A 1-bit transmission-type polarization multiplexing multifunctional terahertz digital coding metasurface

Abstract

The present invention is a 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface. The metasurface unit structure is composed of two layers of subwavelength metal microstructures, a nematic phase liquid crystal and a quartz substrate, and the working frequency band is designed to be in the terahertz frequency region. Among them, the subwavelength metal microstructure is two pairs of symmetrically distributed arc-shaped slotted structures, and each pair of slotted structures includes two sections of non-equal length arc structures. The control medium is a large birefringence nematic phase liquid crystal suitable for the terahertz frequency band, and the medium substrate is quartz glass. Based on the subwavelength metal microstructure, the present invention changes the dielectric constant of the liquid crystal layer by changing the square wave voltage signal loaded on the metal microstructure, thereby regulating the amplitude and phase of the transmission spectrum of the metasurface unit under the irradiation of the X-polarized wave and the Y-polarized wave, thereby realizing the switching of the metasurface coding state and the flexible manipulation of the metasurface transmission beam. The present invention has broad application prospects, especially in terahertz holographic imaging and wireless communications.

Description

Technical Field

[0001] The present invention belongs to the technical field of novel electromagnetic metamaterials, and relates to the structure, design method and application of a 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface. Background Art

[0002] Terahertz waves are electromagnetic waves with a frequency range of 0.1-10 THz, and have broad application prospects in fields such as terahertz holographic imaging and high-speed wireless communications. Traditional RF components are difficult to operate in the terahertz frequency band. Electromagnetic metamaterials are artificial structural materials that can flexibly manipulate terahertz waves by periodically or non-periodically arranging sub-wavelength-scale structural units. Electromagnetic metasurfaces are low-profile, lightweight, and easy-to-integrate two-dimensional metamaterials. Due to their excellent electromagnetic control properties, they have attracted widespread attention from researchers in the fields of electromagnetism, electronic information, and physics.

[0003] By independently designing the size parameters of the metasurface unit, each unit can independently control the physical parameters such as the phase, amplitude and polarization of the electromagnetic wave, so as to obtain the desired complex electromagnetic field distribution on demand to meet specific functional requirements. Based on the phase discontinuity or amplitude discontinuity distribution of the metasurface unit, drawing on the thinking of the digital information field, the unit with a specific electromagnetic wave response can be discretized into a finite type and represented by the numbers "0" and "1". For the phase discontinuous 1Bit electromagnetic surface, the phase difference between the two corresponding basic units is 180°, and the amplitude response is basically the same, so two encodings of "0" and "1" can be obtained; for the 2Bit phase discontinuous electromagnetic surface, it corresponds to 4 basic units, which can be encoded with four encoding states of "00", "01", "10" and "11". Correspondingly, for the amplitude discontinuous electromagnetic surface, high transmittance is represented by the number "1" and low transmittance is represented by the number "0", so the amplitude modulation metasurface can also be designed with the coding thinking. The functions of traditional metasurfaces are fixed once they are processed, which is obviously not conducive to practical applications. In order to realize the dynamic control of metasurface functions, control methods such as PIN diodes, varactor diodes, phase change materials and liquid crystal materials are widely used. The idea of ​​digital coding provides a simple and effective solution to design metasurfaces and make them programmable. Combining metasurfaces with programmable devices, such as microcontroller units (MCUs) or field programmable gate arrays (FPGAs), can quickly customize the required functions and dynamically switch them in real time.

[0004] In recent years, with the development of technologies such as artificial intelligence, neural networks, and machine learning, emerging concepts such as intelligent metasurfaces (IRS) and reconfigurable intelligent metasurfaces (RIS) have been proposed one after another, building a bridge between the physical world and the information world, and playing a vital role in promoting the development of B5G and 6G wireless communication technologies. Since the operating frequency of the present invention is in the terahertz band, traditional PIN diodes and varactors have large parasitic effects in the terahertz band and are difficult to work in the terahertz band. Liquid crystal, as a material with a large birefringence effect in the terahertz band, provides an ideal solution for terahertz dynamic reconfigurable devices, while the current liquid crystal-based transmissive terahertz devices have the disadvantages of low transmission amplitude, small phase modulation range, single polarization, and lack of functions. Summary of the invention

[0005] Technical issues:

[0006] The purpose of the present invention is to provide a 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface. The present invention fills a control medium liquid crystal layer in the cavity formed by the upper and lower metal microstructures, and uses FPGA programming to control the voltage in real time and dynamically control the metasurface unit, thereby changing the electromagnetic transmission response characteristics of the metasurface unit. In particular, the designed coding metasurface has independent electromagnetic responses to X-polarized incident waves and Y-polarized incident waves. It is a transmission polarization multiplexing multifunctional digital coding metasurface within the terahertz frequency band, which is very innovative and feasible.

[0007] Technical solution:

[0008] The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface proposed in the present invention is composed of a two-dimensional array structure of multiple sub-wavelength units, and the units arranged longitudinally and transversely are separated from each other to ensure that each unit is independently fed, and the structure of the sub-wavelength unit mainly includes an upper and lower quartz substrate, an upper and lower metal microstructure, and an intermediate control medium liquid crystal layer. Among them, the upper and lower metal microstructures are deposited on the inner surfaces of the upper and lower quartz substrates respectively, and the control medium liquid crystal layer exists in the cavity formed by the upper and lower quartz substrates.

[0009] The subwavelength metal microstructure is a pair of arc-shaped symmetrical slotted structures, each pair of arc-shaped slotted structures is composed of two arcs of unequal length, and the corresponding arcs are 7π / 12 and 5π / 12 respectively, and a liquid crystal layer is filled between the upper and lower metal microstructures.

[0010] The material used for the sub-wavelength metal microstructure is gold, and its operating frequency is in the terahertz frequency band.

[0011] The regulating medium liquid crystal layer is nematic liquid crystal.

[0012] The upper and lower dielectric substrates are made of quartz glass, with a dielectric constant of 3.78 and a loss tangent of 0.002.

[0013] The upper and lower metal microstructures are the same patterned metal layers, which can effectively improve the transmittance of electromagnetic waves. Therefore, the 1-bit terahertz electrically controlled digital coding metasurface is a transmissive metasurface.

[0014] The dielectric constant of the control medium liquid crystal layer is adjusted by a square wave voltage signal loaded on the upper and lower metal microstructures. In the terahertz frequency range, when the square wave voltage increases from the threshold voltage to the saturation voltage, the dielectric constant of the liquid crystal gradually increases.

[0015] When the electrically controlled digital coding metasurface is vertically incident with an X-polarized wave, the transmission properties of the subwavelength unit are significantly different when it is not loaded and when it is loaded with voltage, that is, the transmission amplitude is similar and the phase difference is 180° within the designed operating frequency range, thereby obtaining two digital coding units of "0" and "1" and being able to switch dynamically in real time.

[0016] When the electrically controlled digitally coded metasurface is vertically incident with a Y-polarized wave, the transmission properties of the subwavelength unit are significantly different when not loaded with voltage and when loaded with voltage, that is, the transmission phase is similar within the designed operating frequency range, while the amplitude differs by 0.66, thereby realizing "0" / "1" coding control of the transmission amplitude.

[0017] The electrically controlled digitally coded metasurface can calculate different phase coding sequences according to different functional requirements, and use the field programmable gate array FPGA to encode and control the square wave voltage loaded on the metal microstructure, so as to realize real-time switching of the transmission phase distribution, and then realize flexible control of the terahertz transmission wave to achieve beam scanning, beam shaping and holographic imaging.

[0018] The electrically controlled digitally coded metasurface can calculate different amplitude coding sequences according to different functional requirements, and use the field programmable gate array FPGA to encode and control the square wave voltage loaded on the metal microstructure to achieve real-time switching of the electromagnetic wave transmission amplitude distribution, and then design a large modulation depth (MD) and low insertion loss (IL) liquid crystal spatial light modulator.

[0019] The metasurface unit is a subwavelength structure, and its period size p is approximately equal to 0.52λ (λ represents the working wavelength). The phase difference and transmittance can be adjusted by changing the outer radius r1, the inner radius r2 and the line width w of the connecting line between the units. The phase modulation frequency and the amplitude modulation frequency can be changed by changing the arc size of the arc slot (corresponding to the angle Ro1).

[0020] Beneficial effects:

[0021] The present invention has the following advantages:

[0022] 1. The present invention provides a 1-bit transmissive polarization multiplexing multifunctional terahertz digital coding metasurface. By carefully designing a double-layer subwavelength metal microstructure, compared with the existing transmissive terahertz liquid crystal coding metasurface with a single function, the present invention can simultaneously work under the irradiation of X-polarized incident waves or Y-polarized incident waves on a single metasurface platform, and has the characteristics of large working bandwidth, rich working frequency points, etc.

[0023] 2. The present invention adopts a double-layer sub-wavelength metal microstructure, which has the advantages of large MD, small IL, large transmission amplitude, and wide phase modulation range compared to other transmission coding metasurfaces.

[0024] 3. The present invention expands the operating frequency of the programmable metasurface to the terahertz frequency band by designing an electrically controlled digitally coded metasurface unit, thereby greatly improving researchers' ability to manipulate terahertz waves. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The array, external control circuit and coding unit structure diagram of the designed coding metasurface. The middle part of the upper and lower quartz substrates is hollowed out to form a thinner quartz area with an area of ​​2cm*2cm and a thickness of 0.2mm, which is used to deposit subwavelength metal microstructures. The edge of the hollow area extends a certain distance along the X and Y directions, and the thickness is set to 1mm (which can be thickened according to actual needs). The extended area is used to deposit external control circuits and pads. This design can ensure the transmittance of electromagnetic waves, and ensure that each unit is independently fed and meets the required mechanical strength. The coding unit adopts a metal-dielectric-metal (MDM) structure, such as Figure 1 As shown in the illustration in the upper right corner.

[0026] Figure 2 It is a detailed schematic diagram of the coding unit structure. The unit adopts a square structure, the period length is p, the thickness of the upper and lower quartz layers is Tq, the thickness of the control medium liquid crystal layer is Tp, the inner radius of the arc slot is r2, the outer radius is r1, the line width connecting the circular patch inside the metal microstructure is w, and the angle between the connecting line near the horizontal direction and the horizontal radius is Ro1.

[0027] Figure 3 Represents the transmission coefficient S21 amplitude spectrum of the coded metasurface under vertical incidence of X-polarized wave, zero bias voltage (0 state, e0=2.547) and saturated bias voltage (1 state, e1=3.75).

[0028] Figure 4 Represents the transmission coefficient S21 phase spectrum of the coded metasurface under vertical incidence of X-polarized wave, zero bias voltage (0 state, e0=2.547) and saturated bias voltage (1 state, e1=3.75).

[0029] Figure 5 Represents the transmission coefficient S21 amplitude spectrum of the coded metasurface under vertical incidence of Y-polarized wave, zero bias voltage (0 state, e0=2.547) and saturated bias voltage (1 state, e1=3.56).

[0030] Figure 6 Represents the transmission coefficient S21 phase spectrum of the coded metasurface under vertical incidence of Y-polarized wave, zero bias voltage (0 state, e0=2.547) and saturated bias voltage (1 state, e1=3.56). DETAILED DESCRIPTION

[0031] The present invention is a 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface, whose array, external control circuit and coding unit structure are as follows: Figure 1 As shown. Its structure is mainly composed of two layers of quartz substrate, two layers of sub-wavelength metal microstructure, a control medium liquid crystal layer, an external control circuit and a pad Pad. The sub-wavelength metal microstructure designed by the present invention is a pair of arc-shaped symmetrical slotted structures, each pair of arc-shaped slotted structures is composed of two segments of non-equal length arcs, and the corresponding arcs are about 7π / 12 and 5π / 12, that is, the corresponding angle Ro1 is about 15°, and its operating frequency is the terahertz frequency band. The array pattern can be obtained by ultraviolet lithography. The material used for the array pattern, control circuit and pad Pad is gold, and the thickness is about 0.5 microns. The upper and lower metal microstructures are also used as electrodes for applying square wave voltage signals. The dielectric substrate is quartz glass, with a thickness of 200 to 900 microns, a dielectric constant of 3.78, and a loss tangent of 0.002. The unit adopts a double-layer sub-wavelength metal microstructure, which can effectively enhance the transmittance of electromagnetic waves, so this is a transmission-type electrically controlled digital coding metasurface.

[0032] The working principle of the designed 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface is as follows. Under the condition that the X-polarized wave irradiates the metasurface array vertically and no bias voltage is applied, the dielectric constant of the control medium liquid crystal is the minimum value. At this time, the corresponding resonant frequency of the coding unit is f1, and the transmission coefficient is S21(1). After the saturated bias voltage is loaded, the dielectric constant of the liquid crystal increases, so the resonant characteristics of the coding unit change, and the resonant frequency point shifts to f2, and the corresponding transmission coefficient is S21(2). The required operating frequency range is designed so that the transmission amplitude of the coding unit is as large and consistent as possible under the two bias voltage states, and the phase difference is 180°, so two discrete digital coding units can be obtained, represented by the numbers "0" and "1" respectively. Similarly, under the condition that the Y-polarized wave irradiates the metasurface array vertically, the corresponding resonant frequencies of the coding unit under the two bias voltage states are f1 and f2 respectively. ′ and f2 ′ , the corresponding transmission coefficients are S21′ (1) and S21 ′ (2) In the design of the working frequency band, the corresponding low transmittance under the two bias voltage states is as small as possible, the high transmittance is as large as possible, and the phase is basically the same, so the amplitude can be encoded with the numbers "0" and "1". By applying a specific voltage sequence to the coding unit under the incidence of two polarized waves, the specific required function can be realized, and the same patterned array greatly simplifies the array arrangement process. In addition, the present invention intends to integrate a field programmable gate array (FPGA) onto the metasurface, store the coding matrix corresponding to the specific function obtained by simulation calculation into the FPGA, and realize rapid control of the digital coding unit by programming the FPGA, thereby realizing the required function, such as terahertz holographic imaging based on phase modulation or amplitude modulation.

[0033] Example 1: Figure 1 The digital coding unit shown has a period length of p = 309 μm, a thickness of the upper and lower quartz layers Tq = 200 μm, a thickness of the control medium liquid crystal layer Tp = 30 μm, an inner radius of the arc slot r2 = 105 μm, an outer radius r1 = 149 μm, a line width w = 20 μm connecting the circular patch inside the metal microstructure, an angle Ro1 = 15° between the connecting line near the horizontal direction and the horizontal radius, and a thickness of the metal layer in the sub-wavelength metal microstructure, the external control circuit, and the pad Pad is 0.5 μm. The control medium liquid crystal layer is filled in the cavity formed by the upper and lower quartz substrates. Under the condition of X-polarized waves incident vertically on the metasurface, when the applied bias voltage is 0 V, the dielectric constant of the liquid crystal is 2.547, and when the saturated bias voltage is applied, the dielectric constant of the liquid crystal is 3.75, so the two bias states can be represented by the discrete numbers "0" and "1".

[0034] The electromagnetic response characteristics of the digital coding unit are simulated and calculated using a commercial electromagnetic simulation software package, and the scattering parameters of the coding unit under the incidence of X-polarized waves and Y-polarized waves can be obtained. Figure 3 and Figure 4 The electromagnetic response characteristics of the coding unit under X-polarized wave irradiation are demonstrated. Figure 3 It can be seen that around 500.5 GHz, the amplitude of the transmission coefficient in the zero bias voltage and saturated bias voltage states is basically the same, about 0.65 and 0.63 respectively. For the zero bias voltage state, the amplitude -6dB (corresponding linear value is 0.5) bandwidth covers from 371.2 GHz to the simulation termination frequency. For the saturated bias voltage state, the amplitude -6dB bandwidth covers two frequency ranges, 361.3 to 428.1 GHz and 493.1 to 507.5 GHz respectively. For the transmission-type coding metasurface, high transmittance is crucial to the performance of the metasurface, and the high transmittance in the present invention benefits from the ingenious design of the metasurface. Figure 4It can be seen that at the designed operating frequency of 500.5 GHz, the phase difference between the two bias states is 181°, and in the range of 474.6 to 520 GHz, the phase difference remains above 130°, with a large phase bandwidth, which meets the requirements of 1-bit transmission phase-modulated coding metasurface.

[0035] Example 2: Figure 1 The digital coding unit shown has a period length of p = 309 μm, a thickness of the upper and lower quartz layers Tq = 200 μm, a thickness of the control medium liquid crystal layer Tp = 30 μm, an inner radius of the arc slot r2 = 105 μm, an outer radius r1 = 149 μm, a line width w = 20 μm connecting the circular patch inside the metal microstructure, an angle Ro1 = 15° between the connecting line near the horizontal direction and the horizontal radius, and a thickness of the metal layer in the sub-wavelength metal microstructure, the external control circuit, and the pad Pad is 0.5 μm. The control medium liquid crystal layer is filled in the cavity formed by the upper and lower quartz substrates. Under the condition of the Y-polarized wave incident vertically on the metasurface, when the applied bias voltage is 0 V, the dielectric constant of the liquid crystal is 2.547, and when the saturated bias voltage is applied, the dielectric constant of the liquid crystal is 3.56, so the two bias states can be represented by the discrete numbers "0" and "1".

[0036] Similarly, the electromagnetic response characteristics of the coding unit under the incidence of Y-polarized waves were obtained using an electromagnetic simulation software package, such as Figure 5 and Figure 6 As shown. Figure 5 It can be seen that at the designed operating frequency of 458.6 GHz, when zero bias voltage is applied, the dielectric constant of the liquid crystal is 2.547, and the corresponding transmittance T max =0.9, when the saturation bias voltage is applied, the dielectric constant of the liquid crystal is 3.56, and the corresponding transmittance T min =0.24, for liquid crystal spatial light modulator, MD and IL are its most critical performance indicators, which can be obtained by the following formula:

[0037]

[0038] IL=-10log(T max )#(2)

[0039] According to formulas (1) and (2), it can be calculated that at 458.6 GHz, MD = 73.3%, IL = 0.45 dB. For the two states of the liquid crystal dielectric constant of 2.547 and 3.56, in the range of 458.6 to 492 GHz, MD remains above 60%. The modulation performance can be improved by refining the liquid crystal dielectric constant value. Figure 6The transmission phase spectrum of the coding unit under Y-polarized wave illumination is shown. It can be seen that the phase difference between zero voltage bias and saturated bias at 458.6 GHz is 33°. The above results show that the designed metasurface meets the requirements for high-performance spatial light modulators. Furthermore, a metasurface multifunctional terahertz holographic imaging and wireless communication system based on phase modulation and amplitude modulation can be designed.

Claims

1. A 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface, characterized in that: The metasurface is composed of a two-dimensional array structure of a plurality of sub-wavelength units, each of which is independently powered, and the structure of the sub-wavelength units comprises an upper quartz substrate (4), upper and lower metal microstructures (3), a control medium liquid crystal layer (6), and a lower quartz substrate (1); wherein the upper layer and the lower layer of the metal microstructure (3) are deposited on the inner surfaces of the upper quartz substrate (4) and the lower quartz substrate (1), respectively, and the control medium liquid crystal layer (6) exists in a cavity formed between the upper quartz substrate (4) and the lower quartz substrate (1); The metal microstructure (3) is a pair of symmetrical arc-shaped slotted structures, each pair of arc-shaped slotted structures is composed of two arcs of unequal length, and the corresponding arcs are and ; The digital coding metasurface can calculate different phase coding sequences according to different functional requirements, and use the field programmable gate array FPGA to encode and control the square wave voltage loaded on the metal microstructure (3), so as to realize real-time switching of the transmission phase distribution, thereby realizing flexible control of the terahertz transmission wave, achieving the effects of beam scanning, beam shaping and holographic imaging, or realizing real-time switching of the electromagnetic wave transmission amplitude distribution, thereby designing a large modulation depth and low insertion loss liquid crystal spatial light modulator.

2. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 1, characterized in that: The material used for the metal microstructure (3) is gold, and its operating frequency is in the terahertz frequency band.

3. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 1, characterized in that: The control medium liquid crystal layer (6) is nematic liquid crystal.

4. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 1, characterized in that: The material used for the upper quartz substrate (4) and the lower quartz substrate (1) is quartz glass, which has a dielectric constant of 3.78 and a loss tangent of 0.

002.

5. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 1, characterized in that: The upper and lower layers of the metal microstructure (3) are the same patterned metal layers, which can effectively improve the transmittance of electromagnetic waves. Therefore, the 1-bit terahertz electrically controlled digital coding metasurface is a transmissive metasurface.

6. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 4, characterized in that: The dielectric constant of the control medium liquid crystal layer (6) is adjusted by a square wave voltage signal loaded on the upper and lower metal microstructures (3); within the terahertz frequency range, when the square wave voltage increases from a threshold voltage to a saturation voltage, the dielectric constant of the liquid crystal gradually increases.

7. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 1, characterized in that: When the digital coding metasurface is vertically incident with an X-polarized wave, the subwavelength unit has similar transmission amplitudes and a phase difference of 180° within the designed operating frequency range when it is not loaded and loaded with voltage, thereby obtaining two digital coding units of "0" and "1" and being able to switch dynamically in real time.

8. The 1-bit transmission polarization multiplexing multifunctional terahertz digital coding metasurface according to claim 1, characterized in that: When the digital coding metasurface is vertically incident with a Y-polarized wave, the subwavelength unit has similar transmission phases within the designed operating frequency range when it is not loaded and loaded with voltage, while the amplitude differs by 0.66, thereby realizing "0" and "1" coding control of the transmission amplitude.

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