Multiphoton absorption of 1.3 mu m wavelength femtosecond laser pulses focused inside Si and SiO2
Résumé
We investigate the non-linear absorption of 1.3 mu m 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 nonlinear absorption at 1.3 mu m 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.