Research On Terahertz Ultrashort Pulse Generation And Detection Technology Based On Nonlinear Crystal

Terahertz wave spectrum is between 0.1 and 10 THz.There is low photon energy,it has high penetration for some nonlinear materials,and has strong dispersion and absorption in many organic macromolecules.So for,it widely applied in many fields,including medical imaging,security detection,material spectrum identification,optical communications,and so on.The paper focuses mainly on the THz pulse generation and detection with nonlinear crystals.The research work consists of three parts as followings:1.Theoretical and experimental studies of the single-shot THz electro-optical sampling(EOS)with orthogonal balance detection.Here we propose and demonstrate experimentally an innovative design that has combined the existent single-shot electro-optical detection technology based on spectrum coding and our orthogonal balanced detection technology reported in 2014,in order to realize single-shot detection with high signal-to-noise ratio(SNR),high modulation,as well as large linear dynamic range.Compared with the existent design,our experiments show that our design has much larger dynamical range.What is more,we have also improved the optical modulation depth by 2.0 times,the SNR by 2.2 times(588: 1 pv.267: 1).2.Theoretical and experimental studies of the vortex THz pulse generation based on the optical difference frequency.Here,to our knowledge,we present the first experimental realization to generate THz vortex pulse from nonlinear frequency conversion.According to the angular momentum conservation of the optical frequency difference,one can generate the vortex terahertz pulse through two vortex light with different topological charges,which has been verified with our experiment.In the experiment,the two pulses with orthogonal polarizations but conjuged topological charges are used to pump collinearly a <110> Zn Te crystal.The pumps are chirped enough to aviod the THz generations from their own optical rectifications.Our measurements show that the collinear pump configuration allows to the stable generation of THz vortex pulses.Our design needn't any THz modulator on the beam path for inducing sphiral phases,which,instead,are carried out with some phase modulators in near infared region,thereby works more effectively and easily.3.The polarization spectral measurements based on rotatory optically dispersion(ROD).The technology utilizes an optically active crystal to induce ROD,so the different frequency components of THz pulse propagate with different polarization orientation.Then,the output terahertz wave is focus on the sample so that the sample information can be obtained for terahertz waves with different polarization directions.By using two balanced detectors,we can obtain the horizontal and vertical polarization components of the terahertz wave at the same time,and then the terahertz spectrum of each polarization direction can be obtained by properly processing the detection results.The innovation of this method is that only one time scanning can obtain the spectral characteristics of samples under different polarization directions and different terahertz wave incidence.