Zhao, Jingjing; Zhang, Yizhu; Gao, Yanjun; Li, Meng; Liu, Xiaokun; Liu, Weimin; Yan, Tian-Min; Jiang, Yuhai

COMMUNICATIONS PHYSICS

2024, 7,

Zhao, Jingjing; Zhang, Yizhu; Gao, Yanjun; Li, Meng; Liu, Xiaokun; Liu, Weimin; Yan, Tian-Min; Jiang, Yuhai

Mixing the fundamental (omega) and the second harmonic (2 omega) waves in the gas phase is a widely employed technique for emitting terahertz (THz) pulses. The THz generation driven by bi-chromatic fields can be described by the photocurrent model, where the THz generation is attributed to free electrons ionized by the omega field, and the 2 omega field provides a perturbation to break the symmetry of the asymptotic momentum of free electrons. However, we find that the THz radiation is amplified by one order of magnitude when driven by bi-focal bi-chromatic fields, which cannot be explained only using the photocurrent model. Meanwhile, present measurements demonstrate that the THz radiation mainly originates from the plasma created by the 2 omega pulses instead of the omega pulses. Energy transfer from the 2 omega beam to the THz beam during the THz generation has been observed, validating the major contribution of the 2 omega beam. Furthermore, the THz bandwidth has been observed to extensively exceed the bandwidth of the pump pulse, not be explained by the photocurrent model as well. These counterintuitive results present a significant challenge for understanding strong-field nonlinear optics and simultaneously expanding various applications.