Long-wavelength multiphoton ionization inside band-gap solids

Using various band-gap materials and tightly focused femtosecond laser pulses with wavelengths in the range 1200--2200 nm, we show that nonlinear absorption is independent of the wavelength except for narrow gap semiconductor materials. This observation corresponds to a transition between multiphoton ionization and tunnel ionization for an adiabaticity parameter of about 3, which compares favorably with Keldysh predictions. Our results indicate that long wavelengths must open up an alternative to pulse shortening for ultraprecision optical breakdown in dielectrics.