Characterization Of THz Wave Generated From Gas Plasma

Terahertz,as a cutting-edge technology,has unique physical properties.It has great potential in spectrum imaging,biomedicine,radar remote sensing,national defense safety and modern communication.Terahertz radiation source is an important foundation for terahertz technology development.Developing high energy and wide spectrum terahertz radiation source is very important for terahertz technology development and application.Therefore,the development of wide-band and adjustable strong terahertz source is an urgent problem to be solved.There are several methods to generate terahertz wave.The way of femtosecond laser induced gas plasma can produce high efficiency and wide spectrum terahertz wave.It has become a popular research field and attracts a large number of scholars' attention.At present,the main way to increase the energy of the terahertz wave generated from femtosecond laser is to improve the pulse energy of laser.However,this method is limited by the performance of the laser system.The traditional method can not continue to enhance the output energy of the terahertz wave.To solve this problem,this thesis studies the generation of terahertz wave from the laser induced gas plasma,includes the characteristics of terahertz wave generated from long-wavelength laser induced air plasma,alkali metal vapor plasma,auto-focusing laser induced air plasma and two-color laser with unusual frequency ratio induced air plasma.The concrete contents are as follows:1.Characterization of terahertz wave generated from air plasma induced by two-color lasers.It is found that the thickness,position and rotation angle of BBO crystals which are used to produce two-color laser can cause the change of the radiation efficiency of the terahertz wave.This is because the thickness and position of the BBO crystal will affect the phase difference of the two-color laser,and the rotation angle will affect the polarization direction of the two-color laser.It is found that the wavelength of the pumped laser will affect the energy of terahertz wave,that is,the longer wavelength can generate terahertz wave with higher energy,and it will also cause the change of the emission angle and polarization state of terahertz wave.All above results can be well explained with the photocurrent model.2.Characterization of the terahertz wave produced by laser induced alkali metal vapor plasma.It is found that the variety of the gas medium will affect the production efficiency of the terahertz wave.Within low vapor pressure region,the efficiency of terahertz wave produced by alkali vapor is about one order of magnitude higher than that of nitrogen.This is because the alkali metal possesses the lowest ionization energy of all elements.It is also found that the length of the wavelength of the pump laser can further increase the efficiency of the generation of the terahertz wave.Meanwhile,the trend of the energy of terahertz wave with the pressure of the gas is also studied.All above results are well explained by the photocurrent model.3.Characterization of the terahertz wave generated by auto-focusing laser based on a spatial light modulator.The Gauss beam was transformed into an auto-focusing beam by using a spatial light modulator,to produce the terahertz wave.It is found that the energy of the terahertz wave produced by the auto-focusing laser is up to 13 times as high as that produced by the Gauss laser under the same condition.And the writer captured the fluorescence distribution of the plasma filament using a CCD camera,it is found that the length of plasma filament produced by auto-focusing laser is longer than that of gaussian beam,and the space distribution of terahertz wave is also different.4.Characterization of the terahertz wave produced by two-color laser with unusual frequency.The writer carried out the experimental verification that the two-color laser with frequency ratio of 1:4 and 2:3 can also produce terahertz wave,and their spectral and polarization properties were studied.