Study On Broadband THz Source In Organic DASTCrystal Via Different Frequency Generation

Terahertz(THz)wave has been widely used in various applications,including non-destructive testing,biomedical science and radar detection,due to its advantages such as low photon energy,wide spectral range and strong penetration.As a key element of THz science and technology,THz source has been one of the research hotspots.A broadband THz wave can be generated by nonlinear difference frequency technology,and it is widely used in spectrum analysis and imaging.For the sake of application requirements of broadband THz source,the dissertation studies theoretically and experimentally in some aspects including growing high-quality DAST crystal,building high-performance broadband THz source and investigating of THz wave amplitude modulation characteristics by light-controlled method based on DAST crystal.The main contents and the creative points are as follows:1.The high-performance DAST crystal was prepared by optimizing crystal purification and growth processes.The growth raw material powder of DAST crystal was prepared by organic synthesis method,and the purity of growth raw material was improved by optimizing the purification process including filtration and multiple recrystallization.Based on the growth habit of DAST crystal,large-size,high-quality DAST crystal was grown using spontaneous nucleation method and seed crystals fixing method,respectively,and the single crystal size reached 4.5×3.5×0.4 mm³.Moreover,the effects of the concentration of saturated growth solution on crystal growth morphology and optical quality during crystal growth process were investigated.The chromophore vibration and rotation characteristics of DAST crystal were analyzed by the Raman spectrum.2.A compact and room-temperature work broadband THz source was demonstrated based on the self-grown DAST crystal.Accorrding to the coupled-wave equation,the energy transfer among three waves was explored in the nonlinear difference frequency process.The effects of the crystal absorption,phase mismatching,crystal length and dual-wavelength pump energy ratio were theoretically studied to the performance of THz wave.The structure and pumping parameters of the broadband THz source were also optimized.A broadband THz wave of 0.1-20 THz was obtained based on the self-grown DAST crystal by the 0-phase phase-matched external cavity difference frequency generation and electronically controlled galvanometer fast scanning technology.When the energy of dual-wavelength pump was 15 m J/pulse,at 18.9 THz,the maximum output energy reached 3.59?J/pulse,and the conversion efficiency was 2.39×10^-4.3.Based on the absorption characteristics of THz wave by the photo-generated carriers,the modulation of the THz wave amplitude was realized in DAST crystal by applying the excitating optical field.The exciton model was used to theoretically analyze the carrier generation mechanism in DAST crystal.The photocurrent response characteristics of DAST crystal under different wavelengths of laser irradiation were investigated.Based on this,the wavelength of the applied excitating optical field was optimized to 532 nm.The terahertz time-domain spectroscopy technique was used to investigate the dielectric properties of DAST crystal under external optical field excitation.When the incident power of 532 nm laser was 80 m W,the transmitted THz intensity was reduced by 26%and the absorption coefficient was increased by 13.7%at 0.68 THz.In addition,photorefractive phenomena caused by photo-generated carriers was also observed in DAST crystal.