  Contribution of plasma oscillation to the broadband terahertz emission is
revealed by interacting two-color laser pulses with a supersonic jet of
nitrogen molecules. Temporal and spectral shifts of THz waves are observed as
the plasma density varies. The former owes to the changing refractive index of
the THz waves, and the latter correlates to the varying plasma frequency.
Simulation of considering photocurrents, plasma oscillation and decaying plasma
density explains the broadband THz spectrum and the varying THz spectrum.
Plasma oscillation only contributes to THz waves at low plasma density owing to
negligible plasma absorption. At the longer medium or higher density, the
combining effects of plasma oscillation and absorption results in the observed
low-frequency broadband THz spectra.
