Femtosecond Laser-Induced Modifications of Frequency Tripling Mirrors

We have studied laser induced material modification in a frequency tripling mirror (FTM) consisting of alternating hafnia and silica layers. The third-harmonic signal generated by a train of femtosecond laser pulses (791 nm, 55 fs, 110 MHz) drops over time until it reaches about 20% of the initial value. From the observed changes in reflection and transmission of the mirror a refractive index change of 0.07 was estimated, which occurs in the layer with the highest field enhancement. This index change triggers a drop in the field enhancement, which reduces the efficiency of nonlinear optical processes. The estimated value of Delta n allowed us to explain the 80% reduction in conversion efficiency and as well as an observed decrease in two-photon absorption.