| Terahertz (THz) radiation is gradually providing an ability to bring the new opportunity for the research of science and the application of technology, Terahertz radiation itself proposes a huge value of research, which attracts more attentions of many resear chers from several fields. The ultrafast laser technology develops quickly as long as ten or more years. It provides a stable and credible excitation source for the generation of terahertz pulses, which brings a booming development of the mechanism research, the measuring technology and the application technology of THz radiation. Terahertz spectra of materials, including the emission, reflection and transmission, are very important. They imply the abundant physical and chemistry information. To study the THz spectra of materials has a great significance for investigating the structure of materials. Terahertz radiation can be a complementariness of infrared and X-ray spectroscopy in many ways, which can bring this kind of technology to many important application fields in the future.It is important to research THz transverse mode distribution in THz field, especially the research of transverse mode distribution near the position measurement of THz spectroscope and THz imaging. The focus point of light beam is very important. Its relative energy density is large. The distinguish ability at the focus point is strong. The signal-to-noise ratio there is also high. It is the location of sample in the spectroscopic experiments. Meanwhile it is the location of imaged object in the imaging experiment of point-by-point scanning of THz pulse. The research of propagation characteristics of THz pulse in this location is significant. THz radiation can not be observed by the eyes directly, and it has not sensitive film for THz wave band only recently, so particular instance of THz transverse mode distribution can not be obtained directly. The propagation characteristic of terahertz wave, which is located near the focus point in the terahertz spectroscopy system, is studied in this paper, also sample imaging is researched at this position, the concrete work and related conclusion are as follows:(1) The research object is a slit, whose width is approximate to the central wavelength of terahertz pulse, and transverse mode distribution of terahertz pulse beam is researched near the focus point. As analyzed, the size of terahertz beam is different at the different frequency. At the higher frequency, the size of terahertz beam is smaller. In the condition of same frequency, when the width of slit is narrow, the size of terahertz beam is larger. The obvious diffraction effect is shown.(2) The research object is a slit, whose width is approximate to the central wavelength of terahertz pulse, and spatial frequency distribution of terahertz pulse beam is researched near the focus point, as analyzed by the experimental data, we find that, in the cross section of focus point of terahertz beam, when the distance away from the center of terahertz beam is large, the peak value of terahertz spectrum is small. The frequency of THz peak appears red shift.(3) The research object is a square edged orifice in the experiment of terahertz pulse scanning imaging, the square edged orifice is scanned point by point in the terahertz pulse scanning imaging system, more points are scanned to increase information content of image; the length in every scanning point is increased to improve the smoothness for the imaging at different frequency. Using the broad band characteristic of terahertz pulse and the technology of terahertz multi-spectra imaging, the square edged orifice image obtained at corresponding frequency point, and which can be shown dynamic, consequently, mutative rule of diffraction influence is observed at irradiation of different frequency monochromatic light. At the same time, a ideal image frequency spot is obtained compared with the size between actual sample and the image, we can use this frequency as the selected frequency for the size of square edged orifice.(4) The research object is a card including four different samples, the card is scanned point by point in the terahertz pulse scanning imaging system, using terahertz multi-spectra imaging technology and the terahertz multi-spectra sample identified mean, the image is picked up at different frequency. Different samples have different absorptions at some frequency, as the color depth is different in the image, the sample image changes continually following increasing frequency. Comparing the trend of different curves, the samples can be identified according to the database of fingerprint spectroscopy.We obtain the rules of the transverse mode distribution and the spatial frequency distribution of terahertz pulse beam through the research of spectroscopy characteristics of terahertz beam and multi-spectra imaging, and diffraction influence is observed at irradiation of different frequency monochromatic light, also another identified sample way is obtained, These results provide an important reference for the location of sample in the experiment of spectroscopy and the location of object in the experiment of imaging, It has an important significant for the further THz multi-spectra imaging. |
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