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Abstract
This paper reports a multi-functional terahertz (THz) metamaterial based on a nano-imprinting method. The metamaterial is composed of four layers: 4 L resonant layer, dielectric layer, frequency selective layer, and dielectric layer. The 4 L resonant structure can achieve broadband absorption, while the frequency selective layer can achieve transmission of specific band. The nano-imprinting method combines electroplating of nickel mold and printing of silver nano-particle ink. Using this method, the multilayer metamaterial structures can be fabricated on ultrathin flexible substrates to achieve visible light transparency. For verification, a THz metamaterial with broadband absorption in low frequency and efficient transmission in high frequency is designed and printed. The sample’s thickness is about 200 µm and area is 65 × 65 mm2. Moreover, a fiber-based multi-mode terahertz time-domain spectroscopy system was built to test its transmission and reflection spectra. The results are consistent with the expectations.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metamaterials are artificial materials composed of subwavelength geometric structures, which have properties that natural materials do not have, such as high-resolution imaging [1,2], stealth [3,4], perfect absorption [5–7] and so on. Realization of multi-functional, large-area, flexible substrate metamaterials can open a door for new generation electromagnetic functional devices. For example, the metamaterial cloak [8] and carpet [9] combined with flexible substrates can realize curved surface stealth. Moreover, flexible substrate metamaterial biosensors [10] can be wrapped on any curved surface, which could be integrated with human skin for health monitoring. In recent years, flexible substrate metamaterials have achieved significant progresses, including wireless strain sensing metamaterials [11], terahertz (THz) polarization converter [12], polypropylene substrate-based metamaterials [13], flexible metasurfaces and metamaterials [14], wearable microwave meta-skin [15], dual-band band-stop flexible metamaterials filter [16], flexible microwave frequency selective metamaterials [17], transient and flexible hyperbolic metamaterials [18], flexible printed optical metamaterials [19] and so on. Among them, flexible substrate-based absorbers have received much attention due to their applications in spectroscopy and stealth. In 2011, Krzysztof Iwaszczuk et al. reported flexible terahertz metamaterial absorber for stealth applications [20]. In 2013, Riad Yahiaoui et al. reported an ultra-flexible multiband terahertz metamaterial absorber [21]. In 2019, H. Lin et al. demonstrated a 90-nm-thick graphene metamaterial absorber for broadband absorption [22]. In 2021, Y. Zhao et al. proposed an ultra-wideband and wide-angle optically transparent flexible metamaterial absorber [23].
However, these flexible substrate-based metamaterials usually have a single function, with only absorption function but no transmission function. In practical application, it may block the electromagnetic waves emitted by its own devices. For this reason, researchers have proposed and fabricated some multi-functional metamaterial absorbers with transmission windows. In 2011, X. Chen et al. presented a multi-layer metamaterial absorber which behave as a dielectric slab in transmission band and act as an absorber in another lower band [24]. In 2017, Q. Chen et al. proposed a frequency-selective metasurface with high in-band transmission at high frequency and wideband absorption at low frequency [25]. In 2018, S. Zhong et al. reported a multifunctional metamaterial composite structure that not only provides the broad-band radar and thermal infrared bi-stealth function but also possesses an in-band microwave transmission window and high optical transparency [26]. In 2021, H. Lin et al. proposed a dual-polarized bidirectional three-dimensional metamaterial absorber with transmission windows [27]. In the same year, K. Wen et al. presented a radar-infrared bi-stealth absorber that not only provides broad microwave absorptivity and low infrared emissivity but also possesses a microwave transmission window at low frequency [28].
However, due to the limitations of processing method, the above metamaterials are non-flexible, and the work frequencies are mostly in microwave band. With the development of communication and high-frequency radar, the working range of electromagnetic absorption devices has gradually developed from microwave to millimeter wave or even THz band, and the shape of devices has also begun to develop towards non-planar and flexible. Therefore, we propose a flexible substrate metamaterial device with both wide-band absorption and high-frequency transmission in the THz band, and report a multilayer structure fabrication method based on nano-imprinting. It is a material that can not only meet the electromagnetic wave absorption but also have a transmission channel. Its functional schematic is shown in Fig. 1(c). When the combat equipment covers the designed multifunctional absorbing material, on the one hand, it can effectively absorb the radar detection wave of the enemy to achieve stealth; on the other hand, it can detect the enemy equipment through the designed transmission channel, so as to obtain a dominant position in the war. For verification, the metamaterial sample is designed and printed. Its thickness is 200 µm and the area is 65 × 65 mm2. Moreover, the sample has flexibility and transparency in visible light.
Fig. 1. (a) The top layer structure of the metamaterials’ unit-cell; (b) 3D schematic diagram of the four-layer structure of the unit-cell; (c) multifunctional schematic diagram of transmission and absorption of the metamaterials.
2. Design and principle
The structure of the proposed transmission-absorption multi-function terahertz metamaterial is shown in Fig. 1(a) and Fig. 1(b). It is composed of four layers: “metal resonance layer - dielectric layer (UV glue) - frequency selective surface - dielectric layer (UV glue)”. The top layer is metal resonant layer composed of 4 L asymmetric structures, which can realize broadband absorption function of electromagnetic wave. The specific structural parameters are shown in the table in Fig. 1; the second layer is dielectric layer composed of UV glue with a thickness of 10µm. The UV glue has good flexibility, and its dielectric constant in the terahertz band is 3.05; the third layer is frequency selective surface composed of a silver square hole with a thickness of 2µm, and the opening size is 84µm. The structure realizes the bandpass function to form the transmission channel of electromagnetic wave; the bottom layer is UV glue with a thickness of 10µm. Through the combination of metal resonance layer and frequency selective surface, the metamaterial achieves broadband high absorption and high transmission. In the following, we will describe in detail how the broadband absorption and high transmission is achieved. We use the commercial electromagnetic simulation software CST 2016 for the simulation. In the simulation process, the boundary conditions are set to unit cell in x and y directions and open (add space) in z direction based on the floquet mode. The tetrahedral meshing and frequency domain solver based on finite element method are used for simulation. After establishing the metamaterial geometry model in the software, the background material is set to air and its dielectric constant is set to 1; the dielectric constant of UV glue is set to 3.05. Since we are using silver nano-particle ink, its conductivity after sintering is different from that of pure silver, we set the conductivity of the metallic structure of the metamaterial to 6.3 × 106 S/m [29].
In order to expand the absorbing bandwidth of the metamaterial and realize broadband absorption, the metal resonant layer is designed as 4 L ring asymmetric structures. The basic idea of the 4 L structures can be explained in such a simple way that multiple different “L” ring structures in one unit cell will lead to the coupling of multiple resonant frequencies, which effectively expands the bandwidth. Figure 2(a) shows the design flow of the 4 L asymmetric structures. We divide the metal square ring into four L sections and change the geometric size of each section to form 4 L asymmetric structures, and then combine these 4 L ring structures into a new unit cell. The schematic diagram of the 4 L asymmetric structure can be shown in Fig. 1(a).
Fig. 2. (a) Design flow of the 4 L asymmetric resonant structures; (b) schematic diagram of wideband absorption structure and absorption spectra; (c) schematic diagram of narrowband transmission structure; (d) transmission spectra of narrowband transmission structure.
In general, the structure of metamaterials is complex, and it is difficult to obtain their electromagnetic characteristics by analytical method. Considering that the size of metamaterial is much smaller than the wavelength, it can be regarded as an equivalent medium with complex dielectric constant ε and complex permeability µ. When the metal resonant layer is strongly coupled with the electric and magnetic fields, the equivalent impedance of the whole metamaterial Z = (µ/ε)1/2 is matched with the free space to minimize its reflectivity. The reflectivity and transmittivity of the metamaterial can be realized by simulating the complex frequency related S parameters S11 and S21 using the electromagnetic simulation software based on numerical simulation, and the absorption rate can be calculated by Eq. (1):
Accordingly, the absorptivity of the 4 L asymmetric structure metamaterial is calculated, and the result is shown in Fig. 2(b). There are four absorption peaks in the absorption curve, and each absorption peak corresponds to a separate sectional square ring, which also confirms the feasibility of using the 4 L asymmetric structure to improve the absorption. The single 4 L structure has been able to form a wide multi-peak absorption, and it needs to further shorten the distance of the peak to achieve broadband absorption.
The next step is to design a frequency selective surface, which is a periodic array of square holes on a silver layer that can achieve high transmission at a specific frequency. Figure 2(c) shows the schematic diagram of the frequency selective surface and Fig. 2(d) depicts its transmission spectrum. It can be seen that the frequency selective surface can achieve high efficiency transmission in specific frequency band. Then, by integrating the 4 L ring asymmetric structure and the frequency selective surface structure into the metamaterial as shown in Fig. 1(b), broadband absorption and high transmission windows can be achieved simultaneously as shown in Fig. 3(b). Accordingly, the frequency selective surface layer has a significant impact on absorptivity. Its function is to work with the 4 L resonant structure to form broadband absorption with higher absorptivity. The reason is that the frequency selective surface has lower conductivity than the complete metal surface. The lower conductivity will increase the absorptivity and make the four peaks close to each other, forming broadband absorption. In addition, it should be noted that after integration the transmission window will have a red shift because of the effect of UV glue on the frequency selective surface.
Fig. 3. (a) The structures’ electric field distribution and current distribution corresponding to the four absorption peaks; (b) absorption and transmission spectra of the transmission-absorption multi-function terahertz metamaterial; (c) the electric field distribution and current distribution corresponding to the transmission peak.
We analyze the absorption and transmission properties of the metamaterial. Figure 3(b) shows the corresponding spectra. There are four absorption peaks in the absorption spectrum, corresponding to the resonant frequencies of 0.83 THz, 0.87 THz, 0.89 THz and 0.92 THz, respectively. There is one transmission peak in the transmission spectrum, and the corresponding resonant frequency is 1.16THz. It shows that the metamaterial can achieve broadband absorption with absorptivity of more than 80% in the range of 0.82THz ∼ 0.93THz, and transmittivity of more than 80% at 1.16THz.
In order to further analyze the absorption and transmission mechanism of the proposed metamaterial, we simulated the electric field distribution and current distribution of five key frequency points, including four absorption frequency points (0.83 THz, 0.87 THz, 0.89 THz, 0.92 THz) and one transmission frequency point (1.16 THz), as shown in Fig. 3(a) and Fig. 3(c); Different colors are used to represent the intensity of electric field and current. Red represents strong electric field and current and blue represents weak electric field and current. According to the electric field and current distribution at 0.83 THz, the electric field are mainly distributed in ring 1 under the action of terahertz waves, indicating that a strong electrical resonance is generated at the ring 1. At the same time, both the metal resonance layer and the frequency selective surface generate induced currents, which are parallel and in opposite directions, thus forming a magnetic moment and leading to the generation of magnetic resonance, as shown in the 0.83THz current distribution. When the magnetic resonance of the metamaterial reaches the strongest, the energy of the terahertz wave will be consumed in a large amount. Under the joint action of electric resonance and magnetic resonance, the metamaterial absorber can realize the strong absorption of terahertz wave; Similarly, the ring 2, ring 3 and ring 4 respectively generate strong electric resonance and form strong magnetic resonance with the frequency selective surface at 0.87THz, 0.89THz and 0.92THz, thus achieving strong absorption of terahertz waves, as shown in Fig. 3(a).
Then the transmission mechanism is analyzed. According to the distribution of electric field and current at 1.16THz, the metamaterial has a weak electric field distribution and current distribution on the 4 L rings, leading to weak magnetic resonance, so the metamaterial has a low absorption at the transmission frequency point. However, the metamaterial has a strong electric field distribution and current distribution at the edge of the square hole, which indicates that the frequency selective surface has a strong electric resonance at 1.16THz and forms a transmissive window.
Considering that the electromagnetic waves are incident from various directions and the polarization angle also changes in practical applications, we studied the influence of the incident angle and the polarization angle on the absorption and transmission performance. Figure 4(a) and Fig. 4(b) show the effect of the incident angle on the absorption and transmission performance, respectively. The absorption performance does not change significantly and has good stability when the electromagnetic wave is incident at an angle from 0° to 30°. However, the absorptivity and absorption bandwidth will be reduced when the incident angle is greater than 30°. With the increase of the incident angle, the performance will gradually decline, and it is difficult to achieve the broadband absorption function. The transmission performance remains good at incidence angles from 0° to 20°, while the transmission peak shifts 0.06 THz towards lower frequencies as the incidence angle increases to 30°, but the overall transmittance remains above 73%. Therefore, the structure can maintain a good performance under a certain angle of incidence; Fig. 4(c) and Fig. 4(d) show the effect of the polarization angle on the absorption and transmission performance, respectively. Since the designed metal resonance layer is asymmetric, the polarization angle has some influence on the absorption performance. The absorption bandwidth decreases gradually when the polarization angle is changed, finally producing a strong absorption of 96% at 0.85 THz. However, the transmission performance is not affected by the change of polarization angle because the designed frequency selective surface has center symmetry.
Fig. 4. Effect of incident angle and polarization angle on transmission and absorption properties: (a) effect of incident angle on absorption; (b) effect of incident angle on transmission; (c) effect of polarization angle on absorption; (d) effect of polarization angle on transmission.
3. Fabrication and measurement
Ag nano-particle ink imprinting (Nano-imprinting) technology is proposed for the processing of designed metamaterials. Figure 5 illustrates the flow of nano-imprinting process, which starts with the spin-coating of UV glue (12 µm) onto a transparent PET substrate (200 µm). The specific printing process is as follows: (1) Cure the UV glue and prepare the Ni mold required for the imprinting process using electroplating; (2) Grooves are formed on UV glue by imprinting process, which are consistent with the microstructure of the metamaterial; (3) silver nano-particle ink was filled into the groove by the blading method in screen printing process and sintered at 150℃ for 15 min; (4) Clean and polish the samples in the previous step to complete the frequency selection of metasurface structure. The polishing process adopts silica solution polishing method. Place the silica solution (silica colloidal particle size is 50 nm) on the sample surface and make it rotate at high speed to realize the polishing of the surface; (5) Continue to spin-coat a layer of 12µm UV glue on the frequency selection surface and cure UV glue; (6) Use another designed Ni mold for imprinting to obtain the 4 L asymmetric structures; (7) Fill with Ag nano-particle ink and sinter at 150 °C for 15 min; (8) Polish and clean the sample to obtain the designed metamaterial.
Figure 6 depicts the sample prepared by the nano-imprinting process. Figure 6(a) is the 3D schematics diagram of periodic structure, Fig. 6(b) shows the curved sample and Fig. 6(c) shows the transparency of the sample. The sample has a size of 100 × 100 mm2, and a very thin thickness of 216 µm which is lightweight in practical applications. According to Fig. 6(c) that the proposed metamaterial has certain visible light transparency characteristic, so it can be applied to the stealth of transparent windows, which also greatly in-creases its application scenarios.
Fig. 6. The metamaterial sample prepared by nano-imprinting process: (a) schematic diagram of periodic structure; (b) sample in the bending state, the inset shows 2 × 2 unit cells observed by microscope; (c) the transparency of the sample.
Next, a fiber-based multi-mode terahertz time-domain spectroscopy system (THz-TDS) was constructed to measure the spectra of samples. The THz-TDS has two measurement modes, one is transmission measurement mode as shown in Fig. 7(a), and the other is reflection measurement mode as shown in Fig. 7(b). For the transmission measurement, the femtosecond laser enters the optical fiber after passing through the reflector, half wave plate and lens, and is divided into two beams by the optical fiber splitter. One beam as the pump light is incident to the terahertz antenna at the transmitting end to generate terahertz waves, and the other beam as the detecting light is incident to the terahertz antenna at the detecting end to detect terahertz pulses. The transmitting antenna radiates terahertz wave, which is collimated by the off-axis parabolic mirror, and then the radiation passes through the sample, focused by the second off-axis parabolic mirror, so that the receiving antenna receives terahertz signals. With this system, the transmissivity of samples can be measured.
Fig. 7. Schematic diagram of the fiber-based multi-mode terahertz time-domain spectroscopy system: (a) the transmittivity measurement mode; (b) the reflectivity measurement mode.
The optical fiber provides the flexibility to adjust the optical path, so the angle of the terahertz transmitter and receiver of the THz-TDS can be adjusted at will. The transmission measurement mode can be adjusted to the reflection measurement mode, as shown in Fig. 7(b). The terahertz wave collimated by the off-axis parabolic mirror is reflected by the sample. The reflected light is focused by the second off-axis parabolic mirror to make the receiving antenna receive the signal. Therefore, the reflectivity of samples can be measured and the absorbance can be obtained according to Formula (1).
In order to ensure the reliability of the data, there are two steps to measure the absorption spectrum. The first is to measure the reference mirror with near 100% reflectivity (gold reflector) as the reference spectrum Rref. The second is to measure the metamaterial sample as the signal spectrum Rsam. The absorption spectrum can be calculated by
The biggest challenge to be solved in the measurement is to calibrate the position and angle of the reference mirror and sample. Only when their positions and angles are completely consistent, can the radiation received by the terahertz antenna not shift during the two measurements. Since the terahertz waves are not visible, we use a calibration method with He-Ne laser for assisted alignment in the experiments. As shown in Fig. 7(b), we drilled a small hole in the center of the first off-axis parabolic mirror, and the collimated He-Ne laser was reflected by the sample through the small hole and then reflected by the second off-axis parabolic mirror to the terahertz receiving antenna. Because the He-Ne laser is visible, we can observe whether the reflected He-Ne laser illuminates the center of the terahertz antenna each time we change the sample to measure the spectrum. This ensures that the terahertz radiation received by the antenna will not shift.
Then, the absorption and transmission spectra of the metamaterial are measured. Figure 8(a) shows the transmission spectrum and Fig. 8(b) is the absorption spectrum. The measurement results show that the absorptivity of the metamaterial exceeds 90% from 0.83 THz to 0.92 THz, and the transmissivity reaches 80% at 1.15 THz. It is proved that the absorption transmission multifunction is effective. Compared with the measured results, the simulation results are consistent with the experiment in the measurement of transmissivity, but the measurement of absorption spectrum is broadened compared with the simulation results. We think that one reason is the errors in the nano-imprinting process. These errors come from two aspects: 1) The imprinting method will lead to slight bending and tilting of the metal structure edges and there are particle voids in the metal structure since it is filled with ink composed of silver nano-particles; 2) Because of the multi-layer structure, the first metal 4 L resonant structure and the third frequency-selective surface layer have alignment errors during the imprinting process. The above two factors result in more complex resonant structures, which introduce more resonant absorption peaks, thus widening the bandwidth. However, these above errors are difficult to simulate with electromagnetic simulation software, thus leading to differences between simulation and experiment.
Fig. 8. Measured results and simulation results of the metamaterial: (a) transmission spectra; (b) absorption spectra.
Moreover, we found that the UV glue is easy to absorb moisture in air. Due to our mistake, the samples were not placed in the drying cabinet during the storage process, resulting in moisture. Because water has a significant absorption effect on terahertz wave, the absorption spectrum measured in the experiment is higher and wider than the simulated one, and the transmission spectrum is lower and narrower than the simulated one. Although there are some differences, the experimental results are basically consistent with the simulation results.
Furthermore, the proposed structure has adaptability to other frequencies. For example, the structure can be expanded to a higher frequency band through size scaling. We simulate the structure after scaling it down by a factor of 10. In the simulation process, the conductivity of the silver is set to 6.3 × 107 S/m. Other conditions remain unchanged. Figure 9(a), 9(b) and 9(c) show the structure and size after scaling, and Fig. 9(d) shows the simulation results after scaling the structure. It can be seen that the proposed structure can be extended to other bands and achieve corresponding functions.
Fig. 9. (a) Top structure of the metamaterial; (b) 3D schematic of the four-layer structure of the metamaterial; (c) dimensions of the metamaterial unit; (d) absorption and transmission spectra of the metamaterial after scaling the structure.
4. Conclusion
In summary, we designed and prepared a flexible substrate transmission-absorption multi-functional terahertz metamaterial based on nano-imprinting method. Its transmission is 80% at 1.15THz, and its absorption is more than 90% at 0.83-0.92THz. The nano-imprinting method combines electroplating of nickel mold and printing of silver nano-particle ink. It has the advantages of simple process, low cost and large area preparation. Using the method, the multilayer metamaterial structures can be fabricated on ultrathin flexible substrates. The overall thickness of the metamaterial is 216µm, which has good flexibility and is easy to conform to the curved surface target. Furthermore, the metamaterial also has certain visible light transparency, which greatly expands its application range. Finally, a fiber-based multi-mode terahertz time-domain spectroscopy system was built to test its transmission and reflection spectra. The measured results were in good agreement with the simulated results. We believe that this multi-functional THz metamaterials and nano-imprinting method will have a wide prospect in the field of THz communication and stealth.
Acknowledgments
The authors would like to extend sincere gratitude towards the Nanofabrication facility in Suzhou Institute of Nanotech and Nano bionics (CAS), Nanjing Institute of Astronomical Optics & Technology, National Astronomical Observatories (CAS), and the respective reviewers for their help and support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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