Theoretical Study On The Terahertz Radiation Generated From Gas Driven By Combined Elliptically And Linearly Polarized Laser Field

Terahertz waves have attracted much attention because of their high transmission,low energy,transient and coherent properties.Terahertz waves have shown important application prospects in many fields,including physics,medicine,biology,security imaging,information transmission and so on.For terahertz radiation sources,increasing the intensity of terahertz and regulating the polarization state of terahertz are very interesting research topics.In this paper,a two-color frequency-doubled femtosecond laser field is used to drive Ar atoms to generate terahertz radiation.The effects of phase difference and polarization angle of two laser pulses on the intensity and polarization state of terahertz waves are simulated by the photocurrent model theory.The main work carried out in this paper and the results obtained are as follows:Firstly,the radiation characteristics of terahertz waves generated by a two-color linearly polarized laser field(800 nm+400 nm)with polarization angle are simulated.For a given laser phase,the terahertz radiation decreases with the increase of the angle of polarization between two laser beams.When the phase difference is about 0.1π,the polarization state of terahertz radiation gradually changes from linear polarization to elliptical polarization.The ellipticity of terahertz radiation reaches its maximum at85° and decreases rapidly to 0 at 90°.These elliptical terahertz waves are weak and differ by a factor of 10 from the strongest linearly polarized terahertz waves produced at an angle with the laser.Secondly,the polarization and intensity characteristics of terahertz waves radiated by a combined field consisting of a linearly polarized pulse(800 nm)and an elliptically polarized laser(400 nm,with an ellipticity of 0.3)acting on Ar atoms are studied.The results show that the terahertz of radiation is strongest when the laser phase difference is ～0.5 at any angle of laser polarization.When the polarized angle of the two laser beams increases to ～60°,the elliptical terahertz radiation appears,and the ellipticity reaches its maximum at the angle of ～85°.At any laser phase,the terahertz yield decreases monotonically with the increase of laser angle.The total laser intensity and intensity ratio do not change the variation trend of terahertz ellipsometry and yield with laser angle.The simulation results well explain the experimental phenomena presented in Optics Letters,43,90(2018).