Nano-indentation and laser-induced damage testing in optical multilayer-dielectric gratings

We demonstrate how a nanomechanical test identifies areas of mechanical field concentration as being comparable to areas where optical fields are known to be concentrated, in the special context of laser-induced damage testing (LIDT) of diffractive gratings of silica deposited on optical multilayers. The nano-indentation response of the diffraction gratings is measured in a new mode that allows for the extraction of a measurable metric characterizing the brittleness of the gratings, as well as their ductility. We show that lower LIDTs are positively correlated with an increased grating brittleness, and therefore identify a nanomechanical approach to describe LIDTs. We present extensive numerical simulations of nano-indentation tests and identify different deformation modes including stretching, shear concentration, and bending as precursors to mechanical failure in the nano-indentation test. The effects of geometrical inhomogeneities on enhanced stress generation in these gratings are specifically examined and addressed, and we show the agreement between nanomechanical testing and analytical interpretation of these inhomogeneities.