Terahertz Time Domain Spectroscopy System Performance

The terahertz (THz) radiation occupies a large portion of the electromagnetic spectrum between 0.1THz and 10THz (1THz=10¹²Hz). Compared with X-ray, it can be used to make non-destructive imaging. The interacting energy level among molecules . vibrational and rotational energy level of many big bio-molecules located in THz range, so the THz time-domain spectroscopy becomes the complementarity of the infrared spectrum technolgy in many fields. THz wave still has transient state wide band and strong penetrability for nonpolar material. These make it proposes the big applied potential in biology medicine microeletronics , agricultrue national defence voyage and so on.Recently many research institutions in the world have started the deep research to the THz technolgy, and has gotten many important progresses. In China, many universities and graduate school also set up THz laboratories. They maily pay attention to the applying of THz technolgy, only a few institutions have interests in the propertys of THz wave. The whole work of this paper was done in THz lab of departments in physics of Capital Normal University. Firstly we measured the transverse mode distribution of THz wave by THz imaging technolgy, which make people have a intuitionistic understanding to the THz wave; in the same time ,we measured the width of THz wave by the slit method. Secondly we study the characteristics of terahertz radiation from the surface of an (100) InAs crystal excited by laser pulse with different wavelengths from750nm to850nm . Finally, we studied the relationship between the pump and probe power and the signal of THz wave.(1). The characteristics of terahertz radiation from the surface of an (100) P -InAs crystal excited by laser pulse with different wavelengths from750nm to850nm are investigated experimentally and theoretically. This kind of THz radiation mainly has a closer relation with Dember field. The THz radiation from InAs can be interpreted as radiated from accelerated photo-carriers in the Dember field and be detected by using the free-space electro-optic sampling method. The results of experiment show that the Dember field in the surface of InAs, concentration of excited carriers, intervalley scattering and the carriers concentration of different excited state, these factors changed when the wavelength of excited pulse changed, so theradiation efficiency and the effective spectrum width of THz wave were different. This investigation will be useful for the measure of time domain spectrum of samples and the optimization of experiment system. It will also make us understanding explicitly the physical principle of the ultra-fast procedure which THz wave radiates from InAs surface excited by femtosecond pulse.(2). When the power of pump beam keep in a constant, we vary the power of probe beam. We found the signal increases linearly with the increasing of the power of probe beam. The normalized signal waveforms are basicly coincident in different power of probe beam. When the power of probe beam keep in a constant, we vary the power of pump beam. We found the signal alse increases linearly with the increasing of the power of pump beam. But the normalized signal waveforms are not coincident in different power of pump beam. When the power of pump beam is small, the width of spectrum of normalized signal is more narrow. When the power of pump beam is big, the waveforms of normalized signal are coincedent again in different power of pump beam.(3). By using THz point-by-point imaging technolgy, THz transverse mode distribution in the positions, where samples are usually placed, is analyzed qualitatively. By applying the"slit method", the beam width of THz wave was measured . The results show that the shapes of the transverse modes of THz wave in different positons around the focus point have much difference. The minimum diameter of THz beam at the focus point is around 1.0mm, which shows the focusing of THz wave has a limit. The results provide a basic reference for further application of THz spectroscopy and THz imaging.In a word, we confirmed the best position of placing samples in experiments and the results provide a basic reference for further application of THz spectroscopy and THz imaging by the studing of the transverse modes distribution of THz wave. The study of the characteristics of terahertz radiation from the surface of an (100) P-InAs crystal excited by laser pulse with different wavelengths from750nm to850nm and the influence of the power of pump and probe beam to the THz siganl, not only it is good for the measure of time domain spectrum of samples and the optimization of experiment system, but also it will make us understanding explicitly the physical principle of the ultra-fast procedure which THz wave radiates from InAs surface excited by femtosecond pulse.