| Terahertz waves are electromagnetic waves that fall between infrared and millimeter waves in the frequency domain spectrum.Over the past few decades,terahertz technology has played an important role in safety inspection,clinical diagnosis,non-destructive testing,environmental monitoring,and information communications for its unique characteristics such as low energy,wide spectrum,and great penetrability.Scientists pay great attention to the terahertz technology.As a key component of terahertz science and technology,terahertz detection holds promise for various potential applications.There are still urgent demands in developing tunable terahertz wave detection technology with high sensitivity,fast response speed and room-temperature operation.With the assistment of relatively mature detectors in near-infrared and visible wavebands,highly sensitive terahertz wave detection can be achieved at room temperature,transforming terahertz wave to near-infrared or visible light through nonlinear optical frequency conversion.In recent years,terahertz frequency up-conversion detection has developed rapidly,but there is still a lack of comprehensive theoretical research to be employed as experimental instruction.This dissertation conducts a theoretical research and preliminary experimental investigation on the weak terahertz wave detection via a 4-dimethylamino-N-methyl-4-stilbazolium tosylate(DAST)crystal based on frequency up-conversion.The main research contents of this dissertation,as well as the innovations are summarized as follows:1.The theoretical optimization of weak terahertz wave detection via a DAST crystal is demonstrated,using the nonlinear frequency up-conversion technique.Firstly,the three-wave coupling equation is proposed using nonlinear optics.Independent programing and detailed simulation are undertook using the Matlab.Secondly,the theoretical optimization of weak terahertz wave detection is carried out,including the selection of conversion mode(difference-frequency process or sum-frequency process),pumping intensity,crystal thickness,pumping polarization direction(along the a-axis or b-axis of DAST crystal)and pumping wavelength.The performance is different between InGaAs-based detectors and Si-based detectors.The advantage of situation with pumping polarization along the b-axis of DAST crystal in terahertz wave detection and imaging are exhibited,taking the noise of actual detectors into consideration.Finally,the terahertz single-photon detection and imaging are further explored theoretically.Theoretical simulations prove that using single photon detectors(SPDs)even avalanche photodiodes(APDs)can realize terahertz single photon detection,and the same for terahertz single photon imaging using single photon camera.2.In terms of experiments,through independent design and construction of optical paths,the terahertz wave detection based on a DAST crystal was realized with room-temperature operation and high sensitivity,with the selection of relevant experimental parameters based on theoretical simulation results.The cutting angle of KTiOPO4(KTP)crystal and coating of mirrors in the optical parametric oscillator(OPO)were designed.The angle of KTP crystals was controlled by Labview programming,and thus the wavelength-tunable infrared laser could be realized.The output lights of double-KTP OPO pumped the DAST crystal,and a Golay cell was employed for measuring the difference-frequency generated terahertz wave.The average power of OPO output was about 25 mW,and the average power of terahertz wave generated through the difference-frequency conversion is about the order from 100 nW to 1 μW.After some pieces of filters,the up-converted signal was collected by a positive-intrinsic-negative(PIN)diode,realizing the tunable terahertz wave detection with high sentivity at room temperature.The detection frequency was adjustable in a wide range of 3～25 THz,and the detection sensitivity was 4 orders of magnitude better than the commercial Golay cell at 4.3 THz.In order to explore a simpler and more portable terahertz source,a stable dual-wavelength laser was tried by applying the novel Yb:YSr3(PO4)3 crystal.3.Further exploration of the frequency up-conversion theory for weak terahertz wave detection is proposed.The four-wave interaction equations of coupled difference-frequency process and sum-frequency process are derived for the first time,considering the joint result of difference-frequency and sum-frequency process.The theoretical simulations of weak terahertz wave detection are carried on,including interaction process of pumping laser.terahertz wave,difference-frequency signal and sum-frequency signal.The influence of sum-frequency signal on detection results under conditions of different crystal thicknesses and terahertz frequencies.The optimization parameters such as pumping intensity and pumping wavelength,and the single photon detection of terahertz wave have been analyzed as well.In addition,the experimental results of sum-frequency and difference-frequency signals in the weak terahertz wave detection were obtained,for different terahertz frequencies and DAST thicknesses.The experimental results were basically consistent with the theoretical calculations,further proving the accuracy of theoretical simulations.In this dissertation,the frequency up-conversion for weak terahertz wave detection based on a DAST crystal is theoretically studied,and further exploration is carried on to enrich the relevant contents.Through designing and constructing the experiment,highly sensitive and tunable terahertz wave detection at room temperature was achieved using a PIN detector. |
