Study On The Techniques Of Terahertz Time-domain Spectroscopy

The terahertz wave refers to an electromagnetic wave within the frequency region from 0.1 to 10 THz,which sits between microwave and infrared radiation.The terahertz wave band sits in a very special position in the electromagnetic spectrum,and has many unique properties,such as low energy,penetrability,"fingerprint" characteristics etc.,making terahertz wave very important application prospects in safety inspections,environmental monitoring,military communications,and biomedical sciences,astronomical observations,and so on.Among them,the terahertz wave time-domain spectroscopy has shown important applications in many fields by virtue of its unique technological advantages,and has attracted the attention of the majority of scientific researchers at home and abroad.This thesis focuses on the terahertz time-domain spectroscopy and has carried out the following research work:(1)The thesis has summarized the basic characteristics and typical applications of terahertz waves,and introduced the principles of terahertz wave spectroscopy,its development and applications,including terahertz time-domain spectroscopy,terahertz time-resolved spectroscopy and terahertz emission spectroscopy.The thesis focuses on the introduction of commercial terahertz time-domain spectrometers and its characteristics developed by a number of foreign companies,and points out the necessity and urgency of independently developing an integrated terahertz time-domain spectroscopy system.(2)The thesis elaborates the design scheme of the integrated terahertz time-domain spectroscopy system.A femtosecond fiber laser used as an excitation source for the integrated terahertz time-domain spectroscopy system was independently developed according to the design scheme.The laser spectrum and far-field spot of the femtosecond fiber laser output were experimentally tested.The shortest pulse duration of the fiber laser reached 60 fs,which can fully meet the demand of the excitation laser source used for the terahertz time-domain spectroscopy system;encapsulation of the excitation laser source and time delay module in the integrated terahertz time-domain spectroscopy system was also completed;an optically controlled time delay and data acquisition program for the integrated terahertz time-domain spectroscopy system was made using the graphical programming language LABVIEW.(3)The terahertz time-domain spectral system based on the two-color laser-induced air plasma had been further optimized.The peak-to-peak amplitude of the terahertz time-domain signal after system optimization has increased by 3 times,and the frequency range has been expanded from 0.1~5THz to 0.1~9THz.The effect of two-color laser wavelength on the terahertz characteristics of plasma radiation was studied based on the photocurrent model.It is found when the fundamental laser wavelength is in the range of 400-1600 nm,the intensity of terahertz signal increases with the increase of the wavelengths of the two-color laser.However,as the fundamental laser wavelength increases further upto the frequency range of 1600-5000 nm,the intensity of the terahertz radiation will increase in oscillation as the wavelength of the fundamental laser increases.