The Development And Application Of Terahertz Time Domain Near-Field System

Terahertz technology is very promising to be used in the biomedical field due to its high safety for biological samples.In particular,terahertz imaging is a completely new label-free approach for the diagnosis of diseases such as tumors.At present,the traditional far-field terahertz imaging technique has a problem of poor spatial resolution,which severely limits the in-depth application of terahertz imaging technology in the biomedical field.The development of terahertz near-field imaging technique can help solve this problem.Based on the theory of photoconductive antenna-based near-field detection and the far-field terahertz time-domain spectroscopy system,in this project we replaced the photoconductive antenna in the traditional terahertz time-domain system by two types of photoconductive antenna microprobes,and built up two sets of terahertz time-domain near-filed systems,i.e.,the transmission mode system and the reflection mode system,by which biological samples were measured.The fulfillment of this project involves quite a lot of work,such as near-field theory study,optical path design,signal modulation and demodulation,control system programming,operation realization interface,sample preparation and tests,data analysis and interpretation,etc.This thesis is structured this way: first,the theory of generation and detection of terahertz waves with photoconductive antennas,the principle of coherent detection,the principle of optical parameter extraction,the principle of near-field high-resolution imaging as well as imaging techniques are introduced;second,technical details of the optical path and circuit design,and the construction and control of the scanning modules are elaborated;third,the establishment of the transmission mode terahertz time-domain near-field system and the reflection mode terahertz time-domain near-field system is demonstrated,respectively;finally,the application of the transmission mode system to image individual cells of the pulp of watermelon,and the application of the refection mode system to distinguish the muscle and fat regions in the porcine tissue by spectroscopy were detailed.The spatial resolution of the transmission mode near-field system was calibrated to be 3 μm,nearly 2~3 orders higher than the traditional far-field terahertz system.Measuring single watermelon pulp cell using the developed transmission mode near-field system is a pioneering work of this kind research.In addition,the established reflection mode near-field system is also a sophisticated instrument.Overall,the results achieved through this project greatly promoted the development of terahertz near-field techniques as well as the application of terahertz technology in biological detections,which strengthened our capabilities to compete with developed countries in this field.