Thickness-dependent ultrafast dynamics and nonlinear responses of indium tin oxide structured by femtosecond laser

Indium tin oxide (ITO) exhibits unique epsilon-near-zero (ENZ) effect with attractive linear and nonlinear optical phenomena which have been extensively explored for various applications. Here, we report the thickness-dependent ultrafast dynamics and nonlinear response of ITO film. The variation in permittivity originating from film thickness leads to different electron-lattice interactions, and induces three distinct ablation morphologies as film expansion, collapse, and crater formation. Through ultrafast imaging and optical field simulation, the transient optical patterns reveal time-evolving diffraction rings that gradually weaken with increasing film thickness, indicating the formation of final structure in hundreds of picoseconds and modulation of probing optical fields. The nonlinear absorption coefficient of laser-structured ITO is approximately 15 times that of the pristine ITO in the ENZ region, which is consistent with contributions from morphology-induced local-field confinement and the accompanying laser-induced permittivity shift. The controllable fs laser patterning method allows an effective, flexible and scalable approach for tailoring the optical nonlinearity of ITO film, which offers potential application in widespread fields.