Microtopography‐Guided Double‐Layer Cross Structure for a Terahertz Multiband Amplitude Modulator

Terahertz (THz) modulator can be used to modulate the amplitude and frequency of THz wave. A THz multiband amplitude modulator based on temperature control is proposed in this study. The metamaterial structure of proposed modulator is a double‐layer cross structure which attached on a silicon substrate, and the cross‐distribution of different materials in top and middle layer is Cu @ SiO2 and SiO2 @ VO2, respectively. To prepare the modulator, a microtopographic substrate‐guided method with low‐cost and high accuracy capacities is proposed. Finally, the proposed modulator exhibits above 70% transmittance in 1.31–1.36 THz, 1.55–1.60 THz, and 1.76–1.79 THz, respectively, at 35 °C After temperature rises to 70 °C, transmittance decreases below 0.1 in 1–2 THz. The similarity of the experimental and simulated transmission is up to 85.67%, and the mean modulation depth (MD) is 0.73. The performance can fulfill the applications in THz amplitude modulator, and due to the large modulation depth of transmission, the proposed modulator can be used as a filter switch. It also indicates the proposed method can be effectively applied in other multilayer composite materials preparation.


Microtopography-Guided Double-Layer Cross Structure for a Terahertz Multiband Amplitude Modulator
Tong Li, Hang Chen, Jia Zhang,* and Zhenlong Wang* DOI: 10.1002/admi.202202210 electromagnetic waves change from transmission to reflection, and the change process is reversible. Due to the phase change properties, VO 2 is often embedded in a metamaterial structure to fabricate a THz modulator, and other materials which have the same phase change properties (e.g., bismuth, graphene, and gyroelectric) can be used to fabricate the THz modulator too. [12][13][14][15][16] As ref. [17] shown, an active and smart electro-optic THz modulator based on a strongly correlated electron oxide VO 2 was proposed. The metamaterial structure of the THz modulator is composed od three layers, the materials of each layer are silver (Ag), silicon dioxide (SiO 2 ), and VO 2 , respectively. With milliampere current excitation on the VO 2 film, the transmission, reflection, absorption, and phase of THz waves can be modulated efficiently. In particular, the antireflection condition can be actively achieved and the modulation depth reaches 0.99. As ref. [18] shown, an optically and thermally controlled THz modulator based on silicon (Si) and VO 2 hybrid meta surface is proposed to improve the ability of flexibly manipulating THz waves, and the modulator can bring a deep MD of 0.97 at 0.9 THz. There are other phase-change materials used in the modulator to modulate THz waves except VO 2 . As ref. [19] shown, a faraday modulator based on graphene, gyroelectric (InSb), SiO 2 , and Si materials was proposed. Finally, the THz signal transmission can be modified from 0 to 0.8 by varying applied static magnetic fields. As ref. [20] shown, a high-performance, broadband THz modulator based on the photo-induced transparency of carbon nanotube films was proposed. The modulation depth of this modulator can reach +0.8 with modulation speeds of 340 GHz under femtosecond pulsed illumination. In summary, the metamaterial structures are used to realize the transmission and reflection, and phasechange materials (PCM) are inserted in the metamaterial structures to modulate the transmission and reflection. Generally, all the structures are prepared by photolithography, etching, or liftoff technologies. As the feature size of metamaterials reduces to micro and nano scale, the cost rises at an exponential rate, and the accuracy of insertion of PCM is difficult to control.
A deep MD of the modulator will bring a large-scale modulation, which means lots of signals can be controlled, thereby the diversity of modulation can be improved. Meanwhile, high rising and falling slopes of the filtering band, leading to a short time from filtering pass to blocking thereby an improvement of transmission response. Herein, a THz multiband amplitude Terahertz (THz) modulator can be used to modulate the amplitude and frequency of THz wave. A THz multiband amplitude modulator based on temperature control is proposed in this study. The metamaterial structure of proposed modulator is a double-layer cross structure which attached on a silicon substrate, and the cross-distribution of different materials in top and middle layer is Cu @ SiO 2 and SiO 2 @ VO 2 , respectively. To prepare the modulator, a microtopographic substrate-guided method with low-cost and high accuracy capacities is proposed. Finally, the proposed modulator exhibits above 70% transmittance in 1.31-1.36 THz, 1.55-1.60 THz, and 1.76-1.79 THz, respectively, at 35 °C After temperature rises to 70 °C, transmittance decreases below 0.1 in 1-2 THz. The similarity of the experimental and simulated transmission is up to 85.67%, and the mean modulation depth (MD) is 0.73. The performance can fulfill the applications in THz amplitude modulator, and due to the large modulation depth of transmission, the proposed modulator can be used as a filter switch. It also indicates the proposed method can be effectively applied in other multilayer composite materials preparation.

Introduction
The frequency of THz waves is between electromagnetic waves and visible light, [1][2][3] due to the particular frequency of electromagnetic spectrum, there are some unusual electromagnetic properties in THz waves which can be widely used in non-destructive testing, imaging technology, and medical Diagnosis. [4][5][6] THz modulator is a kind of THz device, which can be used to modulate the amplitude or frequency of THz waves by physical means (e.g., temperature and electricity). [7][8][9][10] Vanadium dioxide (VO 2 ) is a kind of metal oxide with phase transition, [11] and the non-metallic state of VO 2 changes to metallic state after temperature rises to 68 °C, which can lead modulator with a double-layer cross structure was designed and fabricated, which can realize a multiband amplitude modulation in 1-2 THz. A microtopographic substrate-guided method was proposed to fabricate the modulator with low-cost and high accuracy capacities. Finally, the modulator exhibits above 70% transmittance in 1.31-1.36 THz, 1.55-1.60 THz, and 1.76-1.79 THz, respectively, at 35 °C. After temperature (T) rises to 70 °C, transmittance decreases below 10% in whole range of 1-2 THz, and the mean MD of three transmission peaks is 0.73. In addition, the similarity of the experimental and simulated transmission is up to 85.67%. As a result, the large MD and high transmission prove not only the probability of usage of the modulator in the larger amplitude modulation, but also an effective method for preparing THz microstructures and metamaterials.

Results and Discussion
The proposed multiband amplitude modulator is a kind of periodic metamaterials, and the cell is schematically illustrated in Figure 1. It contains three layers: a Si bottom layer, middle layer with VO 2 @ SiO 2 cross-distribution, and top layer with C u @ SiO 2 cross-distribution. Specifically, the top layer is a composited of Cu and SiO 2 , and Cu is distributed in the central cross area, SiO 2 is distributed around the central cross area. Analogously, the middle layer is a composited of VO 2 and SiO 2 , SiO 2 is distributed in the central cross area, and VO 2 is distributed around the central cross area (Figure 1c). Thickness of both top and middle layers are l 1 , and the thickness of bottom layer is l 2 (Figure 1a). The cross area consists of two rectangular areas with length of x and width of d, and the period of the metamaterial structures is p (Figure 1b).
Generally, the transmission can be calculated by T(ω) = S 2 , in which S is the transmission coefficient. According to the matrix transmission principle, the transmission matrix (T) of electromagnetic wave in metamaterials can be calculated as below.

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where, n is the refractive index of material, k is the wave vector of incident wave, h is the thickness of uniform tablet, z is the impedance of the material. Since the structure size of metamaterials is much smaller than the working wavelength, the metamaterials can be considered as homogeneous materials. As a result, S can be calculated as below.
where γ is the conductivity, ∆l and ∆s are the unit length and area of material, ε and µ are permittivity and permeability, and the detailed derivation of S is shown in the Supporting Information. According to Equations 1-3, for the same THz waves and structure parameter, k and h is invariable. Therefore, the independent variable of transmission (T(ω) = S 2 ) is only the ε and γ.
The design principle of proposed modulator is shown in Figure 1d,e. Cu is distributed in the central cross area of top layer, and VO 2 is distributed around the central cross area of middle layer, Cu and VO 2 are not in the same layer, but structures of Cu and VO 2 can be joined together to form a square structure (Figure 1c). After temperature rises to 70 °C, VO 2 transfer to a conductive state, two complementary structures (Cu and VO 2 ) form a metal plane in the lateral direction. For THz transmission, if the thickness of the metal plane is more than the wavelength of THz waves, it means this metal plane is a perfect electromagnetic shield, THz waves can't be transmitted by the metal plane. Figure 1d illustrates the fluid electric field of proposed modulator after temperature (T) rises to 70 °C, the red line illustrates the direction of fluid electric field, finally, a closed ring electromagnetic field is composited of the fluid electric fields of Cu and VO 2 (blue and green part in Figure 1d), which can stop the THz waves transmitting. Figure 1e illustrates the fluid electric field of single-layer cross modulator, this modulator is composed of three layers which are bottom Si layer, middle VO 2 layer, and top Cu @ SiO 2 cross structure layer, respectively. Different from the proposed modulator, the material which is below top Cu area is VO 2 while not SiO 2 ( Figure 1e). Meanwhile, the γ of SiO 2 is almost 0, while the γ of VO 2 at 35 °C is several hundred. According to Equations 1-3, the transmission coefficient (S) of SiO 2 is close to 1, while of VO 2 is less than 1, resulting in transmittance of SiO 2 is close to 1 which is more than VO 2 . As a result, the THz waves which transmit the top layer can pass through the SiO 2 middle layer completely (Figure 1d), while part of THz waves decay in the VO 2 middle layer ( Figure 1e). Therefore, transmission can be improved by double cross structure of proposed modulator at 35 °C, and after T rises to 70 °C, the transmission of both modulators decreases to 0 due to the electromagnetic shielding effect. As a result, the MD can be improved by the double cross structure, all the analyses can be proved in the followings.
To realize multiband amplitude modulation in the range of 1-2 THz, the mentioned geometric parameters and materials above are optimized via the High-Frequency Structure Simulator (HFSS) with the optimization targets of bandwidth and transmission. Figure 2 illustrates the transmission curve with respect to structural parameters of the modulator via simulation. If the material of the top cross area is SiO 2 (orange curve in Figure 2a), the multi-narrowband transmission peaks (metal materials in Figure 2a) have been changed to a broadband transmission peak at 35 °C, and the transmittance is a little higher than which of metal materials after T rises to 70 °C, resulting in a decrease of MD (Figure 2b). For the SiO 2 of the top cross area, the low scope of transmission curve leads a low transmission response, indicating the metal materials fit for the material of the top cross area. Meanwhile, the transmittance decreases with the increasing permeability of metal (Cu > W > Ti > Ni), as a result, metal material is set as Cu ( Figure 2a). Following, the three transmission peaks of double-layer cross structure is higher than that of single-layer cross structure ( Figure 2c) at 35 °C, the mean difference of peaks between two structures is 0.19, and the transmittance of both structures is 0 at 70 °C, indicating the double-layer cross structure can improve the MD. By the way, after T increases to 70 °C, the transmittance all decreases to 0 in Figure 2c-h, there won't appear transmission curves at 70 °C in the following figures. Figure 2d shows the first and second transmission peak decrease slightly with the thickness of top and middle layer (l 1 ) increased from 50 to 300 nm, while only l 1 is set as 100 nm, the slope of transmission curve is highest. As a result, l 1 is set as 100 nm. In addition, the thickness of silicon substrate (l 2 ) is optimized at 10-40 µm, resulting in a relatively high transmittance of three peaks at 30 µm (Figure 2e). Figure 2f shows the first transmission peak (1.3-1.4 THz) changes with the width of cross structure (d), from which it increases first and then decreases as continuous increasing d from 10 to 40 µm. As a result, the largest MD and highest slope of proposed filter are achieved at d of 20 µm in the simulation. Furthermore, the length of cross structure (x) is also optimized. The second transmission peak changes little with x increasing from 40-60 µm, while the third transmission peak decreases sharply with x increased, meanwhile, there is highest slope of first peak at x of 50 µm. As a result, the largest MD and highest slope of proposed filter are achieved at x of 60 µm. Finally, the period (p) of unit is optimized at 60-120 µm. The transmission curve shifts left as p increases (Figure 2h), meanwhile, the transmittance increases slightly with p increased, while the slope of transmission curve decreases sharply as p exceeding 80 µm. Deep MD brings a large-scale modulation, resulting from a diversification of modulation, and high slopes of the filtering band, leading to a short time from filtering pass to blocking thereby an improvement of transmission response. Therefore, p is set as 80 µm to achieve deep MD and high slope at the same time. Finally, the optimized parameters size and materials are accumulated in Table 1. Since the minimum size of the specimen for the test in our experiments is 5 × 5 mm -2 , the modulator has a scale of 75 × 75 with 5625 patterns. Above all, the optimized structural parameters of proposed modulator are accumulated in Table 1. Upon the optimization, the MD at 1.34, 1.57, and 1.78THz are 0.73, 0.77, and 0.68, respectively, the mean MD can achieve 0.73.
Generally, the designed double-layer cross structure can be prepared on the plane by optical lithography and physical vapor deposition (PVD) for two circles. However, there is an inevitable error at the interface between different cross structures of two layers coming from inaccuracy of mask alignment. The structural error will result in high decrease in transmission. To overcome these drawbacks, a microtopography-guided method has been employed for multi-materials deposition reported by our group before [21] to prepare the double-layer cross structure. In brief, a VO 2 cross hollow structure (green part in Figure 3a) covered on the silicon substrate was prepared by a standard lithography and physical vapor deposition (PVD) process. The length of cross hollow structure is x (x = 60 µm), and the width

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is d (d = 20 µm). Following, a 100 nm SiO 2 film was sputtered on the VO 2 substrate to fill in the gap of VO 2 hollow structure (Figure 3b,e). Third, a 100 nm Cu film was sputtered on the deposition body of SiO 2 to fill in the gap of SiO 2 hollow structure (Figure 3c,f). Fourth, the top side materials were removed by grinding process (Figure 3g) until to revealing the section of z = l 1 on the deposition surface (Figure 3h). The sign of ending for the grinding process is the brightness changes from light to dark at the area around cross structure under the white microscope. This ending sign has been further proven by AFM characterization in the following. After grinding, two different cross-distribution of Cu @ SiO 2 and SiO 2 @ VO 2 for a doublelayer cross structure was revealed (Figure 3g). Finally, the silicon substrate at the bottom was thinning to 30 µm (l 2 ) by diamond wheel grinding (Figure 3i). As a result, the proposed modulator was successfully prepared. To sum up, after twice PVD process, SiO 2 and Cu are deposited to the designed position of different layers with only one optical lithography, mask alignment of twice optical lithography is avoided, and the accuracy of fabrication can be improved. Figure 4a shows an optical image of VO 2 substrate, which exhibits uniform cross hollow structure achieved on the whole substrate. The enlarged optical image (Figure 4a inset) shows clear distribution of VO 2 structure, and the VO 2 film is fabricated by annealing the vanadium (V) film in vacuum, the fabrication process of VO 2 film is mentioned in the Supporting Information.
Bottom layer l 2 = 1 µm Silicon ε 4 = 11.9, µ 4 = 1 Note: ε means permittivity, and µ means permeability. After SiO 2 and Cu films are deposited, the deposition body does not change very much under the microscope (Figure 4b), only the brightness of deposition body surface has been enhanced.
After removing the upside deposition materials, an array Cu @ SiO 2 distribution in the plane is achieved on the whole substrate (Figure 4c), the brightness of cross area is brighter than

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the areas around the cross area, it's the sign of ending for the grinding process. Scanning electron microscope (SEM) images exhibit a unit VO 2 substrate (Figure 4d inset), SiO 2 and Cu deposition body on VO 2 substrate (Figure 4e inset), and double-layer cross structure (Figure 4f inset), respectively. As a result, whatever the VO 2 substrate, deposited body, or the double-layer cross structure after grinding, the outline of the cross structure can be clearly revealed under the electron microscope. Figure 4g illustrates the cross-section SEM image of VO 2 substrate, in which there is a clear relationship between foreground and background of two bumps (Figure 4g inset), it proves the VO 2 substrate unit is a hollow cross structure. After twice deposition and once grinding, the cross-section of Cu @ SiO 2 composite structure and SiO 2 @ VO 2 composite structure are shown in Figure 4h, and there is a dividing line between different materials (Figure 4h inset). To further prove the different materials distribution is fit for the designed distribution, the element mapping of a double-layer cross structure unit (Figure 4i) gives isolate Cu, V, and Si distribution (Figure 4l-n). Figure 4o exhibits the height AFM image of the unit, which shows a very smooth surface with roughness (Ra) of 0.72 nm after grinding. Furthermore, the profiles of cross-section are plotted in Figure 4p, resulting in a maximum fluctuation of 5 nm on the grinding surface. As a result, a double-layer cross structure with Cu @ SiO 2 and SiO 2 @ VO 2 distribution materials on the silicon substrate have been successfully prepared. Figure 5a shows a specimen for the THz modulation test, in which the red box gives the active area of the modulator. The modulation test was carried out in a THz time-domain spectrometer (CCT-1800) with the spectral region from 0.06 to 4 THz and an incidence angle of 35°. Figure 5b plots the transmission curves both from the experiment and simulation. The transmittance of experimental modulator is larger than 70% in 1.31-1.36 THz, 1.55-1.59 THz, and 1.76-1.79 THz at 35 °C. First of all, the coupling electric field of the double cross layers generates the first transmission peak at 1.3-1.4 THz. Following, the coupling electric field between the top cross layer and bottom Si layer generates the second transmission peak at 1.5-1.6 THz. Finally, the coupling electric fields between the middle cross layer and bottom Si layer generates the third transmission peak at 1.8-1.9 THz. After T rises to 70 °C, the transmittance decreases below 10% in 1-2 THz (blue curve in Figure 5b). In addition, the heating process was realized by an annular hightemperature ceramic heating plate, the detailed parameters and heating effect of high-temperature ceramic heating plate are mentioned in the Supporting Information. Significantly, there is still deviation between two transmission curves at 35 °C (Figure 5b). Herein, the transmission error (Err.) is employed to define the similarity of experimental and simulated transmission curves, which can be calculated via Equation 4. Q(t) and R(t) are experimental and simulated transmission curves, respectively. To calculate the Err., equivalent area method is employed here. And then, seven areas are defined as S 1-7 shown in Figure 5c. As a result, Equation 4 can be rewritten as Equation 5. Finally, the value of Err. is calculated to be 14.33%, which identifies the similarity of two transmission curves up to 85.67%. Figure 5d shows VO 2 is in non-metallic state at lower temperature (35-45 °C), which leads transmission peaks are ≈0.8, and the transmission curves change little at 35-45 °C. As T rises above 60 °C, VO 2 is in a metallic state which leads To further prove the performance of the proposed modulator, two evaluation indicators: modulation depth (MD) and scope (S) were proposed. MD and S of the second crest for the transmission curve are defined as Equations 6 and 7, respectively. Where p 1 , p 2 , p 3 , and p 4 are four points on the experimental transmission curve (red and blue curves) in Figure 5b, in which p 1 is the peak point of the second wave crest, p 3 and p 4 are the half crest points at transmission of 0.5 in the second wave crest, p 2 is on the experimental transmission curve (blue curve) at 70 °C, meanwhile, the horizontal coordinates of p 1 and p 2 are the same. The MD and S of the other two crests for the transmission curve can be defined by the same way, and all the values of MD and S are shown in Figure 6. The greater the modulation depth is, the better the modulation effect is. Meanwhile, the performance of transmission response is determined by the scope of transmission curves. The greater the scope is, the faster the transmission response is. Compared with the recent literature ( Table 2), some THz modulator has high modulation depth but low transmission response, and not all the modulators were measured before. The experimental modulation depths of proposed modulator at 1.34, 1.57, and 1.78 THz are 0.73, 0.77, and 0.68, respectively. The mean modulation depth is 0.73 which is higher than the cut-off transmittance (0.7), the proposed modulator can be used as a switching device of the THz filter. Meanwhile, the scopes of the three peaks of transmission curve are 4.38, 7.29, and 6.33, respectively, the mean scope is 6.00. Above all, a multiband amplitude modulation is proven, which indicates not only the probability of usage in the THz modulator, but also a feasible and scale of our method for preparing multilayer microstructures and metamaterials. Figure 6a-h illustrates the transmittance and two evaluation indexes of different parameters for proposed modulator in experiment. "VS" is short for VO 2 @ SiO 2 in Figure 6a, which stands for a cross-distribution of VO 2 @ SiO 2 of middle layer of the proposed modulator, and "VO 2 " in Figure 6a stands for a single VO 2 middle layer (green part in Figure 2d). After T rises to 70 °C, the transmittance will decrease below 0.1 in 1-2 THz whatever how the structural parameters change (Figure 6a,c,e,g). First of all, the three transmission peaks of proposed modulator is higher than that of the single-layer cross structure modulator 35 °C, which can lead to a deeper MD, and all the three scopes of proposed modulator are higher than single-layer cross structure modulator (Figure 6a). Figure 6b illustrates the values of MD and slope for each crest, obviously, the mean MD of double-layer cross modulator (0.73) is deeper than that of single-layer cross modulator (0.58) by 0.15 (Figure 6b). Meanwhile, the mean scope of proposed modulator (6.00) is higher than which of single-layer cross structure modulator (4.74) by 1.26, as a comprehensive consideration of the two evaluation indicators, the modulation performance of doublelayer cross structure modulator is better than single-layer cross structure modulator. Following, the first and third peaks (marked from left to right in Figure 6b) of transmission curve decrease with width of cross structure (d) increased. In particularly, transmittance of the first and third peaks decrease sharply as d exceeding 20 µm, resulting from a steep drop in MD (Figure 6d) of the first (0.26) and third peaks (0.21). Meanwhile, the slope of each crest increases firstly and then decreases with d increased (Figure 6d), finally, the highest slope of each peak can be achieved at d of 20 µm, which leads a best transmission response. Furthermore, there is almost no change of the first crest value with the length of cross structure (x) increased, and the second crest value decreases sharply as x increases to 70 µm, the third crest value changes a lot with x increased at the same time (Figure 6e). As a result, the difference of MD in the second and third crest is more than 0.1 (Figure 6f) between the two x values of 60 µm and 70 µm. Meanwhile, the average slope increases first and then decreases with x increased, finally, the largest average slope is achieved at x of 60 µm (Figure 6f). Finally, As thickness of the first and middle layer (l 1 ) increase, transmittance of all three peaks decrease, in particular, there is a sharp decrease of transmittance at the third peak as l 1 increases to 0.2 µm (Figure 6g). As a result, the MD decreases with l 1 increased. Meanwhile, as l 1 increases, the average slope increases firstly and then decreases, finally, the largest average slope can be achieved at l 1 of 0.1 µm. Figure 7 illustrates the electric field mapping of the modulator with different structures. The red color represents the strong electric field, which can lead to a high transmission. Figure 7a,b, illustrates the electric field of proposed modulator at 35 and 70 °C, respectively. As temperature (T) rises to 70 °C, the electric field intensity of proposed modulator decreases sharply, it means transmittance at 1.6 THz decreases sharply after T rises to 70 °C, it proves the deep amplitude modulation of proposed modulator. In the same way, it can also prove the deep amplitude modulation of single-layer cross structure modulator in Figure 7b,c. However, the electric field intensity of proposed modulator is stronger than the single-layer cross structure modulator at 35 °C. After T rises to 70 °C, the electric field intensity of proposed modulator is weaker than the single-layer cross structure modulator. As a result, the difference of electric field intensity between two T of proposed modulator is more than the single-layer cross structure modulator, which means the MD of proposed modulator is deeper than single-layer cross structure modulator. Figure 7e,f, illustrates the cross-section electric field distribution of the top and middle layer of two modulator, the material of middle layer www.advmatinterfaces.de of proposed modulator is SiO 2 in Figure 7e, while the material of middle layer of single-layer cross structure modulator is VO 2 in Figure 7f. The color of middle layer in Figure 7e is red, while it is blue in Figure 7f, it means the electric field intensity of SiO 2 is stronger than VO 2 , and strong electric field intensity means high transmission which proves the transmission of THz wave of SiO 2 is higher than VO 2 , it's fit for the speculation in Figure 2.

Conclusion
In conclusion, a double-layer cross structure with cross-distribution of Cu @ SiO 2 and SiO 2 @ VO 2 in two layers is presented. The structure exhibits a multiband of transmission in the THz range, and the amplitude of transmission can be modulated based on temperature control, which can work as a THz multiband amplitude modulator. In addition, a microtopography substrateguided method has been proposed to fabricate the modulator with low-cost and high accuracy capacities. As a result, the modulator exhibits above 70% transmission in 1.31-1.36 THz, 1.55-1.60 THz, and 1.76-1.79 THz, respectively, at 35 °C. After T rises to 70 °C, transmission decreases below 0.1 in 1-2 THz. The mean MD of proposed modulator is 0.73, which can be used in switching the device of filter. The similarity of the experimental and simulated transmission is as high as 85.67%, which could be improved by two points. One is to improve the crystalline and purity of the VO 2 by optimizing the rapid annealing process of V film via controlling annealing temperature and time more precisely. Other is to improve fabrication accuracy including size accuracy, surface roughness, and multi-materials (e.g., Cu, SiO 2 , and VO 2 ) distribution accuracy. Finally, it proves the microtopography substrate-guided method can be used to fabricate multilayer structure with multi-materials.

Experimental Section
Electronic Simulations: The high frequency structure simulator (HFSS) soft was employed to design and determine the feature size of the proposed THz modulator.
Fabrication of Double-Layer Cross Structure: The V films were deposited on the silicon substrate by two-target horizontal magnetron sputtering system (YWC-450, Shenyang Yujie Vacuum Equipment Co., Ltd). Then VO 2 films were prepared by rapid annealing of V film in a tube furnace (TF55030C-1, Thermo Fisher Scientific), and silica and copper films were sequentially deposited on the VO 2 film. The aluminum oxide power (Al 2 O 3 , 50 nm in diameter) was used to grind the deposition body (Cu). After removing a certain thickness of upside material, the double-layer cross composite structure was revealed.
Fabrication of THz Absorber: The silicon substrate was thinned to 30 µm by diamond wheel grinding. Above all, the proposed THz modulator was fabricated. In addition, the phase properties of VO 2 was changed by controlling the temperature, an annular high temperature ceramic heating plate was used to control the temperature.
Characterization: Optical images were obtained using Digital Single Lens Reflex (D5600, Nikon). The surface morphology images were taken from scanning electron microscopy (SEM, SU8010, HITACHI) and atomic force microscope (AFM, Bruker Dimension ICON-PT). X ray diffractometer (XRD, X'Pert Pro) was used to test the crystal structure of VO 2 film. The THz transmission tests were carried out with a THz time domain spectrometer (CCT-1800, Huaxun Ark Co., Ltd).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.