A fully spatially integrated terahertz metasurface unit based on liquid crystal

Abstract

The present invention discloses a full-space integrated terahertz metasurface unit based on liquid crystal, which is used for multifunctional dynamic control of full-space terahertz waves; the metasurface unit realizes 3-bit state encoding in a specific frequency band (0.4-0.44THz, 0.7-0.72THz), and realizes a maximum phase shift of nearly 315° in transmission and reflection modes respectively, thereby realizing dynamic programmable manipulation of full-space terahertz waves; the present invention adopts an electrically tunable liquid crystal material combined with a metal metasurface for phase shift, and when the incident terahertz wave interacts with the metasurface, the coding mode of the metasurface is actively regulated by an external bias voltage to realize flexible switching of three functions, including beam steering, orbital angular momentum and holographic imaging.

Description

Technical Field

[0001] The present invention relates to the field of metasurface design technology, and in particular to a full-space integrated terahertz metasurface unit based on liquid crystal. Background Art

[0002] Terahertz waves have the advantages of low energy, high bandwidth and rich biological resonance information, and have broad application prospects in wireless communications, biomedicine and non-destructive testing. The development of terahertz technology has led to an increase in demand for efficient functional devices adapted to this band, especially miniaturized and integrated devices. Multifunctional terahertz devices that can dynamically reconfigure this function are essential to further improve the integration of the system and expand application scenarios. Metasurfaces, a class of metamaterials with two-dimensional structures composed of sub-wavelength artificial structural elements, are widely used in terahertz devices to enhance their interaction with terahertz waves. They provide unprecedented freedom for manipulating the amplitude, phase, polarization and orbital angular momentum (OAM) properties of terahertz.

[0003] However, for traditional metasurface designs, the unit structure is fixed, and the limited electromagnetic (EM) properties cannot meet the urgent need to develop multifunctional integrated terahertz devices. In practical applications, multifunctional dynamic control of terahertz waves is highly anticipated. Programmable metasurfaces based on digitally coded metasurfaces that have emerged in recent years provide a promising solution. By actively switching the coding sequence, programmable metasurfaces can exhibit excellent programmable EM properties. In general, traditional programmable metasurface devices can be controlled by electrical, mechanical, optical, and thermal methods. Among them, electrical tuning is always efficient and cost-effective, especially for miniaturized and integrated systems. In the microwave band, programmable metasurfaces that use diodes to switch between "0" and "1" elements have been widely used. However, due to the limitations of structure size and parasitic effects, diodes are not suitable for terahertz devices. Therefore, various tunable materials such as graphene, vanadium dioxide, superconductors, and liquid crystals are integrated into metasurfaces to achieve active control of terahertz waves. Compared with other electrically controlled devices, the fabrication of liquid crystal-based terahertz devices is compatible with standard liquid crystal display processes, making mass production economical and easy. In particular, due to the birefringence effect of liquid crystals, the phase delay can be changed by changing the orientation of liquid crystal molecules, achieving dynamic manipulation of terahertz waves. In addition, liquid crystal-based coded metasurfaces can be pixelated. Each pixel can be electrically addressed, providing the possibility of designing multi-bit programmable metasurfaces.

[0004] According to the propagation direction of EM waves, metasurface devices can be divided into transmission and reflection. The multiplexing of transmission and reflection of EM waves endows metasurfaces with rich dual-channel manipulation capabilities. Therefore, the proposal of full-space metasurfaces provides a promising strategy to further improve information capacity and maximize the use of space resources. However, existing research mainly focuses on the dynamic control of EM waves in transmission or reflection mode, while the research on the other half-space is still blank. This inevitably limits the application potential of metasurfaces, especially in the realization of multifunctional terahertz devices.

[0005] In summary, the metasurface devices currently used in the terahertz band mainly have problems such as single function, low information carrying capacity, low space resource utilization, and lack of efficient design methods. Summary of the invention

[0006] Purpose of the invention: The purpose of the present invention is to provide a fully spatially integrated terahertz metasurface unit based on liquid crystal to solve the problems raised in the background technology.

[0007] Technical solution: The liquid crystal-based full-space integrated terahertz metasurface unit described in the present invention includes, from top to bottom, a quartz substrate, a square ring cross structure metal patch, an orientation layer polyimide medium, a liquid crystal medium, polyimide, a metal grating, polyimide, a liquid crystal medium, polyimide, a strip metal patch, and a quartz substrate.

[0008] Furthermore, the quartz substrate is used to support the metasurface structure; the metal is gold, with a dielectric constant of 3.75, a loss tangent of 0.0004, and a conductivity of 4.561×10 7 .

[0009] Furthermore, the square ring cross structure metal patch acts as a resonator, so that the structure has high transmittance or reflectivity.

[0010] Furthermore, the metal grating acts as a filter layer, transmitting x Polarized waves, reflection y Polarized waves are used to ensure efficient transmission and reflection of terahertz waves in different frequency bands;

[0011] Furthermore, the strip metal patch, the metal patch and the metal grating are used together as electrodes for applying bias voltage; the medium adopts flexible polyimide material with a dielectric constant of 3.0-3.6 and a loss tangent of 0.0001-0.05. Its surface has grooves in the same direction and is used as an orientation layer to align the liquid crystal molecules in the direction of the grooves.

[0012] Furthermore, the liquid crystal medium has a dielectric constant of 2.55-3.65 and a loss tangent of 0.01-0.03.

[0013] Furthermore, the optimized metasurface unit period is 310-330 μm, the thickness of gold is 0.8-1.2 μm, the thickness of polyimide medium is 1-10 μm, the thickness of liquid crystal medium is 210-230 μm, the thickness of liquid crystal medium is 1150-1250 μm, and the thickness of quartz substrate is 280-320 μm; the outer diameter of the square ring patch in the square ring cross structure metal patch is 310-330 μm, and the inner diameter is 280-300 μm; The long arm length of the character patch is 280-300μm, and the width is 25-35μm; the short arm length is 120-140μm, and the width is 25-35μm; the metal grating length is 310-330μm, the width is 31-33μm, and the gap is 31-33μm; the length of the strip metal patch is 310-330μm, and the width is 50-100μm; by changing the bias voltage and adjusting the dielectric constant of the liquid crystal medium, a phase coverage of 315° is achieved.

[0014] The optimization design method of a full-space integrated terahertz metasurface unit based on liquid crystal described in the present invention adopts linear polarization wave incident to calculate the amplitude and phase of the optimized metasurface unit.

[0015] Furthermore, the optimized phase is quantized to 8th order, that is, it is quantized to a phase distribution containing only equivalent phases of 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° to form a 3-bit coding unit. Two units with a phase difference of 180° are randomly selected from the 3-bit coding unit to form a 1-bit coding unit. They are arranged according to the quantized phase distribution, with a total of 40×40 units. Each column represents a subarray, including 2×40 units.

[0016] The present invention discloses a liquid crystal-based full-space integrated terahertz metasurface unit holographic imaging method, which is characterized in that holographic imaging is achieved by changing the coding state of the liquid crystal metasurface unit device through an external bias voltage.

[0017] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention combines the phase control of transmission and reflection, and designs a metasurface from two dimensions to achieve dynamic control of the full-space terahertz wave. (2) The present invention is based on electrically tunable liquid crystal materials. The proposed programmable metasurface is characterized by achieving 3-bit state encoding within a specific frequency band (0.4-0.44THz, 0.7-0.72THz) and achieving a maximum phase shift of nearly 315° in transmission and reflection modes, respectively. (3) The present invention proposes a full-space integrated terahertz metasurface device based on liquid crystal, which integrates dual-beam steering, OAM beam and holographic imaging technology. The three functions can be flexibly switched by actively controlling the arrangement of the switching coding sequence, and has broad application prospects in the fields of miniaturized integrated communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the full-space metasurface unit designed for the present invention;

[0019] Figure 2 Result diagram of amplitude and phase of the full-space metasurface unit of the present invention;

[0020] Figure 3 Phase encoding and far-field calculation results of the full-space metasurface of the present invention to achieve dual-beam steering function;

[0021] Figure 4 The results of the 8th-order quantization and far-field calculation of the phase distribution of the full-space metasurface of the present invention to realize the OAM beam function;

[0022] Figure 5 A schematic diagram of the target far field of the full-space holographic imaging of the present invention;

[0023] Figure 6 The results of 8th-order quantization and far-field calculation of the phase distribution of the full-space metasurface of the present invention to realize the holographic imaging function. DETAILED DESCRIPTION

[0024] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0025] Example 1 A full-space integrated terahertz metasurface device based on liquid crystal realizes a full-space dual-beam steering function by changing the coding state of the liquid crystal metasurface unit through an external bias voltage.

[0026] The present invention provides a liquid crystal-based full-space integrated terahertz metasurface device to achieve dual-beam steering function and a design method thereof, wherein the structure of the unit device is as follows: Figure 1As shown, it specifically includes: a quartz substrate 1, a square ring cross structure metal patch 2, an orientation layer polyimide medium 3, a first liquid crystal medium 4, polyimide 5, a metal grating 6, polyimide 7, a second liquid crystal medium 8, polyimide 9, a strip metal patch 10, and a quartz substrate 11; in more detail: a quartz substrate with a dielectric constant of 3.75 and a loss tangent of 0.0004, a quartz substrate for supporting a metasurface structure; gold is used for the metal, with a conductivity of 4.561×107, and a square ring cross structure metal patch 2 is used as a resonator to ensure that the structure has High transmittance or reflectivity; the metal grating 6 serves as a filter layer, transmitting x-polarized waves and reflecting y-polarized waves, so as to ensure that terahertz waves are efficiently transmitted and reflected in different frequency bands; the strip metal patch 10 and the metal patches 2 and 6 are used together as electrodes for applying bias voltage; the medium is made of flexible polyimide material with a dielectric constant of 3.5 and a loss tangent of 0.0027, and its surface has grooves in the same direction, which are used as an orientation layer to align the liquid crystal molecules in the direction of the grooves; the liquid crystal medium has a dielectric constant of 2.55-3.65 and a loss tangent of 0.02. The optimized metasurface unit period is 320μm, the thickness of gold is 1μm, the thickness of polyimide medium is 5μm, the thickness of the first liquid crystal medium 4 is 225μm, the thickness of the second liquid crystal medium 8 is 1200μm, and the thickness of the quartz substrate is 300μm; the outer diameter of the square ring patch in the square ring cross structure metal patch 2 is 320μm, and the inner diameter is 290μm; the long arm length of the cross patch is 290μm, and the width is 30μm; the short arm length is 130μm, and the width is 30μm; the length of the metal grating 6 is 320μm, the width is 32μm, and the gap is 32μm; the length of the strip metal patch 10 is 320μm, and the width is 80μm. By changing the bias voltage, the dielectric constant of the liquid crystal medium is adjusted to achieve a phase coverage of 315°.

[0027] Furthermore, the amplitude and phase of the optimized metasurface unit are calculated by using linear polarization wave incidence, as shown in Figure 2 As shown, it is proved that 315° phase coverage can be achieved by adjusting the dielectric constant of liquid crystal by changing the bias voltage.

[0028] In order to reduce the computational complexity, the optimized phase is quantized to 8th order, that is, quantized to only include the phase distribution with equivalent phases of 0° (S0), 45° (S1), 90° (S2), 135° (S3), 180° (S4), 225° (S5), 270° (S6), and 315° (S7), forming a 3-bit coding unit. In the above 3-bit coding units, any two units with a phase difference of 180° are selected to form a 1-bit coding unit. Here, this design selects "S2" and "S6" and arranges them according to the quantized phase distribution. The phase coding distribution is as follows: Figure 3As shown in a and d, there are 40×40 units in total, and each column represents a subarray, including 2×40 units. Figure 3 As shown in b, c, e, and f, dual-beam steering effects are obtained in the far-field space of transmission and reflection.

[0029] By applying different bias voltages to each subarray in the metasurface, the coding sequence is generated by Figure 3 a to Figure 3 The steering angle of the beam is actively modulated by periodically changing the bias voltage so that the coding sequence “00001111000011110000” ( Figure 3 a, sequence one) becomes "00110011001100110011" ( Figure 3 b, sequence 2), in the range of 0.4-0.44THz, the transmitted beam turns from θ=±7.5° to θ=±15°; at the same time, in the range of 0.7-0.72THz, the reflected beam turns from θ=±5° to θ=±10°. Therefore, this design example can dynamically control the steering angle of the terahertz wave in the whole space by externally connecting a bias voltage without changing the metasurface structure.

[0030] The transmitted or reflected power efficiency is defined as:

[0031] ;

[0032] P main and P full Represent the total power of the main beam and the total power transmitted or reflected into space respectively, and the power efficiency represents the sidelobe suppression capability and the power proportion of the main beam. Simulation calculations show that at 0.4-0.44THz, the transmission power efficiency of the first coding sequence is 68.17%, and the maximum power difference between the main beams is 0%; the transmission power efficiency of the second coding sequence is 68.38%, and the maximum power difference between the main beams is 2.9%. At 0.7-0.72THz, the reflection power efficiency of the first coding sequence is 74.51%, and the maximum power difference between the main beams is 0%; the reflection power efficiency of the second coding sequence is 75.95%, and the maximum power difference between the main beams is 0.91%, which has high power efficiency and good uniformity. The dual-beam steering implemented in this design can significantly improve the flexibility and efficiency of the communication system, and support dynamic beamforming and multiple-input multiple-output (MIMO) technology in multi-user environments.

[0033] Example 2: A fully spatially integrated terahertz metasurface device based on liquid crystal realizes the OAM beam function by changing the coding state of the liquid crystal metasurface unit device through an external bias voltage.

[0034] In this example, based on the metasurface device in Example 1, only the unit device ( Figure 1 a) to realize active control of its phase. M × N The metasurface is composed of units, and the unit ( m , n ) can be expressed as:

[0035] ;

[0036] in, θ mn For unit ( m , n ), φ 0 is the initial phase, r mn and r f They are units ( m , n ) and the position vector of the source.

[0037] After 8th order quantization phase, such as Figure 4 As shown in a and d, the metasurface units designed in Example 1 are arranged in phase to simulate the application of different bias voltages to change the orientation of liquid crystal molecules, thereby achieving phase control and encoding of each unit. The metasurface in this example can achieve Figure 4 The coding of a and d is switched. The metasurface consists of 40×40 units. The metasurface is simulated by using linearly polarized wave incidence. The phase distribution in the transmission and reflection modes rotates clockwise to cover a spiral phase of 2π, which is consistent with the preset phase distribution. The purity of the OAM beam satisfies the following transformation relationship:

[0038] ;

[0039] in, W l The topological charge is l The spectral mode weights, Ψ represents the angular distribution of the topological mode, azimuth φ is a periodic function. For the topological charge l = +1, the mode purity of the vortex waves generated in the transmission and reflection modes is 83.5% and 78.4%, respectively. l= +2, the mode purity of transmission and reflection is 85.2% and 76.9% respectively. The results show that the method proposed in the present invention can generate high-quality OAM beams with different topological charges in both transmission and reflection modes. The OAM beams implemented in this example can carry multiple independent information channels, greatly improving the capacity and data transmission rate of the communication system. In future optical fiber communications and free space optical communications, OAM technology is expected to become an important means to achieve ultra-high-speed and large-capacity communications.

[0040] Example 3 A fully spatially integrated terahertz metasurface device based on liquid crystal realizes a holographic imaging function by changing the coding state of the liquid crystal metasurface unit device through an external bias voltage.

[0041] In this example, based on the metasurface device in Example 1, only the unit device ( Figure 1 a) to realize active control of its phase. The far-field image of the target is shown in Figure 5 (a) As shown. According to the target image, the GS algorithm is used for optimization calculation, and finally the phase distribution of the designed target image is obtained. After 8-order quantization phase, as shown in Figure 5 As shown in (b), the metasurface units designed in Example 1 are arranged in phase, with a total of 40×40 units. By applying different bias voltages, the phase distribution of the metasurface can be switched arbitrarily, realizing the change of phase encoding. The far-field results of transmission and reflection are shown in Figure 2. Figure 6 As shown, it is highly consistent with the target image. According to the power efficiency calculation method in Example 1, in the transmission mode within the range of 0.4-0.44THz, the imaging efficiencies of the characters "N" and "J" are 69.6% and 71.3%, respectively; in the reflection mode within the range of 0.7-0.72THz, the imaging efficiencies are 73.1% and 74.2%, respectively. It can be seen that the method proposed in the present invention can achieve far-field images of arbitrary effects, and can also be extended to the field of high-resolution holographic imaging by increasing the number of units. Combining Example 1 with Example 2, the dual-beam steering technology implemented by the present invention improves the efficiency and quality of multi-user communications and adapts to dynamic network environments. OAM beam technology provides a new way for high-capacity, broadband transmission and quantum communication. Holographic imaging technology shows important application value in big data storage, immersive communication and complex signal processing. The present invention integrates these functions into one, which is expected to promote the development of communication systems in a more efficient, smarter and safer direction.

Claims

1. A fully spatially integrated terahertz metasurface unit based on liquid crystal, It is characterized in that The structure comprises, from top to bottom, a first quartz substrate (1), a square ring-shaped cross-structured metal patch (2), an orientation layer polyimide medium (3), a first liquid crystal medium (4), polyimide (5), a metal grating (6), polyimide (7), a second liquid crystal medium (8), polyimide (9), a strip metal patch (10), and a second quartz substrate (11); wherein the square ring-shaped cross-structured metal patch comprises a square ring patch and a cross patch, wherein the cross patch is located inside the square ring patch; and the strip metal patch (10), the metal patch (2), and the metal grating (6) are used together as electrodes for applying a bias voltage.

2. A liquid crystal-based fully spatially integrated terahertz metasurface unit according to claim 1, It is characterized in that The quartz substrate (1) is used to support the metasurface structure; the metal used is gold, with a dielectric constant of 3.7-3.8, a loss tangent of 0.0002-0.0005, and a conductivity of 4.5×10 7 -4.6×10 7 .

3. The liquid crystal-based fully spatially integrated terahertz metasurface unit according to claim 1, It is characterized in that The square ring cross structure metal patch (2) acts as a resonator, making the structure have high transmittance or reflectivity.

4. The liquid crystal-based fully spatially integrated terahertz metasurface unit according to claim 1, It is characterized in that The metal grating (6) acts as a filter layer, transmitting x Polarized waves, reflection y Polarized waves are used to ensure efficient transmission and reflection of terahertz waves in different frequency bands.

5. The liquid crystal-based fully spatially integrated terahertz metasurface unit according to claim 1, It is characterized in that The polyimide medium of the orientation layer adopts a flexible polyimide material with a dielectric constant of 3.0-3.6 and a loss tangent of 0.0001-0.

05. Its surface has grooves in the same direction and is used as an orientation layer to align the liquid crystal molecules in the direction of the grooves.

6. The liquid crystal-based fully spatially integrated terahertz metasurface unit according to claim 1, It is characterized in that The first liquid crystal medium (4) has a dielectric constant of 2.55-3.65 and a loss tangent of 0.01-0.

03.

7. The liquid crystal-based fully spatially integrated terahertz metasurface unit according to claim 1, It is characterized in that The optimized metasurface unit period is 310-330 μm, the thickness of gold is 0.8-1.2 μm, the thickness of the polyimide medium is 1-10 μm, the thickness of the first liquid crystal medium (4) is 210-230 μm, the thickness of the second liquid crystal medium (8) is 1150-1250 μm, and the thickness of the first quartz substrate (1) is 280-320 μm; the outer diameter of the square ring patch in the square ring cross structure metal patch (2) is 310-330 μm, and the inner diameter is 280-300 μm; the long arm length of the cross patch is 280-300 μm, and the width is 25-35 μm; the short arm length is 120-140 μm, and the width is 25-35 μm; the metal grating (6) has a length of 310-330 μm, a width of 31-33 μm, and a gap of 31-33 μm; The length of the strip metal patch (10) is 310-330 μm and the width is 50-100 μm; by changing the bias voltage and adjusting the dielectric constant of the liquid crystal medium, a phase coverage of 315° is achieved.

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