Wavelength dependent terahertz wave modulation based on preformed air plasma

Terahertz wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. For further understanding of the mechanism of the terahertz wave generation in laser-induced plasma as well as the terahertz modulator base on pre-formed air plasma, the generation of terahertz radiation using an effective wavelength scaling mechanism is examined when two-color laser fields are mixed in pre-formed plasma created by synchronized 800nm laser pulse. In our experiment, the effect of preformed plasma is investigated using an orthogonal pumping geometry. With a preformed plasma, both the modulation depth of terahertz radiation energy and the change of terahertz radiation polarization increases with increasing excitation laser wavelength. We found that the terahertz modulation depth and terahertz polarization changes increase as a function of the energy of the 800nm-prepulse. Some possible reasons are discussed. We attribute the terahertz polarization rotation to additional relative phase of the two-color fields introduced by the preformed air plasma. This provides a practical way to control the polarization and energy of terahertz pulses for potential applications.