of the beam in a two-dimensional plane, which is very innovative and feasible. Summary of the invention

[0005] Technical problem: The present invention provides a liquid crystal controlled terahertz two-dimensional programmable metasurface designed based on an electromagnetic metasurface and a metal reflector feeding array. This electrically controlled programmable metasurface has the advantages of small size, light weight, adjustable function and low cost, and therefore has good application prospects.

[0006] Technical solution: The present invention proposes a liquid crystal controlled terahertz two-dimensional programmable metasurface, wherein the metasurface is a layered structure, and the layer order is a dielectric substrate, an array composed of a plurality of metal metasurface units, a first polymer film orientation layer, a nematic liquid crystal layer, a second polymer film orientation layer, an array composed of a plurality of metal reflector feeding units, and a dielectric substrate; wherein the metal metasurface unit and the metal reflector feeding unit are symmetrically arranged on two sides of the nematic liquid crystal layer to form a reflective metasurface; and the initial orientation of the liquid crystal molecules in the nematic liquid crystal layer is determined by the first polymer film orientation layer and the second polymer film orientation layer.

[0007] The planar shape of the metal metasurface unit is a symmetrically distributed metal slotted circular ring structure, that is, a slotted inner circular ring is arranged inside a slotted outer circular ring, and is composed of two layers of metal slotted circular rings, namely, two symmetrical slotted outer semicircular rings on the left and right and two symmetrical slotted inner semicircular rings on the left and right, wherein the slotted outer semicircular ring is concentric with the slotted inner semicircular ring, and the slotted outer semicircular ring and the slotted inner semicircular ring are separated by a certain distance of r1-R2, and the two symmetrical slotted outer semicircular rings on the left and right and the two symmetrical slotted inner semicircular rings on the left and right are symmetrically arranged with respect to a metal wire, and the width of the metal wire is w, wherein the inner diameter of the slotted outer circular ring is r1, and the outer diameter of the slotted inner circular ring is R2; each 2*2 metal metasurface units constitute a super subunit.

[0008] The metal reflector feeding unit is composed of a square metal patch and a metallized via connected thereto. Each metal reflector feeding unit corresponds to 2*2 metal metasurface units. The metal reflector feeding units are independent of each other. Each metal reflector feeding unit corresponds to a metallized via. Different control voltages are input through the metallized vias to control each metal reflector feeding unit, thereby realizing the regulation of liquid crystal molecules between the metal reflector feeding unit and the metal metasurface unit.

[0009] The nematic liquid crystal layer is a liquid crystal layer made of a mixture of nematic liquid crystal molecules, and the initial orientation of the liquid crystal molecules is determined by a first polymer film orientation layer and a second polymer film orientation layer located on both sides of the nematic liquid crystal layer.

[0010] The dielectric substrate is used as a substrate material for preparing the super surface structure, and quartz glass is used as the dielectric substrate, and its terahertz dielectric constant is 3.822, the loss tangent is 0.008, and the thickness is 200 microns.

[0011] The dielectric substrate is used as a substrate material for preparing a metal reflector feeding array, and an FR4 plate is used as the dielectric substrate, and the thickness is 2000 microns.

[0012] The metal metasurface unit and the metal reflector feeding unit serve as the positive and negative electrodes for applying a bias voltage, respectively; the spatial orientation of the liquid crystal molecules is changed by adjusting the bias voltage, thereby dynamically regulating the dielectric constant of the liquid crystal; and independent real-time control of the encoding state of the metasurface unit is achieved.

[0013] The metasurface changes the dielectric constant of the liquid crystal filled between the metal metasurface unit and the feeding unit by the presence or absence of voltage applied to the metal reflector feeding unit, thereby changing the unit resonance state, generating a phase difference of about 180° in the terahertz working frequency band, that is, satisfying the 1-bit coding condition, and then designing the coding pattern of the metasurface; the coding pattern is switched according to different needs to obtain different far-field radiation patterns.

[0014] The material used for the metal metasurface unit is gold, and the designed operating frequency of the metasurface is in the terahertz region.

[0015] The metasurface uses a field programmable gate array FPGA and a waveform generator to generate multi-channel bias voltages, and performs real-time manipulation of the phase response of each unit, thereby changing the phase distribution of the metasurface in real time, and realizing an electromagnetic metasurface constructed based on a metal metasurface and a metal reflector feeding array, which is a two-dimensional programmable metasurface.

[0016] Beneficial effects:

[0017] 1. The present invention reports a terahertz two-dimensional programmable metasurface designed based on an artificial electromagnetic metasurface and a metal reflector feeding array. Compared with the existing coding metasurface, it has the characteristics of electrically programmable, reconfigurable, and independently controllable units.

[0018] 2. The present invention uses quartz glass as a dielectric substrate and prepares an ultra-thin metal structure thereon. Therefore, compared with traditional metamaterial devices, the present invention has the advantages of being ultra-thin, small in size, and light in weight.

[0019] 3. The present invention designs the metal reflective backplane in an array to form an independent feeding unit, which can realize independent regulation of the metasurface unit, thereby realizing the two-dimensional programmability of the metasurface. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the unit structure of the liquid crystal controlled terahertz two-dimensional programmable metasurface of the present invention. The overall dimensions of the metasurface unit in the x and y directions are both p, the inner diameter and outer diameter of the slotted outer ring are r1 and R1 respectively, the inner diameter and outer diameter of the slotted inner ring are r2 and R2 respectively, and the metal wire width is w; the slotted outer semicircular ring is 2.1, the slotted inner semicircular ring is 2.2, and the metal wire is 2.3.

[0021] Figure 2 The cross-sectional schematic diagram of the super subunit of the present invention includes the following structures from left to right: dielectric substrate 1, metal super surface unit 2, first polymer film orientation layer 3, nematic phase liquid crystal layer 4, second polymer film orientation layer 5, metal reflector feeding unit 6, dielectric substrate 7.

[0022] Figure 3 The figure is a schematic diagram of the layered structure of the two-dimensional programmable metasurface proposed in the present invention. The metasurface can independently control the 2*2 metal metasurface units.

[0023] Figure 4 It is the reflection coefficient S11 amplitude curve of the metasurface unit when the dielectric constant of the liquid crystal layer is 2.55 (0 state) and 3.65 (1 state) respectively.

[0024] Figure 5 It is the reflection coefficient S11 phase curve of the metasurface unit when the dielectric constant of the liquid crystal layer is 2.55 (0 state) and 3.65 (1 state) respectively.

[0025] Figure 6 It is the metasurface coding pattern and the three-dimensional far-field scattering pattern of the metasurface under this coding pattern, and the operating frequency is 0.22THz.

[0026] Figure 7 It is the metasurface coding pattern and the three-dimensional far-field scattering pattern of the metasurface under this coding pattern, and the operating frequency is 0.22THz.

[0027] Figure 8 It is the metasurface coding pattern and the three-dimensional far-field scattering pattern of the metasurface under this coding pattern, and the operating frequency is 0.22THz. DETAILED DESCRIPTION

[0028] The present invention is based on a terahertz two-dimensional programmable metasurface designed based on an artificial electromagnetic metasurface and a metal reflector feeding array, and its unit structure is as follows: Figure 1 As shown, it is mainly composed of a dielectric substrate 1, a metal metasurface unit 2, a first polymer film orientation layer 3, a nematic liquid crystal layer 4, a second polymer film orientation layer 5, a metal reflector feeding unit 6, and a dielectric substrate 7, and the designed operating frequency is in the terahertz region. Among them, the metal metasurface unit 2 and the metal reflector feeding unit 6 are symmetrically arranged on both sides of the nematic liquid crystal layer 4 to form a reflective metasurface; the initial orientation of the liquid crystal molecules in the nematic liquid crystal layer 4 is determined by the first polymer film orientation layer 3 and the second polymer film orientation layer 5.

[0029] The planar shape of the metal metasurface unit 2 is a symmetrically distributed metal slotted circular ring structure, that is, a slotted inner circular ring is arranged inside a slotted outer circular ring, and is composed of two layers of metal slotted circular rings, namely, two symmetrical slotted outer semicircular rings 2.1 on the left and right and two symmetrical slotted inner semicircular rings 2.2 on the left and right, wherein the slotted outer semicircular ring 2.1 is concentric with the slotted inner semicircular ring 2.2, and a certain distance r1-R2 is separated between the slotted outer semicircular ring 2.1 and the slotted inner semicircular ring 2.2, and the two symmetrical slotted outer semicircular rings 2.1 on the left and right and the two symmetrical slotted inner semicircular rings 2.2 on the left and right are symmetrically arranged with respect to a metal wire 2.3, and the width of the metal wire is w, wherein the inner diameter of the slotted outer circular ring is r1, and the outer diameter of the slotted inner circular ring is R2; each 2*2 metal metasurface units 2 constitute a super subunit. The metal reflector feed units 6 are independent of each other, and each metal reflector feed unit corresponds to a metallized via. Different control voltages are input through the metallized vias to control each metal reflector feed unit, thereby realizing the regulation of the liquid crystal molecules between the metal reflector feed unit and the metal metasurface unit. The dielectric substrate 1 is made of quartz glass with a thickness of T'; the thickness of the metal metasurface unit 2 is h; the first polymer film orientation layer 3 and the second polymer film orientation layer 5 are made of polyimide film with a thickness of l; the thickness of the nematic phase liquid crystal layer 4 is d; the thickness of the metal reflector feed array 6 is H, the thickness of the dielectric substrate 7 is T, and the initial optical axis direction of the liquid crystal molecules is along the x-axis.

[0030] The super surface unit selected by the present invention is a symmetrically distributed slotted ring structure (such as Figure 1 The material used is gold. The metal reflector feeding array can suppress the transmission of terahertz electromagnetic waves, so this is a reflective artificial electromagnetic surface. The cross-section of a super subunit is shown in Figure 2 shown.

[0031] The working principle of this terahertz two-dimensional programmable metasurface designed based on an artificial electromagnetic metasurface and a metal reflector feeding array is as follows. The metal reflector feeding unit and the metal metasurface unit serve as the positive and negative electrodes for applying bias voltage, respectively. Under the conditions that the terahertz wave is perpendicular to the plane of the slotted ring and the TM wave mode (the electric field is along the x direction), when no bias voltage is applied, the initial spatial orientation of the liquid crystal molecules is determined by the first polymer film orientation layer and the second polymer film orientation layer. At this time, the resonant frequency of the unit structure is ω1, and the corresponding reflection parameter is S11(1). When a certain AC bias voltage is loaded between the metal reflector feeding unit and the metal metasurface unit, the spatial orientation of the liquid crystal molecules will be deflected, and the dielectric constant will also change accordingly. At this time, the resonant frequency of the unit structure will shift to ω2, and the corresponding reflection parameter is S11(2). By designing a specific structure, the reflection amplitude of the unit in the two resonant states remains approximately equal, and the phase difference is close to 180°. The two phase responses are defined as the "0" state and the "1" state, respectively, thereby obtaining a 1-bit coding unit.

[0032] For the programmable metasurface proposed in the present invention, in order to achieve two-dimensional flexible control of the terahertz beam, the key technology is to realize independent control of each unit and real-time switching of the coding state. The present invention arrays the metal reflective backplane to obtain a metal reflector feeding array. Each unit in the feeding array is connected to the external electrode through a metallized via, that is, the feeding unit has both reflection and feeding functions, thereby realizing independent control of each unit. A field programmable gate array (FPGA) and an arbitrary waveform generator are used to generate multi-channel bias voltages to control the phase response of different units, thereby changing the phase distribution of the metasurface in real time, and realizing real-time and flexible regulation of the electromagnetic beam.

[0033] Example 1: Figure 1 The structural unit shown has a period of p=400μm in the x and y directions, and the unit structure parameters are: R1=150μm, r1=130μm, R2=100μm, r2=80μm, w=20μm. The thickness of the quartz glass is T'=200μm, the thickness of the metal metasurface unit structure is h=300nm, the thickness of the polyimide film is l=100nm, the thickness of the liquid crystal layer is d=60μm, the thickness of the metal reflector feeding array is H=18μm, the size of the metal reflector feeding unit is q=680μm, and the thickness of the dielectric substrate is T=2000μm. When no bias voltage is loaded, the dielectric constant of the liquid crystal layer is 2.55. When the bias voltage is increased to 10V, its dielectric constant is about 3.65.

[0034] The unit S parameters obtained by using the electromagnetic simulation software CST are as follows Figure 4 and 5As shown in the figure, near 0.22THz, the amplitude of the reflection coefficient S11 in the two states is very close, both around 0.73, and the phase difference is close to 180°, which meets the 1-bit encoding condition.

[0035] When the electromagnetic wave is incident vertically, when the coding sequence of the metasurface array is Figure 6 When shown in Figure 1 (each coded state represents a super subunit composed of 2*2 metal metasurface units), the CST full-wave simulation method is used to analyze its far-field beam. In the TM wave mode (the electric field is along the x direction), when the operating frequency is 0.22THz, the three-dimensional far-field scattering pattern of the metasurface is as follows: Figure 6 As shown in the figure, the regular reflection beam is significantly suppressed and becomes two oblique scattered beams in the yoz plane. The angle between the scattered beam and the z-axis is 58.4°.

[0036] Example 2: Figure 1 The structural unit shown has a period of p=400μm in the x and y directions, and the unit structure parameters are: R1=150μm, r1=130μm, R2=100μm, r2=80μm, w=20μm. The thickness of the quartz glass is T'=200μm, the thickness of the metal metasurface unit structure is h=300nm, the thickness of the polyimide film is l=100nm, the thickness of the liquid crystal layer is d=60μm, the thickness of the metal reflector feeding array is H=18μm, the size of the metal reflector feeding unit is q=680μm, and the thickness of the dielectric substrate is T=2000μm. When no bias voltage is loaded, the dielectric constant of the liquid crystal layer is 2.55. When the bias voltage is increased to 10V, its dielectric constant is about 3.65. At around 0.22 THz, the amplitude of the reflection coefficient S11 in the two states is very close, both around 0.73, while the phase difference is close to 180°, satisfying the 1-bit encoding condition.

[0037] When the electromagnetic wave is incident vertically, when the coding sequence of the metasurface array is Figure 7 When shown in Figure 1 (each coded state represents a super subunit composed of 2*2 metal metasurface units), the CST full-wave simulation method is used to analyze its far-field beam. In the TM wave mode (the electric field is along the x direction), when the operating frequency is 0.22THz, the three-dimensional far-field scattering direction diagram of the metasurface is as follows: Figure 7 As shown in the figure, the normal reflection beam is significantly suppressed and becomes two oblique scattered beams in the xoz plane. The angle between the scattered beam and the z-axis is 25.2°.

[0038] Example 3: Figure 1The structural unit shown has a period of p=400μm in the x and y directions, and the unit structure parameters are: R1=150μm, r1=130μm, R2=100μm, r2=80μm, w=20μm. The thickness of the quartz glass is T'=200μm, the thickness of the metal metasurface unit structure is h=300nm, the thickness of the polyimide film is l=100nm, the thickness of the liquid crystal layer is d=60um, the thickness of the metal reflector feeding array is H=18μm, the size of the metal reflector feeding unit is q=680μm, and the thickness of the dielectric substrate is T=2000μm. When no bias voltage is loaded, the dielectric constant of the liquid crystal layer is 2.55. When the bias voltage is increased to 10V, its dielectric constant is about 3.65. At around 0.22 THz, the amplitude of the reflection coefficient S11 in the two states is very close, both around 0.73, while the phase difference is close to 180°, satisfying the 1-bit encoding condition.

[0039] When the electromagnetic wave is incident vertically, when the coding sequence of the metasurface array is Figure 8 When shown (each coding state represents a 2*2 super subunit), the CST full-wave simulation method is used to analyze its far-field beam. In the TM wave mode (the electric field is along the x direction), when the operating frequency is 0.22THz, the three-dimensional far-field scattering direction diagram of the metasurface is as follows Figure 8 As shown in the figure, the regular reflection beam is significantly suppressed and becomes two oblique scattered beams in the phi=135° plane, and the angle between the scattered beam and the z-axis is 31.0°.

Claims

1. A liquid crystal controlled terahertz two-dimensional programmable metasurface, characterized in that The metasurface is a layered structure, the layer order of which is a dielectric substrate (1), an array consisting of a plurality of metal metasurface units (2), a first polymer film orientation layer (3), a nematic liquid crystal layer (4), a second polymer film orientation layer (5), an array consisting of a plurality of metal reflector feeding units (6), and a dielectric substrate (7); wherein the metal metasurface units (2) and the metal reflector feeding units (6) are symmetrically arranged on two sides of the nematic liquid crystal layer (4) to form a reflective metasurface; and the initial orientation of the liquid crystal molecules of the nematic liquid crystal layer (4) is determined by the first polymer film orientation layer (3) and the second polymer film orientation layer (5); The planar shape of the metal super surface unit (2) is a symmetrically distributed metal slotted circular ring structure, that is, a slotted inner circular ring is arranged inside the slotted outer circular ring, and is composed of two layers of metal slotted circular rings, namely, two symmetrical slotted outer semicircular rings (2.1) on the left and right and two symmetrical slotted inner semicircular rings (2.2) on the left and right, wherein the slotted outer semicircular ring (2.1) and the slotted inner semicircular ring (2.2) are concentric, and the slotted outer semicircular ring (2.1) and the slotted inner semicircular ring (2.2) are spaced a certain distance r1-R2, and the two symmetrical slotted outer semicircular rings (2.1) on the left and right and the two symmetrical slotted inner semicircular rings (2.2) on the left and right are arranged symmetrically with respect to a metal wire (2.3), the width of the metal wire is w, wherein the inner diameter of the slotted outer circular ring is r1, and the outer diameter of the slotted inner circular ring is R2; each 2*2 metal super surface units (2) constitute a super sub-unit; The metal reflector feeding unit (6) is composed of a square metal patch and a metallized via connected thereto, each metal reflector feeding unit (6) corresponds to 2*2 metal metasurface units (2), the metal reflector feeding units (6) are independent of each other, each metal reflector feeding unit (6) corresponds to a metallized via, and different control voltages are input through the metallized vias to control each metal reflector feeding unit, thereby achieving regulation of liquid crystal molecules between the metal reflector feeding unit and the metal metasurface unit; The nematic liquid crystal layer (4) is a liquid crystal layer made of a mixture of nematic liquid crystal molecules, and the initial orientation of the liquid crystal molecules is determined by a first polymer film orientation layer (3) and a second polymer film orientation layer (5) located on both sides of the nematic liquid crystal layer (4).

2. The liquid crystal controlled terahertz two-dimensional programmable metasurface according to claim 1, characterized in that The dielectric substrate (1) is used as a substrate material for preparing a super-surface structure, and quartz glass is used as the dielectric substrate, and its terahertz dielectric constant is 3.822, the loss tangent is 0.008, and the thickness is 200 micrometers.

3. The liquid crystal controlled terahertz two-dimensional programmable metasurface according to claim 1, characterized in that The dielectric substrate (7) is used as a substrate material for preparing a metal reflector feeding array, and an FR4 plate is used as the dielectric substrate, with a thickness of 2000 micrometers.

4. The liquid crystal controlled terahertz two-dimensional programmable metasurface according to claim 1, characterized in that The metal metasurface unit (2) and the metal reflector feeding unit (6) serve as positive and negative electrodes for applying a bias voltage, respectively; the spatial orientation of liquid crystal molecules is changed by adjusting the bias voltage, thereby dynamically regulating the dielectric constant of the liquid crystal; and independent real-time control of the encoding state of the metasurface unit is achieved.

5. The liquid crystal controlled terahertz two-dimensional programmable metasurface according to claim 1, characterized in that The metasurface changes the dielectric constant of the liquid crystal filled between the metal metasurface unit and the feeding unit by the presence or absence of voltage applied to the metal reflector feeding unit, thereby changing the unit resonance state, generating a phase difference of about 180° in the terahertz working frequency band, that is, satisfying the 1-bit coding condition, and then designing the coding pattern of the metasurface; the coding pattern is switched according to different needs to obtain different far-field radiation patterns.

6. The liquid crystal controlled terahertz two-dimensional programmable metasurface according to claim 1, characterized in that The material used for the metal metasurface unit is gold, and the designed operating frequency of the metasurface is in the terahertz region.

7. The liquid crystal controlled terahertz two-dimensional programmable metasurface according to claim 4, characterized in that The metasurface uses a field programmable gate array FPGA and a waveform generator to generate multi-channel bias voltages, and performs real-time manipulation of the phase response of each unit, thereby changing the phase distribution of the metasurface in real time, and realizing an electromagnetic metasurface constructed based on a metal metasurface and a metal reflector feeding array, which is a two-dimensional programmable metasurface.

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