Excitation-wavelength-dependent terahertz wave modulation via preformed air plasma

We report excitation-wavelength-dependent terahertz wave modulation via a preformed air plasma created by a synchronized 800-nm laser pulse traveling orthogonal to the pump pulses. With a preformed plasma, both the modulation depth of the terahertz energy and the change in terahertz wave polarization increase with the increasing excitation-laser wavelength. The results are well explained by the photocurrent model with a wavelength-dependent ionization rate. We also experimentally detect the refractive index of the two-color field changed by the preformed air plasma. These results improve our understanding of the theoretical mechanism and expand the practical applications of terahertz wave generation and modulation.