Theoretical And Experimental Studies On Terahertz Emission From Laser-induced Plasma

Terahertz(THz)waves(0.1 — 10THz)are electromagnetic radiation which lies between the microwave and infrared region of the electromagnetic spectrum.As this frequency band is very unique in the electromagnetic spectrum,THz waves have many unique features,such as low-energy,penetrating,transient and "fingerprint" characteristics.THz waves have a great many applications,such as safety inspection,environmental monitoring,military communication,biomedicine,celestial observation,and so on.As THz waves have so many potential applications,the THz generation technology(THz source)becomes a very important topic with much concern.THz source based on laser induced air plasma has attracted considerable attention due to its significant advantages,such as ultra-broad bandwidth,no damage threshold,high radiation power,and easily for remote generation.This dissertation mainly focuses on the plasma-based THz source.The factors which affect the THz radiation generation seriously have been studied deeply and detailedly,while the THz emission profile from laser-induced air plasma with different plasma lengths and the major factors that influence THz detection have also been investigated.The detailed contents are shown as follows:(1)The theoretical models of THz wave generation are analyzed in detail.Influence of the laser pulse energy,relative phase and polarization angle on intense THz generation from two-color laser have been simulated based on the transient photocurrent model,and the optimal values of these factors have been obtained,providing the theoretical basis for the later experiment.(2)The THz time-domain spectroscopy system(THz-TDS)based on laser-induced air plasma has been set up and optimized.A number of variables that can obviously influence THz generation and detection have been investigated systematically.The dependence on experimental parameters,including pulse energy(pump pulse and probe pulse),distance between BBO crystal and laser-induced plasma,rotation angle of BBO crystal,chopper frequency and plasma length,and the optimal values of these parameters have also been obtained experimentally.At last,the peak to peak amplitude of the THz signal from the THz-TDS has been optimized up to 15 times higher than it had previously,and the frequency spectrum bandwidth has also been extended from 0.1~2.5THz to 0.1~5THz.(3)By using the Cherenkov model,the characterization of the emission profile of THz wave generated by one color laser-induced air plasma was studied,and the relationship between the radiation angle and the ratio of the wavelength of the THz wave and the plasma length has been deduced.Meanwhile,we have also investigated the relationship between the THz amplitude and the radiation angle with different plasma lengths.