| Defined by the the frequency range of 0.1 to 10 THz(1012 Hz) and located between far-infrared and microwave in electromagnetic spectrum, terahertz radiation is commonly hard to generate and detect. However, since the THz part of the spectrum is energetically equivalent to many important physical, chemical and biological processes including superconducting gaps and protein dynamical processes, it is of great interest to facilitate experimental research in this region and has stimulated many methods to fill this gap in the past two decades.This thesis concentrate on the terahertz time-domain spectroscopy (THz-TDS). The experiment implement optical rectification to generate terahertz radiation with femtosecond laser and install a pump-probe system. The time shapes of terahertz radiation with and without sample are recorded, after fourier transform, the complex refractive index standing for the absorption and dispersion in THz region is extracted. According to Lorentz oscillator disperse model the complex refraction index can be used to figure out the resonant absorption frequency(ωto) of the sample.Chapter 1 is a breif review of terahertz optics, with emphasis on its fundmental physical significance. In Chapter 2 an ultrafast dynamics in the semiconduct is scrutinised, some key techniques in femtosecond laser system are surveyed. Furthermore fourier transform is detailed in physical and mathematical angle. Chapter 3 introduces the experimental scheme of THz-TDS.Chapter 4 and Chapter 5 are my work. In Chapter 4 I present a new calculating method of time-shape of terahertz radiation in the relaxation from kinematic view which give an well fit to existed experimental data, in the later thesis discuss the electromagnetic signal delay in the the ultrafast dynamic process and point out it is unadvicable to calculate time-shape...
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