Research On Terahertz Time-domain Spectroscopy Of GaSe And Difference Frequency Generation Of Terahertz Radiation In GaSe

Terahertz sources and terahertz time-domain spectra are two important fields interahertz technology and attract great interests of research in recent years. We have builtup a terahertz time-domain spectral system and use it to investigate the absorptionspectrum of GaSe crystals combined with the calculation of density-functionalperturbation theory. In addition, we have realized a tunable terahertz wave source with anarrow line-width by difference-frequency generation in GaSe crystals. The majorresults of this thesis are outlined as following:1,We have set up a terahertz time-domain spectral system. After extensiveexperimental comparison of many kinds of terahertz sources generated byfemtosecond laser, the terahertz time-domain spectral system is finally based onthe optical rectification and electro-optic sampling of ZnTe (110) crystal with thethickness of 1 mm. The geometrically optimized system is a good experimentalplatform for the study of optical properties of many samples in terahertz rangeswith a signal-to-noise ratio more than 1000, a spectral range in 0.1-2.5 THz and aspectral resolution about 50 GHz.2,We have validated the optimization of the terahertz time-domain spectral system.Firstly, terahertz waveforms were analyzed by solving nonlinearly coupled waveequations. The different influence of group-velocity mismatch on the ZnTeemitter and the sensor was pointed out and the optimal balance between bandwidth and sensitivity of the system was experimentally demonstrated using 1 mmthick ZnTe as an emitter and 3.5 mm thick ZnTe as a sensor. Secondly, thebalanced cross-polarization detection mode was proposed by the noise analysisof the system and the improvement of the signal-to-noise ratio wasexperimentally confirmed to 2.5 times higher in the same condition.3,We have investigated the absorption spectra of GaSe crystals in the terahertzrange. Abundant absorption structure beside the reststrahlen band of GaSecrystals was found when the FTIR absorption spectrum was measured in theroom temperature. The absorption peaks of GaSe crystals in the terahertz rangewere assigned by time-domain spectrum at low temperatures combined with thecalculation of density-functional perturbation theory and the stack faults in theGaSe sample were predicted.4,We have demonstrated the tunable, narrow line-width terahertz source based ondifference frequency mixing in GaSe. Employing an Nd:YAG laser and a narrowline-width OPO, the tunable terahertz radiation was realized by collinearphase-matching. The absolute pulse energy was measured with a bolometeraccording to its response to the stable radiation by constructing of a long terahertz pulse from its single response to a nanosecond terahertz pulse. Thefrequency of terahertz radiation is measured with a FTIR using the step-scanfunction. The all-solid terahertz source produces 17.3 W peak power at 3.07 THzat a repetition of 10 Hz in the tunable range of 0.5-4.7 THz with a line-width lessthan 0.3 cm-1 and has a potential application in high-resolution spectralinvestigations.5,We also carried out some work related to this thesis. The transmission scanterahertz imaging was introduced following the terahertz time-domain spectrum.The dual-wavelength OPO was developed and the tunable mid-infrared radiationwas obtained in the GaSe before the terahertz source was realized.