Transflective terahertz ultrafast large-field-of-view superstructure scanning array and design method thereof

Abstract

The invention discloses a transflective terahertz ultrafast large-field-of-view superstructure scanning array which comprises a patterned metal layer, a response material layer and a substrate layer. The patterned metal layer comprises m * n patterned metal units with the same size, and each patterned metal unit comprises an annular metal hole and a Z-shaped structure arranged in the annular metal hole; the gradient change direction corresponding to the patterned metal layer is the x direction, and the adjacent patterned metal units on the x axis are arranged at equal difference rotation angles; the response material layer is arranged on the silicon layer below the patterned metal layer; and the substrate layer is arranged below the response material layer as a substrate, and the substrate layer is made of sapphire. According to the invention, large-field-of-view transflective beam scanning can be carried out on incident terahertz waves, and ultrafast energy regulation and control can be carried out on beam energy through pump light control.

Description

Technical Field

[0001] This invention belongs to the field of terahertz modulator design, and in particular relates to a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array and its design method. Background Technology

[0002] In recent years, to realize next-generation 6G communication networks, the terahertz band (0.1–10 THz) has been regarded as a key technology for achieving Tbit / s peak rates and microsecond-level latency in 6G communication due to its abundant spectrum resources and narrow beam characteristics. However, the application of terahertz communication is still limited by problems such as line-of-sight transmission, atmospheric loss, and difficulties in beam scanning. Terahertz beam scanning arrays based on metasurfaces have become a research hotspot because they can dynamically switch beam pointing and have multiple electromagnetic degrees of freedom, which can be used to cope with multiple transmission scenarios in the future.

[0003] However, traditional terahertz beam scanning arrays still suffer from drawbacks such as narrow field of view, single transmission / reflection mode, slow dynamic response, and narrow operating bandwidth, which limit their engineering applications. Therefore, a terahertz meta-beam scanning array that can achieve a large field of view, transmission / reflection duplex, ultrafast modulation, and a large operating bandwidth is of great significance for improving the performance of terahertz communication systems and has important engineering application value.

[0004] Transmittance-reflection metasurface scanning arrays are based on artificially designed metasurfaces that can phase-modulate terahertz waves in a wide range, thereby achieving beam scanning with a large field of view in both transmission and reflection spaces. Furthermore, the transmittance-reflection beams have high synchronization symmetry, and the beam energy can be excited by external pump light to achieve ultrafast energy modulation. Summary of the Invention

[0005] This invention applies a broadband electromagnetic modulation method to a transmissive and reflective optically modulated ultrafast terahertz meta-scanning array, and discloses a meta-scanning array and its design method that simultaneously satisfies transmissive and reflective dual-beam modulation, ultrafast energy controllability, and a large field of view.

[0006] This invention discloses a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array, which includes a patterned metal layer, a responsive material layer, and a substrate layer.

[0007] The patterned metal layer comprises m×n patterned metal units of the same size, each patterned metal unit including an annular metal hole and a Z-shaped structure disposed within the annular metal hole;

[0008] The gradient change direction corresponding to the patterned metal layer is the x-direction, and adjacent patterned metal units on the x-axis are set with equal rotation angles.

[0009] The responsive material layer is disposed in a silicon layer beneath the patterned metal layer;

[0010] The substrate layer is positioned below the responsive material layer, and the substrate layer material is sapphire.

[0011] The transmitted-reflection terahertz ultrafast large field-of-view meta-scanning array can modulate the incident x-polarized terahertz wave into four modulated beams: a right-hand circularly polarized transmitted wave, a left-hand circularly polarized transmitted wave, a left-hand circularly polarized reflected wave, and a right-hand circularly polarized reflected wave. When the operating frequency of the incident wave varies between 0.51 THz and 1.21 THz, the deflection angles of the four beams can change, namely -157° to -115°, -63° to -23°, 23° to 65°, and 115° to 157°, respectively, with a total field of view of 4×42° and a spatial coverage of 47.8%. Simultaneously, when the pump light energy is between 0 and 100 μJ / cm², the array can also modulate the wave into four beams. 2 When varying within the range, the beam energy modulation depth can reach nearly 100%, and the change response time period is approximately 250 ps.

[0012] Furthermore, gold is selected as the material for the patterned metal layer, and the thickness of the patterned metal layer is 0.2 μm;

[0013] The patterned metal unit has a period of 80 μm, the inner diameter R of the annular metal hole is 38 μm, the bottom side length of the Z-shaped unit is L = 58.5 µm, the side width is 44.5 µm g, and the line width is s = 8 µm.

[0014] Adjacent patterned metal units have eight different rotation angles, with a difference of 22.5° between them: 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°.

[0015] The silicon layer has a thickness of 0.6 μm;

[0016] The dielectric constant ε of the sapphire r The value is 9.5, and the substrate thickness is 500 μm.

[0017] A method for designing a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array is also provided, which includes the following steps:

[0018] Step 1: Design of a multi-pole resonant broadband tunable terahertz transmissive and reflective unit;

[0019] Step 2, design patterned metal layers;

[0020] Step 3: Select a material as the response material layer;

[0021] Step 4: Design of ultrafast energy controllable terahertz frequency-scanning metasurface array for transmission and reflection.

[0022] Furthermore, in step 1, the terahertz transmission and reflection unit is composed of three layers stacked from top to bottom: a metal layer, a responsive material layer, and a substrate layer.

[0023] Furthermore, in step 2, the scattering intensity of each multipole in the unit can be calculated from the surface current of the metal, and the specific calculation formula is as follows:

[0024] ;

[0025] Where c is the speed of light, and ω is the angular frequency;

[0026] An electric dipole P is represented as:

[0027] ;

[0028] The magnetic dipole M is represented as:

[0029] ;

[0030] The cyclic dipole T is represented as:

[0031] ;

[0032] electric quadrupole Q e Represented as:

[0033] ;

[0034] Magnetic quadrupole Q m Represented as:

[0035] ;

[0036] Where j is the current density and r is the displacement vector. The current density is directly exported from Comsol software, and the scattering intensity value of each multipole can be calculated by Matlab software.

[0037] In order to improve the Z-shaped unit with electric dipole as the main resonant mode, a ring-shaped metal hole structure is introduced to achieve the original resonant mode of the Z-shaped structure without destroying it, and to improve the disadvantage of the Z-shaped unit having high resonant frequency at both ends and low frequency in the middle, which meets the rotation requirements of the PB structure.

[0038] Furthermore, in step 2, the optimization selection of parameters includes the following steps:

[0039] Determine the period size p of the unit cell; the period directly determines the upper and lower limits of the operating frequency of the entire unit cell, and the resonant frequency follows the formula. Here, c is the speed of light in a vacuum, and ε rThe dielectric constant of the substrate is ε; the substrate is selected as ε r Using 9.5 sapphire material as the substrate, the frequency was designed to be 1 THz according to actual needs, and the period was selected as 80 μm through calculation;

[0040] The inner diameter R of the annular metal hole structure is determined. The inner diameter R determines the parameter size of the Z-shaped structure. In order to provide sufficient rotation and parameter selection space for the Z-shaped structure, and to avoid increasing the coupling between adjacent units and affecting the array function, the inner diameter R of the metal hole is 38 μm.

[0041] The base length L, side width g, and line width s of the Z-shaped unit were determined. To achieve the design goal of efficient transmission and reflection of electromagnetic waves and 360° phase coverage over the widest possible operating frequency band, parameter scanning simulation was conducted. Based on the criteria of the highest transmission and reflection amplitude, the flattest line shape, and the widest bandwidth, the optimized parameters were selected as L = 58.5 µm, g = 44.5 µm, and s = 8 µm.

[0042] Furthermore, in step 3, the responsive material layer should meet the following characteristics:

[0043] The material is isotropic;

[0044] The material must possess ultrafast optical response characteristics;

[0045] The photoresponse time needs to be at the picosecond level and the range of photoconductivity variation needs to be as large as possible.

[0046] The intrinsic band gap of the material must be less than the photon energy of the 800 nm wavelength pump light required for experimental conditions;

[0047] Response materials need to be easy to process and low in cost;

[0048] Based on the above characteristics, a silicon material with a thickness of 0.6 μm was ultimately selected as the response material layer.

[0049] Furthermore, in step 4, the phase modulation of the unit is achieved as follows:

[0050] When the Z-shaped structure rotates 180° counterclockwise, its transmission phase and reflection phase With rotation angle There is a 2x relationship, represented as follows: and The negative sign indicates that the transmitted and reflected waves propagate in opposite directions;

[0051] The rotating unit enables the transmission and reflection phases to cover 360°.

[0052] The angle of array beam scanning depends on the phase gradient. It follows Snell's theorem:

[0053] ;

[0054] ;

[0055] Where, θ t Let θ be the transmission angle. r Let n be the reflection angle. i n is the refractive index of the space for reflection. t The refractive index of the transmission space is . dx represents the phase difference per unit distance, where dx is the period p; λ0 is the wavelength of the electromagnetic wave in vacuum, which can be derived from... The calculation yields a value where c is the speed of light in a vacuum, approximately 3 × 10⁻⁶. 8 m / s, f0 is the operating frequency;

[0056] The angle Δθ within the 0.51-1.21 THz operating range with a period p of 80 μm was calculated to ensure the selected phase gradient. The obtained deflection angle difference Δθ is largest in the range of 0.51-1.21 THz, while... and It needs to be less than 1. According to calculations, a phase gradient interval of 45° is optimal.

[0057] The array size generally needs to meet the requirement of 8 wavelengths to avoid sidelobes, and should be larger than the experimental THz spot size. The gradient change direction is the x-direction, and the rotation angle θ between adjacent Z-shaped units differs by 22.5°, specifically 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°.

[0058] The beneficial effects achieved by this invention are:

[0059] This invention provides a design method for a terahertz transmission and reflection large field-of-view beam scanning array, which can simultaneously achieve large field-of-view (4×42°) control of the deflection angle of transmitted and reflected waves, and the deflected beams can be efficiently energy-controlled by pump light (modulation depth nearly 100%).

[0060] By introducing a nested Z-shaped improved structure with a metal annular hole, the working bandwidth of this invention is broadened, and the stability of transmission and transmission amplitude is better in the broadband range.

[0061] By using silicon as the responsive material, this invention enables the array to have picosecond-level ultrafast optical control capabilities, allowing for dynamic control of beam energy. Attached Figure Description

[0062] Figure 1 Schematic diagram of a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array;

[0063] Figure 2 Traditional Z-shaped element analysis. (a) Typical structure, (b) Current distribution at high and low frequencies, (c) Multipole scattering behavior, (d) Transmission and reflection amplitudes.

[0064] Figure 3 Nested Z-shaped element analysis. (a) Structural model, (b) Current distribution at high, medium and low frequencies, (c) Comparison of multipole scattering performance, (d) Comparison of transmission and reflection amplitudes.

[0065] Figure 4 Characterization of photogenerated carrier dynamics in silicon films. (a) Spectral modulation curve of silicon film conductivity; (b) Numerical variation of silicon film conductivity with pump light energy at 0.5 THz, 1 THz, and 1.5 THz; (c) Ultrafast excitation and relaxation processes of transmitted and reflected waves.

[0066] Figure 5 The amplitude and phase change with rotation angle θ. (a) Relationship between transmission phase and PB element rotation angle, (b) Relationship between reflection phase and PB element rotation angle. (c) Transmission phase at 0 S / m (d) Reflection phase at 0 S / m (e) Transmission phase at 3000 S / m (f) Reflection phase at 3000 S / m;

[0067] Figure 6 Transmission and reflection amplitudes and phases vary with silicon conductivity. (a) Broadband effect of conductivity variation on transmitted energy; (b) Broadband effect of conductivity variation on reflected energy.

[0068] Figure 7 Transmitted / reflected terahertz ultrafast large field-of-view meta-scanning array. (a) Deflector microscope image, scale bar 50 μm. (b) Broadband two-dimensional far-field distribution map (xoz plane). (c) Comparison of theoretical and experimental beam deflection angles. (d) Normalized intensity of transmitted / reflected beams at 0.56 THz: pump flux at ±35° (position of maximum intensity) is 0 (no pump), 5, 10, 50, and 100 μJ / cm. 2 Within the range of 10°~60° / -10°~-60°. Detailed Implementation

[0069] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0070] like Figure 1 As shown, this invention provides a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array, which is divided into a three-layer integrated structure, including a patterned metal layer (upper layer), a responsive material layer (middle layer), and a substrate layer (lower layer).

[0071] Gold is selected as the material for the patterned metal layer, and the thickness of the patterned metal layer is 0.2 μm. The patterned metal layer includes m×n patterned metal units of the same size. Each patterned metal unit includes an annular metal hole and a Z-shaped structure disposed in the annular metal hole. The period of the patterned metal unit is 80 μm, the inner diameter R of the annular metal hole is 38 μm, the base length L = 58.5 µm, the side width g = 44.5 µm, and the line width s = 8 µm.

[0072] The gradient change direction corresponding to the patterned metal layer is the x-direction, and adjacent patterned metal units on the x-axis are set with equal rotation angles; adjacent patterned metal units have 8 different rotation angles, with a rotation angle difference of 22.5°, namely 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°.

[0073] The responsive material layer is disposed on a silicon layer beneath the patterned metal layer, and the silicon layer has a thickness of 0.6 μm.

[0074] The substrate layer is positioned below the responsive material layer. The substrate material is sapphire, and the dielectric constant of sapphire is ε. r The value is 9.5, and the substrate thickness is 500 μm.

[0075] The transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array can modulate incident x-polarized terahertz waves into four modulated beams: a right-hand circularly polarized transmitted wave, a left-hand circularly polarized transmitted wave, a left-hand circularly polarized reflected wave, and a right-hand circularly polarized reflected wave. When the incident wave operating frequency varies between 0.51 THz and 1.21 THz, the deflection angles of the four beams can change to -157° to -115°, -63° to -23°, 23° to 65°, and 115° to 157°, respectively, with a total field of view of 4 × 42° and a spatial coverage of 47.8%. Simultaneously, when the pump light energy is between 0 and 100 μJ / cm², the array can achieve the same effect. 2 When varying within the range, the beam energy modulation depth can reach nearly 100%, and the change response time period is approximately 250 ps.

[0076] This invention also provides a design method for a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array, including the following steps:

[0077] Step 1: Design of a multi-pole resonant broadband tunable terahertz transmissive and reflective unit;

[0078] To facilitate the design of large-angle beam scanning arrays, the metaarray unit needs to possess broadband controllability and dynamic response capabilities in both transmission and reflection spaces. The metaarray unit consists of three layers stacked from top to bottom: a metal layer, a responsive material layer, and a substrate layer.

[0079] Step 2, Design patterned metal layer

[0080] The patterned metal layer on top of this unit determines the amplitude and phase characteristics of the modulated terahertz wave, requiring efficient transmission of electromagnetic waves and 360° phase coverage across the widest possible operating frequency band. Existing units typically employ a multi-layer metal-dielectric stacked structure, which usually has a narrow bandwidth, cannot achieve large field-of-view beam scanning, and the multi-layer metal makes fabrication difficult and expensive. Therefore, this paper proposes using a single-layer PB metal unit as the upper structure, which can achieve ultra-wideband frequency response and amplitude-phase modulation.

[0081] Secondly, PB single-layer metal structures typically feature Z-shaped, H-shaped, and open-ring patterns. Z-shaped metals generally offer a wider control bandwidth compared to other shapes, making them more suitable for subsequent broadband scanning array designs. Here, we first examine traditional Z-shaped units (such as...) Figure 2 The current distribution, multipole scattering, and transmission / reflection amplitude shown in (a) are used to guide the optimal design of the unit. Figure 2 (b) To obtain the current distribution of the element at low and high frequencies through simulation, it can be seen that the Z-shaped element excites two different resonant modes at low and high frequencies, as shown by the green arrows. To guide the subsequent optimization direction, quantitative calculation and analysis are performed through multipole scattering decomposition, as shown... Figure 2As shown in (c), the scattering intensity of each multipole in the unit cell can be calculated from the surface current of the metal. The specific calculation formula is as follows:

[0082] (1)

[0083] Where c is the speed of light and ω is the angular frequency.

[0084] Among them, the multipoles P, M, T, Q e Q m It can be represented as:

[0085] Electric dipole (ED).

[0086] (2)

[0087] Magnetic dipole (MD)

[0088] (3)

[0089] Toroidal dipole (TD).

[0090] (4)

[0091] Electric quadrupole (EQ).

[0092] (5)

[0093] Magnetic quadrupole (MQ)

[0094] (6)

[0095] Where j is the current density and r is the displacement vector. The current density can be directly exported from Comsol software, and then the scattering intensity value of each multipole can be calculated using Matlab software.

[0096] Multipole scattering calculations revealed that the Z-shaped unit cell is dominated by the ED electric dipole mode at both high and low frequencies, followed by the TD toroidal dipole and MQ magnetic quadrupole modes. Here, the electric dipole manifests as a flow of positive charge from one end to the other, thus generating a current. Therefore, the minimum resonant frequency f of the superstructure unit cell... min The maximum length l of the flowing current max Specific decision. Figure 2(d) represents the transmission and reflection amplitudes of the Z-shaped unit, which are consistent with the linear shape of the multipole scattering intensity variation. Therefore, this means that the bandwidth and linear shape of the transmission and reflection spectral lines are directly determined by primarily changing the electric dipole and secondarily changing the intensity of the cyclic dipole and magnetic quadrupole.

[0097] Observing the resonance of a traditional Z-shaped element reveals that its modulation effect is controlled only by two strong resonant modes at low and high frequencies. This results in a "high at both ends and low in the middle" characteristic in the overall transmission or reflection amplitude, which is highly detrimental to achieving stable array functionality over a wide bandwidth. Furthermore, the traditional Z-shaped element has a relatively narrow transmission bandwidth, which can be further broadened.

[0098] Based on the above analysis, the Z-shaped unit uses electric dipoles as its primary resonant mode. To optimize bandwidth, either the low-frequency resonant frequency needs to be lower or the high-frequency resonant frequency needs to be higher. Furthermore, the current path reveals that the high-frequency and low-frequency resonances share a common resonant current path and are not independent. Simply increasing the length of the Z-shaped structure will only shift the overall operating frequency band and will not achieve the desired bandwidth increase. Therefore, an additional structure is needed. This structure must meet the following conditions: first, it must not disrupt the original resonant mode of the Z-shaped structure; second, it needs to introduce a new multipole mode resonance at the mid-frequency to improve the original "high at both ends and low in the middle" drawback; and third, it must not affect the rotation requirements of the original PB structure and must be strictly symmetrical, without disrupting the original PB amplitude-phase relationship.

[0099] In summary, adding an annular metal hole structure to the outside of the Z-shaped unit constitutes a structure as follows: Figure 3 (a) shows a nested Z-shaped structural unit, and this ring structure satisfies all the above requirements. Firstly, the ring-shaped metal hole structure is placed outside the traditional Z-shaped structure; the two are independent and do not disrupt the original resonant mode, which cannot be achieved by adding additional metal branches. Secondly, simulations show that due to the introduction of the ring-shaped metal hole, such as... Figure 3 As shown in (b), the surface current on the metal structure not only maintains the original paths at low and high frequencies (green arrows), but also generates new current paths (red arrows). This not only extends the path of the electric dipole, but also excites a new magnetic field, as shown in the figure. As shown. The simulation was performed using Comsol software. During the simulation, the x and y directions were set as periodic boundary conditions, the +z direction as free space convenient conditions, and the -z direction as absorbing boundary conditions. This can be used to simulate a semi-infinite space medium, with the element excited by a waveport. Thirdly, the annular hole structure possesses both axisymmetry and rotational symmetry, i.e., isotropic, without disrupting the original PB amplitude-phase relationship.

[0100] For parameter optimization, the first step is to determine the element period size p. This directly determines the upper and lower limits of the entire element's operating frequency, and the resonant frequency follows the formula... Here, c is the speed of light in a vacuum, and ε r Let ε be the dielectric constant of the substrate. The substrate needs to be stable, isotropic, low-cost, and easy to process; therefore, ε is chosen here. r A 9.5 sapphire crystal was used as the substrate. Therefore, based on actual needs, a frequency of 1 THz was designed, and a period of 80 μm was calculated to be reasonable.

[0101] The second step is to determine the inner diameter R of the metal hole. The inner diameter R determines the parameter size of the Z-shaped structure in the next step, so sufficient rotation and parameter selection space must be provided for the Z-shaped structure; therefore, the inner diameter should be as large as possible. On the other hand, an excessively large inner diameter can increase the coupling between adjacent units, affecting the array's functional performance, and also reduce the minimum edge linewidth, which is detrimental to manufacturing. Therefore, considering all factors, the inner diameter R of the metal hole is set at 38 μm.

[0102] The third step is to determine the base length L, side width g, and linewidth s of the Z-shaped element. These three parameters jointly determine the amplitude and phase performance of PB. To achieve the design goal of efficient transmission and reflection of electromagnetic waves and 360° phase coverage over the widest possible operating frequency band, after detailed parameter scanning simulation, the optimized parameters are obtained as follows: L = 58.5 µm, g = 44.5 µm, s = 8 µm. At this setting, the transmission and reflection amplitudes are the highest, the line shape is the flattest, and the bandwidth is the widest.

[0103] In addition, gold (Au) was chosen as the metallic structural material, and its conductivity was set to 4.56 × 10⁻⁶ during simulation. 7 S / m is high because it is easy to process and obtain and has high stability. The selection of the response material layer (middle layer) will be introduced separately in the next section. Here, the simulation only considers the performance of metal (upper layer)-substrate (lower layer).

[0104] In summary, nested Z-shaped elements were designed based on optimized parameters. Figure 3 (b) shows the current distribution of the element when a circularly polarized electromagnetic wave is incident along the +z direction. Three typical frequencies represent the element's broadband performance: 0.51 THz at the lowest frequency, 0.85 THz at the middle frequency, and 1.21 THz at the highest frequency. Resonant currents are present in both the Z-shaped structure and the external metal aperture, distinguished by green and red arrows, respectively. This indicates that the element responds within a broadband range of 0.51–1.21 THz, with the strongest resonance at the lowest frequency. Circular and linear currents excite different modes of magnetic fields, as shown in the figure. As shown. The current distribution of the nested circular hole structure and the traditional Z-shaped structure is compared ( Figure 3 (b) and transmission and reflection amplitudes Figure 3(d) As can be seen, at the lowest frequency, due to the absence of the circular hole structure, the current (green) on the Z-shaped structure cannot continue to extend, i.e., the current path is shortened. On the other hand, at the high frequencies of 0.81 THz and 1.21 THz, the ring magnetic field formed by the circular hole metal structure also does not exist.

[0105] Furthermore, through multipole scattering analysis ( Figure 3 (c) It was found that the nested circular aperture structure significantly improved the unit scattering intensity, expanded the unit resonant bandwidth, and enhanced the modulation amplitude. The top three multipoles contributing to scattering power remained the electric dipole (ED), vortex dipole (TD), and magnetic quadrupole (MQ). Simultaneously, the intensity of all pole components in the nested Z-shaped structure increased exponentially, especially higher-order modes such as TD and MQ, due to the new field introduced by the external metal aperture. This enhancement of resonant modes directly improved the unit amplitude performance in two ways. Firstly, the presence of the metal circular aperture increased the length of the longest current path l. max The increase extends the resonant frequency from 0.62 THz to 0.51 THz, an increase of 0.11 THz. On the other hand, due to the presence of the metal aperture, the resonant fields of each multipole are enhanced over a wide range, especially near 0.85 THz, where the transmission and reflection amplitudes no longer exhibit a linear shape that is high at both ends and low in the middle, but rather a relatively flat shape.

[0106] Step 3, determine the intermediate layer response material

[0107] The upper metal layer determines the amplitude, phase, and polarization characteristics of the modulated wave of the unit cell, while the middle layer needs to be dynamically controlled through suitable response materials. Common dynamic response materials include semiconductor materials such as Si (silicon), Ge (germanium), MoS2 (molybdenum disulfide), WS2 (tungsten disulfide), BP (black phosphorus), GaAs (gallium arsenide), and quantum well structure materials; two-dimensional carbon materials such as graphene and carbon nanotubes; as well as VO2 (vanadium dioxide), perovskite materials, organic conjugated materials, liquid crystal materials, and topological insulators.

[0108] The following aspects were considered when selecting materials. First, the selected materials must be isotropic, meaning their performance for x-polarized and y-polarized electromagnetic waves must be completely consistent. This is because the previously designed upper metal layer is anisotropic, which has determined the PB amplitude-phase relationship between x-polarized and y-polarized electromagnetic waves. The intermediate layer material cannot disrupt this relationship; for example, materials with polarization differences, such as liquid crystal materials, were excluded. Second, the material must possess ultrafast photoresponse characteristics. This is because the dynamic control performance of the array depends entirely on the response of the intermediate layer material, so materials such as VO2, which lack photoresponse, can be excluded. Third, the photoresponse time needs to reach the picosecond level, and the range of photogenerated conductivity variation needs to be as large as possible. Materials such as Ge, MoS2, WS2, and BP, although possessing ultrafast responses, have too small a range of photoexcited conductivity variation and can be excluded. Fourth, the intrinsic band gap of the material must be smaller than the photon energy (1.55 eV) of the 800 nm wavelength pump light available under experimental conditions. Finally, the response material must also be easy to process and low in cost.

[0109] We need to examine whether the α-type epitaxial Si material meets the design requirements. First, the material is isotropic, which meets the design requirements.

[0110] Second, a 0.6 μm thick silicon material is fabricated on a sapphire substrate. Given that silicon's intrinsic band gap is 1.12 eV, it can be excited by pump light, and its photoconductivity can be calculated from the measured power intensity. Figure 4 (a) The photoconductivity of the silicon layer under different pump light intensities was derived through experiments.

[0111] Photoconductivity It can be calculated using the following formula:

[0112] (7)

[0113] Where ε0 is the free space permittivity, c is the speed of light in free space, and d is the thickness of the silicon epitaxial layer. and The refractive indices of the media on both sides of the sample (air and sapphire) are 1 and 9.5, respectively. It is the transmission signal of the sample without optical excitation. It is a difference in transmission spectrum. Figure 4 (b) shows that the silicon film exhibits similar trends across a wide bandwidth of 0.5, 1.0, and 1.5 THz, conforming to logarithmic growth, with a maximum conductivity reaching as high as 1.5 × 10⁻⁶. 4 S / m meets design requirements.

[0114] Third, the transmission spectrum can also reveal the dynamic relaxation behavior of charge carriers within the silicon film, allowing for the calculation of the silicon's response time to pump light. For example... Figure 4As shown in (c), it can be seen that at 100 μJ / cm 2 The relative change in the amplitude of the main peak of the time-domain signal of a THz pulse under pump light intensity ( The pump pulse arrives at the sample at t = 0 ps and then reaches its maximum value at t0 = 2 ps. The relaxation process can be fitted by the following exponential decay function:

[0115] (8)

[0116] Where A0 and A1 are constants. This refers to the lifetime of photocarriers. When the pump light intensity is 200 μJ / cm²... 2 hour, The value was extracted to 246.3 ps, as shown by the fitted black curve in the figure. Therefore, the entire excitation-to-recovery cycle can be completed within an ultrafast speed (approximately 250 ps). Correspondingly, terahertz waves in the reflection space also exhibit a synchronous ultrafast response. Thus, silicon film materials can achieve ultrafast response, meeting design requirements.

[0117] Finally, Si is a common semiconductor material that is easy to process and fabricate. Therefore, Si was chosen as the response material for the intermediate layer of the unit cell. The thickness needs to be as thin as possible while ensuring a large range of conductivity variation. Through simulation and experiments, a thickness of 0.6 μm was determined to meet the design requirements.

[0118] Step 4: Design of Transmissive and Reflective Ultrafast Energy Controllable Terahertz Frequency-Scanning Metasurface Array

[0119] The ability of a cell to independently control its amplitude and phase is a prerequisite for array functional design.

[0120] On the one hand, phase control is achieved by rotating the Z-shaped structure inside the circular hole. First, the designed Z-shaped element is a PB element, which has a geometric phase-rotation angle relationship. Figure 5 This demonstrates the transmission phase when the Z-shaped structure is rotated 180° counterclockwise. and reflection phase With rotation angle There exists a 2x relationship, which can be represented as follows: and The negative sign indicates that the transmitted and reflected waves propagate in opposite directions. Therefore, rotating the element can cover the transmission and reflection phases by 360°. This can be demonstrated through CST simulation. Figure 5The phase difference stability under different rotation angle differences is demonstrated, showing that both transmission and reflection phases exhibit good phase stability. Phase fluctuations remain controlled within ±5° over a wide bandwidth (0.51-1.21 THz), as shown in the gray box in the figure. The offset remains stable even when the silicon film conductivity changes from 0 S / m to 3000 S / m. The proposed design unit meets the 360° phase requirement.

[0121] On the other hand, varying the pump energy flux density can dynamically control the transmission and reflection amplitudes. For example... Figure 6 As shown. At 1600 μJ / cm 2 At the pump light intensity, the conductivity is approximately 15000 S / m, at which point both the transmission and reflection amplitudes are close to 0. The modulation depth at this point is calculated. Approximately 100%. And by observing the transmission amplitude ( Figure 6 (a) and reflection amplitude ( Figure 6 (b) shows that when the conductivity is greater than 3000 S / m, the amplitudes of both are already at a low level and change relatively slowly. Further detailed calculations can be made to determine the transmittance T of the modulated wave. -+ and reflectivity R ++ When the conductivity of the silicon film increases from 0 to 3000 S / m, the metasurface can achieve a high modulation depth of 97% to 83% within a wide bandwidth of 0.51 THz to 1.21 THz, and the pump light threshold is only 100 μJ / cm. 2 .

[0122] Therefore, the proposed method of combining photoresponsive silicon material and Z-shaped structure rotation can achieve independent control of amplitude and phase, which is beneficial for the design of metasurface arrays.

[0123] Next, the angle of array beam scanning depends on the phase gradient. It follows Snell's theorem, as follows:

[0124] (9)

[0125] (10)

[0126] Where θ t Let θ be the transmission angle. r Let n be the reflection angle. i n is the refractive index of the space for reflection. t The refractive index of the transmission space is . Let dx be the phase difference per unit distance, where dx is the period p. λ0 is the wavelength of the electromagnetic wave in vacuum, which can be obtained from... The calculations yielded the following result: where c is the speed of light in vacuum, approximately 3 × 10⁸ m / s, and f₀ is the operating frequency. Here, n... i For 1, n t for .

[0127] According to the formula, the angle Δθ within the operating range of 0.51-1.21 THz at a period p of 80 μm can be obtained. It is known that the deflection angle changes exponentially with frequency, meaning the lower the frequency, the larger the deflection angle. Therefore, it is necessary to ensure the selected phase gradient... The obtained deflection angle difference Δθ is largest in the range of 0.51-1.21 THz, while... and It needs to be less than 1. According to the formula, the optimal phase gradient interval is 45°.

[0128] On the other hand, the array size generally needs to accommodate 8 wavelengths to avoid sidelobes, and must be larger than the experimental THz spot size to ensure testing results. Therefore, a transmissive and reflective ultrafast energy-controllable terahertz frequency-scanning metasurface array was designed based on the aforementioned optically controlled broadband transmissive and reflective metasurface unit. The array consists of 200×200 units, with a size of 16mm×16mm, as shown below. Figure 7 As shown in (a). The gradient change direction is the x-direction, and the rotation angle θ of adjacent Z-shaped elements differs by 22.5°, specifically 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°. The transmission angle and reflection angle can be calculated using formulas (12) and (13):

[0129] As shown in the above formula, the beam deflection angle is dispersive to the design frequency (λ0). This means that the ultra-wideband modulation frequency will result in a large deflection angle range, which is precisely what is used to achieve large field-of-view beam scanning. Given that the operating range of the metasurface is 0.51-1.21 THz, substituting this into the formula, we can calculate the transmission deflection angle in the range of 23° to 65°. When the incident wave is a right-hand circularly polarized wave (σ+), simulations can obtain the scattering circle diagram of the transmitted wave in the wide frequency range, specifically at frequencies of 0.51, 0.56, 0.66, 0.80, and 1.21 THz. Figure 7 As shown in (b), it can be observed that the lower the frequency, the larger the deflection angle, which is consistent with the theoretical formula.

[0130] On the one hand, because the metasurface array proposed in this invention possesses PB phase characteristics, the scanning field of view will be multiplied when the incident wave is replaced by an x-polarized wave. Linear polarization can be considered as the synthesis of two circular polarizations with different rotation directions. Therefore, for the same gradient phase, the x-polarized beam will be split into two right-handed (σ+) and left-handed (σ-) circularly polarized beams with symmetrical energy distribution and opposite deflection angles. The scanning field of view coverage is multiplied from the first quadrant to both the first and second quadrants.

[0131] On the other hand, since the unit can operate simultaneously in both transmission and reflection spaces, the scanning beam can be further expanded exponentially to the full spatial range on the xoz surface. Due to spatial symmetry, the rotation direction of the deflected beam also reverses. Therefore, when an x-polarized terahertz wave is used as the incident wave, the metasurface array can divide it into four modulated waves, distributed in four quadrants of the entire space: the first quadrant (transmitted right-handed spiral wave), the second quadrant (transmitted left-handed spiral wave), the third quadrant (reflected left-handed spiral wave), and the fourth quadrant (reflected right-handed spiral wave). Figure 7 (c) The full-quadrant transmission / reflection spectra of theoretical and measured values ​​are given. The continuous coverage angles of the scanning beam are -157° to -115°, -63° to -23°, 23° to 65°, and 115° to 157°, with spatial coverage of [missing information]. The beam steering coverage ratio (within 360° omnidirectional space) reaches as high as 47.8%. Therefore, this deflector can generate four beams simultaneously in four Cartesian quadrants, with a total field of view of 4×42°, and its steering angle can be modulated synchronously with the frequency. The test results and theoretical results are in perfect agreement.

[0132] Finally, the energies of the four deflection beams located in different quadrants can be obtained from 0 to 100 μJ / cm. 2 The pump light is continuously and dynamically controlled. For example... Figure 7 As shown in (d) and (e), when the pump flux threshold is as low as 100 μJ / cm 2 Taking a ±35° deflection beam as an example, its intensity can range from the maximum value to the off state, with energy modulation efficiencies of 91.2% and 88.6% for transmission and reflection, respectively.

[0133] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the scope of protection of the present invention; all technical solutions formed by equivalent transformations or equivalent substitutions fall within the scope of protection of the present invention; the parts of the present invention not described in detail are well-known technologies to those skilled in the art.

Claims

1. A transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array, characterized in that, The transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array includes a patterned metal layer, a responsive material layer, and a substrate layer; The patterned metal layer comprises m×n patterned metal units of the same size, each patterned metal unit including an annular metal hole and a Z-shaped structure disposed within the annular metal hole; The gradient change direction corresponding to the patterned metal layer is the x-direction, and adjacent patterned metal units on the x-axis are set with equal rotation angles. The responsive material layer is disposed in a silicon layer beneath the patterned metal layer; The substrate layer is positioned below the responsive material layer, and the substrate layer material is sapphire. The transmitted-reflection terahertz ultrafast large field-of-view meta-scanning array can modulate the incident x-polarized terahertz wave into four modulated beams: a right-hand circularly polarized transmitted wave, a left-hand circularly polarized transmitted wave, a left-hand circularly polarized reflected wave, and a right-hand circularly polarized reflected wave. When the operating frequency of the incident wave varies between 0.51 THz and 1.21 THz, the deflection angles of the four beams can change, namely -157° to -115°, -63° to -23°, 23° to 65°, and 115° to 157°, respectively, with a total field of view of 4×42° and a spatial coverage of 47.8%. Simultaneously, when the pump light energy is between 0 and 100 μJ / cm², the array can also modulate the wave into four beams. 2 When varying within the range, the beam energy modulation depth can reach nearly 100%, and the change response time period is approximately 250 ps.

2. The transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array according to claim 1, characterized in that, The patterned metal layer is made of gold, and the thickness of the patterned metal layer is 0.2 μm. The patterned metal unit has a period of 80 μm, the inner diameter R of the annular metal hole is 38 μm, the bottom side length of the Z-shaped unit is L=58.5 µm, the side width is 44.5 µm g, and the line width is 8 µm. Adjacent patterned metal units have eight different rotation angles, with a difference of 22.5° between them: 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°. The silicon layer has a thickness of 0.6 μm; The dielectric constant ε of the sapphire r The value is 9.5, and the substrate thickness is 500 μm.

3. A method for designing a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array, characterized in that, The design method for the transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array includes the following steps: Step 1: Design of a multi-pole resonant broadband tunable terahertz transmissive and reflective unit; Step 2, design patterned metal layers; Step 3: Select a material as the response material layer; Step 4: Design of ultrafast energy controllable terahertz frequency-scanning metasurface array for transmission and reflection.

4. The design method for a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array according to claim 3, characterized in that, In step 1, the terahertz transmission and reflection unit is composed of three layers stacked from top to bottom: a metal layer, a responsive material layer, and a substrate layer.

5. The design method for a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array according to claim 3, characterized in that, In step 2, the scattering intensity of each multipole in the unit cell can be calculated from the surface current of the metal, and the specific calculation formula is as follows: ； Where c is the speed of light, and ω is the angular frequency; An electric dipole P is represented as: ； The magnetic dipole M is represented as: ； The cyclic dipole T is represented as: ； electric quadrupole Q e Represented as: ； Magnetic quadrupole Q m Represented as: ； Where j is the current density and r is the displacement vector. The current density is directly exported from Comsol software, and the scattering intensity value of each multipole can be calculated using Matlab software. To improve the Z-shaped unit's main resonant mode of electric dipole, a ring-shaped metal hole structure is introduced to achieve the original resonant mode of the Z-shaped structure without destroying it, thus improving the disadvantage of the Z-shaped unit's resonant frequency being high at both ends and low in the middle, and satisfying the rotation requirements of the PB structure.

6. The design method for a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array according to claim 5, characterized in that, In step 2, the optimization selection of parameters includes the following steps: Determine the period size p of the unit cell; the period directly determines the upper and lower limits of the operating frequency of the entire unit cell, and the resonant frequency follows the formula. Here, c is the speed of light in a vacuum, and ε r The dielectric constant of the substrate is ε; the substrate is selected as ε r Using 9.5 sapphire material as the substrate, the frequency was designed to be 1 THz according to actual needs, and the period was selected as 80 μm through calculation; The inner diameter R of the annular metal hole structure is determined. The inner diameter R determines the parameter size of the Z-shaped structure. In order to provide sufficient rotation and parameter selection space for the Z-shaped structure, and to avoid increasing the coupling between adjacent units and affecting the array function, the inner diameter R of the metal hole is 38 μm. The base length L, side width g, and line width s of the Z-shaped unit were determined. To achieve the design goal of efficient transmission and reflection of electromagnetic waves and 360° phase coverage over the widest possible operating frequency band, parameter scanning simulation was conducted. Based on the criteria of the highest transmission and reflection amplitude, the flattest line shape, and the widest bandwidth, the optimized parameters were selected as L = 58.5 µm, g = 44.5 µm, and s = 8 µm.

7. The design method for a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array according to claim 3, characterized in that, In step 3, the responsive material layer should meet the following characteristics: The material is isotropic; The material must possess ultrafast optical response characteristics; The photoresponse time needs to be at the picosecond level and the range of photoconductivity variation needs to be as large as possible. The intrinsic band gap of the material must be less than the photon energy of the 800 nm wavelength pump light required for experimental conditions; Response materials need to be easy to process and low in cost; Based on the above characteristics, a silicon layer with a thickness of 0.6 μm was ultimately selected as the response material layer.

8. The design method for a transmissive and reflective terahertz ultrafast large field-of-view meta-scanning array according to claim 3, characterized in that, In step 4, the phase modulation of the unit is achieved as follows: When the Z-shaped structure rotates 180° counterclockwise, its transmission phase and reflection phase With rotation angle There is a 2x relationship, represented as follows: and The negative sign indicates that the transmitted and reflected waves propagate in opposite directions; The rotating unit allows the transmission and reflection phases to cover 360°. The angle of array beam scanning depends on the phase gradient. It follows Snell's theorem: ； ； Where, θ t Let θ be the transmission angle. r Let n be the reflection angle. i n is the refractive index of the space for reflection. t The refractive index of the transmission space is . dx represents the phase difference per unit distance, where dx is the period p; λ0 is the wavelength of the electromagnetic wave in vacuum, which can be derived from... The calculation yields a value where c is the speed of light in a vacuum, approximately 3 × 10⁻⁶. 8 m / s, f0 is the operating frequency; The angle Δθ within the 0.51-1.21 THz operating range with a period p of 80 μm was calculated to ensure the selected phase gradient. The obtained deflection angle difference Δθ is largest in the range of 0.51-1.21 THz, while... and It needs to be less than 1. According to calculations, a phase gradient interval of 45° is optimal. The array size generally needs to meet the requirement of 8 wavelengths to avoid sidelobes, and should be larger than the experimental THz spot size. The gradient change direction is the x-direction, and the rotation angle θ between adjacent Z-shaped units differs by 22.5°, specifically 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5°.

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