Pulse Broadening Effect Based on the Focal Region Size in Plasma-Based Terahertz Sources

There is an issue throughout the research of the THz generation mechanism. The pump laser pulse duration in the experiment is on the order of femtoseconds, but the radiated THz pulse duration reaches the order of picoseconds. [1] It is interpreted as relaxation of carrier momentum in the photoconductive antenna method. [2] As for the plasma-based THz radiation, we believe that the spatial propagation of the laser pulse is responsible for this phenomenon. For a long pulse, the pulse length is much larger than the focal region; hence, the focal region can be treated as a point and classical breakdown model should be employed. When the pulse duration approaches the femtosecond regime, the pulse will not fill the entire focal region and instead occupies only a small fraction of the focal region at any given instant; Thus, a femtosecond breakdown model should be employed. [3] This model can explain the pulse broadening phenomenon in the terahertz generation. However, it indicates some kind of frequency conversion from a spatial perspective.