Nanostructured silica based optical coatings for high power laser systems

Most of optical coatings are formed using at least two materials with different refractive indexes. Silicon oxide is one of the materials with large band gap value. Therefore, high laser fluency is necessary to induce... [ view full abstract ]

Most of optical coatings are formed using at least two materials with different refractive indexes. Silicon oxide is one of the materials with large band gap value. Therefore, high laser fluency is necessary to induce dielectric breakdown in it. Unfortunately, other typical high refractive index materials have low laser resistivity and limit the overall multilayer coating laser induced damage threshold.

A novel approach is presented to form the multilayer optical coatings based on nanostructured silica thin films. Glancing angle deposition method was employed to form silica sculptured thin films with different porosity. Tailoring the refractive index of individual layers, multilayer antireflection and high reflection coatings have been manufactured. Optical and structural characterization of nanostructured and standard coating, produced by sputtering technology, designs was performed and compared. Spectral performance of silica based antireflection coatings was found to exceed the two-material based coatings (see Fig. 1). Measured resistivity to laser radiation indicate the damage threshold of more than 15 J/cm² for silica coatings at the wavelength of 355 nm in nanosecond regime. High reflection all-silica mirrors also showed increased laser resistivity compared to standard dielectric mirrors. A possibility to obtain laser induced damage threshold of 65 J/cm²at the wavelength of 355 nm in nanosecond regime is demonstrated and presented in Fig. 2. The graph shows the possibility to damage optical component at different laser fluency. Optical losses of such multilayer nanostructured coatings were investigated by light scattering measurements and possible sources were analyzed. Also, combined techniques of standard and glancing angle deposition methods were used to form more spectrally favorable mirrors. Optical and structural analysis of such coatings indicate possible applications of nanostructured silica films for upgrading standard designs.

All-silica based multilayer coatings, produced by glancing angle deposition, were characterized by measuring optical performance and resistivity to laser radiation. Analysis indicate the advanced properties of nanostructured silica thin films. Tailored porosity of individual layers allows to form anti- and high-reflection coatings capable to withstand extremely high laser fluency.