1. Introduction

Terahertz (THz) spectroscopy has emerged as a potential technique for qualitative and quantitative detection due to the properties of non-ionization, fingerprint spectrum, and non-destruction [1–3]. In recent years, some researches have focused on investigating the transmission responses of samples in pellet form using THz spectroscopy for molecular mode identification and concentration analysis, such as antibiotics [4,5], pesticides [6,7], additives [8], and other chemical and biological materials [9–13], which lays the basis for sample detection in the compound. However, the low photon energy of THz radiation and its weak interaction with samples limit, to some extent, the sensing sensitivity of THz spectroscopy techniques. To highly enhance THz sensing sensitivity, metamaterials, artificially designed electromagnetic materials arranged with subwavelength structures, have been introduced and various types of THz metamaterials have been engineered with a satisfied sensitivity achieved in the sensing applications [14–18].

Unfortunately, because of the strong absorption of water in the THz region [19], researches mentioned above mainly performed using samples in dry state with water totally evaporated, which hinders the widespread use of THz sensing technique in practice. In order to overcome these deficiencies, attenuation total reflection THz spectroscopy has been demonstrated for direct detection of aqueous solutions [20,21]. Besides, devices combining THz metamaterials with microfluidics [22–24] or nanofluidics [25] in transmission mode have also been tried for sensitive analysis of liquid sample by reducing solution volume. But these devices have stringent requirements for fabrication and are quite expensive. Therefore, other rapid and simple analysis methods for aqueous solutions based on THz spectroscopy are still in great need to realize the full potential of THz sensing technology as well as to expand its application fields.

Owing to the reduction of strong absorption of water and the remarkable enhancement of THz electric field, THz reflection spectroscopy combined with metamaterials is also increasingly explored for rapid analysis of solutions in a convenient way [26]. However, as far as we know, these researches mainly focused on qualitative detection of solutions [27], or quantitative detection with poor sensitivity [28]. The quantitative detection of sample in aqueous solutions using THz metamaterial-based reflection spectroscopy reminds further study to improve THz sensing performance. Among these THz metamaterials, the metallic mesh devices (MMDs) are thin metal films with apertures arranged periodically. Due to their extraordinary optical transmission (EOT) effect, ease of fabrication, flexible structure, and no need of substrate, MMDs have been used for sensing applications [29–32].

In this paper, we demonstrated the MMD as a sensor for aqueous solution analysis in the THz region. THz spectral response and electric field distribution of MMDs under different azimuth angles (ϕ) in oblique reflection mode were investigated. Based on the angle characteristics, a rapid solution analysis platform based on the MMD was set up and the aqueous solution of doxycycline hydrochloride (DCH), a kind of TCs which was widely used due to their broad-spectrum antibacterial activity and low production costs [33], was directly and rapidly detected through THz reflection spectra. The results show that the DCH solution with a concentration as low as 1 mg L⁻¹ can be successfully determined, suggesting the potential applications of this platform for the analysis of strongly absorptive solution in the THz region.

2. Materials and methods

2.1. Materials

Doxycycline hydrochloride (DCH, >94%) was purchased from Sangon Biotech (Shanghai, China). Deionized water was obtained from the Milli-Q SP reagent water system (18 MΩ•cm⁻¹, Millipore, Billerica, MA, USA). All of these chemical reagents were used without further purification.

For the DCH solution detection by the MMD-based platform, a series of DCH aqueous solutions with concentrations of 0 mg L⁻¹ (deionized water), 1 mg L⁻¹, 10 mg L⁻¹, 100 mg L⁻¹, 1000 mg L⁻¹, and 10000 mg L⁻¹ were prepared by dissolving appropriate amount of DCH powder in the deionized water. A polypropylene film was used as a sensor for comparison. For solution detection by the polypropylene film-based platform, various DCH aqueous solutions with concentrations of 0 mg L⁻¹ (deionized water), 5 mg L⁻¹, 10 mg L⁻¹, 20 mg L⁻¹, 100 mg L⁻¹, and 500 mg L⁻¹ were prepared.

The THz MMD used in this study was fabricated from nickel by the electroforming method and free of substrate. The grid interval of the MMD was about 250 µm, with an opening length of 180 µm and a thickness of 60 µm. The polypropylene film used in this study has a thickness of about 60 µm.

2.2. Experimental setup

All the THz time-domain waveforms were collected by a commercial THz time-domain spectroscopy (THz-TDS) system (TAS7500SP, Advantest Corporation, Tokyo, Japan) in oblique reflection mode with a spectral range from 0.1 to 4.0 THz. The schematic illustration of the reflection THz-TDS integrated with an MMD was shown in Fig. 1. In this strategy, two optical fiber lasers at 1550 nm with pulse durations less than 50 fs were used as a pump source and a probe source at a repetition rate of 50 MHz, respectively. The photoconductive antenna (PCA) method with a hyper-hemispherical silicon lens was both used for THz generation and detection. The emitted THz signal in transverse magnetic (TM) polarized mode was illuminated on the lower surface of an MMD or a polypropylene film arranged horizontally at a fixed incident angle (θ) of 10°. Through controlling the two optical fiber lasers utilizing phase-modulated dual-laser-synchronized control technology [34], the time-domain waveforms carrying the information of samples could be obtained at a high speed. All the measurements were carried out at 23 °C (±0.5 °C) with the relative humidity less than 1% after nitrogen purging to avoid the influence of water vapor.

 

Fig. 1. Schematic of the THz MMD-based reflection platform with the polarization of THz electric field $\vec{E}$ being parallel to latitudinal direction of the periodic apertures of the MMD (incident angle θ=10°).

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2.3. Spectra acquisition and data analysis

For the angle characterization, the MMD was attached on a rotation stage to make sure a stable spectral acquisition at a desired azimuth angle. For DCH aqueous solution detection through the MMD or polypropylene film, a 100 μL liquid sample was pipetted carefully onto the MMD or polypropylene film using a micro pipette from a fixed position. To avoid some uncertain effects from the ambient humidity or airflow on the liquid samples, a hollow plastic cup was used for every liquid sample measurement, including DCH aqueous solution detection through the MMD or polypropylene film, to isolate the liquid sample from the ambient conditions. Each collected spectrum was an average of 1024 scans and three measurements were conducted for each sample.

The reflection signal of a flat and smooth metal block was acquired as reference, and was collected before each sample measurement. After the reflected time-domain waveforms of samples and corresponding references were recorded, a conventional fast Fourier transform (FFT) was applied to get their reflected THz electric field intensity in the frequency-domain, namely E_sample and E_reference. The reflectance of sample was defined as R = (E_sample/E_reference)² [35].

2.4. MMDs simulations

The simulations in our study were carried out using the 3D finite-difference-time-domain (FDTD) method (FDTD Solutions 8.19.1584, Lumerical Solutions, Inc. Vancouver, Canada). The nickel was modeled as perfect electric conductor. Broadband Fixed Angle Source Technique (BFAST) based plane wave source in the range of 0.6 to 1.3 THz was chosen to match the periodic boundary condition in the oblique incidence geometry (θ=10°). The reflection spectra and electric field distributions of MMD under different azimuth angles (ϕ ranging from 0° to 90°) were simulated. In the part of investigating the influence of immersion depth, “H₂O (Water)-Palik” in the material database was chosen as the water sample, the thickness of which above the MMD surface was set to 1000 μm.

3. Results and discussion

3.1 Angle characteristics of THz MMDs

Since the THz MMD has an effective dielectric constant in a plasma form and can support spoof surface plasmon polariton, its spectral response should be sensitive to the direction of the incident beam, which has been well elucidated for surface plasmon polariton in the infrared region [36–38]. To demonstrate the angle-dependent properties of MMD in the THz region and lay the foundation for solution analysis, THz reflection response of the MMD under various azimuth angles was investigated.

Figure 2(a) shows the geometric diagram of an incident beam to the MMD, with the incident angle θ and azimuth angle ϕ of the THz wave vector $\vec{k}$ marked. The azimuth angle ϕ indicates the angle between the projection of THz wave vector $\vec{k}$ onto the MMD plane with the latitudinal direction of the MMD apertures. The polarization of THz electric field $\vec{E}$ lies in the incident plane. Considering the symmetry and periodicity of the MMD geometry structure relative to the THz electric polarization, azimuth angle ranging from 0° to 45° at an interval of 5° was selected for measurement. The temporal waveforms and measured reflection spectra were recorded and analyzed, as presented in Figs. 2(b) and (c). For the temporal waveforms, the main pulses for different ϕ show a good consistency, while the slave pulses get gradually attenuated as ϕ increases. This can be interpreted as a model of comprehensive effect from the nonresonant mode which describes signal response from individual holes and corresponds to the main pulse, and resonant surface mode which describes signal traverses the grating through adjacent holes and corresponds to the slave pulse [39]. As ϕ decreases from 45° to 0°, the nonresonant mode remains and the resonant surface mode increases to dominate the signal response. This is also demonstrated by the reflection spectra in Fig. 2(c). When ϕ equals to 45°, there is a very broad dip around 1.0 THz, corresponding to EOT in transmission. As ϕ decreases, the dip splits and a reflection peak (∼0.95 THz) grows up, which is consistent with the change of the slave pulse, indicating a strong resonant surface mode. Moreover, corresponding reflection spectra were also simulated by the FDTD method (see Fig. 2(d)); an agreement is achieved between the results obtained from experiments and simulations. The differences between peak reflectance are probably due to the limited length of time-delay line and the loss caused by metallic materials [40] in the experiment. Generally, the slave pulses of THz MMD with a strong resonance could last a long time [39], and shortening the time-delay length could dampen the reflectance peak. While the differences between peak frequency can be attributed to the size variation of the fabricated apertures [41] in the experiment.

 

Fig. 2. (a) The geometric diagram of an incident beam to the MMD in TM polarized mode (incident angle θ, azimuth angle ϕ, THz wave vector $\vec{k}$, THz electric field $\vec{E}$, THz magnetic field $\vec{H}$). (b) Measured temporal waveforms of the THz MMD under different azimuth angles; the inset shows their time-domain slave pulses enlarged. (c) Measured and (d) corresponding simulated reflection spectra of the THz MMD under different azimuth angles.

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To further confirm the angle-dependent reflection of the THz MMD, the reflection spectra as a function of frequency (from 0.6 to 1.3 THz) and azimuth angle (from 0 to 90 degree) were simulated; the results are plotted in Fig. 3(a). The simulated data illustrates that the angle-dependent reflection spectra show a good symmetry along ϕ=45°, as marked by a dashed line. The broad dip gradually splits into two dips and these two dips move away from each other with ϕ getting away from 45°, which accords well with the results on metallic hole arrays at THz frequencies [42]. In addition, the reflectance peak (∼0.98 THz) between two split dips comes up to the highest when the projection of THz electric field polarization onto the MMD plane gets parallel to latitudinal direction of the MMD apertures, namely, ϕ=0° or 90°. To reveal the near-field features corresponding to far-field THz spectra, typical cases of electric field distributions were also simulated. Figures 3(b)–(d) present the top views of electric field distribution of the MMD at the reflection resonant frequency (∼0.98 THz) for 0°, 30°, and 45°, respectively. In all cases, the maximum electric field, that corresponds to the hot spots in the figures, is considerably localized along the hole edges, which suggests the greatly enhanced electric field.

 

Fig. 3. (a) Simulated reflection spectra as a function of frequency (from 0.6 to 1.3 THz) and azimuth angle (from 0° to 90°); the color bar indicates THz reflectance and the dashed line indicates the symmetry line of ϕ=45°. Simulated electric field distributions of the MMD at the reflection resonant frequency (∼0.98 THz) for azimuth angles of (b) 0°, (c) 30°, and (d) 45°, respectively; the color bar indicates the normalized electric field strength.

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3.2 Concentration analysis of DCH aqueous solutions

Based on the analysis above, the configuration of ϕ=0° or 90° was selected for concentration analysis of DCH aqueous solutions in reflection mode. Herein, a strong resonant surface mode can be obtained, which helps to enhance the interaction between THz waves and the liquid samples, including interaction strength and interaction area.

The schematic diagram of solution sensing is shown in Fig. 4(a), with a volume of 100 μL droplet covering the MMD surface and no leakage. In this case, the droplet can fully cover the detection aperture of this THz-TDS system with a thickness of more than 2 mm, which is larger than the penetration depth of THz waves into the aqueous solution, thus enabling no transmission THz signal and fully focusing on reflection spectra for analysis. Various concentrations of DCH aqueous solutions, ranging from 0 (deionized water) to 10000 mg L⁻¹, were detected. Figure 4(b) presents how the reflection spectra of the MMD change with the concentrations of DCH solutions. When the droplet was deposited onto the MMD, the overall reflectance shows a little increase but the sharp peak gets down obviously, which indicates the droplet surely has an effect with THz waves and can significantly modulate MMD-based reflection THz spectra. Overall, these spectra can be distinguished from each other from the changes of the dip reflectance. To emphasis the differences, dip reflectance was extracted and plotted as a function of the DCH concentration, see Fig. 4(c). The dip reflectance increases monotonically as the concentration increases and the DCH aqueous solution with a concentration as low as 1 mg L⁻¹ can be detected directly, reflecting the potential for highly sensitive detection of substances in aqueous liquids in the THz region. In addition, a polypropylene film with a thickness of about 60 μm was used as a senor for comparison. Figure 4(d) shows the reflectance spectra of DCH aqueous solutions with concentrations of 0 mg L⁻¹ (deionized water), 5 mg L⁻¹, 10 mg L⁻¹, 20 mg L⁻¹, 100 mg L⁻¹, and 500 mg L⁻¹. The results indicate that the polypropylene film-based platform shows a rather low reflectivity of less than 0.1 and cannot distinguish these DCH aqueous solutions, revealing the superiority of THz MMD for aqueous solution analysis.

 

Fig. 4. Concentration measurement of DCH aqueous solutions: (a) the schematic diagram of DCH aqueous solutions detection; (b) experimental THz reflection spectra of different concentrations of DCH aqueous solutions through the MMD; (c) Dip reflectance versus DCH concentrations; the error bars are the standard deviations of three replications; (d) experimental THz reflection spectra of different concentrations of DCH aqueous solutions through the polypropylene film.

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3.3 Mechanism exploration from numerical simulation

Due to the strong absorption of water in the THz region, THz spectroscopy has the superiority in sensing the minor changes of aqueous solutions over other spectroscopy technologies. To further illuminate the underlying mechanism of DCH aqueous solution detection using MMD-based reflection THz spectroscopy, numerical simulations were conducted by the FDTD method.

As for various concentrations of DCH aqueous solutions, the extinction coefficient and kinetic properties of water molecules, as well as surface tension, may differ. Notably, the droplet deposited on the MMD can slightly immerse into the hole at a certain depth. Therefore, droplets in different concentrations may present different shapes inside the MMD periodic holes. For simplification, the surface curvature of the droplet was treated as flat and the equivalent immersion depth was taken into consideration here. Figure 5(a) shows how the THz reflection spectra change with the immersion depth of water. The trend of reflection spectra versus immersion depths accords well with that versus solution concentrations in Fig. 4(b) with a higher concentration corresponding to a deeper immersion depth. The results demonstrate that THz spectral differences of DCH aqueous solutions with various concentrations, to some extent, can be attributed to differences in immersion depth. In addition, the extinction coefficient is another important parameter, especially for solution sensing in the THz region. According to some researches [20,43], DCH aqueous solution with a higher concentration has a smaller extinction coefficient. Therefore, we also simulated the THz reflection spectra of droplets with different extinction coefficients ranging from 0.4 to 1.2 at an interval of 0.2, with the immersion depth set to 18 μm and the refractive index set to 2.2. As shown in Fig. 5(b), the dip reflectance increases with a decreasing extinction coefficient, which agrees well with the measured results in Fig. 4(b). The results confirm that the extinction coefficient of DCH aqueous solutions with different concentrations can also account for the change of THz reflection spectra.

 

Fig. 5. The effect of aqueous solution on the MMD reflection spectra: (a) simulated reflection spectra of the THz MMD with aqueous solutions immersed into the MMD holes in different depths; (b) simulated reflection spectra of the THz MMD with aqueous solutions in different extinction coefficients; the immersion depth was set to 18 μm and the refractive index was set to 2.2.

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In a word, the immersion depth and extinction coefficient of liquid can account for the concentration-dependence of THz reflection spectra. A higher concentration of DCH solution has a smaller extinction coefficient and can induce a deeper immersion depth into the MMD holes, thus resulting in an increase at the dip reflectance.

4. Conclusions

In summary, we proposed a THz MMD-based reflection platform for DCH aqueous solution detection in a rapid and convenient way. We experimentally and theoretically determined the angle characteristics of the MMD and observed its electric field distribution at some typical azimuth angles in the oblique incidence geometry. The results demonstrate that a strong resonant surface mode with a high area ratio of strong electric field can be obtained when the projection of THz electric field polarization onto the MMD plane gets parallel to latitudinal direction of the MMD apertures (ϕ=0° or 90°), which can be helpful for sensing applications. Furthermore, this platform was utilized for DCH aqueous solution analysis and corresponding sensing mechanism was explored by numerical simulations. The results show our platform can be used to directly detect the DCH solution with a concentration as low as 1 mg L⁻¹, and the immersion depth into the MMD holes and the extinction coefficient of the solution can account for the concentration-dependence of THz reflection spectra. We expect that our work can provide a new method for the microanalysis of chemicals or biomolecules in strongly absorptive solutions in the THz region and the THz spectroscopy technology could play a more important role in material analysis.