  The two-color strong-field mixing in gas medium is a widely-used approach to
generate bright broadband terahertz (THz) radiation. Here, we present a new and
counterintuitive method to promote THz performance in two-color scheme. Beyond
our knowledge that the maximum THz generation occurs with two-color foci
overlapped, we found that, when the foci of two-color beams are noticeably
separated along the propagation axis resulting in cascading plasmas, the THz
conversion efficiency is surged by one order of magnitude and the bandwidth is
stretched by more than 2 times, achieving $10^{-3}$ conversion efficiency and
$>$100 THz bandwidth under the condition of 800/400 nm, $\sim$35 fs driving
lasers. With the help of the pulse propagation equation and photocurrent model,
the observations can be partially understood by the compromise between THz
generation and absorption due to the spatial redistribution of laser energy in
cascading plasmas. Present method can be extended to mid-infrared driving
laser, and the new records of THz peak power and conversion efficiency are
expected.
