Wavelength dependent terahertz wave modulation based on preformed air plasma

Abstract

Terahertz wave generation in a filament pumped by a two-color femtosecond laser has been widely used as a source for nonlinear terahertz spectroscopy. Various optical lasers and gases were explored to further optimize the source with broader radiation bandwidth and higher output pulse energy, whereas few attempts were made for this purpose with longer pump wavelengths. We examine the optical excitation-wavelength scaling behavior of terahertz waves generated from two-color laser filament in air, with particular focus on the terahertz wave frequency bandwidth and polarization. Terahertz wave generation from the photocurrent surge driven by lasers with relatively longer wavelengths yields the shift to higher central frequency with rapid increase in the output energy with increasing pump power, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, we find the terahertz polarization undergoes a continuous rotation as a function of the laser wavelength. Our results contribute to the understanding of terahertz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.