  Although ultrafast laser materials processing has advanced at a breakneck
pace over the last two decades, most applications have been developed with
laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared
(MIR) femtosecond laser source development may create novel capabilities for
material processing. This is because, at high intensities required for such
processing, wavelength tuning to longer wavelengths opens the pathway to a
special regime of laser-solid interactions. Under these conditions, due to the
${\lambda}^2$ scaling, the ponderomotive energy of laser-driven electrons may
significantly exceed photon energy, band gap and electron affinity and can
dominantly drive absorption, resulting in a paradigm shift in the traditional
concepts of ultrafast laser-solid interactions. Irreversible high-intensity
ultrafast MIR laser-solid interactions are of primary interest in this
connection, but they have not been systematically studied so far. To address
this fundamental gap, we performed a detailed experimental investigation of
high-intensity ultrafast modifications of silicon by single femtosecond MIR
pulses (${\lambda}$ = 2.7 - 4.2 ${\mu}$m). Ultrafast melting, interaction with
silicon-oxide surface layer, and ablation of the oxide and crystal surfaces
were ex-situ characterized by scanning electron, atomic-force, and transmission
electron microscopy combined with focused ion-beam milling, electron
diffractometry, and ${\mu}$-Raman spectroscopy. Laser induced damage and
ablation (LIDA) thresholds were measured as functions of laser wavelength. The
traditional theoretical models did not reproduce the wavelength scaling of the
damage thresholds. To address the disagreement, we discuss possible novel
pathways of energy deposition driven by the ponderomotive energy and field
effects characteristic of the MIR wavelength regime.
