The Study On The THz-Electromagnetic-Wave Emitter Based The Polariton Of LiNbO₃ Crystal

THz-wave is the electromagnetic wave, which the frequency is from 0.1THz to 10THz (1THz=10¹²Hz), and the wavelength is between microwave and infrared light. THz-wave is undeveloped due to the difficulty in both generation and detection. However, in last 10 years, sub-millimeter wave THz has become one of attractive area of scientific researchs. THz wave can pass through most of the matter, and its THz finger-spectrum can be got easily. THz wave is applying in the regions of condensed matter physics, astrophysics, biology, medicine, security detection. In this paper, the significance of THz research, status quo and THz development direction in the future is analyzed. In addition of these, the research about THz-wave generator which has the tunable ability based on the LiNbO₃ crystal is reported. The research of these papers is as follow:(1) The theroy of the polariton in the solid is dicussed, and the princible of submillimetre THz-generator based on polariton stimulated Raman-scattering is analysed. Combined the theory with experimental research, the performance of the THz-generator which the 1064 nm Nd: YAG laser use as pump, LiNbO3 crystal use as nonlinear medium is study.(2) 85mm-long short resonator cavity TPO which use 5%mol MgO:SLN as nonlinear medium is firstly proposed and demonstrated. Compare with the traditional TPO of 160mm cavity, the threshold values decrease by 22.3%, energy output increase by 170%, the tunable ability of 0.5THz～3.1THz expand to 1.0～2.8THz, and the linewidth of a typical 109 GHz is also measured by using a Fabry-Perot interferometer. Moreover, by using the 85mm short cavity TPO, experiment under the high energy pump beam is performed, and the THz output with the peak energy of 420pJ/pulse at 1.2THz is observed. Up until 2006, this experiment result of 420pJ/pulse is the higest THz output in the same type THz-generator(3) THz output character comparative experimental research of CLN and SLN, which were grew by using two different method are performed.The experiments indicated the output of CLN showed high stability than that of SLN, and the strong photorefractive effect in SLN will cause the phase-mismatch, which is the reason of the un-stability. However, the MgO doped in SLN crystal can restrain the photorefractive effect of SLN, improve its output stability, and improve the threshold of damaged dued to the laser.(4) A THz-surface-emitter(SE-TPO) with symmetric resonator cavity structure is firstly designed and demonstrated. The experiments show that the terahertz radiation beam quality is high, and the THz beam spatial show the Gaussian distribution with the beam quality factor of Mx²=1.15, Mz²=1.25, the tunable frequency range of 0.4～3.19THz, tuning bandwidth of 2.79THz, and the THz beam shown the high focusing properties with focus area less than 0.4mm². The peak output energy of SE-TPO is 104pJ/pulse at the pump energy of 20.3mJ/pulse, and the threshold is lower than 11mJ/pulse, the THz wave linewidth Av is 92GHz.(5) Based on the SE-TPO, THz transmission imaging system which the resolution close to the wavelength of terahertz wave, the signal-to-noise ratio up to 6000, the dynamic range of 37dB is builted. Using this system, the preliminary experiment of non-destructive detacting and online terahertz waves testing are firstly performed. At the frequency of 1.8～1.9THz, this system can imaging clearly for dehydration leaves, the metal blade in packages, the broken metallic wires in plastic tube, and these experiments show SE-TPO has the application potential in many areas.(6) A THz absorption spectra system is builted based on the SE-TPO. THz spectra of some samples included standard riboflavin, maltose and sucrose were measured by using this system. The finger absorption spectra of riboflavin match with the standard spectrum. The experiment results also show that the absorption finger spectra are obvious difference between sucrose and maltose absorption.