  In the world of epsilon-near-zero (ENZ) materials, the plasma is unique for
its natural ENZ properties at the plasma frequency (wp). However, for the air
plasma during femtosecond laser filamentation with wp in terahertz (THz) band,
which is also known as a broadband THz emitter, the possible ENZ effect has
long been neglected. In this work, interactions between the laser plasma in air
and the radiated THz waves were investigated, and the THz resonance absorption
at the ENZ point, which gave rise to the generation of surface plasmon waves,
has been theoretically and experimentally demonstrated for the first time, to
the best of our knowledge. Specifically, this ENZ effect was accompanied with
THz modulations along the plasma filament in multiple domains, including the
linear-to-elliptical THz polarization conversion and the multi-ring pattern of
far-field THz profiles, etc. Thanks to these novel ENZ-induced phenomena, the
understanding of this important plasma-based THz source has been enriched.
Moreover, by extending the laser plasma to the role of ENZ material, it is
promising to expedite related applications benefiting from the ENZ nature,
e.g., the strong spatial confinement of THz waves inside the ENZ region between
the bi-filaments array, which realized all-optical time-domain integration of
broadband THz pulses as we recently displayed in [arXiv].
