Two-color strong-field terahertz amplification in the non-identical focusing condition

High optical-to-terahertz (THz) conversion efficiency is crucial for generating THz waves. Strong-field ionization using a bi-focal bi-chromatic geometry produces cascading plasmas, leading to amplified THz generation and extended THz bandwidth. We investigate the effect of focal length in this bi-focal geometry on THz intensity. The results show that the THz intensity produced with non-identical focal lengths of the bi-chromatic fields is 25% higher than that of identical focal lengths, and 15 times higher than traditional bi-chromatic THz generation. The electro-optic sampling measurements reveal that the THz-electric-field intensity generated by the bi-focal bi-chromatic field exhibits periodic oscillations, which results from the modulation of the photoelectron's asymptotic velocity by the relative phase of the bi-chromatic pulses, and are consistent with the photocurrent model. The photocurrent simulations indicate that the self-compression of the second harmonic pulse significantly enhances THz amplification and extends the THz bandwidth. These findings deepen understanding of the THz generation mechanism and suggest potential avenues for optimizing THz sources.