Sensitive detection of chlorpyrifos pesticide using an all-dielectric broadband terahertz metamaterial absorber

Journal Pre-proof Sensitive detection of chlorpyrifos pesticide using an all-dielectric broadband terahertz metamaterial absorber Pengcheng Nie, Dongying Zhu, Zijian Cui, Fangfang Qu, Lei Lin, Yue Wang

PII:

S0925-4005(19)31841-6

DOI:

https://doi.org/10.1016/j.snb.2019.127642

Reference:

SNB 127642

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

29 July 2019

Revised Date:

5 December 2019

Accepted Date:

27 December 2019

Please cite this article as: Nie P, Zhu D, Cui Z, Qu F, Lin L, Wang Y, Sensitive detection of chlorpyrifos pesticide using an all-dielectric broadband terahertz metamaterial absorber, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127642

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Sensitive detection of chlorpyrifos pesticide using an alldielectric broadband terahertz metamaterial absorber Pengcheng Nie, 1,2,3 Dongying Zhu,4,5 Zijian Cui,4,5 Fangfang Qu,1,2 Lei Lin,1,2 and Yue Wang 4,5,* 1

College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058,

China 2

Key Laboratory of Spectroscopy Sensing, Ministry of Agriculture and Rural Affairs, Hangzhou

3 State

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310058, China

Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027,

China 4

Key Laboratory of Ultrafast Photoelectric Technology and Terahertz Science in Shaanxi, Xi’an

5

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University of Technology, Xi’an 710048, China

Key Laboratory of Engineering Dielectric and Its Application, Ministry of Education, Harbin

*Corresponding Author: Yue Wang

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Graphical abstract

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E-mail: [email protected]

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University of Science and Technology, Harbin, Heilongjiang 150080, China

Highlights 

An all-dielectric broadband THz absorber on a highly-doped silicon-based sensing chip was proposed.



It exhibits an interaction efficiency of ∼99% at 1.33 THz and a broad bandwidth covers 1

600 GHz of the center frequency. 

It is a THz sensor with high sensitivity and stability under different temperature, humidity and time conditions.



This absorber sensor can be used to detect chlorpyrifos pesticides with a trace content of 0.1 mg ∙ L−1.

ABSTRACT: Metamaterial absorbers consisting of metal, metal-dielectric, or dielectric materials display

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properties that make them feasible for use as signal enhancement tools to quantitatively detect traces amounts of samples. We propose an all-dielectric broadband terahertz absorber on a highly-doped silicon-based sensing chip. This semiconductor metamaterial absorber exhibits an experimental

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absorption of ∼99% at 1.33 THz and a broad bandwidth (absorption of ≥90%) that covers 600 GHz

of the center frequency. The measurement agreed well with the simulations and calculations and the

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wide tuning ability of the absorber was verified by optical excitation. Furthermore, we demonstrated that this metamaterial absorber, as a highly stable THz sensor to temperature, humidity and time,

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shows promise in detecting trace pesticides. Regression coefficients between chlorpyrifos concentrations (0.1, 1, 10, 50 and 100 mg ∙ L−1 ) and the corresponding spectral peak intensities and the frequency shifts near 0.93 THz were 0.9943 and 0.9750, respectively. The detection limit of

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chlorpyrifos reached a level of 0.1 mg ∙ L−1 , and the frequency response of the absorber to 10 mg ∙ L−1 chlorpyrifos solution was 4.6 GHz. These results indicate that the combination of THz

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spectroscopy and metamaterials can be used in the detection of chemical and biological materials with high sensitivity and stability, providing a new strategy for future applications in the fields of

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food and agriculture.

KEYWORDS: all-dielectric broadband absorber, terahertz spectroscopy, semiconductor metamaterial, signal enhancement, pesticide detection

1. Introduction The development of perfect absorbers, which are designed and fabricated according to the unique properties of metamaterials, has become a hot topic of research in recent years [1,2]. These absorbers

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offer broad potential to the application of surface-enhanced molecular spectrometers [3], biosensors [4,5]

, thermal emitters [6,7], stealth technologies [8,9], and optoelectronic devices [10,11]. Dual- [12] ,triple-

[13]

, and multi-band [14] perfect absorbers of different sizes or shapes integrated into one absorption

unit have been widely proposed. Generally, most metamaterial absorbers are made of metals. They are difficult to use for sensing due to their low melting points, large ohmic loss and high thermal conductivity

[15]

. Furthermore, most current absorbers are intrinsically narrowband due to their

resonance characteristics [16]. To obtain a broader range of resonance frequencies, the inclusion of more electrical ring resonators of differing sizes and shapes are required. By including a variety of

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materials with different resonances to broaden the absorption band, restrictions are imposed on resonator couplings, resulting in a lower level of performance than that of a black body absorbing

all incident electromagnetic radiation [17]. Many biomolecules have broad dielectric responses in the

THz range and sensors based on broadband absorbers can detect biomolecules and other substances

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in low concentrations with high sensitivity [18] .Therefore, the development of a broadband, perfect absorber with a simple structure is a critical problem that requires attention [19].

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The limitations described above prompted us to propose an alternative candidate for THz-wave manipulation: the use of an all-dielectric based broadband perfect absorber. We focused on a

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broadband nearly perfect absorber in the THz range, which is realized by etching a grating structure on phosphorus-doped all-silicon with a resistivity of 0.013 Ω·cm. We numerically and

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experimentally demonstrated a broadband absorption that can achieve about 600 GHz of bandwidth. The resonators have the advantage of avoiding Joule heating, which may occur in a comparable metallic-based system, as silicon offers good temperature stability. We demonstrated, both

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experimentally and by simulation, the photo-tunable ability to control various properties of electromagnetic waves efficiently. Furthermore, the fact that spatial dispersion in semiconductors at

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THz frequencies is similar to that of metals in the optical regime could be exploited in their use as highly sensitive sensors for precise detection-based applications. To explore the sensing potential of the proposed all-dielectric broadband THz metamaterial

absorber, we used THz time-domain spectroscopy (THz-TDS) with the absorber for chlorpyrifos (C9H11Cl3NO3PS, CAS: 2921-88-2) detection. Chlorpyrifos is an efficient, broad-spectrum organophosphorus pesticide, which is toxic and hazardous, especially to aquatic organisms [20]. The unavoidable abuse of chlorpyrifos has caused long-term adverse effects in agriculture and the 3

environment and affected a potential hazard to human health

[21]

. The detection of chlorpyrifos

residue in food products is extremely important in the field of food safety. Conventional methods, such as high-performance liquid chromatography, gas chromatography, and ultra-performance liquid chromatography–tandem mass spectrometry, are highly sensitive but time-consuming

[22]

.

Spectroscopic methods of analysis, such as visible-infrared, near-infrared, and mid-infrared, are rapid but less accurate

[23]

. Therefore, there is an urgent requirement to develop an accurate,

convenient, and sensitive method of detecting chlorpyrifos. THz-TDS and surface enhanced Raman spectroscopy are advanced techniques for molecular fingerprint characterization, which have been

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used widely in the biosciences and food safety industry. However, the liquid enhancer for Raman spectroscopy is unstable, which leads to low repeatability of test results

[24].

The solid-state

metamaterials utilized in the THz region have proven to be highly efficient for improving the detection sensitivity of chemical and biological substances [25].

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Here, we present a highly sensitive method for the detection of chlorpyrifos using THz-TDS

combined with a pre-designed metamaterial absorber. THz absorbance spectra were measured using

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the designed absorber structure on which different concentrations of chlorpyrifos were placed. Both the spectral frequency shift and absorbance intensity showed sensitive resonances to trace amounts

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of chlorpyrifos concentration. This THz metamaterial absorber may be used as a rapid and accurate sensing tool for the determination of trace substances, which provides feasibility for various

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applications in the biosciences.

2. Materials and Methods

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2.1. Structural design of metamaterial absorber

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A coaxial ring and cylinder etched into a doped silicon wafer are illustrated in Figure 1. We used heavily-doped silicon to implement the all-dielectric absorber with a carrier concentration of 2.91 × 1018 cm−3 and electron mobility of 174 cm2 /(Vs). The individual resonators had grating heights of t1 = 60 µm, substrate heights of t2 = 300 µm and radii of r = 75 µm, r1 = 60 μm, and r2 = 32 μm; the unit cell had a periodicity of p = 200 µm. According to the specification, the silicon wafers were (100) oriented and n-type doped with phosphorus. In the experiment, the carrier concentration was the only free parameter, whilst the electron mobility was determined from this concentration through

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an empirical model. The gratings were fabricated using conventional microfabrication techniques involving photolithography and deep reactive ion etching. Figure 1b shows the microscope image of the arrays imaged from the top view.

2.2. Preparation of chemical materials Chlorpyrifos standard (C9H11Cl3NO3PS, analytical grade > 99.4 %) and petroleum ether (CAS 8032-32-4) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Chlorpyrifos and petroleum ether were used as solute and solvent, respectively. Firstly, a sample of chlorpyrifos in solution with

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a concentration of 100 mg ∙ L−1 in petroleum ether was prepared by mixing 5 mg of solid state chlorpyrifos standard powder in 50 ml of petroleum ether. Chlorpyrifos solutions at concentrations

of 0.1, 1, 10 and 50 were then prepared by dilution with the 100 mg ∙ L−1 solvent. Finally, the

2.3. Acquisition of THz time-domain spectra

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prepared chlorpyrifos solutions were uniformly mixed with a centrifugal oscillator.

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A THz-TDS system CCT-1800 (China Communication Technology Co., Ltd, Shenzhen, China), was used to measure the spectra of the chlorpyrifos solutions. The frequency range of CCT-1800 is

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0.1~4 THz, its spectral resolution is 30 GHz, the spot size is 3 mm, and the signal-to-noise ratio (SNR) reaches 70 dB at 0.5THz. Before measurement, the metamaterial absorber was immersed in a beaker containing 50 ml of anhydrous ethanol solution and cleaned with a supersonic instrument

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for 3 mins. The absorber was then placed in the sterile cover (room temperature, humidity < 5%). During measurement, 100 μL of chlorpyrifos solution (at concentrations of 0.1, 1, 10, 50, and 100

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mg ∙ L−1 , respectively) were dropped on the surface of the metamaterial absorber with three replicates and dried at room temperature (25 ℃). Finally, the THz time-domain spectra of the

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chlorpyrifos solutions were measured (five repeated scans for each measurement) by THz-TDS in reflection mode. Terahertz laser pulses that were directly incident on the gold-plated mirror and then reflected back were obtained as reference signals. Those incident on the metamaterial absorber carrying samples and then reflected back were obtained as sample signals. According to the Fourier transform, the time-domain spectra of the reference signals and the sample signals were transformed into the frequency-domain spectra. Based on the frequency-domain spectra, the absorption spectra of the samples were obtained by subtracting the sample signals from the reference signals. 5

3. Results and Discussion 3.1. Absorption characteristics and mode analysis In Figure 2, numerical simulation of reflection and absorption for the absorber was performed using a finite-integral time-domain solver provided by the CST Microwave Studio software (Computer Simulation Technology Ltd., Germany). The absorption was calculated as A = 1 − 𝑆11 2 − 𝑆21 2 , where S21 is zero across the entire frequency range. The A > 90% absorption bandwidth was larger than 0.6 THz and the A > 85% absorption bandwidth was about 1 THz. There were two

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absorption peaks at 0.93 THz and 1.33 THz. The absorption at 1.33 THz was approaching 100%. In the simulated models, the dielectric properties of the silicon are described by the Drude model with the plasma frequency and scattering rate estimated earlier [26] ε = ε∞ −

ω2p

(1)

ω(ω+ⅈΓ) 𝑁 ⅇ2

which is 1.89×1014 rad/s. 𝜀∞ = 11.7

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where 𝜔𝑝 is the plasma frequency defined by 𝜔𝑝2 = (𝜀 𝑐

0𝑚

∗)

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and 𝜀0 is the permittivity in a vacuum, Γ = 1⁄𝜏 is the carrier scattering rate, 𝑁𝑐 = 2.91 × 1018 cm−3 is the carrier density and e is the electronic charge. The experimentally measured

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electromagnetic response of the fabricated absorber is shown in Figure 2. The measurement and simulation results are in general agreement, with slight discrepancies, due to fabrication tolerances, system noise, and sample alignment.

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To further investigate the underlying absorption mechanism, we computationally monitored energy dissipation in the all-dielectric absorber structure. In Figure 3 we present the distribution of

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the electric field, current densities, and power flow at the resonance frequencies of 0.93 and 1.33 THz. The grating should no longer be treated only as a dielectric film, but instead as a periodic

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waveguide array [27], which can be explained by a two-dimensional rigorous coupled-wave analysis method [28]. The electric field distribution (Figure 3b) and current density (Figure 3d) of the xz-plane, proved that the electromagnetic wave was concentrated in the air gap of the grating layer at a high frequency. As shown in Figure 3e and f, the spatial distribution of the power loss density is in the xz-plane. As can be observed, for the lower frequency at 0.93 THz, most of the power was dissipated on the grating layer, while the power loss was dissipated in the grating gap at 1.33 THz. Further, the observed experimental and numerical results can be explained by the plasmonic modes[29,30]. THz 6

wave is incident on the structures and then diffracted by the periodic grating. The diffraction satisfied phase matching and excited the cylindrical surface plasmon polaritons supported by the bound mode along the cylindrical sidewalls[31,32]. In order to investigate the performance of the absorber for different structural parameters, numerical simulations were used to demonstrate the dependence of reflection spectra on the period p and radius r. By increasing the value of p, a blue shift of both absorption peaks occurred (Figure 4a) and a red shift occurred after increasing r (Figure 4b). It can be seen that the resonance frequencies are almost independent of p due to the non-radiative mode in the proposed structure.

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Consequently, in order to get a broadband absorption at a pre-designed frequency range, the geometric parameters of the absorber should be adjusted at the correct frequency.

3.2. Dynamic behavior under photoexcitation

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We evaluated the absorber’s overall photo-tuning ability in light harvesting and optical detectors. It was also meaningful to calculate the dynamic behavior arising from optical excitation. We

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measured the absorption spectra at the optical powers of 400 mW (707 μJ/cm2) and 900 mW (1591 μJ/cm2) as shown in Figure 5a and b. The THz beam was at a 30 oblique incidence and the optical

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pump beam was at a 25 oblique incidence. The pump beam was 800 nm near infrared light with a pulse duration of 0.2 ms. We considered that the silicon, including the metamaterial layer and

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substrate, had homogeneously increased carrier density induced by the optical excitation of the pump in the simulation. The thickness of the doped layer was determined using the penetration depth of 800 nm light in silicon along the z direction. The excited carrier density was calculated by

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[33,34]

𝑛(z) =

1 𝐸𝑝ℎ

∙

𝑑𝑓(𝑧) 𝑑𝑧

=

𝛼𝑓0 ⅇ −𝛼𝑧 𝐸𝑝ℎ

(2)

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where 𝐸𝑝ℎ (= ℎ𝑣) is the photon energy of the pump beam, α (1020 cm-1) is the absorption coefficient and 𝑓0 is the incident fluence. In order to facilitate the understanding of our experimental results, Figure 5c shows the simulated THz electric field absorption amplitude as a function of frequency for different optical excitation fluences. The black curve (0 J/cm2) shows the absorption without photoexcitation. We divided the layer into ten layers; each layer had a thickness of 6 μm as shown in Figure 5c. We analyzed different carrier densities for each layer to

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approximate the gradient distribution of the carrier density. With no pump beam, the spectral results are shown in Figure 5d, even though the probe beam was obliquely incident, which was verified by the simulation results shown in the supplemental information. The peak absorption frequency was approximately 1.26 THz with a bandwidth of 550 GHz, which was slightly different to the result for normal incidence. As the fluence of the pump beam increased to 20 000 J/cm2, peak absorption frequencies blue-shifted to 1.4 THz due to the photo-excitation of the carriers in silicon. At the same time, the peak absorption at 0.93 THz decreased from 97.1% to 87%, as illustrated in Figure 5d.

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3.3. Stability of metamaterial absorber Under the influence of temperature, humility and time period, the stability of the absorber sensor were tested, and the corresponding results are shown in Figure 6. The absorbance spectrum of the

absorber with 50 mg ∙ L−1 of chlorpyrifos solution dropped on its surface was obtained and

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averaged by five repeat scans. As shown in Figure 6a, the peak intensities of 0.93 THz were measured at a humility of 60 % at six different temperatures (22.7, 23.0, 23.7, 24.7, 25.7, 26.7 and

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27.6 ℃). It illustrates that the peak intensity of the absorber sensor increases with the rise of temperature in some extent. Because the thermal movement of molecules increases with the rise of

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temperature, thus resulting in an increase in polarization of the peak intensity. However, the curve changes only in a small slope and the relative difference of absorbance from maximum to minimum is 0.079. It indicates that the sensor can work stably at room temperature around 24 ℃. Figure 6b

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shows the results measured at a temperature of 24 ℃ at seven different humidities of 49.3, 52.5, 55.9, 59.3, 63.0, 66.9, 69.8 and 71.3 %. The maximum drift value of peak intensities obtained under

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different humidity values was 0.038, which proved that this sensor had satisfactory stability to humidity. Figure 6c and d depict the spectra and their peak intensities obtained every 10 minutes

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over a continuous time period of 500 minutes, respectively. These data were monitored at the temperature of 24 ℃ and humidity of 60 %. The spectra were stable over time and the peak absorption intensities at 0.93 THz were concentrated on the average value of 2.009 (a.u.) in the range of 1.970-2.091 (a.u.) . The quantile-quantile map of the average peak intensity at each time point (Figure 6d) displayed an approximate linear plot drawn in a normal distribution, which implied the stability of the average data of peak intensity. Under the control of temperature and humidity, the error fluctuation in five consecutive scans at the same time point was mainly caused by internal 8

instability of the THz-TDS system. Over all, these experimental results demonstrated that the metamaterial absorber (with chlorpyrifos solution dropped on its surface) has good stability under the influence of temperature, humidity and time.

3.4. Enhancement of chlorpyrifos sensing In the present work, we developed a THz all-dielectric structure that can be utilized for qualitative and quantitative biological sensing. We demonstrated a doped-semiconductor operating at THz frequencies and acting as a highly sensitive and stable sensor. In the experiment, samples of

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chlorpyrifos solution at different concentrations were dropped on the surface of the metamaterial absorber and dried before spectral measurement in the reflection mode (Figure 7a). The average absorbance spectra of the metamaterial absorber with and without chlorpyrifos solutions were

calculated by three parallel experiments and five repeated scans in each measurement, as shown in

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Figure 7b. There was a clear change of spectrum on adding chlorpyrifos solutions onto the surface of the metamaterial absorber. Although, as reported previously by Qu et al.

[35],

three absorption

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peaks of chlorpyrifos were located at 1.47, 1.93, and 2.73 THz, they were not around the peaks of the metamaterial at 0.93 and 1.33 THz. However, chlorpyrifos located in the gratings induced a

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change in the dielectric properties of the metamaterials. As a result, changes in the amplitude and frequency of the absorption peak could be observed. By using this semiconductor metamaterial

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structure, a trace amount of chlorpyrifos (down to 0.1 mg ∙ L−1 ) could be detected successfully (Figure 7b). The signal-noise-ratio of the absorption peak at 0.93 THz was much higher than that at 1.33 THz. Hence, the spectral characteristics at 0.93 THz were used to explore the sensing

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enhancement effect of the semiconductor metamaterial. The results illustrate that the resonant peak shifted to a higher frequency, while the peak intensity decreased to a lower value with the increase

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on chlorpyrifos concentration. Therefore, the intensity values and frequency shifts could be considered as key measurements to detect the concentration of chlorpyrifos. Good linearity between the concentration of chlorpyrifos and the intensity values, as well as frequency shifts, can be seen in in Figure 7c and d. The regression coefficients achieved between chlorpyrifos concentration and the peak intensity and the frequency shift were 0.9943 and 0.9750, respectively. These results showed that both spectral peak intensity and peak frequency shift had good resonances with chlorpyrifos concentrations ranging from 0.1 to 100 mg ∙ L−1 when deposited onto the surface of 9

the semiconductor metamaterial. In addition, the peak intensity response of the absorber to 0.1 mg ∙ L−1chlorpyrifos solution was 0.144 (a.u.), and the frequency response of the absorber to 10 mg ∙ L−1 chlorpyrifos solution was 4.6 GHz. Under the action of metamaterials, the amplitude of the absorption peak was more sensitive to changes in concentration, so the detection result obtained using amplitude variation was better than that using frequency shift. To further improve sensitivity and selectivity, the THz metamaterial could be fabricated by designing a suitable grating that fits the response of the sample. This study demonstrated that the proposed semiconductor metamaterial can be used as a signal enhancement biosensor for quantitative detection in the field of THz

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applications. It has sensing potential, which provides the possibility for rapid and accurate detection of compounds in the field of food safety.

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4. Conclusions

In conclusion, we have designed and fabricated an ultra-compact, polarization-insensitive THz-

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wave absorber by using artificially doped free carriers. The absorber is tunable and acts well as a highly sensitive sensor. This broadband perfect THz absorber is based on an all-dielectric structure

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and is independent of polarization and has been theoretically and experimentally demonstrated to be highly efficient. This absorber highlights a new strategy in the control of dielectric metasurfaces. In all, a novel semiconductor metamaterial fitted for THz-devices based on extraordinary reflection

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has been developed, which has good stability and can be used effectively to enhance sensing in the trace molecular detection of biological and chemical substances. By using this metamaterial

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absorber combined with THz technology, a trace amount of chlorpyrifos pesticide was detected. Specifically, both the spectral intensity and frequency shifts were sensitive to the concentration of

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chlorpyrifos, which illustrates the effectiveness of the metamaterial absorber. It has profound significance for developing a cheap, stable, sensitive, and precise THz biosensor, which is a key step toward real applications in the biosciences.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements This research was supported by the National key R & D Program (2018YFD0700704), and by the Natural Science Foundation of China (61975163), and by Open Project of Key Laboratory of Engineering Dielectrics and Its Applications, Ministry of Education (KEY1805) .The authors would like to acknowledge the technical support provided by Shenzhen Institute of Terahertz technology and Innovation.

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Author Biographies Pengcheng Nie is an Associate Research Fellow in the college of Biosystems Engineering and Food Science of Zhejiang University. He received M.S degree in Electronic Information Technology from East China Jiaotong University (2008) and PhD degree in Agricultural mechanization from Zhejiang University (2012), respectively. His main research interests are Advanced Spectral Analysis and Spectral Instruments, Digital Agriculture and Agricultural Sensor Technology. E-mail: [email protected]. Dongying Zhu received bachelor degree from Zhengzhou Aviation Industry Management College Zhengzhou (China) in 2017. Currently Master's degree studying at Harbin University of Science and Technology with research experience in design of nano/micro photonics, optics, and electromagnetics devices. E-mail: [email protected].

focused

on

the

analysis

and

modelling

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Zijian Cui is a postgraduate student at Harbin University of Science and Technology. His research is metamaterial

[email protected].

and

metasurface.

E-mail:

Fangfang Qu is a Ph.D student in the college of Biosystems Engineering and Food Science of Zhejiang

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University. She received her M.S. degree in Computer Science and Technology from China Three Gorges

University in 2016. Her main current research interest is Digital Agricultural Information Processing and

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Terahertz Technology. E-mail: [email protected].

Lei Lin is a Ph.D student in the college of Biosystems Engineering and Food Science of Zhejiang University. He received his M.S. degree in Agricultural Mechanization from Jiangxi Agricultural [email protected].

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University in 2014. His main current research interest is Raman Spectroscopy Technology. E-mail:

Yue Wang received the Ph.D. degree in microelectronics and solid-state electronics from Harbin Institute

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of Technology, Harbin, China, in 2011. He has been a professor in the Department of Applied Physics, Xi’an University of Technology, Xi’an, China. He was a research scholar in College of Engineering, Boston University, Boston, MA, USA. His research interest focuses on the terahertz pulse spectroscopy

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of carbon-based nano-materials, antenna and propagation, surface plasmon polaritons, terahertz

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metamaterial and applications. E-mail: [email protected]

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Figure 1. The structure of the sample. (a) Schematic diagram of structural array. The t1=60μm，t2=300μm (b) top

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view of microscope photograph which the p=200μm，r = 75 µm, r1 = 60 μm, and r2 = 32 μm

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Figure 2. The reflection and absorption spectra for the simulated and measured results of the all-dielectric terahertz

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absorber.

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Figure 3. The distribution of (a), (b) the electric field. (c), (d) current densities. (e), (f) power flow for the two

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absorption peaks and surface current densities at resonant frequencies of 0.93 and 1.33 THz.

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Figure 4. The dependences of the absorption spectra on grating period p (a), and grating outer radius r (b).

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Figure 5. (a), (b) Experimental and simulated absorption amplitude at optical powers of 400mW and 900mW,

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respectively. (c) Simulated absorption amplitude for various optical fluences. (d) The peak absorption frequency and

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absorbance at 0.93 THz versus the pump fluence.

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Figure 6. Stability properties of absorber sensor under different factors. (a) stability of peak intensity under temperature, (b) stability of peak intensity under humidity, (c) absorbance spectra over a continuous testing, (d) peak intensity over a continuous time respond.

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* Note: The error bars in Figure 6(a) and (b) were the peak (0.93 THz) intensity errors among five

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repeat scans of spectra under different points of humidity (6a) and temperature (6b).

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Figure 7. Schematic diagram of terahertz metamaterials detection of chlorpyrifos. (a) the measured absorption

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spectra of the metamaterial with (w/) and without (w/o) chlorpyrifos. (b) the regression curves established based on

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the spectral intensity variations (c) and the frequency shifts (d).

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