multilayers with gradient refractive index decreasing from the substrate index to that of air. [8][9][10] So the key challenge in the fabrication of broadband AR coatings lies in fi nding fi lm materials with tunable refractive index especially a low-index coating for top layer. In high power laser system, the frequency conversion process in which the fundamental beam of 1053 nm is converted to the second harmonic beam of 527 nm fi rstly, and fi nally converted to the third harmonic beam of 351 nm by two frequency conversion crystals makes many optical components subject to transmission of three wavelengths laser beams simultaneously. To reduce the energy loss, it is highly desirable to develop AR coatings exhibiting high transmittance simultaneously at 351, 527, and 1053 nm. [ 11 ] This broadband AR coating working in several specifi c wavelengths faces greater challenges compared to the common broadband AR coating because the peak position must be tailored in specifi c wavelengths by precisely controlling the refractive index and fi lm thickness of each layer. Any slight deviation of refractive index or fi lm thickness will lead to the shift of peak position and thus fail in reaching the required optical performance. For all above diffi culties, many approaches cannot already meet the requirement such as reactive ion etching process [ 12,13 ] and the heteroepitaxial growth of nanorods because of the complex process, incapable precise control of refractive index and fi lm thickness, high cost of these techniques, and non-availability in a large area substrate. [ 14 ] Fortunately, sol-gel process provides a preferable choice due to the great advantage on precise control of refractive index by tailoring the sol microstructure and chemical composition, excellent adhesion to glass surface, low process temperature, and good homogeneity. [15][16][17][18] The sol-gel derived silica and magnesium fl uoride coatings, characterized by low refractive index of 1.45 and 1.38 respectively, low dispersion in UV region, and good stability, have become the best choice of fi lm materials in high power laser systems. [19][20][21][22] There have been many reports devoted to tune the refractive index of silica coating based on the acid-catalyzed and base-catalyzed procedure. For instance, Thomas prepared