Study On The Terahertz Time-Domain Spectroscopy System Based On Nonlinear Crystals And Air Plasmas

Different regions of electromagnetic waves in human life and production have a wide range of applications,such as microwave communications,infrared monitoring,X-ray imaging.Terahertz located between microwave and infrared with transient,low energy,penetration,coherence,fingerprint identification and other characteristics has potential applications in the biomedical,semiconductor,security,defense and military,information and communication technology,Earth and space science,basic research and other fields.As a technology gap in electromagnetic waves,terahertz technology has not yet been widely applied.Terahertz-Time Domain Spectroscopy(THz-TDS)contains the generation,transmission,detection and acquisition of terahertz radiation.It is the basic platform for studying the basic properties and applications of terahertz radiation.The generation and detection of terahertz pulses mainly includes photoconductive antenna radiation and sampling,optical rectification and electro-optical sampling based on nonlinear crystal,terahertz radiation and detection based on air plasma.The terahertz waves generated and probed by different principles have many differences in power,bandwidth,signal-to-noise ratio,dynamic range and polarization,and have advantages and disadvantages in specific sample measurements and application development.Based on the last two kinds of radiation and detection methods,this thesis analyzes the influencing factors of the generation and detection of broadband terahertz waves in THz-TDS based on various experiments and theories,and simulates the terahertz wave transmission and transformation in a THz-TDS and the reflection on a rough surface.The specific research contents are as follows:(1)Build the THz-TDS system based on the optical rectification&electro-optical sampling principle of nonlinear crystal;Build the whole air plasma THz-TDS system based on the transient photocurrent principle;Acquire THz signals from the two systems respectively and compare the differences in the radiation mechanism,detection principle,signal characteristics,system composition and some specific details about construction process of the two THz-TDS systems.(2)Analyze the impact of pump laser pulses parameters,detection laser pulses parameters,nonlinear crystal,chopper modulation parameters and other major optical components on the performance of the THz-TDS system based on the optical rectification&electro-optical sampling using relevant experimental and theoretical methods.(3)Analyze the impact of pump laser pulses parameters,detection laser pulses parameters,frequency doubling crystal,bias voltage,parameters of detector and other major optical components on the performance of the THz-TDS system based on the air plasmas using relevant experimental and theoretical methods.(4)Analyze and simulate the radiation,collimation and focusing of THz waves generated by all the three mechanisms based on the principle of propagation and transformation for Gaussian beam and the principle of ABCD matrices.(5)Utilize the Rayleigh scattering factor and the modified Fresnel formula based on the Kirchhoff scattering theory to determine the reflection coefficients of the TE wave and the TM wave respectively on a rough surface at different incident angles and THz frequencies.