Terahertz Sources Based On GaSe Crystal And Metal Films

Terahertz（THz） radiation, benefiting from its unique property, has a promising application on basic and applied science, such as spectroscopy, material science, biology, communication and international anti-terrorism. THz sources play a key role in THz application. In this thesis, the phase-matching types in THz difference frequency generation（DFG） process are calculated and compared based on the most potential nonlinear crystal on THz source- Ga Se. The broadband THz radiation is obtained from Ga Se based on the optical rectification and the pump intensity dependence of THz output is studied. Besides, the metal films are used to emission broadband THz radiation and the mechanism is discussed. This work applys the theoretical and experiment basis for the high power and varies THz sources.Following is the summary of main research work:1. Second harmonic generation and dispersion property of Ga Se crystal are researched by a polarized and tunable, acousto-optically Q switched CO2 laser. Among the available Ga Se dispersion equations, the most suitable dispersion equations in IR range for Ga Se is determined, according to the comparison between measured and calculated external phase-matching angle results in varies fundamental wavelengths from 9.2-10.7μm.2. Based on CO₂ laser and Ga Se crystal, the phase-matching parameters in the THz-DFG process, including the phase-matching angle, THz output intensity, acceptance parameters, are calculated and compared under collinear propagation situation. The result shows oo-e phase-matching type can realize THz generation with a phase-matching angle cut Ga Se crystals. Besides, the oo-e phase-matching type has the optimum angle-tunable property, acceptance parameters and operation feasibility.3. The 0-3THz THz radiation is realized in Ga Se crystal based on the optical rectification of ultrafast laser pulses. Owing to the two-photon absorption the pump intensity dependence of THz output changes from linear at lower pump intensity to sub-linear at higher pump intensity. The two-photon absorption can deplete the pump intensity to reduce the THz radiation; at the same time, it excites the free carriers that increase the THz absorption. To simulate the sub-linear pump intensity dependence of THz output, the two-photon absorption coefficient and the dielectric function of Ga Se at THz range are measured. With Drude-Lorentz model, the relaxation time of free carriers is obtained to be 0.38 fs and the pump intensity dependence of THz output is simulated which agrees with the experiment result. At last, the Er and In doped Ga Se crystals are prompted to reduce the effect of two-photon absorption.4. The gold film and nanohole structures are studied as THz emitters excited by ultrafast laser pulses and 0-3THz radiation is detected in far-field. The relationship between THz output and incident angle, pump intensity and pump wavelengths is studied with a seirs of experiments. With laser excitation, the ionization and motion of photoelectrons results in a femtosecond current surge in near-field and terahertz electromagnetic radiation in far-field. The laser pulses can also excite the surface plasmon polaritons that enhance the THz emission through the ponder motive acceleration of photoelectrons. This leads to the strongest THz emission in the angle that surface plasmon is optimally coupled. With the increasing pump intensity, the change of relationship between pump intensity and THz emission shows the transformation of electron ionization from mulitphoton ionization to tunnel ionization. Using different pump wavelengths, the lifetime of surface plasmon polaritons can modulate the THz output spectrum range. In conclusion, surface Plasmon polaritons can modulate the THz output enlectric amplitude and spectrum range.