Tea is the second most popular non-alcoholic beverage in the world, ranked below water, and it is popular in China, Japan, the United Kingdom, and many other countries because of its health benefits [1]. However, tea plants are susceptible to a variety of pests and diseases, which affect tea production and economic benefits. Chlorpyrifos is a broad-spectrum organophosphorus pesticide that is widely used to control various pests and diseases on tea plants. Previous studies have shown that low levels and long-term exposure to chlorpyrifos can lead to diseases such as developmental defects, memory problems, and subclinical neuropathy [2–4]. As a result, maximum residue limits have been established for tea by Japan, China, and the European Union [5–7]. At present, traditional methods such as gas chromatography and liquid chromatography-mass spectrometry are the most commonly used methods in detecting the pesticide residues in tea. However, these methods are time and solvent consuming, and they often involve several complicated procedures. Thus, they are inapplicable to the quality and safety supervision of the tea market. Spectroscopic methods such as near-infrared spectroscopy [8] and Raman spectroscopy [9] have also been applied to the detection of pesticides in recent years. These methods have the characteristics of being simple, fast, and convenient, but the sensitivity is limited. Therefore, it is of great significance to develop new technologies for pesticide detection in tea with good reliability and high sensitivity.

Terahertz (THz) spectroscopy has the advantages of low photon energy, strong penetration, and strong anti-interference ability [10], and it is widely used in non-destructive testing [11], genetic diagnosis [12], and biomedical imaging [13]. However, the sensitivity of terahertz spectroscopy is very low because the intermolecular or intramolecular vibrations of analytes in the terahertz band are extremely weak. To improve the sensitivity of terahertz spectroscopy, metasurface structures have been proposed and used in the terahertz technology. However, the increase of sensitivity is limited because of the low $Q$-factor of metasurface structures [14–18]. The concept of bound states in the continuum (BICs) has been introduced into the design of metasurface, making it possible to achieve high $Q$-factor terahertz devices [19,20]. BIC behaves as a special mode characterized by energy being bound in a continuum and is unable to couple with free space radiation in photonic systems. In theory, BICs deliver infinite lifetime with infinite radiation $Q$-factor, which exhibits the nonradiative property of spectral linewidth disappearance [21,22]. The introduction of external perturbations can make BICs produce limited leakage and become an observable quasi-bound state in the continuum (QBIC) [23–25]. The QBICs have extremely narrow linewidths observed in the far field and the ability to capture more electromagnetic energy for longer time in the resonant cavities, which can achieve strong enhancement of the local light field [26,27].

This paper demonstrates a QBIC-based terahertz metasurface structure for the detection of chlorpyrifos in tea. The metasurface structure consists of a periodic array of two microrods that support the BIC mode, and the BIC mode can be transformed into the QBIC mode by changing the length of the microrod. The characteristics of the BIC mode and QBIC mode are investigated by analyzing their electromagnetic field and surface currents. The sensing of analytes with different refractive indices shows the ability of this metasurface to achieve highly sensitive detection. Finally, the sensing performance of the metasurface for detecting the chlorpyrifos concentration in tea is experimentally studied. As the concentration of chlorpyrifos increased, the transmittance of the resonance shows a red-shift trend. The results show that the proposed terahertz metasurface provided a new idea for the detection of chlorpyrifos in tea.

A two-microrod structure is chosen as the unit cell used for designing the metasurface due to its multiple advantages, including easier parameters optimization, higher symmetry and periodicity, and simpler fabrication processes. The perspective view of the proposed THz metasurface is depicted in Fig. 1(a), and the unit cell of the metasurface is illustrated in Fig. 1(b). The metasurface structure consists of an array of two copper microrods periodically arranged on a quartz substrate. The geometry parameters of the unit structure are as follows: the period in both $x$ and $y$ directions is $P_x=P_y=200\mathrm{\mu m}$, the distance between two microrods is $d=75\mathrm{\mu m}$, the width of the microrods is $w=30\mathrm{\mu m}$, the height of the microrods is $h=0.2\mathrm{\mu m}$, and the length of the right microrod is fixed to be $L_1=130\mathrm{\mu m}$, while the length of the left microrod $L_2$ can be changed. Numerical simulations on the electromagnetic responses of the metasurface structure were performed by using the finite element method. The periodic boundary conditions are imposed on both $x$ and $y$ directions, and the open boundary condition is applied to the $z$ direction. The conductivity of copper is set to be $5.8\times {10}^7\text{S/}\mathrm{m}$ [28]. The frequency domain finite element method is employed to simulate the sensing characteristics of the proposed metasurface structure.

The Longjing tea (Hangzhou, China) was purchased from the local supermarket, and chlorpyrifos powder (99%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). In order to determine the content of trace chlorpyrifos, 0.20 g chlorpyrifos powder was dissolved in acetone to produce the stock solution of chlorpyrifos. And then the acetone was absorbed by a pipetting gun to dilute the chlorpyrifos solution. The tea was brewed by weighing 5.00 g tea into a beaker, adding 200 mL boiling water to brew for 15 min, and then quickly straining into another beaker. After the tea was cooled to room temperature, five different tea samples with chlorpyrifos at concentrations ranging from 0 to 1000 ppm (0, 0.1, 1, 100, 1000 ppm; ppm, parts per million) were obtained by adding an appropriate amount of the stock solution of chlorpyrifos to pure tea. For THz spectroscopy measurement, the tea sample was dropped with 10 μL on the surface of the metasurface, and the measurements were repeated four times to ensure the stability of the results. Sample spectra were collected when THz waves passed through the metasurface coated without and with tea sample. The reference spectra were collected when THz waves passed through the air. By this way, the transmission spectra of the sample (metasurface coated without and with tea sample) can be extracted by comparing the sample spectrum with the reference spectrum. More importantly, the spectrum of the metasurface coated with the tea sample at a concentration of 0 ppm could be set as the reference spectrum to eliminate the impact of the background (tea).

The proposed metasurface structure was fabricated by using the laser direct writing process, thermal evaporation process, and lift-off process. First, the quartz substrate was cleaned with deionized water and acetone solution and then dried in the nitrogen environment. Second, AZ 5214-E photoresist was spin-coated on the quartz substrate at the speed of 2000–4000 r/min for 90 s. The photoresist coated quartz substrate was then baked on a 100°C heating stage for 90 s. Next, the photoresist coated quartz substrate was patterned using a laser direct writing system (Microlab 4A100, Suzhou Sudaweige Optoelectronic Technology Co., Ltd., China). The carved structure was then rinsed with tetramethylammonium hydroxide solution for 30 s. Then, a 200-nm-thick copper film with a 10-nm-thick chromium film (adhesive layer) was deposited on the carved structure by thermal evaporation. Finally, the sacrificial layer was rinsed off with N-n-methylpyrrolidine-2-one solution and rinsed with deionized water during lift-off process. The photograph of the fabricated metasurface sample, which consists of $70\times 70$ units, is shown in Fig. 1(c). Figure 1(d) displays the optical microscopy image of the metasurface structure. The thickness of the copper layer on the quartz substrate was measured by using a 3D profilometer (Dektak XT, Bruker Corporation, Germany), and the results are shown in Fig. 1(e). The copper has good uniformity, and the measured thickness is 200 nm.

The effect of altering the dimension of the metasurface on the spectral response of BIC to QBIC is investigated in both simulations and experiments, and the results are shown in Figs. 2(a)–2(c). In Figs. 2(a) and 2(b), there are two resonances observed in the simulated and experimental transmission spectra of the metasurface structure with different values of $L_2(L_1=130\mathrm{\mu m})$. One is the BIC resonance located at around 0.6 THz, and the other is the dipole resonance located at around 0.8 THz. Of particular note is the discrepancies between the results from simulation and experiment, including the intensity in the resonance, the shift in the resonance as $L_2$ increases, and the position of the dipole resonance. The potential reasons for these discrepancies are the machining error in the fabrication, the absorption loss of the quartz substrate, and the limited resolution in THz spectroscopy system. The two dashed lines presented in Figs. 2(a) and 2(b) are the evolutions of the BIC/QBIC resonances in the simulation and experiment, the trends of which are consistent with that shown in Fig. 2(c). The BIC/QBIC resonance displays an evident red-shift as $L_2$ increases. Meanwhile, the linewidth of the QBIC resonance gradually narrows as $L_2$ approaches $L_1$, indicating the decreasing leakage rate of QBIC mode. When $L_2=L_1$, the linewidth disappears, and the QBIC resonance vanishes, in which the perturbation is reduced toward zero, and the symmetry-protected BIC mode (black star) with infinite $Q$-factor appears. By inducing an asymmetric parameter $\alpha =L_1-L_2$, the relationship between the $\alpha$ values and $Q$-factors retrieved from the BIC/QBIC resonance in Figs. 2(a) and 2(b) is shown in Fig. 2(d). Here, the $Q$-factor is calculated as the ratio of resonance frequency to the full width at half-maximum in the resonance. It is obvious that the measured data agree well with the simulated data. When $\alpha =0\mathrm{\mu m}$, the $Q$-factor distinctly tends to infinity, indicating that there is no energy radiating into free space in the symmetric-protection BIC mode.

When the THz waves are vertically incident on the metasurface, the metallic surface electrons with the same oscillation frequency as the incident waves will generate an enhanced electromagnetic field confined to a small area on the metallic surface [29]. Moreover, the energy leakage of THz waves can be clearly shown by monitoring the magnetic field component in the $z$ direction [30]. Figure 3 compares the surface electric field and the magnetic field distribution at 0.64 THz [BIC mode with $L_2=130\mathrm{\mu m}$ ($L_1=L_2$) and QBIC mode with $L_2=120\mathrm{\mu m}$]. It can be found that the surface electric field and the magnetic field excited by the $y$-polarized THz waves are distributed around the microrods in the $y$ direction. According to the trend of the surface currents, an electric dipole resonance occurs on the microrods can be observed. In this case, the surface currents run from the positive electrode to the negative electrode of the microrods. For the BIC mode [see Figs. 3(a) and 3(b)], the electric field is clearly confined to the top and bottom areas of the microrods, and the magnetic field is mainly confined to the left side of the two microrods. While for QBIC mode, the electric field is significantly enhanced compared to that of the BIC mode due to the fact that the symmetry of the structure is destroyed. The magnetic field is symmetrically distributed on both sides of the coupled microrods, which is stronger than that of the BIC mode. This magnetic field map for QBIC mode indicates that there is obvious magnetic field leakage, and a sharp resonance appears in this situation.

The $Q$-factor and resonance intensity $I$ are two particularly important parameters for the performance of metasurface [31,32]. Here, the $I$ is defined as the difference between maximum and minimum in transmission amplitude of the resonance. Usually, the performance of QBIC resonance in the metasurface can be optimized by adjusting the structure parameter to give a high $Q$-factor. However, QBIC resonance with high $Q$-factor suffers from low intensity $I$, which limits the performance parameter $Q\times I$. Therefore, it is significant to determine the optimal structure parameter to achieve high $Q\times I$. In Fig. 4(a), the $Q$-factor and resonance intensity for different asymmetric parameters $\alpha$ are calculated and presented. As the $\alpha$ increases, the $Q$-factor gradually decreases while the resonance intensity gradually increases. The value of $Q$-factor is equal to the value of resonance intensity when $\alpha =12.5\mathrm{\mu m}$. The comprehensive performance parameter $Q\times I$ is shown in Fig. 4(b). As the $\alpha$ increases, $Q\times I$ value rapidly increases and reaches a maximum value of 59.5 at $\alpha =12.5\mathrm{\mu m}$. Then it decreases and reaches a value of 15.9 at $\alpha =40\mathrm{\mu m}$. Therefore, $\alpha =12.5\mathrm{\mu m}$ is the optimized value for the comprehensive performance parameter $Q\times I$.

The sensing performance of the metasurface is investigated by varying the analyte thickness and maintaining the refractive index of the analyte. According to the fact that the refractive index of biomolecules is usually between 1.4 and 2.0 [33,34], the analytes (dielectric layer) with refractive index of 1.6 are widely used as representative samples deposited on the metasurface for exploring the relationship between the frequency shift of resonance and the analyte thickness. Figure 4(c) shows the frequency shift versus analyte thickness with a refractive index of 1.6 in the case of $\alpha =12.5\mathrm{\mu m}$ at the QBIC mode. The frequency shift rapidly increases to a value of 51.7 GHz when the analyte thickness increases from 0 to 10 μm. Then the frequency shift experiences a slow increase, followed by saturation as the analyte thickness continues to increase. These phenomena indicate that the metasurface can be used to detect the analyte thickness and can achieve high sensitivity when the analyte thickness is lower than 10 μm. Next, the detection of the refractive index based on the frequency shift is demonstrated in Fig. 4(d). An evident frequency shift of 121.4 GHz in the resonance can be observed while varying the refractive index from $n=1$ to $n=2$. The sensitivity of the metasurface structure is defined as $S=\mathrm{\Delta }f/\mathrm{\Delta }n$, where $\mathrm{\Delta }f$ is the frequency shift due to the coating of the analyte on the metasurface and $\mathrm{\Delta }n$ is the change of the refractive index [35]. Table 1 compares the sensitivity of our proposed metasurface to that of the other THz metasurface reported in the previous works. The calculated sensitivity of our metasurface is 121.4 GHz/RIU. The equivalent sensitivity per micron of analyte thickness is 8.09 GHz/RIU. It can be seen that our metallic metasurface structure shows great advantage in sensing application over the reported THz metallic metasurface. The sensitivity of the all-dielectric metasurface is higher than that of our metallic metasurface; however, compared with the all-dielectric metasurface, the metallic metasurface has the superiority of a simple manufacturing process, high manufacturing precision, and notable ductility, which is more suitable for a wide range of applications [39].Table 1.

Sensitivity Comparison of Our Metallic Metasurface and Reported THz Metasurface Structures

Further, the sensing performance of the metasurface is also experimentally evaluated by detecting chlorpyrifos concentration. A continuous-wave THz spectroscopy system (TeraScan 1550, TOPTICA Photonics AG, Germany) is used for spectral measurement [see Fig. 5(a)], and its schematic diagram of the working principle is shown in Fig. 5(b) [40]. In our experiments, the optical path of the THz spectroscopy system is covered by a plastic box to ensure a constant environment for spectral measurements. Additionally, dry air is charged into the plastic box, and the amount of dry air can be controlled by a glass rotor flowmeter to maintain constant relative humidity for spectral measurements. By this way, the spectral measurements are performed at a room temperature of 25°C ($\pm 0.1°\mathrm{C}$) with a relative humidity of less than 4.0% ($\pm 0.1\%$) in the experiments. Figure 5(c) depicts the schematic representation of the sensing measurement. Three replicates were prepared for each concentration to reduce random errors. The spectral measurements are performed only after the complete evaporation of the solution. Therefore, the tested object is a layer of chlorpyrifos film deposited on the metasurface. The presence of a trace amount of external chlorpyrifos induces changes in the dielectric environment of the metasurface surface, leading to enhanced localized electric fields on the surface. These changes will be reflected in variations of the resonance frequency, which enables highly sensitive detection of chlorpyrifos concentrations. In Fig. 5(c), as the concentration of chlorpyrifos increases from 0 to 1000 ppm, the QBIC resonance shows a red-shift from 637.1 to 629.7 GHz, and the frequency shift reaches up to 7.4 GHz. The response of the frequency shift to the chlorpyrifos concentration is nearly linear in the concentration range of 0–1000 ppm. The detailed variations of the chlorpyrifos concentration with the frequency shift are linearly fitted with a determination coefficient of 0.99. The fit equation is estimated to be $C=3.11+1.39\times F$, where $C$ is the chlorpyrifos concentration and F represents the frequency shift. The result demonstrates a strong linear correlation between the frequency shift and the chlorpyrifos concentration ranging from 0.1 to 1000 ppm, indicating the satisfactory capability of the metasurface for chlorpyrifos detection.

A QBIC-based terahertz metasurface structure composed of periodic arrays of two microrods is proposed to theoretically and experimentally verify the feasibility of detecting chlorpyrifos in tea. The BIC resonance at 0.64 THz can be excited by breaking the symmetry of the metasurface structure. In this way, the electromagnetic energy can be effectively enhanced, thereby promoting the interaction between the electromagnetic waves and the analyte. Through simulation, the $Q$-factor of the QBIC mode can be adjusted by modifying the structural parameters. In addition, the sensing performance of the metasurface is evaluated by varying the refractive index or thickness of the analyte. The results show that, when the thickness increases from 0 to 32 μm, the frequency shift first increases sharply and then tends to saturate, and the sensitivity is 121.4 GHz/RIU. In the experiment, the sensing performance of the metasurface is analyzed by the detection of chlorpyrifos in tea. The data show that, as the chlorpyrifos concentration increased from 0 to 1000 ppm, the resonance has an evident red-shift, and the frequency shift is 7.4 GHz, which indicated that the metasurface could detect chlorpyrifos at low concentrations, providing a new method for the detection of chlorpyrifos in tea. Our work can provide new references and inspiration for the design and application of QBIC-based terahertz metasurfaces in the future.