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Abstract
Sandwich-type structure based on Salisbury screen effect is a simple and effective strategy to acquire high-performance terahertz (THz) absorption. The number of sandwich layer is the key factor that affects the absorption bandwidth and intensity of THz wave. Traditional metal/insulant/metal (M/I/M) absorber is difficult to construct multilayer structure because of low light transmittance of the surface metal film. Graphene exhibits huge advantages including broadband light absorption, low sheet resistance and high optical transparency, which are useful for high-quality THz absorber. In this work, we proposed a series of multilayer metal/PI/graphene (M/PI/G) absorber based on graphene Salisbury shielding. Numerical simulation and experimental demonstration were provided to explain the mechanism of graphene as resistive film for strong electric field. And it is important to improve the overall absorption performance of the absorber. In addition, the number of resonance peaks is found to increase by increasing the thickness of the dielectric layer in this experiment. The absorption broadband of our device is around 160%, greater than those previously reported THz absorber. Finally, this experiment successfully prepared the absorber on a polyethylene terephthalate (PET) substrate. The absorber has high practical feasibility and can be easily integrated with the semiconductor technology to make high efficient THz-oriented devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) frequency range (i.e., 0.1-10 THz) is considered to be one of the most important parts of the electromagnetic spectrum. THz technology with its strong penetrability, high safety, low energy, fingerprint characteristics, and other advantages has received increasing interest from the attention of the scientific community [1,2]. Particularly during the last decade, with the rapid development of THz applications in fields such as THz communication, THz imaging, THz detection and so on. THz-related electronic components have been widely applied to current technologies. THz absorber can be widely used in THz emitting/detecting devices to reduce the sidelobe radiation or other undesirable radiation, improve their transmission environment, and ensure that the delicate electronic instruments work as normal [3–5]. Another potential use is for radar cross-section (RCS) reduction, which can achieve effective stealth [6]. Moreover, THz stealth materials also play a very important role in the fields of national defense security and personal information protection, which may help to avoid significant losses to individuals and even countries. Therefore, high-performance THz absorbers have the potential for wide applications and prospects for development [7–10].
THz absorbing materials can be divided into several categories such as paint, sparse material, and metamaterials. However, paint is hindered by two main factors: the low viscosity and the lack of strong adhesion force to any substrate [11–17]. Moreover, this issue of environmental protection also needs to be taken into account. Sparse material, whose electromagnetic parameters can be approximately tuned to be those of free space, realizes broadband absorption. But, the sparse structure also has disadvantages such as low adhesion to the substrate and poor resistance to high temperatures [18–21]. The THz metamaterial absorbers consisted of a periodic resonant subwavelength structure on the top of a dielectric spacer layer and with a metallic backside reflector. A metamaterial absorber is capable of frequency-selective absorption in the multiband THz frequency range. However, when it comes to the practical application of quality control and processing design, the metamaterial structure faces the problems such as complex preparation, poor stability, and high cost [22–25]. Thus, most of the research on THz-absorbing metamaterials is based on theoretical simulations. Research-based on THz-absorbing metamaterials has been in urgent need of a breakthrough.
Another kind of THz absorber can be built using a Salisbury screen. The Salisbury screen was initially utilized to hide a target from radar in the microwave frequency ranges. This method was proposed in 1952 [26]. The classical Salisbury screen is composed of a metal resistor sheet, a dielectric spacer layer, and a metal base plate. In this structure, the front resistor sheet is placed at a quarter wavelength from the metal base plate. The absorbance of this kind of THz absorber under normal incidence conditions can be expressed as in the following equation:
As one-atom-thick two-dimensional (2D) material, graphene has attracted much attention with respect to its extraordinary electronic and optical properties such as ballistic transport [28,29]. Graphene, as a two-dimensional material with zero band gap, has very high carrier mobility. Compared with metallic copper and silver low resistance and good electrical conductivity, graphene is more suitable to be a component in precision instruments. Moreover, the poor adsorption properties of metal films are more prone to falling off and poor stability. The adsorption between metal and substrate is purely physical, while graphene can be physically adsorbed (van der Waals forces) plus chemically adsorbed (π chemical bonding) with the substrate. Metals that are too thin will have poor electrical conductivity and those too thick will have limited absorption properties for multilayer stacking. But graphene offers both nanoscale thickness and excellent electrical conductivity. It was found that the carrier concentration and conductivity of graphene can be controlled by electrostatic field tuning or by chemical modification [30,31]. Pu et al. proposed a high efficient electromagnetic absorber based on monolayer graphene [32]. Numerical simulations show that the absorption rate of THz waves can be dynamically modulated by tuning the graphene Fermi energy. Woo et al. proposed that graphene was utilized as a resistive sheet in a Salisbury screen to avoid the influence of nanometer scale disorders in conventional metallic films [26]. The sheet resistance of the graphene film was modified using a chemical doping method to enhance the performance of the absorbers.
In this work, single layer graphene (SLG) was grown by means of chemical vapor deposition (CVD) onto a copper substrate. Graphene as a resistive sheet in a graphene Salisbury shielded THz absorber. It is confirmed by a combination of simulation and experimental demonstration that 1. the number of resonance peaks can be increased by varying the thickness of the dielectric layer in a fixed frequency range. 2. Graphene has a significant absorption enhancement effect on the absorber with the modulation depth of absorbance can reach up to 37%. 3. In addition, we successfully prepared the absorber structure on a flexible PET substrate. Considering its simplicity, flexibility, lightweight and cost-effectiveness, the proposed absorber is the potential to be used for imaging, energy harvesting, and the detection of pesticides.
2. Experimental
2.1 Simulation
The schematic of the proposed THz absorber is illustrated in Fig. 3(c). The device consists of a graphene layer on top of a 25 µm thick PI matrix 200 nm thick Au layer locates underneath the PI, and is supported by the Si substrate. In the simulation, the metal structures were made of Au with a conductivity of 4.56 × 107 S m−1. The real and imaginary parts of the PI were set to $\varepsilon $ = 2.3 and $i$ = 0.098, respectively [30]. The parameter of graphene was described by the Kubo formula with a fixed relaxation time of 100 fs. The Fermi level of graphene was set to 100 meV in the simulation. The basic unit period of the absorber structure is set to 40 µm. It should be noted that, in the simulation, graphene was treated as a 2D material and described by its surface conductivity. Periodic boundary conditions are set in the x and y directions, and two Floquet ports are set in the z-direction, The electromagnetic wave vertical incidence excitation condition is set at port 1. The absorbance is defined as A = 1-R(ω)-T(ω), where the reflectance R(ω)= |S11|2 and the transmittance T(ω)=|S21|2. The S-parameters at different frequencies can be extracted directly from the simulation results. The transmission for this device is 0 as the reflecting layer (in this work, Au) prevents all THz waves from passing through this graphene sensor. So, the absorbance can be simplified to A(ω) = 1-R(ω).
2.2 Device preparation
Fabrication of Graphene absorber. Commercialized polyimide (PI) film with thicknesses 25 µm was used in this work. The tape was purchased from 6 Carbon Technology (Shenzhen, China). The SLG was purchased from XFNANO (Nanjing, China). During fabrication, the silicon wafer is used to support the designed absorber. Cr and Au’s films are deposited on the silicon wafer by magnetron sputtering, and Cr is coated on the wafer to increase the adhesion between the Au and the wafer. The Polydimethylsiloxane (PDMS) solution and curing agent (ratio 10:1) mixture were then spin-coated onto the Au surface at 6000 rpm for 60 s to obtain a PDMS thickness of 7.46 µm. The PI film is stuck on the PDMS surface. Furthermore, the PI/Au/Si substrate was placed into an oven at 80°C for 20 mins. The CVD copper-based SLG was transferred to the PI/Au/Si substrate surface as follows: (1) First, cut the SLG into suitable sizes. (2) A 4% mass fraction, Polymethyl Methacrylate (PMMA) film was spin-coated on the SLG surface. (3) Then the SLG was placed in copper sulfate solution (copper sulfate: hydrochloric acid: deionized water = 10 g:10 ml:50 ml) to completely etch off the copper substrate for about 30 mins. (4) The PMMA-supported SLG was rinsed in deionized water for 30 mins. (5) The PMMA-supported SLG was fished up with the PI/Au/Si substrate and dried naturally. (6) PMMA/SLG/PI/Au/Si structure was put into acetone to remove PMMA from the graphene surface.
2.3 Characterization
The number of CVD graphene layers was determined with a UV-Vis spectrophotometer. Raman spectroscopy of the graphene samples was performed by a Renishaw in spectrometer at 532 nm laser excitation. The presence of graphene on silicon dioxide/silicon substrates was confirmed by G-peaks, D-peaks and 2D-peaks. The fabricated samples were characterized by scanning electron microscopy (SEM), ZEISS SUPRA-55, to obtain the geometric parameters. A commercial THz time-domain spectrometer (Zomega-Z3) was employed to perform the reflection-mode measurements for the samples [33]. The test was performed at room temperature and the reflected signal of 200 nm Au on the silicon wafer was used as the reference signal.
3. Results and discussions
The fabrication procedure of a graphene based THz absorber operating in reflection mode is schematically shown in Fig. 1. The graphene sensor consisted of a graphene monolayer, a thin dielectric layer and Au back-reflector. We used 25 µm PI for the dielectric layer because it has the characteristics of heat resistance as well as chemical stability. By virtue of the semimetallic nature of graphene, this graphene−dielectric−metal sandwich structure satisfied the impedance matching requirement and provided a thin resonant cavity that trapped the incident THz wave [3].
The SLG has 2.3% absorption in the visible spectrum and the absorption rate will linearly increase with the number of layers [34]. In Fig. 2(a), the absorbance of SLG at 550 nm was 2.3%, in the experiments we tested. It indicates that the sample is a single-layer graphene film. Moreover, Raman spectroscopy enables fast and non-destructive detection of samples with accurate capture of all sources of information about carbon-based materials. The sample was found to be free of D-peaks. In Fig. 2(b), indicating that the structure is a highly ordered aligned lattice structure without the introduction of foreign impurities and defects. There is a low-intensity G peak at 1589 cm-1 and a high-intensity 2D peak at 2676 cm-1. In addition to I2D/IG = 2, the results are consistent with the tested in Fig. 2(a) indicating that the sample is the SLG.
To investigate the effect of the sheet resistance of graphene on the performance of the absorber, a schematic diagram of this experiment designing two different structures PI/Au and graphene/PI/Au is shown in Fig. 3(a) and Fig. 3(c). Firstly, the experiment simulates the PI/Au structure with a 50% resonance peak at 1.4 THz through numerical simulation, and a 35% resonance peak is measured at 1.5 THz in the experimental test, as shown in Fig. 3(b). In this case, the absorbing structure can be seen as a simple resonant cavity. In addition, this experiment simulates the graphene/PI/Au structure. The simulation results show that the structure has an absorption of 83% resonance peak at 1.4 THz, and a 72% resonance peak was measured at 1.46 THz in the experimental test, as shown in Fig. 3(e). The absorption mechanism can be drawn from the multilayer interference theory as shown in Fig. 3(d) [35]. In a graphene-based Salisbury screen, assuming negligible losses in the dielectric layer and almost complete reflection by the metallic plate, graphene can only absorb a small percentage of the incident wave. However, there will generate complete absorption if two waves come in opposite phase in a lossy sheet. In this simple structure, one wave is the incident wave and the second wave is the reflected wave from the metallic plate. To clearly explain the enhanced interaction between the THz wave and absorbers induced by the graphene, we simulated the THz electrical field distribution of the absorber with two different structures responding to PI/Au and graphene/PI/Au. The |Ex| distribution on the yoz plane of the two structures in the 1.4 THz frequency is illustrated in Fig. 3(f). It can be seen that without graphene, the field intensity in the air region varies very significantly due to the standing wave effect which indicates a strong reflection at this time. When graphene is embedded in the device, there is a strong electric field between the graphene and air interface, at the same time, the field intensity in the air region is distributed as a smoother profile, which indicates that the incident wave interferes with the reflected wave [1,36]. The absorption is stronger at this time. The introduction of graphene resulted in significant electric field enhancement and led to an increase in absorption from 35% to 72%. The redshift of the resonant frequency of absorber is resulted from the nonzero reactance of graphene.
Fig. 3. (a) Schematic diagram of PI /Au structure. (b) Simulation and experimental test results of PI /Au structure. (c) Schematic diagram of graphene/ PI /Au structure. (d) Multilayer interference theory diagram. (e) Simulation and experimental test results of graphene/ PI /Au structure. (f) Electric field distribution of both structures.
Next, to investigate the role of dielectric layer thickness variation in the Salisbury screen absorber, two different structural schematics of graphene/PI/PI/Au and graphene/PI/PI/PI/Au were experimentally designed as shown in Figs. 4(a) and 4(d). Through the numerical simulation and experimental testing of two different structures as shown in Figs. 4(b) and 4(e), it is found that the absorption peak shows a red shift with increasing PI thickness since the extended travel length of PI changed the impedance. This result is consistent with the work of Xu et al [37]. To clearly explain the enhanced interaction between the THz wave and PI, This experiment simulates the yoz plane |Ex| electric field distribution at different resonant frequency for both structures as shown in Figs. 4(c) and 4(f). When the thickness of PI is 50 µm, the electric field distribution at 0.68 THz is mainly concentrated at the separate surfaces of graphene and air, while the electric field distribution at 2.03 THz is concentrated at the interface between graphene and air and at the partition of the two dielectric layers. Similarly, when the thickness of PI is 75 µm, the electric field distribution at 0.45 THz is mainly concentrated at the separate surfaces of graphene and air, the electric field distribution at 1.35 THz is mainly concentrated at the interface between graphene and air and the boundaries of the bottom dielectric layer, while the electric field distribution at 2.21 THz is concentrated at the interface between graphene and air and the boundaries of the two dielectric layers. The electric field distribution at high frequency increases with the thickness of the dielectric layer increasing. The enhanced loss surface interaction is the main reason for the resonance peak absorption enhancement.
Fig. 4. The designed graphene/PI/PI/Au structure's (a) schematic picture (b) absorption curve of simulation and experimental test results (c) electric field distribution in yoz plane |Ex|. The designed graphene/PI/PI/PI/Au structure's (d) schematic picture (e) absorption curve of simulation and experimental test results (f) electric field distribution in yoz plane |Ex|.
In addition, as the frequency increases, the resonant electric field is not confined to a certain location of the absorber but spreads into the dielectric spacer layer. That is, several neighboring dielectric patches together support a resonant mode. This high-frequency multi-electric field coupling mode causes stronger absorption peaks. In principle, a large number of stacked waveguides may have the potential to incorporate many resonant modes and realize wave absorption with a frequency band as wide as possible [38].
The above results show that increasing the thickness of the PI dielectric layer can produce multi-frequency resonance, and graphene can significantly enhance the interaction with the incident THz wave, which can fuse multiple resonance to achieve the effect of broadband strong absorption. Further, we place the graphene at the same thickness dielectric layer partition, and the structure schematic is shown in Fig. 5(a) and Fig. 5(d). Firstly, the graphene/PI/graphene/PI/Au structure was demonstrated by simulations and experimental tests, and the results are shown in Fig. 5(b). Compared to the graphene/PI/PI/Au structure, increasing the number of layers of graphene still leads to an overall increase in absorbance, with shifts from 68% and 91% to 84% and 93%, respectively. Additionally, the resonance peak appears blueshift with the resonant frequency from 0.53 THz and 1.5 THz to 0.57 THz and 1.74 THz, respectively.
Fig. 5. The designed graphene/PI/graphene/PI/Au structure's (a) schematic picture (b) absorption curve of simulation and experimental test results (c) electric field distribution in yoz plane |Ex|. The designed graphene/PI/ graphene/PI/graphene/PI/Au structure's (d) schematic picture (e) absorption curve of simulation and experimental test results (f) electric field distribution in yoz plane |Ex|.
The analysis of the electric field strength at different resonant frequency in Fig. 5(c) reveals that the introduction of graphene leads to a significant enhancement of the electric field intensity at the resonant frequency, which improves the overall absorption performance of the structure. In the same way, we conduct simulations and experimental tests to verify the structure of graphene/PI/graphene/PI/graphene/PI/Au structure, and the results are shown in Fig. 5(e). In contrast to the test results of 5b, this absorbing structure multi-frequency resonance generates a strong coupling to achieve broadband absorption. The absorber structure has an absorption of more than 81% at 0.25 to 2.25 THz and a relative bandwidth of 160% with a maximum absorption of 100%. In this absorbing structure model, wideband absorption is mainly obtained by stitching the resonant absorption of a series of stacked waveguides in which the PI thickness provides multiple resonance peaks in the composite structure, and the introduction of graphene enhances the coupling between adjacent resonance peaks. Besides, the electrical resonance makes the absorber impedance match well with the free space, and the incident wave is absorbed with low reflection to produce a broadband absorption effect. The absorber structure has the advantages of simple structure, easy fabrication and high absorption strength compared with the pyramid structure [39].
A comprehensive review of the recent terahertz absorber literature indicated that the graphene-based Salisbury screen terahertz absorber is among the most efficient terahertz absorbers in the 0.2 ∼ 2.5 THz range known to date in Table 1. So far, most of the research has focused on metamaterials and Foam. However, absorbers based on graphene Salisbury screen possess not only both efficient absorption and simple preparation with broader universal applicability.
Table 1. Performance comparison of various types of THz absorbers
Today's electronic devices are increasingly becoming more flexible and portable. Based on this, this experiment also successfully prepared graphene-based THz absorbers on PET substrates as shown in Fig. 6(a). The measured result in Fig. 6(c) shows a strong absorption peak located near 1.3 THz, which agrees well with the numerical result in Fig. 3(b). The flexible graphene THz absorber can adhere well to various surfaces without atomic gaps compared to other structures. Figure 6(b) shows the graphene-absorbing structure under scanning electron microscopy. The graphene, dielectric layer, and metal substrate are all clearly captured.
Fig. 6. (a) Optical picture characterization of the absorber. (b) SEM image of the absorber structure. (c) Absorption spectra for a flexible graphene terahertz absorber on PET substrate.
4. Conclusion
Finally, this experiment produces THz absorbers based on graphene Salisbury screen. The experiments combined simulation and experimental tests to demonstrate that 1. Graphene in the absorber will enhance the strong coupling with the incident THz wave and improve the overall absorption performance (wider resonance peak, blue shift of resonance peak, and increased absorption rate); 2. The increase of the thickness of the PI dielectric layer can change the travel impedance of the absorbing structure and increase the number of resonance peaks. 3, The graphene-PI simple stacking design achieves 0.25∼2.25 THz absorption 81% broadband in the relative bandwidth of 160%, with a maximum absorption of 100 perfect absorbers. In addition, this experiment successfully constructed the absorbing structure on a PET flexible substrate, and the absorber can be arbitrarily bent and attached to various curved surfaces. With this simple fabrication via an efficient and low-cost fabrication method, the device can be commercialized with more potential.
Funding
National Natural Science Foundation of China (No. 52205609, No. 52275577); Key Research and Development Project Key Program of Shanxi Province (202102040201007); Research Project Supported by ShanXi Scholarship Council of China (2020-109); Natural Science Foundation of Shanxi Province (No. 20210302123056).
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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