Research On Terahertz Source Based On Laser-plasma Interaction

Terahertz technology has been widely used in defense security,biological diagnostics,medical imaging and so on.With the further development of terahertz technology,there is an active demand for high-power and broadband terahertz source.The traditional terahertz source of photoconductive antenna and optical rectification by crystals,are not only in low power,but also have a narrow band.In recent years,owing to its broadband spectrum and high power characteristics,the terahertz source based on laser plasma interaction has attracted considerable attention from researchers all over the world.Based on the principle of laser plasma interaction,this thesis investigated the terahertz source based on the laser gas target interaction and the laser solid target interaction.The mainly aspects are as follows:(1)The experiment of terahertz emission from two color field was carried out.And the whole optical terahertz system was quickly optimized by the spectral-coding single-shot detection technique.In order to produce high-flux terahertz output,the optimized position of the BBO crystal was 51.8 cm long from the focus lens.(2)Three kinds of coherent detection methods for the broadband terahertz were investigated experimentally.Because the energy of probe beam must be high enough in the air coherent detection method,consequently the related pump beam will be lower.Though the coherent detection will be realized with low energy of probe beam in BBO-air coherent detection method.The photomultiplier(PMT)will be easily saturated or damaged by the high conversion efficiency of the BBO crystal.The result of the Air-biased coherent detection method was better than the other two methods.After the optimization,the signal-to-noise ratio of the terahertz signal is 40 dB and its spectrum bandwidth is 0.1-8 THz.By contrast,we finally choose the air biased coherent method as the detection method of high pressure laser gas target terahertz source.(3)The terahertz generation from Nitrogen in different high pressure conditions was studied in two color laser field.The experiment results show the intensity of the terahertz pulse will decrease with the increasement of pressure and plasma density.But the signal will be suddenly enhanced at the pressure of 193 kpa.The bandwidth of terahertz pulse reaches up 0.1-10 THz,and the signal-to-noise ratio is 43 dB,which is better than the results from air.It indicates that the terahertz pulse was actually enhanced at certain high pressure condition.(4)The related research on terahertz source from laser-solid interactions was carried out.In this thesis,we mainly discuss the preliminary works of this part,include the design and fabrication of continuous moving target based on 1 kHz repetition frequency,the construction of observation system in terahertz wave intensity,and the related motion control program written by the Labview software.