Theoretical And Experimental Research On Collinear Different Frequency Generation THz With CO₂Laser

Terahertz （THz） science and technology attract world-wide research interest for itspotential applications including spectroscopy, pharmaceutical, chemicals, biomedicaldiagnostics, and so on. THz source is one of the key issues. Difference frequencygeneration （DFG） is one of the effective ways to produce stable operation, tunable andcoherent THz radiation at room temperature. The principle of nonlinear optical DFG THz isthe use of two similar wavelength laser beam matching DFG the wavelength of radiation inthe THz region. Due to the same propagation of the two beams, collinear DFG can acquireefficient THz using the area and length of in the crystal.In this thesis, CO₂laser collinear DFG are investigated for producing THz in nonlinearcrystals. The theory of efficient THz acquired for periodic crystals is introduced. Accordingto the Manley-Rowe relation, compared to other lasers, the maximum conversion efficiencycan be improved by mid-infrared CO₂laser for the wavelength running at10μm.Theexperiments are established pumped by one self-made CO₂laser with dual-wavelengthoutput based on collinear DFG in a stacked GaAs wafers and a GaSe crystal. The maincontents are as follows:（1） According to the coupled wave equation, the power and efficiency transfer amongthe three waves has been studied in the process of terahertz wave collinear DFG in quasiphase matched （QPM）. Surface-emitted difference frequency generation （SEDFG） usingQPM has been proposed. We explore the generation of the high power conversionefficiency of THz wave in periodically-inverted crystals with mid-infrared CO₂laser byusing three schemes including perpendicular structure, slant-stripe structure andtwo-dimensional structure. The relations of the angle and the THz wavelength areconsidered. The relation of the grating period is increased when THz wavelength isincreased.（2） In order to obtain various THz waves, the wavelength of the CO₂laser must betuned. So grating tuning technology is used to obtain the wavelength tuning of CO₂laser. Adual-wavelength output, the orthogonal polarization pulses discharge tunable CO₂laser isestablished. Temporal waveforms of dual-wavelength CO₂laser pulses acquired18. Using a compact dual-wavelength CO₂laser around10μm as collinear DFG pumping source, weacquired a compact THz source.（3） Using one self-made CO₂laser with dual-wavelength output, orthogonalpolarization pulses can be tuned range of9.1-10.7μm, the experiment is success tocollinearly phase-matched DFG in a GaSe crystal. THz pulses have been generated at awavelength range of236.3-1104.5μm （0.27-1.3THz）. THz pulses phase-matching angleand phase-matching conditions agrees with theoretical calculations. The maximum singlepulse energy of11nJ was generated at a wavelength of243.6μm （1.23THz）,corresponding to a peak output power182mW.（4） Pumped by one self-made CO₂laser with dual-wavelength output based oncollinear QPM-DFG in a stacked GaAs wafers. GaAs wafers thickness and orientation isselected, and optically-contacted method is bonded to stacked GaAs wafers. We observedthat the QPM-GaAs wafers can effectively increase the THz generation power andefficiency by increasing the number of periods. The maximum single pulse energy of12nJwas generated at a frequency of0.94THz （319μm） by using ten GaAs wafers,corresponding to a peak output power200mW. The maximum single pulse energy isincreasing by10%than that of a GaSe crystal. The scheme for generation of tunable THzwave in fan out structure crystal with CO₂laser is proposed. The length of periodical crystalcan be change by a simple movement, and will be able to achieve tunable THz wave outputin this scheme.