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Abstract
Terahertz sensing of highly absorptive aqueous solutions remains challenging due to strong absorption of water in the terahertz regime. Here, we experimentally demonstrate a cost-effective metamaterial-based sensor integrated with terahertz time-domain spectroscopy for highly absorptive water-methanol mixture sensing. This metamaterial has simple asymmetric wire structures that support multiple resonances including a fundamental Fano resonance and higher order dipolar resonance in the terahertz regime. Both the resonance modes have strong intensity in the transmission spectra which we exploit for detection of the highly absorptive water-methanol mixtures. The experimentally characterized sensitivities of the Fano and dipole resonances for the water-methanol mixtures are found to be 160 and 305 GHz/RIU, respectively. This method provides a robust route for metamaterial-assisted terahertz sensing of highly absorptive chemical and biochemical materials with multiple resonances and high accuracy.
© 2017 Optical Society of America
1. Introduction
Metamaterials are artificially engineered materials with periodically arranged, subwavelength structures and exhibit unique electromagnetic properties that are unavailable among the naturally existing materials [1–7]. Due to strong localization and enhancement of the electromagnetic fields, metamaterials exhibit strong optical response towards the presence of an analyte. Such a response enables material sensing with metamaterials in different spectral range, extending from microwaves to optics [8–15].
Recently, metamaterial-assisted sensing platform integrated with terahertz spectroscopy has attracted a lot of interests and is being increasingly implemented for chemical and biological sensing [11,12,16–30]. The key advantages of this sensing platform include high sensitivity, real-time, and label-free detection. However, at terahertz frequencies, owing to the strong absorption of polar liquids, such as water, most of these studies have typically been limited to dry or partially hydrated specimens. Very few reports on direct liquid sensing based on terahertz metamaterials have been presented [11,17,20,22,23,27]. Since most of the functionalities of chemical and biological materials are expressed in water, the key solvent of most biological substances, therefore, it is important to realize readily available and cost-effective sensing platforms for water-based real biological systems.
In this letter, we demonstrate a cost-effective metamaterial sensor with a simple metallic structure but strong and sensitive multi-modal resonance for highly absorptive water-methanol mixtures. The metamaterial sensor that consists of asymmetric dual-wire arrays offers the ease of fabrication and exhibits multi-modal resonances, including a Fano resonance dip, a dipole resonance dip, and a Fano transmittance peak between the two dip resonances due to structural symmetry breaking. Predominantly, both resonances have strong intensity and are able to sense highly absorptive solutions. Sensing accuracy can be improved by detection of frequency shifts at multiple resonances. In addition, we use free-standing, transparent and thin Mylar substrate with a total thickness of 22 μm. The thin substrate with lower dielectric constant has lower influence on the capacitance of the resonators, thus enhancing the sensitivity [31–35]. We demonstrate that the proposed metamaterial sensor combined with terahertz time-domain spectroscopy (THz-TDS) could be effectively utilized in terahertz sensing of water-methanol mixtures with the capability to identify solvent type and determine the corresponding concentrations. This work would motivate metamaterial-assisted terahertz sensing for chemical and biological substances in highly absorptive aqueous systems.
2. Metamaterial structure and simulation
Figure 1(a) shows the microscopic image of the proposed asymmetric dual-wire resonator (ADWR) with the inset showing the unit cell schematic. The unit structure is composed of two aluminum (Al) wires placed in parallel on the Mylar substrate. The detailed structural parameters are as follows: Px = 80 μm, Py = 126 μm, L1 = 60 μm, L2 = 106 μm, d = 20 μm, and ω = 6 μm. The thicknesses of the Al and Mylar films are 200 nm and 22 μm, respectively.
Fig. 1 (a) Microscopic image of the ADWR with the inset showing the schematic unit structure where Px = 80 μm, Py = 126 μm, L1 = 60 μm, L2 = 106 μm, d = 20 μm, and ω = 6 μm. (b) Schematic diagram of liquid sample measurement by ADWR combined with THz-TDS.
Numerical simulation of the ADWR was carried out using CST Microwave Studio frequency domain solver with tetrahedral mesh [36]. The Mylar substrate was modeled as a lossless dielectric with dielectric permittivity ε = 2.89 and the DC conductivity σ of Al was set to be 3.56 × 107 S/m.
3. Experimental
The ADWR was fabricated using the following processes: the dual-wire structures were first patterned on Mylar substrate by conventional photolithography using positive photoresist. A 200-nm-thick Al film was then thermally deposited and followed by lift-off process.
Double-distilled water (home-made) and methanol with a purity higher than 99.9% (Pharmco-AAper) were used in this work without further purification. The water-methanol mixtures were prepared with 0%, 30%, 50%, 70%, 100% (v/v %) water volume percentage.
The ADWR and water-methanol mixtures were measured by a traditional photoconductive switch-based THz-TDS system [20,37]. Figure 1(b) shows the schematic diagram of liquid sample measurement using ADWR-assisted THz-TDS. The ADWR was placed in a quartz cuvette composed of two parallel, 60-μm-spaced, and 1-mm-thick windows with the side of the Mylar substrate closely attached to one of the quartz windows. The liquid sample was then injected into the cuvette. The cuvette was sealed and placed midway between the transmitter and receiver in the focused beam of the THz-TDS system. The polarization of the incident terahertz field was parallel to the wires. An identical empty quartz cuvette was used as the reference. The amplitude transmission t(ω) was obtained by the following equation:
where and are the fast Fourier transformed (FFT) transmitted electric fields of the sample and the reference pulses, respectively, ω is the angular frequency. The average of three repeated transmission measurements was used for further analysis.4. Discussion
4.1 Analysis of simulation results
Figure 2(a) presents the simulated amplitude transmission of the ADWR and the two individual wire structures with L1 = 60 μm, L2 = 0 μm, and L1 = 0 μm, and L2 = 106 μm. The two individual bar structures resonated at two different frequencies due to their unequal wire lengths. The short wire resonated at a higher frequency, f = 1.695 THz, while the longer wire resonated at a lower frequency, f = 1.068 THz. However, once the two bars were placed together, the resonance in the coupled ADWR exhibited multi-modal resonances including an asymmetric resonance dip at 1.038 THz, a transmittance peak at 1.269 THz, and a symmetric resonance dip at 1.734 THz. It should be noted that the two resonance dips happened to be located near the two resonance dips of the individual wire arrays. The insets of Fig. 2(a) present the surface current distributions at the two resonance dips of the coupled ADWR. We observed Fano-like antiparallel currents at 1.038 THz and a dipole-like parallel currents at 1.734 THz.
Fig. 2 Simulated transmission of (a) the ADWR with L1 = 60 μm, L2 = 106 μm (red solid circle) and the two individual wire structures with L1 = 60 μm, L2 = 0 μm (black triangle) and L1 = 0 μm, L2 = 106 μm (blue square) with the insets depicting the surface current distributions of the ADWR at 1.038 and 1.734 THz, and (b) DWRs with different asymmetries where L2 was increased from 60 to 120 μm, but a constant L1 = 60 μm.
In order to understand the mechanism of these multi-modal resonances especially the Fano resonance, we investigated the transmission responses of dual-wire resonators (DWRs) with different asymmetries. Figure 2(b) shows the simulated amplitude transmission of the DWRs, including one perfectly symmetric dual-wire resonator (SDWR) with L1 = L2 = 60 μm and five ADWRs with L2 increasing from 60 to 120 μm, but a constant L1 = 60 μm. The other parameters of these DWRs remain unchanged as Px = 80 μm, Py = 126 μm, d = 20 μm, and ω = 6 μm. For the SDWR, there was only a single resonance at 1.824 THz observed in the transmission. However, when the two wires had different lengths, that is, the structural asymmetry was introduced, multi-modal resonances would appear. In addition, a gradual red shift was observed for both resonances, and the Fano resonances became significantly broader but stronger with the increase of the asymmetry. These phenomena could be contributed to the destructive interference between the two wires caused by symmetry breaking [38–41]. In the symmetric system, there was no destructive interference between the two wires, and hence only one broad dipole resonance in the transmission spectra is seen. However, once the structural symmetry is broken in the unit cell with two unequal lengths of wires, the destructive coupling at lower frequency leads to antiparallel surface current distribution that gives rise to the sharp Fano resonance. The surface current distribution at higher frequency is parallel in nature thus leads to dipolar resonance mode, as shown in the inset of Fig. 2(a). When the length difference between the two wires was increased, the Q factor of the Fano resonance dip was declined with increasing resonance intensity, as shown in Fig. 2(b).
We further investigated the sensitivity of the Fano and dipole resonance dips in the ADWRs with varying L2 shown in Fig. 2(b) using rigorous simulations. The shifts in the resonance frequency at 4 μm analyte thickness and varying refractive index from n = 1.0 to n = 1.6 with an incremental step of 0.2 were calculated. Table 1 lists the simulated Fano and dipole resonances sensitivities in terms of change in resonance frequency per refractive index unit (RIU) of the analyte and Fano resonance intensities of the ADWRs. The Fano resonance intensity here was defined as the power transmission (the square of the amplitude transmission) difference between the Fano resonance dip and the Fano transmittance peak.
Table 1. Summary of the simulated Fano and dipole resonances sensitivity (unit: GHz/RIU) and Fano intensity of the ADWRs with varying L2 (unit: μm).
When the length difference between the two wires was increased with the increasing L2, the Fano resonance sensitivity was gradually decreased from 268 to 161 GHz/RIU, while the dipole resonance sensitivity was gradually increased from 200 to 258 GHz/RIU and even higher than the Fano resonance sensitivity when L2 exceeded 80 μm, as shown in Table 1.
In a typical Fano resonance, the resonance linewidth is narrower than dipolar resonances. However, sharp Fano resonances also suffer from the problem of having very weak resonance intensities [41]. Since the absorption of the analyte in our case was extremely large, a weakly excited Fano resonance would not be enough to sense the absorptive analytes. Therefore, in order to enhance the intensity of the Fano resonance, we increased the asymmetry parameter in the structure which lead to large broadening of the Fano resonance along with simultaneous enhancement in the resonance intensity. The broadening of Fano resonance even exceeded the linewidth of the dipolar resonance which explains the higher sensitivity of the dipolar resonance, as shown in Table 1. Though the Fano resonance sensitivities of the ADWRs with L2 below 106 μm were higher, their intensities were below 0.8. Therefore, we selected the ADWR with L1 = 60 μm and L2 = 106 μm for further experimental study of highly absorptive water-methanol mixtures. From our observation of the simulated electric fields, the Fano resonance of the selected ADWR shows super-radiative excitation of fields and the higher frequency dipolar resonance shows the sub-radiative fields where the longer metallic bar almost remains unexcited and the intense electric fields are confined on the smaller metal bar, as shown in Fig. 3. Therefore, the dipole resonance here has narrower resonance linewidth and higher sensitivity than Fano.
Fig. 3 Simulated electric field distribution of ADWR with L1 = 60 μm and L2 = 106 μm at (a) 1.038 THz, Fano resonance dip and (b) 1.734 THz, dipole resonance dip without analyte coated on the metasurface.
We also investigated the single dipole resonance sensitivity for single wire structures with varying wire length L increased from 60 to 120 μm. The single dipole resonance sensitivity was gradually decreased from 277 to 161 GHz/RIU with increasing L. Even though the highest dipole resonance sensitivity of single wire structure (277 GHz/RIU) was a little higher than the dipole resonance sensitivity of the selected ADWR (245 GHz/RIU), the selected ADWR has particular importance in material sensing due to its improved sensing accuracy by detection of frequency shifts at multiple resonances. Besides, the Fano resonance dip sensitivity of the selected ADWR (176 GHz/RIU) was significantly higher than the reported terahertz asymmetric split ring (TASR) with identical analyte thickness (36.7 GHz/RIU) [26].
4.2 Analysis of experimental results
We firstly measured the terahertz transmissions of the water-methanol mixtures in quartz cuvette without ADWR and extracted the refractive indices of different water contents for subsequent measurements with ADWR. Figure 4(a) depicts the measured frequency-dependent amplitude transmissions of the water-methanol mixtures with various water contents from 0% (pure methanol) to 100% (pure water) at 0.5-2 THz. The measured amplitude transmission of different water-methanol mixtures was decreased with increasing water content due to stronger absorption of water than methanol. The refractive indices n(ω) of different water-methanol mixtures was derived from the transmission results by the following equation [42,43]:
where Фs and Фr are the phases of the sample and reference, respectively, c is the speed of light, ω is the angular frequency, d is the thickness of the liquid sample, and nr is the refractive index of the reference. Figure 4(b) presents the frequency-dependent refractive indices of different water-methanol mixtures. The refractive indices of the water-methanol mixtures were increased with increasing water content due to higher refractive index of water. The refractive indices of pure methanol and pure water, as shown in Fig. 4(b), agreed well with previous works [44,45].Fig. 4 (a) Measured transmission and (b) refractive indices of the water-methanol mixtures with different water contents ranging from 0% to 100%.
Subsequently, we measured the water-methanol mixtures with ADWR. Figure 5(a) shows the measured amplitude transmission of the bare ADWR metamaterial. The simulated amplitude transmission was also shown for comparison. We can observe that the experimental amplitude transmission of the bare ADWR is in good agreement with the simulation. In order to increase the difference between water-methanol mixtures measured by ADWR-assisted THz-TDS, we used the normalized transmittance method. The normalized amplitude transmission T(ω) of water-methanol mixtures measured by the ADWR-assisted THz-TDS was obtained by the following equation:
where is the amplitude transmission of water-methanol mixtures measured by the ADWR-assisted THz-TDS, is the amplitude transmission of 100% methanol obtained by THz-TDS without ADWR shown in Fig. 4(a), and ω is the angular frequency. Figure 5(b) presents the normalized amplitude transmission T(ω) of the water-methanol mixtures with ADWR. A gradual red shift was observed at all resonances (Fano resonance dip, Fano transmittance peak, and dipole resonance dip) with increasing water contents due to the increasing refractive indices of the water-methanol mixtures under test at the resonance frequency. Besides, the amplitude transmission of water-methanol mixtures at the Fano transmittance peak with ADWR was decreased with the increase of water content, which was similar to the trend obtained with traditional THz-TDS without ADWR shown in Fig. 4(a). Clearer difference in the amplitude transmission between different water contents was obviously observed in the Fano transmittance peak using ADWR, which demonstrates a higher sensitivity of the ADWR-assisted THz-TDS than the conventional THz-TDS.Fig. 5 (a) Experimental and simulated transmission through the bare ADWR metamaterial. (b) Normalized measured transmission of the water-methanol mixtures obtained by ADWR-assisted THz-TDS.
We plotted the resonance frequency shift against the refractive index of the water-methanol mixture for all resonances in Fig. 6(a). Sensitivities of the Fano resonance dip, Fano transmittance peak, and dipole resonance dip turned out to be 93, 160, and 305 GHz/RIU, respectively. More importantly, the amplitude of the Fano transmittance peak fitted exponentially with the changes of the refractive indices or the concentrations of water in the mixtures and their resonance frequencies were linear fitted with the changes in refractive indices when measuring with ADWR, as shown in Fig. 6(b). Thus we are able to realize qualitative and quantitative sensing of methanol and water simultaneously with ADWR-assisted THz-TDS.
Fig. 6 Measured frequency shifts of (a) Fano resonance dip, Fano transmittance peak, and dipole resonance dip with the changes of refractive indices of different water contents. (b) Resonance frequencies and transmission values of the Fano transmittance peak with the changes of refractive indices of different water contents.
5. Conclusion
In conclusion, by breaking the symmetry of dual-wire structures via unequal lengths, we obtained a readily available metamaterial with multi-modal resonances in the terahertz regime. By further optimizing the geometry design, a practicable terahertz metamaterial sensor with strong multi-modal resonances and high sensitivities was achieved for efficient sensing of strongly absorptive water-methanol mixtures. The experimental results of the water-methanol mixtures sensing show that the demonstrated metamaterial achieves frequency sensitivities of 93, 160, and 305 GHz/RIU for the Fano resonance dip, the Fano transmittance peak, and the dipole resonance dip, respectively. This metamaterial presents a promising avenue for highly accurate, cost-effective, label-free, real-time, qualitative and quantitative terahertz sensing of chemical and biological substances in water, methanol, and other types of highly absorptive aqueous systems. As for the real measurements of biological materials, the sensitivity and selectivity of the proposed metamaterial can be further improved by optimizing the structural parameters to match the resonance frequencies of the objective biological molecule and specific chemical reaction between the biological target and binding agents immobilized on the metamaterial. Real-time monitoring can be achieved by integrating with microfluidic devices.
Funding
Natural National Science Foundation of China (NSFC) (31471410); National Science Foundation (NSF) (ECCS-1232081).
Acknowledgments
This work was performed in the Ultrafast THz Optoelectronic Laboratory at Oklahoma State University in Stillwater, Oklahoma. Min Chen acknowledges the sponsorship of the China Scholarship Council.
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