Recently, researchers from the Thin Film Optics Laboratory of the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) has made progress in the measurement of nonlinear absorption coefficients of wide-bandgap oxide thin films. They obtained the nonlinear absorption coefficients of common HfO₂, Al₂O₃, and SiO₂ thin-film materials by suppressing the nonlinear response of the substrate. The study was published in Optical Materials Express.  

With the construction of 100 PW laser in China and the development of EW-level peak power laser, the extremely weak nonlinear optical response of wide-bandgap thin film materials will affect the laser output performance.

In this study, the researchers proposed a highly sensitive method to measure the nonlinear absorption of such thin films by choosing an ultra-thin and ultra-low nonlinear response material as the substrate.  

They analyzed the influence of nonlinear response of substrate material and thickness on that of thin film and selected 50 μm MgF₂ and quartz glass as the substrate for coating. The nonlinear intensity of substrate is suppressed to at least 80% of the whole element or can even be ignored so that the normalized transmittance of the thin film can be obtained effectively. This makes two-photon and three-photon absorption coefficients of HfO₂, Al₂O₃, and SiO₂ thin-film materials extracted.  

The accurate nonlinear absorption response measurements of wide bandgap oxide thin films provide practical guidance for the design and fabrication of ultra-high peak power laser thin films.