Enhancement of Terahertz Radiation from a Transversely Asymmetric Femtosecond Laser Filament

Abstract

Plasma filaments via femtosecond laser ionization in air have been extensively studied as a significant terahertz (THz) source, based on which one particular objective is to enhance the efficiency and intensity of THz radiation. To this end, various strategies have been explored, including modulating the pump laser through temporal asymmetry of the carrier or envelope, or by tailoring the spectral amplitude profile asymmetrically. Apart from the above “asymmetric” operations in time and frequency domains, here we proposed a straightforward and practical method based on the spatial asymmetry. Specifically, an opaque blade was employed to partially obstruct the cross-section of the pump laser beam, resulting in a notable THz power enhancement of up to 60%. To interpret this improvement, we introduced a spatially asymmetric photocurrent mechanism: the created steep gradient of the inhomogeneous laser field enhances the electron drift motion, which in turn generates a stronger transverse current, leading to the observed increase in THz signal strength. To summarize, the proposed experimental method is highly accessible without requiring additional modulation devices, significantly lowering the application threshold. And its mechanism is also versatile, not only enhancing THz transverse wave radiation in both single- and dual-color field configurations, but also potentially applying to other setups, such as tilting focusing lenses or frequency-doubling crystals, warranting a reevaluation of previous studies. Additionally, our approach optimizes energy usage, enabling stronger THz radiation under low-power laser pump conditions and further allowing the blocked laser energy to be redirected for enhanced functionalities.