High-power and high-aspect-ratio optical coatings by atomic layer deposition

Abstract

In high-power applications, optical coatings must meet rigorous thermomechanical and damage threshold standards in addition to performing the desired optical function, which includes filters, beam splitters, anti-reflection coatings, and high-reflectivity mirrors. We investigate several aspects of high-power coatings and the particular suitability of atomic layer deposition (ALD) to meet many of the design goals. After reviewing the origin of thermal expansion in solids, techniques for its measurement in thin films, and the unique characteristics of ALD, we look at the ability to predict a coating's thermal deformation. Coatings using ALD alumina and hafnia are demonstrated to have very consistent refractive indices, growth rates, thermal expansion coefficients, and biaxial moduli, which together enable a priori design of "thermally invariant" mirrors that maintain high reflectivity without changing shape with temperature. We have also characterized the undesired crystallization of ALD hafnia that can lead to roughness at thicknesses relevant to optical coatings. A nanolaminate strategy is explored, where ultrathin layers of alumina---less than 1 nanometer thick---are inserted periodically to disrupt the growth of hafnia crystallites. The hafnia-rich nanolaminates, near 100 nanometers in total thickness, are found to be amorphous and smooth down to very low concentrations of alumina and have a predictable decrease in refractive index with increasing alumina concentration. The thermal conductivity of ALD alumina and hafnia along with a series of nanolaminates is characterized in detail, focusing on the effect of interfaces in the nanolaminate films. The room-temperature thermal conductivity of the partially-crystalline pure hafnia film is 1.7 W/(m K), whereas all nanolaminates fall in the range of 1 to 1.2 W/(m K). Cryogenic measurements to 30 K show that this 30-40% reduction is likely due to the amorphous nature of the nanolaminates rather than the effect of interface resistance, and the thermal conductivity closely follows that expected for fully-disordered hafnia. A unique feature of ALD is its ability to conformally coat very high-aspect-ratio structures, like nanoscale holes and trenches. We investigate this at the mixed length scale of many common optical systems, with at least one dimension on the order of centimeters, another as low as several micrometers, and with nanoscale thickness precision. An example is coating the inside of a hollow glass capillary waveguide. We find that ALD alumina considerably outperforms hafnia under such conditions and quantify the difference using a large-area wedge structure with cross-section varying from about 20 micrometers to over a millimeter. The alumina process hardly notices the constrained geometry, whereas hafnia shows variation in thickness and refractive index consistent with non-ideal ALD growth mechanisms. Both coatings remain quite repeatable, with the resonance of a Fabry-Perot filter behaving as predicted except at the deepest regions of the wedge.