Terahertz radiation generation by three-color laser pulses in air filament.

We demonstrated terahertz (THz) radiation generated from a three-color laser-excited filament in air for a wide tuning range and various wavelength ratios of the pump laser pulses. As a pump source, the radiation of the femtosecond Ti:sapphire laser and infrared output of the optical parametric generator were used. It was found that the spectrum of generated THz pulses spans up to 50 THz, while the overall THz generation efficiency is of an order of magnitude larger than the one produced by two-color pulses in an analogous configuration. Our results support the idea that three-color pulses are more suitable for broadband THz generation, since the corresponding waveforms can produce faster free electrons in larger quantities. We have found that the conversion efficiency is significantly limited due to intensity clamping. The spectral broadening of THz radiation is explained by aperiodicity of ionization events in time. [ABSTRACT FROM AUTHOR]