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Abstract
Multiband terahertz absorbers are essential photonic components for responding to, manipulating, and modulating terahertz waves. In this work, improved electric split resonant ring arrays are used to demonstrate multiband terahertz wave absorption. The proposed design strategy is simple, practical, and significant. Experiments and simulations reveal perfect absorption at 0.918 THz and 1.575 THz for the transverse magnetic (TM) polarization and at 0.581, 1.294, and 1.556 THz for the transverse electric (TE) polarization. In addition, the weak resonant peaks that occurred in the experiments in both polarization states have been verified by the simulations. Furthermore, five concentration gradients of 2, 4-dichlorophenoxyacetic acid solutions and six concentration gradients of chlorpyrifos have been detected using the absorber. The lowest detectable concentration that could be monitored was 0.1 ppm. The absorption, intensity, and frequency shift values for the different solution concentrations at the resonant peaks were analyzed. The highest linear regression coefficients were 0.9862 and 0.9565 for the TE and TM polarizations, respectively. This multi-band absorber was demonstrated to be highly efficient in detecting pesticides for food safety applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterial-based perfect absorbers have attracted considerable research interest and have seen rapid development in recent years because of their ability to obtain perfect or near-unity absorption and wide angles of incidence at specific frequencies. Another distinguishing property of metamaterial absorber is that the absorber structure can be designed to operate over a wide electromagnetic spectrum range from microwaves to the terahertz (THz) and optical bands [1–4]. Thanks to their flexibility and enhanced performance, metamaterial perfect absorbers have many potential applications, including use in photovoltaic cells, sensors, and spectroscopic detectors [5–8].
In particular, major efforts have been made to obtain perfect absorption in the THz range, where it is difficult to find natural materials that exhibit strong absorption. Research into THz metamaterial perfect absorbers is a highly active field because of the potential applications of these devices in biochemical sensing, nondestructive imaging, communication and homeland security [9–14]. To date, various types of resonant absorbers, including single-frequency [15–17], multiband [18–23], broadband absorbers [24–31], and optical tunable absorbers [32–34], which have been constructed using metallic metasurfaces-dielectric space-metal ground structure or doped semiconductor structure, have been demonstrated to operate at THz frequencies. It is expected that THz perfect absorbers will become available to handle the THz spectrum in all kinds of applications.
Here, a simple but efficient strategy is proposed to realize multiband absorbers for operation in the THz range by introducing a topological structure that is based on previous split ring resonators (SRRs). It is found that the improved device structure shows polarization sensitivity. For the transverse electric (TE) polarization, the device has three distinct absorption bands at 0.581, 1.294 and 1.556 THz, with absorptions of 73.57%, 89.16%, and 91.17%, respectively. For the transverse magnetic (TM) polarization, the proposed absorber remains highly absorptive, with values of 90.05% and 94.68% at 0.918 THz and 1.575 THz, respectively. A physical insight into the improved absorber performance can be provided based on interference theory. The experimental results show excellent agreement with the results from both interference theory and simulations. Furthermore, we demonstrate that our proposed device can operate as a THz sensor in pesticide detection application. The positive performance of the designed device in the sensor application may be because of the different sensor sensitivity responses of its different resonant modes.
2. Design, fabrication, and experiments
The metamaterial structure that was used in this study is based on previously presented electric response SRRs. Figure 1(a) shows a schematic depiction of the topological structure of the designed multi-band absorber, which consists of a square array. The unit cell, as shown in the inset of Fig. 1(d), is strongly coupled to a uniform electric field and consists of a triple-layer metal-dielectric-metal thin film laminate, in which only the top metal layer is patterned; the structure serves as an electric inductive-capacitive (LC) resonator. The ground metal layer acts as a reflective mirror to eliminate transmission. Coupling between the two metallic layers results in magnetic resonance, which is dependent on the dielectric constant of the dielectric layer, so the dielectric material that is placed between the metal layers plays an important role in manipulating the resonance conditions to provide perfect absorption. The inset in Fig. 1(d) shows the optimized parameters of the unit cell, which include the lattice constant of 120 μm, the line width w = 5 μm, and the gap h = 3 μm. The other dimensions of the cell are optimized and are given as a = 12 μm, c = 6 μm, d = 50 μm, e = 78 μm, f = 59 μm, and j = 22 μm. The two-layer metallization absorber was fabricated on a 500-μm-thick silicon substrate. A 0.3-μm-thick SiO2 layer was deposited on the silicon substrate by chemical vapor deposition. The top metallization layer (comprising a 30-nm-thick Cr adhesion layer and a 200-nm-thick Au layer) was patterned using standard positive lithography, metal evaporation, and ion beam etching techniques. The two-layer metallization absorber was fabricated on a 500-μm-thick silicon substrate. A 0.3-μm-thick SiO2 layer was deposited on the silicon substrate by chemical vapor deposition. The top metallization layer (comprising a 30-nm-thick Cr adhesion layer and a 200-nm-thick Au layer) was patterned using standard positive lithography, metal evaporation, and ion beam etching techniques. The polyimide dielectric layer (~8 μm thick) was deposited using the spin-coating method and soft cured in a vacuum oven filled with nitrogen for protection. The ground metallization layer (Ti 30 nm adhesion layer and 200 mm Au layer) was deposited using the same process that was used for the top layer. Figure 1(b) shows a photograph of the fabricated metamaterial sample. The experiment was carried out using reflection-mode THz time-domain spectroscopy (TDS) with a pair of emitting/detecting low-temperature (LT)-GaAs photoconductive antennas. The THz beam that is generated is focused on the sample at a 30° oblique angle of incidence. The total scanning duration was 54 ps and the spectrum resolution was 6 GHz. The radiated THz power is 16.4 μW. The experiments were performed entirely within a dry nitrogen chamber to avoid THz radiation absorption by water vapor. For comparison, numerical simulations were performed using the finite-element method based on the CST Microwave Studio.
3. Results and discussions
The absorber sample was examined experimentally via reflection-mode TDS (CCT-1800, China communication technology Co., Ltd, Shenzhen, China)). A THz wave from a photoconductive antenna was reflected from both the absorber sample and a reference Au mirror. The absorber measurements were characterized in the cases of the TE and TM polarizations, and the measured reflection characteristics of the THz waves from the sample and the reference mirror are shown in Fig. 1(d). The THz beam is incident at an angle of 30°. The measured reflection was used to calculate the experimental absorption A (ω).
Fig. 1 (a) Illustration of multiple-band THz metamaterial absorber. (b) Photograph of the metamaterial absorber. (c) Optical microscopy image of the absorber. (d) Reflection time-domain waveforms in the TE and TM polarization cases. Inset: Designed structure and geometric parameters of the unit cell.
Figure 2 shows the measured absorption amplitude as a function of frequency for both polarizations. In Fig. 2(a), two strong resonances can be observed at approximately 0.918 THz and 1.575 THz for the TM polarization, which correspond to absorption amplitudes of 90.05% and 94.68%, respectively. In addition, another two weak resonances occur at 0.581 THz and 1.294 THz. In the TE polarization case, as shown in Fig. 2(b), the obvious resonant absorption peaks occur at 0.581 THz, 1.294 THz, and 1.556 THz, with amplitudes of 73.57%, 89.16%, and 91.17%, respectively. Interestingly, a weak resonance also is observed at 0.918 THz.
Fig. 2 Experimentally measured absorption spectra of samples for (a) TM polarization, and (b) TE polarization. Simulated absorption spectra of absorbers for (c) TM polarization, and (d) TE polarization.
To aid in understanding of the measured results, we performed full-wave electromagnetic simulations to explain the resonant absorption behaviors. The absorption A (ω) of each of the samples can be calculated based on the simulated reflection coefficient S11(ω) and the transmission coefficient S21(ω) using No transmission is detected because of the bottom metallic layer of the structure. Therefore, the absorbance A (ω) is equal to 1−2 for the designed structure. From a practical case viewpoint, the absorbers were often operated with electromagnetic waves at various angles of incidence (θ) and in various polarization states (ϕ). Therefore, the simulations of the absorption as a function of frequency are performed at angles of incidence of 0° and 30° for the TE and TM polarizations. Figure 2(c) shows for the TM case that the absorption spectrum has two distinct peaks, located at 0.922 THz and 1.589 THz in the cases of polarization angles of 0° and 15°, and these peaks correspond to absorption values of 99.0% and 99.6% at ϕ = 15°. For the TE polarization, simulated absorption is demonstrated in Fig. 2(d). At a polarization angle of 0°, the absorption values at the oblique angle of incidence of 30° are 77.7%, 91.7%, and 94.4% at 0.588, 1.28, and 1.559 THz, respectively.
The absorption results in Figs. 2(a) and 2(c) and Figs. 2(b) and 2(d) show the excellent agreement between the experimental and simulation data for both polarizations, apart from some slight deviations from the nonresonant positions as a result of fabrication tolerances and measurement errors. When compared with the results for ϕ = 0° and ϕ = 15° that are shown in Fig. 2(c), the fabricated sample has two weak resonant absorption responses (as shown in Fig. 2(a)) at 0.581 THz and 1.294 THz, which arise from the response of the electric field component Ex to the incident THz wave at a polarization angle of 15°. Similarly, the weak resonant response at 0.918 THz that is plotted in Fig. 2(b) results from excitation of the electric field component Ey of the incident THz wave, which can be demonstrated using the simulated results shown in Fig. 2 (d). Comparison of the experimental data to the simulation results shown in Fig. 2 indicates that for the TE polarization, the first absorption maximum has shifted by 7 GHz, while the second and third absorption maxima have shifted by 14 GHz and 3 GHz, respectively. For the TM polarization case, the two strong resonance peaks have shifted by 4 GHz and 14 GHz. The absorption amplitudes in the experiments are lower than those in the simulations. These differences may be caused by composite factors composed of metallic structure rounding, the optical alignment of the sample, reduced conductivity in the metallization, and higher dielectric losses. Furthermore, a physical insight into the improved absorber performance can be provided based on interference theory. The calculated absorption spectra based on interference theory (not shown here) is in excellent agreement well with results of full wave electromagnetic simulations. The destructive interference of multi-reflection THz wave leads to the perfect absorption at specific resonant peaks [16].
From the absorption maps presented in Fig. 3, it can be seen that the absorption performance can be maintained at more than 85% as θ increases from 0° to 50°. Furthermore, at both 1.28 THz and 1.559 THz, the absorber exhibits better absorption characteristics at larger angles of incidence for the TE polarization. For the TM polarization, the absorption amplitudes are all greater than 90% for angles of incidence of up to 65°. In addition, more resonant peaks can be observed in the absorption spectra because of excitation of the higher resonant modes.
Fig. 3 Absorption maps at oblique incidence for THz waves in the cases of (a) TE polarization, and (b) TM polarization.
To provide a better understanding of the triple-band resonant absorption mechanism, a straightforward and intuitive method is proposed for analysis of the electric field distribution from the absorption peaks on the x-z plane of the absorber. Figure 4 shows the resonant modes at the different absorption peaks for the TE polarization. The resonances that are associated with the LC resonant response are observed in the gap for the three resonant peaks. Figure 4(a) presents the electric field distribution that is associated with A(ω) = 0.777 at 0.588 THz. The Fig. 4(a) clearly shows that the electric field is mainly concentrated at the outmost side. Figure 4(b) shows the electric field intensity distribution that was extracted from the resonant peak at 1.28 THz. The electric field is distributed over the two sides of the short edge. Note here that the electric field distribution has transferred from the outside to the inside. At 1.559 THz, the electric field is mostly localized inside the ‘I’ type structure.
Fig. 4 Electric field distributions at different resonant frequencies. (a) 0.588 THz, (b) 1.28 THz, and (c) 1.559 THz.
Here, to realize the simple sensor properties of the device [35], we performed absorption after coating the sample with 2, 4-dichlorophenoxyacetic acid (2, 4-D) solutions using five different concentrations of 0.1 ppm, 1 ppm, 5ppm,10 ppm, and 15 ppm. 2, 4-D is a plant growth regulator that offers high selectivity, high efficiency and systemic activity. It has been widely used as a pesticide and herbicide to inhibit weed growth, as an auxin analog to induce callus formation, and as a preservative to prolong the storage periods of agricultural products [36,37]. 2, 4-D (purity quotient >99.9%; purchased from Mumu Bio-technology Co., China) solutions were prepared by mixing 2, 4-D crystals into a petroleum ether solvent. The absorption spectra of five different 2, 4-D concentrations (0.5, 1, 5, 10, and 15 ppm), as shown in Fig. 5, were measured by TDS as a function of frequency in the TE polarization case. The resonances at approximately 0.53 THz, 0.85 THz, 1.15 THz, and 1.38 THz shifted to lower frequencies when compared with the corresponding absorption peaks of the samples without the 2, 4-D solutions. The maximum shift of approximately 192 GHz was obtained at 1.556 THz. Absorption amplitude changes can be clearly observed for the different 2, 4-D concentrations. These results showed that our designed metamaterial absorber could be used as a high-sensitivity sensing device.
Fig. 5 Experimentally measured absorption spectra for different 2,4-D concentrations at four resonant frequencies from (a) to (d) in the TE polarization case. (e)–(h) Regression coefficients corresponding to the spectra in (a)–(d), respectively. Insets: Schematics of as-used THz sensor and an illustration of the reflection measurement setup.
Furthermore, in the four resonant peaks shown in Figs. 5(a)–(d), it was noted that the maximum shift among the four resonant peaks was observed for the 15 ppm 2, 4-D solution in the current experimental configuration. As shown in Figs. 5(e)–(h), the resonant absorption shift can be approximated as a linear response that decreases with increasing 2, 4-D solution concentration. The resonant peaks at approximately 0.83 THz obtained a good regression coefficient of R2 = 0.9862. No obvious peak shifts were observed between 0.5 and 1 ppm. One possible reason for such a result may be the minimal concentration of 2, 4-D present in the solution. Within this concentration range, the changes in the value of the 2, 4-D content have no influence on the dielectric parameters of the solution. The changes in the resonance peaks are largely determined by the thickness, volume and dielectric properties of the analyte on the sample. Therefore, it can be inferred that the shift in the resonant frequency can be attributed to the effect of the petroleum ether solvent on the absorption peaks. To satisfy the needs of practical applications, a series of 2, 4-D solutions with varying concentrations must be prepared and measured as part of our future work. In addition, the selectivity and sensitivity of the device could be improved further by designing the metamaterial absorber to match the intrinsic resonant properties of the concerned analyte.
To provide a comprehensive analysis of the sensing performance, the absorption of the device was also tested after coating the sample with chlorpyrifos [O, O-Diethyl-O-(3,5,6-trichloro-2-pyridyl) phosphorothioate] with concentrations varying from 0.1 ppm to 100 ppm (0.1, 1, 10, 20, 70, and 100 ppm) in the TM polarization case. Chlorpyrifos, a broad-spectrum organophosphate insecticide, is widely used commercial insecticides in the cultivation of wheat, corn, rice, tea, vegetable, and fruit trees [38]. High-purity chlorpyrifos, with an analytical grade of >99.4%, was purchased from National Standard Reference Materials Network (China) and was used without further purification. The amplitudes of all resonant peaks obviously increase as the concentration of chlorpyrifos increases, as shown in Figs. 6(a)-(c). When compared with the results shown in Fig. 2(a), the first, second, and third resonant peaks of the absorber with chlorpyrifos are redshifted to lower frequencies. Remarkably, a very minimal concentration of 0.1 ppm has been detected by the designed device. Figures 6(a)-(c) show that the absorption at approximately 0.576 THz increases by 3.45% as the chlorpyrifos concentration increases from 10 ppm to 20 ppm, and by 6.25% when the concentration increases from 20 ppm to 70 ppm. At approximately 0.91 THz, the absorption increased by 1.23%, corresponding to an increase in the concentration from 10 ppm to 20 ppm, and by 1.64% as the concentration increased from 20 ppm to 70 ppm. Obviously, the sensitivity of the device at the first resonant peak is higher than that at the second and third resonant peaks for the same increase in concentration. At 0.576 THz [Fig. 6(d)], the maximum regression coefficient of R2 = 0.9565 was achieved. Figures 6(e) and 6(f) also show good results, as the regression coefficients are more than 80% at the second and third peaks. In order to verify the stability of our THz absorption sensor, the repeatability performance measurement has been carried out, as shown in Fig. 7. It is noted that the THz sensor exhibits the good repeatability in detecting chlorpyrifos with the concentration of 1 ppm and 100 ppm.
Fig. 6 Experimentally measured absorption spectra of different chlorpyrifos concentrations at three resonant frequencies from (a) to (c) in the TM polarization case. (d)–(f) Regression coefficients corresponding to (a)–(c), respectively.
Fig. 7 Ten repeat measurements for detecting chlorpyrifos with the concentration of (a) 1 ppm, and (b) 100 ppm.
In essence, our multiple-band absorber indicates potential for use as a pesticide concentration-selectable detector platform in which detection is based on straightforward reflection measurements. Multiple resonant peaks provide greater convenience for selection of the best matching in the absorption of the pesticide concentration. This advantage will benefit the development of simple, low-cost detectors for food safety and biomedical diagnostics applications.
4. Conclusion
We have developed an efficient strategy to construct multi-band THz absorber devices based on simple electric resonant SRRs. The anisotropic absorption properties of the proposed devices have been demonstrated experimentally using a THz TDS system. Two strong resonances can be observed at approximately 0.918 THz and 1.575 THz for the TM polarization, corresponding to absorption amplitudes of 90.05% and 94.68%. In the TE polarization case, obvious resonant absorption peaks occur at 0.581 THz, 1.294 THz, and 1.556 THz with amplitudes of 73.57%, 89.16%, and 91.17%, respectively. Remarkably, as an example, we demonstrate that our device can operate as a high-sensitivity sensor to detect different concentrations of a pesticide. The multiband strategy presented here has profound significance for the development of low-cost sensors to detect pesticide residues.
Funding
National Natural Science Foundation of China (NSFC) (61201075, 11704310, and 61575158); Project of High Talent Level of Xi’an University of Technology.
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