Research On Terahertz Rapid Detection Of Pesticide Residues Based On Metal Gratings

Most of the vibration information of chemical molecules is in the terahertz band,so terahertz spectroscopy has outstanding advantages in the field of chemical molecule detection.Due to the weak interaction between the terahertz wave propagating in free space and the substance,it is difficult to obtain a strong terahertz detection signal,and the application is greatly limited,which can not meet the detection requirements of trace substances.In order to enhance the interaction between the terahertz wave and the substance to be detected,and to improve the terahertz detection sensitivity,functional materials with excellent performance have emerged.The most common method is to use the metamaterial structure to improve the detection accuracy by enhancing the local electric field.However,most metamaterial structures use photolithography technology,which is complicated and time-consuming to manufacture.In order to improve the accuracy of terahertz detection,a terahertz detection technology based on metal grating is proposed.At the same time,nanosecond pulse laser is used to process the metal grating.The processing is simple and the time is short,which shows good application potential.This paper analyzes the surface plasmon characteristics of metal grating,studies the interaction between terahertz electric fields and matter,establishes a quantitative detection model based on transmissive and reflective terahertz metal grating,with the help of nanosecond pulsed laser processing technology to realize the fabrication of metal grating,which can be used to detect the concentration of copper sulfate(Cu SO₄)and carbendazim,and provide a new idea for pesticide residue detection and biosensing technology.The main content roughly includes the following four parts:(1)Based on the electromagnetic field theory,starting from the properties of surface plasmons at the metal-medium interface,the dispersion relationship between the incident electromagnetic wave and the surface plasmons polarized wave at the metal-medium interface is obtained by using Maxwell equations.The three main characteristic parameters of the wavelength,propagation distance and penetration depth of the surface plasmons are obtained,and the six ways to excite the plasmons from the metal surface are introduced in detail,and the abnormal transmission properties of metal grating surface plasmons related to the principle of terahertz detection are studied.(2)Construct transmissive and reflective metal grating sensing models.According to the parameter changes in the metal grating structure,the relationship between the structure parameters of the metal grating,the material parameters of the dielectric layer and the resonance frequency of the sensor is established,and the important factors affecting the terahertz detection performance are obtained,which provides theoretical support for the terahertz detection technology based on the metal grating.(3)Design a transmissive terahertz sensing system based on metal grating,which is applied to high-precision detection of Cu SO₄.The direct transmission analysis method is used to study the terahertz absorption spectrum of Cu SO₄.Through CST electromagnetic simulation software,a metal grating with a circular structure is selected,and the structural parameters of the metal grating are optimized and adjusted.Using a polyethylene film with a small dielectric constant as the sensing substrate,and use a nanosecond laser to fabricate a circular hole array metal grating to detect the concentration of trace Cu SO₄,and obtain the relationship between the terahertz transmission spectrum and the concentration of Cu SO₄.The results show that the fabricated metal grating improves the sensitivity of terahertz spectroscopy in detecting Cu SO₄,and the detection limit reaches 0.375 mg/L.(4)Comparative study of transmissive and reflective terahertz detection performance based on metal grating,and applied to the detection of trace carbendazim pesticide content.According to the CST simulation results,a square-structured metal grating is selected,and transmissive and reflective terahertz sensing systems are designed,optimized,and fabricated.The polyimide film with low dielectric constant is used as the sensing substrate,and the unique design of analyte channel and liquid inlet and outlet can be used for testing liquid samples.Comparing and analyzing the spectrograms of the transmissive and reflective metal grating,the experimental results are consistent with the theoretical simulation results,and the relationship between the resonance peak frequency shift response and sample concentration is clarified.It is found that the sensitivity of the reflective sensor is increased by about 8 times compared with the transmissive sensor.