Nano-mechanics of Optical Structures for High Laser-damage Threshold Applications

Download or read book Nano-mechanics of Optical Structures for High Laser-damage Threshold Applications written by Karan Mehrotra and published by . This book was released on 2016 with total page 188 pages. Available in PDF, EPUB and Kindle. Book excerpt: "This dissertation focuses on the use of scanning electron microscopy (SEM), nano-indentation and finite-element analysis (FEA) to observe, measure and validate the nano-mechanical properties (elastic modulus, hardness, yield stress, deformation, fracture) of nm-level features in important optical micro- and nano-structures. The optical micro- and nano-structures include single layer oxide films, multilayers comprised of oxide layers, and optical diffraction gratings. In addition to the nano-mechanical properties, we also study the deformation in brittle (amorphous) silica walls that comprise the diffraction grating by suppressing fracture as well as on the nano-mechanics of defects in optical structures. We use this understanding of nano-mechanics in diffraction gratings to show that it naturally complements optical testing (laser-induced damage threshold tests), and mechanical fields (for example, deformation fracture strain) expose the same regions of the grating structure in a manner analogous to optical fields (for example, electric fields). A major conclusion is that deformation can be entirely separated from fracture in patterned surfaces. This result is in distinct contrast to bulk surfaces where fracture is always preceded by deformation. In Chapter 2 we use nano-indentation to perform mechanical characterization of optical oxide single-layer and multi-layer thin films, and the results are interpreted based on the deposition conditions used. These oxide films are generally deposited to have a porous microstructure that is optimized to maximize the laser induced damage thresholds, but changes in deposition conditions lead to varying degrees of porosity, density, and possibly the microstructure of the thin film. Of the four single-layer thin films tested, alumina was observed to demonstrate the highest values of nano-indentation hardness and elastic modulus. We also demonstrate how single-layer thin film data may be used in the analysis of multilayer thin films and present an experimental study of indentation size effects (ISE) on multilayer thin films (silica-hafnia system). These multilayer coatings show a decrease in hardness for an increase in indentation loads when using a Cube-corner tip. The data are interpreted using the Nix & Gao model of gradient plasticity, and predicts an excellent correlation between for the depth dependence of hardness in our silica-hafnia multilayer thin films. In Chapter 3 we characterize "blisters", defects observed in multilayer dielectric (MLD) coatings after exposure to acid cleaning procedures. Nanoindentation is used to make site-specific indentations across blisters to measure the mechanical response, especially their elastic compliance under different conditions of loading. Two regions of statistically different mechanical response are identified within a blister defect and compared to the undisturbed regions of the MLD coating. We conclude that different blisters follow the general trend that maximum compliance is always seen in the "extended region" of the blister, furthest from the blister's initiating nodule/scratch and the coating age might have an effect on the indentation response for larger depths of penetration into the thin film. Additionally, our numerical model is used to estimate the extent of "blistering" in a coating, a result verified through cross-sectional SEM images of the "blister" defect. In Chapter 4 we measure the mechanical response of optical multilayer dielectric (MLD) diffraction gratings, geometries which are constrained in only one transverse direction but free in the other, using nano-indentation. Primarily, 2 types of indentation response were observed: indents almost perfectly centered on a particular grating "wall", without extending to a sidewall or the edge and without disturbing any adjacent walls; and indents made off-center on a "wall" which were catastrophic even at the same low load. The indentation record of load versus displacement uniquely distinguishes these two regimes, and is also correlated to the properties of bulk surfaces. The centered indents allow us to invoke a state of entirely ductile deformation and measure the yield stress of silica at the nm-scale (~4.1 - 4.6 GPa). The direct measurement of yield stress if silica at the nm-level is an exciting result. Off-centered indents at the same loads fracture the grating walls and this is used to hypothesize a fracture mechanism and measure an estimate of fracture stress (~1.1 - 3.3 GPa). Non-linear, 3-D FEA using ABAQUS® validates our experimental results as well as the deformation mechanism. Finally, in Chapter 5 we use the "slightly" off-centered indents on the gratings walls to study a combined response of ductility and fracture. Load-displacement curves in conjunction with observations from SEM images provide estimates of fracture strain. Mechanical field thresholds, represented by fracture strain, are used to correlate nano-mechanical damage in gratings to their optical performance (measured through laser-induced damage thresholds). FEA reveals that nano-indentation tests expose the same regions on the grating structure as an optical test. Here we draw attention to the important effects of inhomogeneities and non-uniformities (geometrical and material) in concentrating mechanical fields. Therefore, nano-mechanical testing complements and could even precede optical testing to gauge the performance of diffraction gratings. In summary, our work reveals that elasticity, ductility and fracture at the nm-level can each be studied separately, in contrast to micromechanical deformation; that SEM plays an important role in identifying relevant features; that in addition to characterization, nano-indentation may be useful as a diagnostic tool to study response to intense light fields; and that numerical simulations naturally complement the experimental nano-mechanics to model the complex nm-level response of optical nanostructures"--Pages xi-xiv.