In the past decades，THz-TDS has been widely used in the biological，chemical and food safety fields^［1-4］. Meanwhile，metamaterial，an artificial electromagnetic material with special electromagnetic characteristics，has been typically used for enhancing THz-material interactions to achieve high sensitivities. Generally，a metamaterial consists of periodic resonance elements providing enhancements on the electric field of specified frequency at certain locations of the sensor^［5-9］. Terahertz sensors based on metamaterial have achieved more and more researchers’ interest because of the full interactions between target samples and the enhanced electromagnetic waves. Any modification in the permittivity of the sample will lead to a clear signature in the electromagnetic response which could be recorded by using the THz-TDS. Thus，the THz sensor based on metamaterial can be utilized for the sensitive detection and distinction of biological or chemical materials which have strong THz responses. Furthermore，the inter-molecular signatures in vast biological and chemical molecules are within the broad THz spectral bands^［10-12］.

With the rapid development of the food industry and the progress of chemical synthesis technology，food additives are widely used in the worldwide^［13-15］. Inositol（IS）plays a specific role as the signaling molecule involves the regulation of vesicular trafficking as well as several other nuclear events^［16-17］. Leucine（LC）and Taurine（TR）are also important amino acids that play important roles in human life activities^［18-19］. IS，LC and TR have been widely added as food additive in the infant formula，health food and drink beverage because of the essential supplements for the human body^［20-21］. The traditional detection methods for these food activities mainly include chemical methods，microbiological methods，spectroscopy methods，chromatography methods including gas chromatography，gas chromatography mass spectrometry，ion chromatography and liquid chromatography ^［22-26］. Terahertz detection technology provides the possibility to achieve high sensitivity，low cost and simple operation in the field of food activities. Especially，terahertz waves show great potential in bio-sensing applications because of the absorption of terahertz waves in these bio-molecular systems are pretty impressive.

In this paper，a terahertz sensor based on metamaterial which is composed of an array of double-gap SRR structure with tip ends has been proposed. The properties of the sensor are studied both numerically and experimentally. Meanwhile，the electric field distribution of the sensor with tip end has been studied. The sensing capability of the metamaterial sensor for the distinguishment of IS，LC and TR has been investigated by using the THz-TDS technology. The rapid and real-time monitoring of IS，LC and TR have been demonstrated based on the proposed metamaterial sensor. This method has the advantage of simple operation and low cost，which provide an effective reference for realizing the application of metamaterial in the field of food safety.

The split ring resonator（SRR）with tip ends is selected as the metamaterial resonator element. Simulations were carried out by using Computer Software Technology（CST）Microwave Studio to study the high frequency properties of the metamaterial sensor. The schematic of the proposed metamaterial sensor is shown in Fig. 1，in which a 400 μm thick quartz wafer with the dielectric constant of 3.75 and loss tangent of 0.000 1 is used as the substrate. On the upper surface of the quartz substrate，a 200 nm thick gold with an electrical conductivity of 4.56 x 10⁷ S/m is deposited to construct the resonators. The tip ends are created by cutting an ellipse off a circle that has the same center and cutting a rectangle with a long side of 140 μm parallel to the x direction. The radius of the circle is 70 μm，the major axis of the ellipse is 136 μm，while the minor axis is 124 μm，the edge of the rectangle going across the center of the circle has a height of 12 μm. The incident THz wave was projected vertically onto the structure，with the E-field polarization perpendicular to the major axis of the proposed metamaterial structure. The inset of Fig. 1 shows the details of a single unit of the metamaterial sensor. The other optimized dimensional parameters of the single unit of the sensor are listed in Table 1.

The simulation results of surface current and E-field distribution have been studied and shown in Fig. 2. The E-field polarization direction is parallel to the y-axis，as indicated by the red arrow in the figure. It can be observed that the electric field of SRR structure with tip ends becomes much stronger compared with ones without tip ends. Stronger E-field will lead more intense surface current accumulate at the tip ends，which means a higher sensitivity of the metamaterial sensor.

Then，the proposed metamaterial sensor based on SRR structure was fabricated by using semiconductor processing technology. Fig. 3 presents a typical optical microscopy image of the fabricated structure and the enlarged demonstration of unit cell.

IS，LC and TR powder with a purity of 99.6% was purchased from Shanghai Macklin Biochemical Co.，Ltd. The Deionized water was purchased form Chengdu Kelong Chemical Co.，Ltd. Then，solutions to concentrations of 0.2 mg/ml，0.4 mg/ml，1.5 mg/ml，2 mg/ml，3 mg/ml and 4 mg/ml were obtained by mixing the IS，LC and TR powder and deionized water，respectively. We focused on these six solutions to concentrations ranging from 0.2 to 4 mg/ml because it was found that the solution was beginning to crystallize when the concentration is greater than 4 mg/ml，leading to an uneven sample film when the water is evaporated. When the concentration of solution was lower than 0.2 mg/ml，the sample thin film could not be detected due to the limited sensitivity of the proposed sensor and the limited frequency resolution of the TH-TDS.

The transmission spectra from 0.01~6 THz were obtained by using the TH-TDS provided by Neaspace Company，which supplies 0.01 THz resolution and 80dB dynamic range. The Ti：sapphire femtosecond laser was used to drive the THz-TDS system. The femtosecond laser beam was dived into pump and probe beam. The pump beam was incident on the photoconductive antenna to generate THz pulse. The generated THz pulse was collimated by parabolic mirrors and focused onto the measurement area by using the 4-methyl-1-pentene（TPX）lens. The proposed metamaterial sensor was placed in the measurement area. Then，the transmitted THz signal through the sensor was measured by electro-optic sampling technique based on ZnTe crystal using time delay between the probe beam and the generated THz pulse. The measurement system is shown in Fig. 4（a-b），sensor assembled with fixture was placed into the holder to ensure the consistency of sensing position. During the experiment，THz time-domain spectra of samples were collected in transmission mode. All collections were carried out at room temperature and humidity less than 2%. The specific measurement process was as follows：firstly，the fabricated metamaterial sensor was cleaned with deionized water and dried with drying oven；then，the sensor was putted into a sample cell，and 10 μl sample solutions were dropped on the surface of the sensor by using Sartorius Mechanical Pipette；after that，the metamaterial sensor was placed into the drying oven for 20 minutes with the temperature at 60 °C to avoid any additional absorption by the water；finally，the sensor was mounted at normal incidence to the THz beam for spectrum collection. After the measurement，the metamaterial sensor coated with samples was rinsed thoroughly with deionized water and dried in drying oven again. The measurement for the clean and dry metamaterial sensor was taken before next deposition of the sample in order to ensure that the metamaterial was completely clean and there was no change in its optical properties. In addition，special attention was taken to ensure that each transmission spectrum was measured from the fixed location with the metamaterial sensor before and after depositing sample in order to avoid the measurement error. Three times of whole measurement setups have been implemented for each sample to reduce the random error.

The whole SRR resonant unit can be regarded as an LC resonant circuit，the resonance frequency of which is defined as f=1/2LCwhere C and L are the capacitance and inductance of the SRR metamaterial sensor，respectively. Our previous work^［27］ has shown that the resonance frequency of the LC circuit model is strongly dependent on the capacitance of the dielectric over-layer coated on the sensor surface. Therefore，the resonance frequency changes when the refractive index of the sample changes. The resonance frequency shifts induced by the dielectric over-layer are primarily due to the changes in the SRR capacitance.

To further characterize the SRR resonators and evaluate the property of the metamaterial sensor，the frequency-dependent transmission of the sensor has been simulated by using the Computer Software Technology（CST）Microwave Studio. The results show that all transmittance spectra of the sensor coated with samples are shifted toward lower frequency regime with the increase of the refractive index of samples. The gaps in SRR structure behave like capacitors in the LC circuit which have resonant frequencies depending on the refractive index of surrounding materials. Thus，it can be possible to detect the refractive index of sample coated on the metamaterial sensor by monitoring the resonance frequency shifts. In the simulation，samples with the same refractive index and different thicknesses have been used to represent the samples with different concentrations. The transmission spectra of metamaterial for different refractive indexes with the sample thickness of 0.4 μm are shown in Fig. 5（a）. The relationship between frequency shift and refractive index is shown in Fig. 6（a），the results show that the frequency shift changes linearly with the refractive index value changing from 1.0 to 2.4 in an incremental step of 0.2. The transmission spectra of SRR with the same refractive index of 1.2 and different thicknesses are shown in Fig. 5（b）. The relationship between frequency shift and the thickness of sample film is shown in Fig. 6（b），The results show that the frequency shift changes linearly with the thickness change from 0.4 μm to 2.4 μm in incremental steps of 0.4 μm. The sensitivity of the SRR metamaterial sensor has been calculated，reaching 32.0 GHz/RIU and 46.0 GHz/RIU for the sample thickness of 0.4 μm and 0.8 μm，respectively. Hence，we realized qualitative and quantitative sensing of the sample concentration by using the proposed SRR metamaterial sensor.

The concentration dependent characteristics of the fabricated SRR metamaterial sensor have been investigated in this section. As mentioned above，the metamaterial sensor is sensitive to the relative index and the thickness of the sample solution coated on the surface. The resonant frequency will get red shift when the relative index and thickness of the solution sample increase，which means the sensor can be used for detecting solution sample with different relative indexes and different concentrations.

The positions of resonant peaks’ shift as the concentrations of IS，LC，and TR solutions change，as shown in Fig. 7（a-c）. To observe more details，the region 0.90-1.02 THz is magnified. As shown in Fig. 7，the position of the resonant peak shifts from 0.980 THz to 0.950 THz，0.980 THz to 0.958 THz and 0.980 THz to 0.961 THz as the concentration of IS，LC and TR changes from 0 to 4 mg/ml，respectively. The corresponding measurement sensitivity is 48.6 GHz/RIU，47.5 GHz/RIU and 37.6 GHz/RIU，respectively. The positions of resonant peaks are all red shifted with the increase of the sample solution concentrations，which agrees very well with the simulations. Moreover，Fig. 7（a）shows that the resonant peaks of IS solutions to concentrations of 0.2 mg/ml and 0.4 mg/ml are almost the same，which means the minimum detectable IS concentration of our proposed metamaterial sensor is determined to be 0.4 mg/ml due to the frequency resolution of the TDS system is 10 GHz. Similarly，Fig. 7（b） shows that the minimum detectable LC concentration of our proposed metamaterial sensor is determined to be 0.2 mg/ml，as the resonant peaks of 0.2 mg/ml and 0.4 mg/ml are different. While Fig. 7（c） shows that the minimum detectable TR concentration of our proposed metamaterial sensor is determined to be 2 mg/ml in our experiment，as the resonant peaks of 0.4 mg/ml，0.2 mg/ml and 0 mg/ml are the same. Fig. 7（a-c）experimentally show how the transmittance spectra of the metamaterial sensor changed with the concentration of IS，LC and TR ranging from 0.2 mg/ml to 4 mg/ml. The transmittance spectra of each concentration could be well-distinguished from each other at the resonance frequency peaks when the IS and LC was under consideration. When the sensor was coated with TR solution samples，the transmittance spectra could not be distinguished clearly.

For further study，the frequency shift values of the resonant peaks versus solution concentrations were plotted in Fig. 8. The experimental results agreed to well with the simulation results. The clear linear relationships between the frequency shift values and the concentration of IS and LC were observed and a fitting model for them was built，as indicated by the line in Fig. 8. The frequency shift values increased as the concentration increased，and the fitting equation was estimated to be：

where ρ is the concentration of solution samples in mg/ml，k and m are the fitting coefficients. The fitting coefficients k and m for IS and LC solution sample are 4.71 and 10.98，3.73 and 6.35，respectively. These results indicate that the concentration of solution samples could be predicted based on the different frequency shift，which means the quantitative analysis of sample could be achieved.

In addition，selective detection of proposed metamaterial sensor for different samples with the same concentration has been studied，and the results were shown in Fig. 9. The black line represents a sensor without any samples. The samples with concentration of 4 mg/ml were successively coated on the sensor surface，and the transmittance spectra were obtained. The green，blue and red lines represent the transmission spectra of IS，LC and TR，respectively. There was a significant difference in the transmittance spectra，because the characteristic frequency of IS is closer to the resonant frequency of the SRR sensor than the characteristic frequency of LC and TR，which means higher resonant coupling strength. Furthermore，the SRR sensor can be specifically designed for other target samples by simply changing its structural parameters to make the resonant frequency of the SRR overlap spectral with the characteristic frequency of the target samples. These features are attractive in the field of biological，chemical and food industry.

In short，we have proved the feasibility of detecting food additives based on a metamaterial sensor in the terahertz range both theoretically and experimentally. The common additives such as inositol（IS），LC and TR have been sensed as examples. The results show that the proposed metamaterial sensor can effectively detect concentration of LC as low as 0.2 mg/ml. The proposed detection method is based on the principle that samples of different thicknesses cause different frequency shifts，and it has its positive side and negative one. The positive side was that this approach could be universally applied to other food additives or even biological materials；while the negative one was that the precision and accuracy of the measured results were quite sensitive to the deposition of any non-target samples. Therefore，we must prevent the metamaterials from being contaminated by the non-target samples during the sample preparation and measurement. The detection method with the advantage of simple operation and low cost，which provides an effective reference for realizing the application of metamaterial in the field of food safety in the future.