  Broadband ultrashort terahertz (THz) pulses can be produced using plasma
generation in a noble gas ionized by femtosecond two-color pulses. Here we
demonstrate that, by using multiple-frequency laser pulses, one can obtain a
waveform which optimizes the free electron trajectories in such a way that they
reach the highest velocity at the electric field extrema. This allows to
increase the THz conversion efficiency to the percent level, an unprecedented
performance for THz generation in gases. Besides the analytical study of THz
generation using a local current model, we perform comprehensive 3D simulations
accounting for propagation effects which confirm this prediction. Our results
show that THz conversion via tunnel ionization can be greatly improved with
well-designed multicolor pulses.
