  A wavelength-degenerate pump-probe spectroscopy is used to study the
ultrafast dynamics of strong-field ionization in femtosecond-laser
subwavelength-structured fused silica. The comparative spectra demonstrate that
femtosecond-laser subwavelength structuring always give rise to great
enhancement for strong-field ionization as well as third-order nonlinear
optical effects, which is the direct evidence of great local field enhancement
in subwavelength apertures of fs-laser highly-excited surface. In short, the
study shows the prominent subwavelength spatial effect of strong-field
ionization in femtosecond-laser ablation of dielectrics, which greatly
contributes to the well-known "incubation effect".
