Terahertz Time-domain Spectroscopy For Water And Biological Sample Imaging

Terahertz radiation is the electromagnetic wave of frequency range from 0.1 THz to 10THz(1THz=1012Hz).It is characterized by high penetration,"fingerprint feature"absorption,low energy,large bandwidth and high signal-noise ratio.Terahertz time-domain spectroscopy(THz-TDS)is a coherent detection technology.The basic principle is to use the femtosecond pulse to produce the THz electric field with picosecond,the amplitude and phase information of the terahertz signal can be obtained,and the absorption coefficient of the sample can be calculated.THz-TDS can play a very important role in the research of semiconductors and superconducting materials,imaging of biological tissues,unmarked detection of genes,nondestructive diagnosis of medicine,broadband communication and so on.This paper first introduces the theory and application of terahertz radiation,and then introduces the typical structures of transmission and reflection around terahertz time domain spectroscopy.Subsequently,the research on the refractive index and absorption coefficient of liquid and the imaging of biological samples are carried out in the following two aspects:First,the refractive index and absorption coefficient of pure water are calculated by the transmission spectrum measurement experiment.The refractive index of water is about 2.6 at 0.2 THz,and then gradually decreases to 2.2 at 1.2 THz.The absorption coefficient increased from 130 cm-1 to 250 cm-1 in the frequency range of 0.2-1.1 THz,and the absorption coefficient exceeded 300 cm-1 at 1.4THz.Deuteroxide also absorbs terahertz radiation strongly.Its refractive index is not significantly different from ordinary water,and its absorption coefficient is about 20 cm-1 lower than that of pure water.Based on the difference between pure water and deuteroxide,the two kinds of water are used as solvents to distinguish the structural differences of different DNA.The second is to scan the tissue section of different parts of mice by terahertz scanning,which is mainly based on the uneven distribution of water content within the sample,and the degree of absorption of THz signals in different regions is different.By means of extreme contrast imaging,ratio contrast,absorption coefficient contrast imaging and other data processing methods,the final image of biological tissue sections is marked with different transmission degree of different terahertz signals with different colors.On this basis,it is hoped that through a large number of repeated experiments,the water content of different tissues is quantified from the absorption coefficient,so as to achieve the purpose of identifying the type of tissue.