Nonlinear thickness dependence of two-photon absorptance in Al2O3 films

Linear and nonlinear absorptance in Al2O3 films of different optical thickn esses are investigated using an ArF laser calorimeter. While the linear abs orptance at 193 nm shows the expected linear increase, nonlinear absorptanc e increases quadratically with increasing film thickness. Thus, it cannot b e described by a constant nonlinear absorption coefficient beta. The experi mental findings are explained by a simple phenomenological approach using e xcited states with a finite interaction length longer than the actual film thickness, a new material constant Gamma is introduced, which describes the nonlinear absorptance behavior correctly.