Characterization of nonlinear absorption in femtosecond laser inscription

Permanent micro-modification of refractive index in dielectrics by means of femtosecond laser pulses is a novel enabling technology in photonics. A wide range of photonic structures manufactured using this method has been recently demonstrated. Nonlinear absorption is a crucial mechanism of efficient energy deposition at the spatial scale down to a fraction of laser wavelength. Two main distinct nonlinear absorption processes are relevant in femtosecond energy deposition and inscription process: multi-photon absorption (MPA) and plasma absorption (PA) due to resistive dissipation in generated electron-hole plasma. Understanding of the relative contribution of these mechanisms is ultimately required for optimization of the inscription procedures. This paper presents detailed experimental characterization of nonlinear absorption and fit numerically the model parameters.