Optical rectification of laser pulses in LiNbO<sub>3</sub> crystal pumped by high power laser is one of the most powerful way to generate this high-peak-power terahertz pulses. It enhances the laser-terahertz transform efficiency by tilted-pulse-front pumping(TPFP) to fulfill phase match in the LiNbO<sub>3</sub> crystal. However, comprehensive theoretical analysis is still lack. In this work, a detailed theoretical model to investigate the THz generation efficiency by using nonlinear susceptibility tensor of LiNbO3 crystal was presented. Based on femtosecond laser system, a setup to generate high-peak-power terahertz pulses and a time domain spectroscopy system are established. The property of generated terahertz pulses was analyzed by using terahertz camera and THz time domain system. We also realized the single-shot measurement of terahertz temporal waveform by using this terahertz source.
The refractive indices, absorption coefficients, and complex dielectric constants of paraffin-embedded brain glioma and normal brain tissues have been measured by a terahertz time-domain spectroscopy (THz-TDS) system in the 0.2- to 2.0-THz range. The spectral differences between gliomas and normal brain tissues were obtained. Compared with normal brain tissue, our results indicate that paraffin-embedded brain gliomas have a higher refractive index, absorption coefficient, and dielectric constant. Based on these results, the best THz frequencies for different methods of paraffin-embedded brain glioma imaging, such as intensity imaging, coherent imaging with continuum THz sources, and THz pulsed imaging with short-pulsed THz sources, are analyzed.
Optical retification of laser pulses in LiNbO<sub>3</sub> by tilted-pulse-front pumping(TPFP) is a powerful way to generate terahertz(THz) pulses. Detailed theoretical analysis of the TPFP THz pulses generation efficiency and polarization with different pump beam polarization was presented. The THz generation efficiency for various of pump beam polarization angle and THz pulse polarization were also researched. The results predict that the magnitude of the THz electric field is maximized when the pump beam electric field vector is parallel to the LiNbO<sub>3</sub> crystal axis.
Recently, Silicon nanowires(Si NWs) have been suggested as a promising candidate for solar energy harvesting. The Si NWs have the advantage of enhancing the optical absorption for broadband spectra, which increases the optical absorption significantly. It is of crucial importance for high efficiency solar cell. We applied the transient optical-pump THz emission spectroscopy to research Si NWs fabricated by chemical etching and to analyse the THz radiation mechanism. The experiment datas indicated that Si NWs enhance the optical absorption significantly. And the lifetime and dynamics properties of photoexcited carriers are important to the photovoltaic conversion efficiency of solar cell. Here the optical-pump terahertz-probe system was employed to study the ultrafast dynamics and transport properties of photoexcited carriers in Si NWs. The experiment results shown that carrier lifetime in Si NWs is approximate 0.7ns, and it reduces optical energy-conversion efficiencies of solar cell. However, compared to other silicon nanostructures, Si NWs fabricated by chemical etching have relative high values for both the carrier lifetime and mobility. The detailed analysis of the optical absorption and carriers dynamics would be significant for optimizing configuration of the silicon nanostructures.