Ultrafast charge emission at terahertz frequency in high-intensity femtosecond laser ablation process

Initial charge emission processes during femtosecond laser ablation are investigated by monitoring the waveforms of terahertz radiation in real time using echelon-based single-shot terahertz time-domain spectroscopy and by measuring the ablated depth profiles using spectral domain optical coherence tomography. High sensitivity of the single-shot terahertz detection system enabled the acquisition of full pulse-to-pulse terahertz waveforms and revealed the changes in the amplitude and phase of the emitted terahertz waves due to the different fundamental processes of laser ablation at femtosecond and picosecond timescales. At low fluence close to the ablation threshold, a cosinusoidal terahertz wave is observed, indicating that the electrons are emitted from the surface, inducing damages to the material and causing incubation of the ablation process. At medium fluence, the terahertz wave changes to sinusoidal, suggesting that electron emissions are soon screened and are accompanied by material removal. At high fluence, time-delayed high-intensity terahertz radiation is observed as a signature of secondary charge emission. The waveform and the comparison with the ablated depth profiles reveal the importance of acoustic wave generation due to rapid lattice heating, demonstrating the importance of monitoring the initial charge dynamics for understanding the subsequent laser ablation processes.