Theoretical And Experimental Study On A High Performance Potassium Titanyl Phosphate Terahertz Parametric Source

Terahertz wave means the electromagnetic radiation wave between 0.1 THz and 10 THz.It is located between the microwave and infrared region.It has many unique properties,and has wide application prospects in the fields of biomedicine,material science,safety monitoring,communication and national defense.Terahertz radiation source are one of the key factors for the development and applications of terahertz technologies.Looking for terahertz radiation source with better performance has always been the pursuit of terahertz researchers.Among them,terahertz parametric sources are one of the important terahertz radiation sources.It has many advantages,such as narrow line-width,continuous tunability,room temperature operation,simple structure,high peak power,good temporal and spatial coherence and so on.The physical basis of terahertz parametric sources is stimulated polarnton scattering(SPS).The process of the stimulated polariton scattering can occur in suitable nonlinear crystals.Currently,it has been found that LiNbO3(LN)crystal,KTiOP04(KTP)crystal,KTiOAs04(KTA)crystal and RbTiOP04(RTP)crystal can be used for terahertz parameter sources.It is a pity that the generated terahertz pulse energy is very limited.The combination of a Stokes parametric oscillator and a Stokes parametric amplifier is one of the important means to solve the problem.The Stokes parametric oscillator generates a Stokes pulse first.It is used as the incident Stokes pulse for the Stokes parametric amplifier.The terahertz pulse can be generated by SPS when the Stokes parametric amplifier amplifies the incident Stokes pulse.In this way,when the energy density of the pump beam is less than the damage threshold of the nonlinear crystal,the size of the pump beam is not limited,so the energy of the pump pulse is not limited.By expanding the beam of the incident Stokes pulse,good spatial overlap of the pump and Stokes beams can be realized.The delay device on the path of the pump beam can ensure the good temporal overlap between the pump pulse and the Stokes pulse.The operation process of terahertz parametric sources can be described by coupled wave equations.They describe the changes of the three waves in their propergating directions.However,in the previous theoretical processing,there are some unreasonable assumptions.First,the pump intensity is regarded as a constant.Second,the propagation directions of the pump wave.Stokes wave and terahertz wave are regarded as collinear.In the process of stimulated polariton scattering,the pump wave is severely consumed,and the non-collinear phase matching angle between the pump and terahertz waves is also very large(up to about 60°).Therefore,the depletion of the pump wave and the large phase matching angle are the necessary factors to be considered in the theoretical processing.In addition,the vertical surface emission structure for the terahertz wave has a great influence on the terahertz wave propagation in the crystal,and the influence of the pulse temporal shapes,beam spot spatial shapes and intensity distributions of the pump and incident Stokes waves on the intensity distributions of the three waves in the crystal can't be ignored.As one of the new terahertz parameter source crystals,KTP crystal has the advantages of mature growth technology,good optical quality and high damage threshold.The terahertz band(mainly 3.3-6.5 THz)produced by KTP crystal is different from the terahertz band(mainly 0.9-3.1 THz)produced by MgO:LiNbO3 crystal.Therefore,it is necessary to explore the potential of KTP crystals in obtaining large energy terahertz pulses.In this paper,a high-energy terahertz wave output is obtained by using the experimental scheme that combines the Stokes parametric oscillator and the Stokes parametric amplifier.The operation characteristics of Stokes parametric oscillator and Stokes parametric amplifier based on KTP crystal are studied respectively.At the same time,in terms of Stokes parametric amplifier,we present a numerical method for solving the coupled wave equations and simulate the operation process.The theoretical results are verified by the experimental results,which are helpful to the design and optimization of terahertz parametric sourcesThe research contents of this thesis are as follows.1.Study on the generation of high energy Stokes pulses from Stokes parametric oscillators.The main purpose of the Stokes parametric oscillator is to generate a higher Stokes wave pulse,which is then amplified by the Stokes parametric amplifier.Therefore,the input and output characteristics of the Stokes parametric oscillator based on KTP crystal are mainly studied,and the effects of angle tuning curve,polarization direction and the output mirror transmission are considered respectively.The experimental results show that when the angle between the pump and Stokes waves is 4.4°,and the pump polarization direction is parallel to z axis of the crystal,and the output mirror transmission is 60%,we can get the biggest Stokes and terahertz wave pulses.2.Study on the generation of high energy terahertz pulses by the combination of Stokes parametric oscillator and Stokes parametric amplifier.Based on the experimental results of the Stokes parameter oscillator,we choose 4.4° as non-collinear matching angle in this experiment.When the energies of the pump wave and the incident Stokes wave are the maximum,the terahertz wave output energy is optimized by adjusting the delay between the pump and incident Stokes wave pulses.When the pump pulse energy is 580.0 mJ,the incident Stokes pulse energy is 36.6 mJ,and the delay between the pumping pulse and the incident Stokes pulse is 1.7 ns,the amplified Stokes pulse energy is 192.1 mJ,and the obtained maximum terahertz pulse energy is 17.0 ?J.In addition,by studying the pump loss characteristics of different time delays,we further confirm that the delay adj ustment is of great significance to generate higher terahertz wave pulses.3.Theoretical simulation research on the Stokes parametric amplifier.Considering the large angle non-collinear phase matching,pump beam depletion,terahertz wave surface-emitted structure.Gaussian pulse distribution and other practical factors,we make a detailed theoretical simulation for the Stokes parametric amplifier through the coupled-wave equations.The theoretical results are in agreement with the experimental results on the whole.We also give a reasonable analysis of the difference between the theoretical and experimental results.