3D laser microfabrication inside narrow band gap solids like semiconductors will require the use of long wavelength
intense pulses. We perform an experimental study of the multiphoton-avalanche absorption yields and thresholds with tightly focused femtosecond laser beams at wavelengths: 1.3μm and 2.2μm. For comparisons, we perform the experiments in two very different materials: silicon (semiconductor, ∼1.1 eV indirect bandgap) and fused silica (dielectric, ∼9 eV direct bandgap). For both materials, we find only moderate differences while the number of photons required to cross the band gap changes from 2 to 3 in silicon and from 10 to 16 in fused silica.
We investigate the non-linear absorption of 1.3 μm femtosecond laser pulses strongly focused inside silicon and fused
silica. Through transmission diagnostics, multiphoton initiated energy deposition is clearly observed inside these two
materials with nanojoules laser pulse energy when using 0.3 numerical aperture objective. For silicon, the non-linear
interaction is strongly dependent on the focusing depth due to the presence of spherical aberration contrarily to fused
silica. Below the surface, we find a difference of three orders of magnitude between the intensity thresholds for non-linear
absorption at 1.3 μm wavelength inside the two tested materials due to the difference of number of photons
required for non-linear absorption. By measuring the transmission of the ionizing pulses during multiple pulse
irradiation, irreversible modifications of the material are monitored inside fused silica in accordance with previous
studies at 800 nm. For similar laser energy deposition, the response of bulk silicon remains unchanged over more than
twenty thousands pulses suggesting no irreversible modifications are initiated.