Tri-wavelength broadband antireflective coating built from refractive index controlled MgF2 films

Regulation of the refractive index over a wide range is very important in the realization of tri-wavelength antireflective (AR) coating in high power laser systems, but many regulation approaches are too complex or violent to satisfy the practical requirements. Here, a simple, template-free sol-gel route was proposed to regulate the refractive index of MgF2 film by heat treating MgF2 sol and hence to control the self-assembly process of colloidal MgF2 nanocrystals. In this self-assembly process, the originally packed nanocrystals gradually evolved into bigger hollow vesicles, which reduced the refractive index of the MgF2 film from 1.38 to 1.2. When the refractive indices of the bottom and top layers were set as 1.34 and 1.2, the tri-wavelength broadband AR coating was finally realized on a quartz substrate, with the transmittance of 99.54%, 98.65% and 98.58% at 351 nm, 527 nm and 1053 nm, respectively.