Temporal-Spatial Manipulation of Bi-Focal Bi-Chromatic Fields for Terahertz Radiations
arxiv(2024)
摘要
Mixing the fundamental (ω) and the second harmonic (2ω) waves
in 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 ω field, and the 2ω 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, contradicting the common
understanding of the photocurrent model. Meanwhile, present measurements
demonstrate that the THz radiation mainly originates from the plasma created by
the 2ω pulses instead of the ω pulses. Energy transfer from the
2ω beam to the THz beam during the THz generation has been observed,
validating the major contribution of the 2ω 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 indicate that undiscovered physical mechanisms are
involved in bi-chromatic THz generation in plasma, presenting a significant
challenge for understanding strong-field nonlinear optics and simultaneously
expanding various applications.
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