THz Research on High-Frequency Waves under Different Ionization Modes

摘要

The high-frequency waves-Terahertz (THz) emission from laser-induced air-plasma is presented. The development of high power and high efficiency THz wave radiation source has significant value. The development of laser technology has led spectroscopic sensing and imaging in the THz region to be widely employed in diverse fields such as material characterization, ultrafast dynamic process studies, and biological science. The emission of THz waves has become one of the most important areas in THz research.In this research, we develop an THz time-domain spectroscopy system(THz-TDS), which is that the input laser pulses are generated from a p-polarized regenerative Ti:sapphire amplififier with a 50-fs pulse duration centered at 800 nm. A 100- μm-thick type-Ⅰ BBO crystal is employed for frequency doubling the pump pulses and subsequently generating the THz wave. By this setup we investigate that when the terahertz emission from only fundamental wave and with second-harmonicwave mixing, the intensity of the terahertz wave obtained is different. The energy of THz wave generated by only fundamental wave is weak, and that generated by four wave mixing is stronger. The energy intensity of THz wave generated by four wave mixing is much higher than that of THz wave generated by fundamental wave (almost an order of magnitude higher). We believe that the different ionization modes causes the different of terahertz wave energy intensity. In addition, we find that the second harmonic wave grows in a quadratic curve with the increase of pump energy, so we believe that when the pump energy increases, the terahertz wave also grows in a quadratic curve.