Terahertz Time-domain Spectroscopic Setup And Measurement With Free Space Electro-optic Sampling

Terahertz (THz) radiation, which occupies a large portion of the electromagnetic spectrum between the infrared and microwave bands, offers innovative imaging and sensing technologies that can provide information not available through conventional methods such as microwave and X-ray techniques. In the last decade, rapid progress in ultrafast laser technology provides a steady and reliable optical source for the THz pulses generation, which greatly promotes the research in the THz waveband that is difficult to access before.Chapter one introduces the character of THz radiation. THz spectra of materials contain abundant physical and chemical information. Therefore, THz technology has widely used in many domains, for example foundational research, industrial and military applications. As THz wave (T-ray) technology improves, new T-ray capabilities will impact a range of interdisciplinary fields, including communications, imaging, medical diagnosis, health monitoring, environmental control, and chemical and biological identification. It offers an opportunity for transformational advances in defense and security.Chapter two is an overview on generation and detection of THz radiation. The two common methods of THz generation are photoconduction and optical rectification. These two methods can generate broad band pulsed THz wave. Photoconductive sampling and free space electro-optic sampling are usually used to detect THz radiation. Next, We give the methods of building a THz time-domain spectroscopic setup and the mechanisms of terahertz wave generation. Because there is not commercial terahertz spectrometer, we must build the experimental setup ourselves. The methods which are introduced in this chapter are the experiences that we obtained in the experiments. The signal to noise ratio and dynamic range are two important parameters to show the capability of a spectroscopic system. I mention the method to calculate the two parameters at the end of this chapter. We have done some experiments on THz wave emission. By using the transmission-type of THz setup and low temperature GaAs (LT-GaAs) as the emitter, I studied the relationship betweenthe THz signal and biased voltage. In addition, LT-GaAs and a semi-insulating GaAs(SI-GaAs) are used to generate THz electromagnetic radiation. The character of THz radiation generated by each one is compared and analyzed. The results show that the spectral region of emitted THz wave is different.Chapter three illustrate the results of the terahertz time-domain spectroscopic measurements that I have done. I present a comparison of THz spectra for several different materials. The complex refractive index of these materials is obtained. The results show that the Polystyrene is a kind of excellent material for the THz application. It possesses very small absorption at low frequency, such as lower than 3 THz, but almost can block all of near-infrared and visible light.We also measured four different samples of explosives. It can be seen that the four explosive samples all have characterized absorption peak in the frequency band between 0.2THz to 2.5THz. These spectroscopic signatures can be used to identify explosives and energetic materials.Chapter four is the summary of this thesis.