Preliminary Investigation Of Terahertz Generation By Difference Frequency Based On CO₂Laser

Terahertz radiation generation by difference frequency has advantages of simplicityfor tuning, straightforward alignment and with high power and coherent THz output.TEA CO₂laser is good pump source for THz-DFG for its high power and multilinetunability. GaSe crystal is one of the crystals having the lowest absorption coefficientwhich makes it potential for THz-DFG. In this thesis, THz-DFG based on CO₂laser isstudied hoping to get THz wave with high power and good tunability. On the theoreticalside, phase-matching parameters for GaAs, ZnGeP2and GaSe DFG based on CO₂laseris computed, and taking effective nonlinearity, infractive index and damage thresholdinto consideration, a comparison of performances of these crystals are made and specialadvantages of each crystal are analyzed. According to power expression of THz wavegenerated by DFG, impact analysis of phase-mismatching and absorption of crystal ismade indicating the importance of exact alignment and designing optimal length ofcrystal. On the experimental side, pump system consisting of TEA CO₂laser andrf-excited waveguide CO₂laser is designed and investigated. The TEA CO₂laser istuned at10.25μm with peak power of tens of kilowatt and the waveguide laser is pulsedwith peak power of more than100W at10.59μm operating at high repetition rate.Experimental setup of THz-DFG based on GaSe crystal is designed and beamcombining and synchronization of the two pump laser pulses and other aspects areinvestigated. The synchronization is bad mainly owing to the uncertainty of dischargingtime of TEA CO₂. The estimated peak power of THz wave is given based on experimentparameters.