The Impact of Dispersion of the Ultrashort Light Pulses on the THz Radiation from Asymmetric Air Plasmas

摘要

One of the emerging methods for broadband terahertz pulse generation is based on focusing two-color ultrashort pulses (e.g. the combination of fundamental and second harmonic beams) into air. The dispersion of these short laser pulses defines their temporal shape; hence it affects the magnitude and spectral quality of the generated terahertz (THz) radiation. The goal of this study is to understand better the role of dispersion of the pulses on THz radiation from asymmetric plasmas. Our key finding is that peak intensity of THz pulses can be significantly controlled through the group delay dispersion (GDD) of the fundamental pulse. The peak of THz pulse envelope shows quite regular oscillations as a function of GDD with a periodicity of approximately 1000 fs2. It has been found that the oscillation is not related to the amount of plasma generated, but is proportional to the asymmetry of the electric charges present in the plasma. Another interesting observation is that the amount of dispersion for the most intense terahertz pulse is shifted away from the transform-limited duration of the fundamental pulse, which is the result of the group-delay mismatch between the fundamental and second harmonic pulses. We anticipate that the spectral control of broadband THz pulses can be utilized in THz spectroscopy.