Laser filaments─plasma channels formed by femtosecond laser ionization in air─have emerged as a prominent source of broadband terahertz (THz) radiation and a versatile platform for free-space THz wave manipulation. However, the underlying near-field interactions, particularly the potential excitation of surface waves at THz frequencies, have remained largely unexplored due to the extreme intensity of the filament, which preclude the use of conventional near-field probes. Here, by combining in situ modulation of the filament using a movable ceramic plate and far-field THz time-domain detection, we achieve theoretical and experimental accesses to near-field THz phenomena, namely, the generation of THz surface plasmon waves (SPW) at the plasma frequency within the filament region. Our findings not only deepen the fundamental understanding of THz creation and confinement along the laser-induced plasma filament, but also establish the filament as a free-space, solid-substrate-free platform for THz SPW studies, with potential of advanced THz wave guiding and signal processing applications benefiting from the SPW nature.
Zhao et al. (Wed,) studied this question.