Study On Control Of Terahertz Wave

The frequency of terahertz electromagnetic wave is range from0.1to10THz and its wavelength is between infrared light to microwave. The terahertz (THz) wave has the capability to transmit through general materials, but it can be strongly reflected by metal materials and absorbed by water or water vapor. There are several methods to generate the terahertz wave, including photo carrier acceleration, the second order of nonlinear effect in electric-optical crystal, plasma oscillation and electronic nonlinear transmission line. People focus on the new methods to generate high power terahertz wave. The propagation of the ultra-short laser pulse has many nonlinear physical phenomenons and the dual-color pulses filaments can generate terahertz wave explained by photo-current model. We can use the change of transmission medium, external electric static field and molecular alignment to influence the terahertz wave.The main work of the thesis is based on filamentation generated by femtosecond laser pulse. Different gas and external electric static field with molecular alignment both have effect on modulation and influence on terahertz wave. The following studies have been performed:1. Different gases and pressures of the gas have influence on terahertz wave. With the same gas as the medium, the femtosecond laser transmits through the gas and its filamentation emits terahertz wave. When the pressure of the gas is changed, the plasma of the filamentation will be influenced with the electron density. So the intensity of the terahertz wave grows with the increased gas pressure. With different gas as the transmission medium, intensity of terahertz wave is related to the property of the gas, when the pressure of the gas is in the same condition. The clamp intensity of filamentation depends on the gas, the transmission medium.So the intensity of terahertz wave is influenced by the gas property because the clamp intensity has effect on the electron density. 2. External electric static field and molecular alignment has influence on terahertz wave. For the femtosecond filamentation ionized the air to generate plasma channel, the intensity of terahertz wave will grow with the increased external electric static field with the molecular alignment pulse effect on the dual-color pulse. And the intensity of terahertz wave can be decreased by the plasma de-focusing when the molecular alignment pulse is forward to the dual-color pulse with the electric static field effect on the filamentation.