  Nowadays, via controlling surface plasmons (SPs) on elaborate man-made
structures, plasmonics aiming at manipulating light beyond the diffraction
limit has aroused great interest. Here, nevertheless, we demonstrate in
short-pulse laser ablation ultrafast active plasmonic structures spontaneously
generate in virtue of plasmonic effects rather than human ingenuity. First, the
splitting of laser-induced subwavelength gratings that is experimentally
evidenced on ZnO, Si, and GaAs, is confirmed to originate in the conversion of
SP modes from the resonant to the nonresonant mode and further to the inphase
or antiphase asymmetric mode. Further, as pulse number increases the universal
scaling-down of laser-induced structures is derived from the conversion of
physical regimes of plasmonic interaction from the optical to the electrostatic
regime, which may arouse quasistatic SPs with interacting scales far beyond the
diffraction limit and result in the ultrafast, non-thermal ablation for
extraordinary electrostatic enhancement. Generally, the plasmonic mechanisms
reveal the link between the deep-subwavelength space-scale and the ultra-short
timescale for ultrafast laser-induced nanostructures: basically, "nanoscale"
tends to eliminate electromagnetic retardation effects and bring an instant
respond to the incident field, and arouse electrostatic interactions with giant
local-field enhancement facilitating ultrafast ablation driven by tremendous
electrostatic forces. Thus, active plasmonic structures provided with
simultaneous "nanoscale" and "ultrafast" is apt to form in ultrafast ablation.
In addition, the plasmonic mechanisms can act as a powerful "evolutionary
force" acting in multipulse ablation and promoting the growth of resonant
nanostructures-various typical plasmonic structures may be self-generated
through multipulse evolution, which can be considered as a kind of natural
plasmonics.
