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Abstract
The gate-tunable absorption properties of graphene make it suitable for terahertz (THz) absorbers. However, the realization of a graphene-based THz absorber faces challenges between the difficulty of patterning graphene for processing and the intrinsically low absorbance of graphene with the high electric field needed to change the conductivity of graphene. This report presents an electrically tunable graphene THz absorber where a single-layer graphene film and a gold reflective layer are separated by a polyimide (PI) dielectric layer to form an easily fabricated three-layer Salisbury screen structure. The carrier density of the graphene layer can be efficiently tuned by a small external electrical gating (-5V-5 V) with the assistance of an ion gel layer. The voltage modulation of the Fermi energy level (EF) of graphene was confirmed by Raman spectra, and the variation of the device absorbance was confirmed using a THz time-domain spectroscopy system (THz-TDS). The measurements show that the EF is adjusted in the range of 0-0.5 eV, and THz absorbance is adjusted in the range of 60%-99%. The absorber performs well under different curvatures, and the peak absorbance is all over 95%. We conducted further analysis of the absorber absorbance by varying the thickness of the PI dielectric layer, aiming to examine the correlation between the resonant frequency of the absorber and the dielectric layer thickness. Our research findings indicate that the proposed absorber holds significant potential for application in diverse fields such as communication, medicine, and sensing.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
THz absorbers dissipate incident THz energy into heat, resulting in an intense attenuation of reflected THz radiation [1–3]. In recent years, THz absorbers have received widespread attention due to their applications in radar stealth [4,5], imaging detection [6,7], biosensing [8,9], and medical care [10–12]. Efficient THz absorption has attracted a great interest. Conventional absorbers are designed as a classic “sandwich” structure with a metallic metamaterial structure at the top and a metallic reflector layer at the bottom, separated by a dielectric layer [13–18]. By adjusting the size of the metallic metamaterial structure, high absorbance at specific frequencies can be realized. Usually, most of the above structures are passive components, and the absorption performance of this device is also determined once it is prepared. The increasingly complex electromagnetic application environment requires an active tunable electromagnetic absorber [19], such as controlling the absorbance of the electromagnetic absorber to electromagnetic waves according to the changes in the electromagnetic environment, which significantly limits its application.
Recently, graphene has attracted much attention due to its extraordinary electrical and optical properties [20]. Graphene is well suited for use as a tunable THz absorber because its EF and carrier mobility can be extensively tuned by external stimuli and it strongly interacts with THz waves [21,22]. Currently, the tunable strategies to investigate graphene for THz absorbers include electrostatic control [23–25], chemical doping [26–30], and optical pumping [31,32]. Among them, electrostatic control has attracted utmost interest from researchers due to its continuously tunable advantages. Y. Jiang et al. proposed a THz absorber based on patterned graphene, which electrostatically modulates the graphene EF with PI as the dielectric layer, with a modulation voltage as high as 60 V [33]; R. Fates et al. proposed an electrostatic modulation of the graphene EF, which using 90 nm SiO2 as the gate dielectric layer with modulation voltages up to 50 V [34]. However, the above studies usually require the application of high-bias voltages. In recent years, effective modulation of graphene EF can be achieved with smaller gate voltages by using solid-state gel electrolytes instead of insulating layers such as PI and SiO2 [35]. J. Ge et al. proposed a nanolayer THz super surface with ion gel to effectively regulate the EF of graphene, which can realize the switching between transmission and absorption modes [36]. However, the practical applications are still easily limited because: (1) Most of the currently reported graphene THz absorbers based on ion gel modulation are metamaterial devices, and similar studies originate more from theoretical simulations and lack experimental validation due to the limitations of the preparation process. (2) They are usually prepared on hard substrate surfaces such as silicon and do not work well on some curved surfaces. Therefore, the development of non-patterned flexible tunable THz absorbers is of great significance to advance the application of THz.
In this work, we developed an easily fabricated flexible tunable THz absorber by employing a single-layer graphene, PI, and a gold reflective layer. The graphene/PI/Au structure can be regarded as a classical Salisbury screen, and the resonance condition at positive incidence can be written as [37]:
where ${\textrm{n}_{\textrm{Pl}}}$ and $\textrm{d}$ are the refractive index and thickness of the PI layer, respectively, and m is an integer. The EF of graphene can be effectively tuned by an external electrostatic field with the assistance of ion gel, and the electrostatic field-regulated EF of graphene was characterized using Raman spectroscopy. In addition, the absorbance of the absorber was simulated and measured using COMSOL Multiphysics software and a THz-TDS. The results show that using a bias voltage of less than 5 V produces a significant modulation effect on the absorber, and the peak absorbance can be continuously adjusted from 60% to 99%. The absorbance of samples with different curvatures is more satisfactory, with the peak absorbance exceeding 95%. We conducted further analysis of the absorber absorbance by varying the thickness of the PI dielectric layer, aiming to examine the correlation between the resonant frequency of the absorber and the dielectric layer thickness. Our structure can be generalized to“3D graphene” such as topological Dirac/Weyl semimetal, and our method can be used to design and fabricate similar graphene-based devices.2. Experimental section
2.1 Simulation
The schematic diagram of the designed flexible tunable graphene THz absorber is shown in Fig. 1(a), which consists of a PI support layer, a gold reflective layer, a PI dielectric layer, a Single-layer graphene, an ion gel, and an electrode. Among them, the gold reflection layer, the PI dielectric layer, and the graphene layer constitute the Salisbury screen. The EF of graphene can be continuously tuned by the ion gel layer. In the THz band, due to the semi-metallic properties of graphene and the highly reflective properties of Au, this graphene-PI-Au structure satisfies the impedance matching condition to trap the incident THz wave inside the thin resonant cavity. The structure has the properties of a horizontal waveguide, which can have a strong interaction with THz waves and realize near-unit absorption at the resonant frequency.
Fig. 1. (a) Schematic of flexible tunable graphene THz absorber. (The right-hand figure shows a cross-section of the XoZ plane of the schematic diagram.) (b) Mechanism of flexible tunable graphene THz absorber. (c) Transmittance of THz absorber. (d) Absorption curves of test results for THz absorber and PI-Au devices.
In this study, which was carried out using COMSOL Multiphysics software, the refractive index of the PI tape was set to 2.08 and the thickness was set to 50 µm. The absorbance of the proposed absorber can be defined as A(ω) = 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. Since the thickness of Au is greater than the skinning depth of THz waves, the THz wave does not pass through the sample, and the T(ω) = 0. The absorbance can be simplified to A(ω) = 1-R(ω). The Au layer was set to be an ideal electrical conductor during the simulation. During the simulation, transition boundary condition was used to describe graphene due to its nanoscale scale, and the thickness of graphene was set to 0.35 nm. The boundary conditions of the THz absorber are periodic in the x and y directions and have a perfectly matched layer in the Z direction.
The surface conductivity of graphene, including both intra-band and inter-band jumps, can be obtained from the Kubo formula [38]:
The relaxation time $\tau $ is defined as:$\tau = \frac{{\mu {E_F}}}{{e\nu _F^2}}$, where the graphene Fermi velocity ${\nu _F} = {10^6}m/s$.
2.2 Materials
The PI tape was purchased from 3 M (USA), poly (vinylidene fluoride)-hexafluoropropylene (P(VDFHFP)) and 1-ethyl-3-methylimidazolium bis(tri-fluoromethylsulfonyl) imide ([EMI][TFSA]) were purchased from Aladdin (USA), and chemical vapor deposition of copper-based single-layer graphene was purchased from Shenzhen Six Carbon Science and Technology (Shenzhen, China), and all the reagents were purchased and used directly without further purification.
2.3 Equipment
A THz-TDS was used to acquire broadband THz transmission and reflectance spectra with a signal-to-noise ratio of more than 40 dB. The system was purged with nitrogen to minimize the effect of water vapor before measurements, and the temperature of the THz test laboratory was maintained at 23 ± 1°C. A Raman spectrometer Renishaw was used to collect the Raman spectra of the samples, which were obtained under the conditions of 532 nm wavelength excitation and a 50× objective lens. The presence of graphene on SiO2/Si substrates was confirmed by G-peaks, and 2D-peaks. The number of layers, defects, and the doping of graphene can also be characterized. Imaging was performed using a scanning electron microscope (SEM) SUPRA-55 (ZEISS). SEM images were used to determine the cross-sectional morphology of the prepared absorber and the thickness of the ion gel layer.
2.4 Device preparation
The THz absorber preparation process is shown in Fig. 2. During the preparation process, silicon wafers are used to support the designed absorber and are removed after final packaging. The specific preparation process is as follows: (1) A commercial PI film is adhered to the surface of the wafer; (2) 10 nm Cr and 200 nm Au are deposited on the surface of the PI film by magnetron sputtering; (the presence of Cr increases the adhesion of the Au to the surface of the PI film) (3) PI tape with a thickness of 50 µm is adhered to the surface of the Au; (4) Commercially available CVD-grown single-layer graphene is transferred onto the PI tape surface through the assisted transfer of polymethyl methacrylate (PMMA) to the surface of the PI tape, and acetone was used to remove the PMMA from the graphene surface; (5) Ag electrode was prepared at the edge of the graphene; (6) An ion gel solution was obtained by dissolving the polymer P (VDF-HFP) and the ion liquid [EMI] [TFSA] in acetone. The mass ratio of polymer, ion liquid, and solvent acetone was 1:4:7. The solution was stirred at 50 °C for 2 hours to form a homogeneous solution. The solution was then rotationally coated on the graphene surface at a rate of 3000 rpm. The sample was baked at 70 °C for 24 h to remove the acetone solvent; (7) Ag electrode was prepared on an ion gel to form a pair of electrodes for a tunable THz absorber; (8) The supporting silicon wafers were removed, and the samples were encapsulated on the surface of different support materials (PCB boards and plastic tubes with different diameters).
3. Results and discussion
We investigated the characterizations of the THz absorber, and the transmission spectrum of the designed absorber is measured in Fig. 1(c), which shows that the absorber transmission is 0. This is due to the presence of a metal reflective layer at the bottom of the absorber with a thickness greater than the skinning depth of the THz wave, which blocks the transmission of the THz wave. Figure 1(d) measures and compares the THz absorbance spectra of the THz absorber and the PI-Au device. The absorbance of the PI-Au device is about 0.2 due to the absorption of the PI layer, and when the impedance (Z0) of the free space matches the impedance (Z) of the transparent THz absorber, the absorbance will reach 100% when the impedance matching requirement is perfectly matched.
The EF of graphene is an important factor affecting the surface conductivity, and the EF of graphene can usually be modulated by chemical doping, electrostatic modulation, and optical pumping. Here, we show the strategy of ion gel modulation of graphene EF. The ion gel modulation of the graphene EF mechanism is shown in Fig. 1(b). In the case of zero bias voltage (Vg = 0), the free cations and anions in the ion gel are randomly distributed. When an external bias voltage is applied to the top and bottom surfaces of the ion gel, the Ag electrodes generate an electric field and attractions with opposite charges from the gel, and the bottom surface of the ion gel undergoes electron transfer with graphene. Therefore, the EF of graphene can be effectively regulated by a small bias voltage (<5 V).
Next, we experimentally measured the typical Raman spectra of ion-gel modulated graphene under different electrically biased conditions using a Raman spectrometer, as shown in Fig. 3. Figure 3(a) shows typical Raman spectra of ion gel and ion gel-graphene, and we note that the red spectral line (ion gel-graphene) has two additional peaks compared to the black spectral line (ion gel), which are located at (1580 cm-1) and (2680 cm-1), respectively. These two peaks are typical characteristic peaks of graphene and are known as G peak (1580 cm-1) and 2D peak (2680 cm-1). The G peak contains the E2 g phonon mode at the Γ point of the Brillouin zone and the 2D peak involves transverse phonon scattering near the Brillouin zone k point. Electrostatic doping of graphene leads to shifts in the EF, which are usually reflected in the Raman spectra by changes in the frequency, intensity, and linewidth of the G and 2D peaks. Here, we focus on the change in the frequency of the G peak, which is the most significant way to reflect the level of graphene doping. As shown in Fig. 3(b), the G peak position of the ion gel-graphene layer on SiO2/Si substrate is extremely sensitive to the doping achieved by applying the bias voltage, with the G peak position ranging from 1585 cm-1 to 1603 cm-1 and the bias voltage ranging from -5 V to 5 V, and the peak position changes significantly. We noticed that the wave number of the G peak is not the minimum at 0 V bias, which is due to the P-type doping of graphene in the air. Under the bias conditions of -1.5 V and -2 V, the wave number of the G peak reaches a minimum value of 1585 cm-1, and it can be assumed that graphene reaches the electrically neutral point at this time, i.e., the EF of graphene is 0 eV. Under a positive bias voltage, as the bias voltage rises, the G peak moves to a higher wave number, which indicates the enhancement of P-type doping of graphene. Under negative bias voltage, the wave number of the G peak decreases and then increases as the bias voltage is increased, indicating that the smaller negative bias voltage acts to counteract the spontaneous P-type doping in air followed by N-type doping of graphene.
Fig. 3. (a) Typical Raman spectra of ion gel and ion gel-graphene on SiO2/Si substrates. (b) Raman shift of the G peak of graphene coated with ion gel on SiO2/Si substrate with bias voltage. (c) The SEM image of the cross section of the flexible tunable THz absorber. (d) Variation of EF of graphene coated with ion gel on SiO2/Si substrate with bias voltage. (The black curve is calculated from the graphene Raman G peak shift, and the red curve is derived from Eq. (5).)
Figure 3(c) shows a cross-sectional SEM image of the prepared flexible tunable THz absorber, in which the sample is divided into several layers, which are ion gel layer, graphene layer, PI dielectric layer, Au reflective layer, PI support layer, and flexible PCB in order from top to bottom. where the thickness of the ion gel layer is 15 µm, and the thickness of the PI dielectric layer is 50 µm. Usually, the EF of graphene satisfy the following relationship [34]:
where $\hbar $ is the reduced Planck constant, ${\nu _F} = {10^6}m/s$ is the Fermi-Dirac velocity, n is the carrier concentration: where ${C_\textrm{i}} = {\mathrm{{\cal E}}_\textrm{i}}/{t_\textrm{i}}$ is the ion gel layer capacitance, ${\mathrm{{\cal E}}_\textrm{i}}$ is the ion gel relative dielectric constant, and ${t_\textrm{i}}$ is the ion gel thickness.Figure 3(d) depicts the relationship between the bias voltage and the EF, with the red curve obtained from the above equation and the black curve derived from experimental measurements of the Raman G peak change, which is derived based on: (1) the G peak wave number reaches the minimum when the bias voltage is -1.5 V or -2 V, at which time the graphene EF is 0; (2) a 1 eV change in graphene EF corresponds to a shift of the G peak wave number of the Raman spectrum by 34.9 cm-1 [39,40]. The results show that under the -5V-5 V bias voltage, the G peak wave number moves 18 cm-1, corresponding to a change of 0.5 eV in the graphene EF. In terms of specific values, there are significant differences between the two, which may be due to (1) the existence of many approximations in the above equations and (2) the fact that some of the material parameters (e.g., ion gel layer dielectric constants and graphene Fermi velocities, etc.) brought into the equations differ from the actual values due to the limitations of the fabrication process. However, the trends of graphene EF changes for both responses show consistency.
We made experimental measurements of the absorbance of THz absorbers under different bias voltage conditions using a typical (THz-TDS) (Fig. 4(a)). Due to the presence of an Au reflective layer at the bottom of the absorber with a thickness greater than the skinning depth of the THz wave (Fig. 1(c)). Simply measure the reflectance of the sample using a reflective optical path, and then obtain the absorbance of the THz absorber by using the formula: A(ω) = 1-R(ω). The absorbance spectra at 0.3-1.3 THz are shown in Fig. 4(c). The resonant frequency fluctuates around 0.7 THz, the peak absorbance ranges from 0.6 to 0.99, and the bias voltage ranges from -5 V to 5 V, with significant absorbance variations. It is noteworthy that the curve with the lowest peak absorbance occurs at -2 V bias voltage, not 0 V, which is because graphene exhibits P-type doping in air. COMSOL Multiphysics is used to simulate the graphene EF in the range of 0-0.5 eV, and the results are shown in Fig. 4(e)-(f). The absorber resonance frequency is at 0.72 THz, and the absorber vertical electric field distribution shows strong absorption at the resonance frequency (Fig. 4(e)). As the EF increases, the absorbance of the THz absorber is enhanced from 0.6 to 0.99 (almost complete absorption), indicating that the impedance mismatch |Z- Z0| between free space (Z0) and the THz absorber (Z) becomes smaller to ∼0. The simulation results show a surprising agreement with the measurement results.
Fig. 4. (a) THz-TDS. (b) Planar tunable graphene THz absorber. (c) Absorbance spectra of the THz absorber at different bias voltages. (d) Measured peak absorbance as a function of bias voltage. (e) Simulated result of vertical electrical field distribution at 0.72 THz. (f) Absorbance spectra of the simulated THz absorber of graphene at different EF.
We further plotted the relationship between the peak absorbance and the bias voltage as shown in Fig. 4(d), where the THz peak absorptivity reaches a minimum (0.6) when the bias voltage is -2 V, indicating that the EF of the graphene is close to the charge neutral point. When the bias voltage is scanned from -2 V to 5 V, the absorptance of the absorber is enhanced from 0.6 to 0.99, and the graphene P-type doping is enhanced, showing the process of getting electrons. When the bias voltage is scanned from -2 V to -5 V, the absorbance of the absorber is enhanced from 0.6 to 0.8, with graphene N-type doping enhancement, showing the process of losing electrons. We also noticed that when the bias voltage reaches ±4 V, the peak absorbance remains unchanged when the bias voltage continues to increase, indicating that the electron transfer between the ion gel and graphene saturates under the bias voltage of ±4 V and the graphene EF cannot be further enhanced by this method. The THz-TDS test results and Raman test results show good agreement.
To better expand the application of the absorber, we tested the THz absorption performance of the samples under different curvature bends. We attached the samples to the outer surfaces of the walls of semicircular transparent plastic tubes with different diameters. The samples are shown in Fig. 5(a), and the absorbance of the test samples is shown in Fig. 5(c). The results show that the peak absorbance of the absorber under different curvatures under the effect of bias voltage modulation is more than 95%, and our prepared THz absorber shows good absorption performance under different curvatures, which can be applied to the surface of complex shaped objects.
Fig. 5. (a) THz absorbers adhered to the outer surface of the wall of a semi-circular transparent plastic tube of different diameters (diameters of 6 cm, 5 cm, 4 cm, and 3 cm, respectively). (b) THz absorbers with different PI dielectric layer thicknesses. (c) Absorbance spectra of flexible THz absorbers with different radii of curvature. (d) Absorbance spectra of THz absorbers with different PI dielectric layer thicknesses.
In addition, we have investigated the absorption performance of THz absorbers with different PI dielectric layer thicknesses. The samples are shown in Fig. 5(b), and the absorbance of the test samples is shown in Fig. 5(d). Based on the Eq. (1), the resonance frequency of the absorber exhibits an obvious redshift to the red when the PI thickness is gradually increased. In the range of 0.1-1.65 THz, and the resonance frequency of the absorber with a PI dielectric layer thickness of 50 µm is 0.72 THz, the resonance frequency of the absorber with a PI dielectric layer thickness of 100 µm is 0.36 THz and 1.08 THz, and the resonance frequency of the absorber with a PI dielectric layer thickness of 150 µm is 0.24 THz, 0.72 THz, and 1.20 THz. The results in Fig. 5(d) have the same trend as the above analysis but the peak positions are different. In the range of 0.1-1.65 THz, the number of absorption peaks of the three absorbers with different PI dielectric layer thicknesses is in perfect agreement with the theoretical analysis and reach the peak and valley at 1.65 THz at the same time. However, the absorber with a PI layer thickness of 50 µm has one complete absorption peak with a resonance frequency of 0.70 THz, the absorber with a PI layer thickness of 100 µm has two complete absorption peaks with a resonance frequency of 0.39 THz and 1.19 THz, respectively, and the absorber with a PI layer thickness of 150 µm has three complete absorption peaks with a resonance frequency of 0.26 THz, 0.80 THz and 1.39 THz, respectively. For the first resonance frequency, the test results can be in good agreement with the theoretical analysis, and for the second and third resonance frequencies, the test results are blue-shifted relative to the theoretical analysis by about 0.1 THz and 0.2 THz, respectively. We introduce a new correction method to calculate the resonance frequencies of the THz absorber with different thicknesses of the dielectric layer: for the resonance frequency obtained by the above equation and set to ${f_1}$ (Unit is THz), the corrected resonance frequency:
where d and m have the same meaning as in Eq. (1).4. Conclusion
In summary, we fabricated a flexible tunable THz absorption device. The Single-layer graphene and Au reflective layer are separated by a PI dielectric layer to form a Salisbury screen structure, and the graphene EF can be effectively tuned under the action of small external bias voltages (<5 V) through the assistance of ion gel, which can in turn enhance THz wave absorbance. The graphene EF changes are characterized by Raman spectroscopy and theoretical analysis with the EF tuning range of 0-0.5 eV. The THz wave absorbance enhancement is characterized by a combination of COMSOL Multiphysics software and THz-TDS, and the absorbance is enhanced from 60% to 99%. We also attach the THz absorber to the surface of objects with different curvature radii, and the results show that the peak absorbance of the absorber under different curvatures is more than 95% under the effect of bias voltage modulation. Our prepared THz absorber shows good absorbance performance under different curvatures, and it can be applied to the surface of objects with complex shapes. We also studied the absorber performance with different PI dielectric layer thicknesses and explored the relationship between the resonant frequency of the absorber and the thickness of the dielectric layer. The proposed flexible tunable THz absorber is helpful for applications on complex surfaces.
Funding
National Natural Science Foundation of China (52205609, 52275577); Key Research and Development Project Key Program of Shanxi Province (202102040201007); General project of Natural Science Foundation of Shanxi Province (20210302123056, 202203021223005); Shanxi Provincial Postgraduate Scientific Research Innovation Project (2023KY597); The 19th graduate science and technology project of North University of China (20231906).
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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