Study On The Terahertz Wave Generation And Detection Characteristics Of ZnTe Crystals

Terahertz wave is an electromagnetic wave between infrared and microwave in electromagnetic spectrum,which is in the cross-band of photonics and electronics.In the last two-decades,with the development of optoelectronics and materials science,the understanding of this wave band is continuously deepening.However,the challenges lie in the production of high power,broad band terahertz radiation source and corresponding high sensitivity detectors.ZnTe crystal is found to be the most suitable laser driving crystal（800 nm）for broadband terahertz wave generation and detection material due to its high first dielectric susceptibility,second nonlinear dielectric susceptibility,and simple collinear phase-matching condition.Although much research has been done on this material,there are still many questions to be clarified about the physical nature of the interaction between optical pulse and the material,the relationship between terahertz wave and material physical properties,and the effect of material uniformity on terahertz wave generation and detection.We observed terahertz wave generation from ZnTe crystals of different thickness by using a 1560 nm femtosecond fiber laser as excitation source.This is the first time that 1560 nm pulsed laser driving terahertz generation from ZnTe is observed experimentally.The phenomenon of terahertz pulses“walk off”at different velocities is experimentally measured.Based on theoretical analysis,the influence of the wavelength of pulse laser,pumped laser group refractive index and phase refractive index in THz range of ZnTe on generated THz wave in time-domain and frequency domain are analyzed.The generation mechanism of terahertz wave in electro-optic crystal is that the pump laser produces a free terahertz pulse at the incident and exit surfaces of the crystal.The velocity of terahertz pulse in the medium is determined by its own frequency.In the region without the coherence length in the electro-optic crystal,the pumped laser produces a moving polarization electric field which propagates with the group velocity of the pump laser,and does not produce terahertz pulses.The intensity and bandwidth of the output THz wave are the result of the interference between two THz pulses in the crystal length.The crystals of Mn incorporated ZnTe with Mn concentrations of 2.8%,3.1%,and 3.4%were grown by the temperature gradient solution growth method（TGSG）.Magnetic,optical and electrical properties of this material have been evaluated systematically.Compared to the intrinsic ZnTe,the transmission of Zn_1-xMn_xTe was improved by 25～40%and 38～56%in IR(1000-4000 cm^-1)and THz range（0-1 THz）,respectively.Moreover,the resistivity increased about 10 times due to the reduced carrier concentration.The lattice imperfection induced by Mn incorporation was identified by Raman spectra,which determines the refractive indices and the phase-matching condition.An obvious enhanced THz response as high as 10.4%～18.9%（emitter）and 16.9～28.0%（sensor）are observed in Mn incorporated ZnTe crystals.Within Mn concentration range investigated,an optimum value for x of～0.028 has been proposed.Due to the different orientations between the twin and ZnTe matrix,the terahertz produced by twin may be in reverse phase with the terahertz wave produced in the matrix,thus weakening the overall terahertz generation efficiency.The electric field distribution on both sides of the twin boundary was characterized by a Pockels extinction detection system.It was proved that the twin boundary would lead to the electric field distortion in the crystal.There was an electric field distortion zone on both sides of the twin interface,and its width could reach 1.2 mm（average electric field was 3.6 kV/cm）.A model of n-n⁺-n type homogeneous junction energy band on both sides of twin boundary is proposed to explain the variation of internal electric field under different bias voltage.By using laser terahertz emission microscopy（LTEM）,both terahertz waveform and its two-dimensional spatial distribution in ZnTe crystals were characterized with two different wavelength excitation sources.The emission efficiency of terahertz decreases by about five times under the incidence of 1560 nm pump laser at Te inclusion.Meanwhile,the defect enrichment zone on both sides of Te inclusion would increase the noise level of the system.For the first time,the terahertz wave produced by Te inclusion under 800 nm pump laser is observed,and its amplitude is more than 5 times higher than that of ZnTe.However,its generated bandwidth is lower（below 0.3 THz）.By measuring the dependence of the amplitude of terahertz wave generated in Te inclusion on the pump laser power and polarization angle,the mechanism of terahertz wave generation in Te inclusion is determined to be photocurrent surge effect.By measuring the terahertz polarization component in Te inclusion,the internal electric field distribution in Te inclusion caused by the heterojunction formed by the contact between p-type ZnTe and Te inclusion was imaged in two-dimensionally.Photogenerated carriers in Te inclusion move back and forth in the inclusion under inhomogeneous internal electric field distribution,resulting in long-term oscillation.The dependence of terahertz amplitude from 800 nm fs laser-pumped ZnTe crystal on azimuth angle of crystal is measured,which verifies the effectiveness of the method of detecting terahertz polarization component by spiral-type photoconductive antenna.The terahertz polarization component data detected by spiral-type photoconductive antenna are corrected by using ZnTe generated terahertz wave data.Using ZnTe crystal as detection crystal,the dependence of terahertz polarization component emitted from GaTe crystal on crystal azimuthal angle,incident angle of laser beam and laser polarization angle was analyzed.A model based on the optical rectification effect and photocurrent surge effect is proposed to describe the THz emission mechanism,which is confirmed by both THz emission spectroscopy and electric property characterization.The results not only demonstrate the THz radiation properties of bulk crystals,but also promise the potential application of THz emission spectroscopy for characterizing the surface properties of layered materials.