Designing Optical Coatings with Incorporated Thin Metal Films

Abstract

In the world of nanomaterials and meta-materials, thin films are used which are an order of magnitude thinner than historically used in optical thin film coatings. A problem stems from the island structure that is seen as the film nucleates and grows until there is coalescence or percolation of the islands into a nearly continuous film. The application problem is that the indices of refraction, n and k, vary with thickness from zero thickness up to some thickness such as 30 or 40 nanometers for silver. This behavior will be different from material to material and deposition process to deposition process; it is hardly modeled by simple mathematical functions. It has been necessary to design with only fixed thicknesses and associated indices instead. This paper deals with a tool for the practical task of designing optical thin films in this realm of non-bulk behavior of indices of refraction; no new research is reported here. Historically, two applications are known to have encountered this problem because of their thin metal layers which are on the order of 10 nm thick: (1) architectural low emittance (Low-E) coatings on window glazing with thin silver layers, and (2) black mirrors which transmit nothing and reflect as little as possible over the visible spectrum with thin layers of chromium or related metals. The contribution reported here is a tool to remove this software limitation and model thin layers whose indices vary in thickness.

Designing black mirrors

“…For the MDMDMD designs, there are 27 combinations. Designs were optimized for each of these 36 cases using the special software [10,11] which was developed to deal with the fact that the indices of very thin layers vary with thickness.…”

Model-free approach to finding index of refraction values of optical coatings from spectral measurements

“…It is further understood that most films start with nucleation on a surface into island structures like raindrops forming on an automobile windshield which coalesce with increasing deposition until more continuous and dense bulk-like films are formed. Thin films of silver [5,6], for example, show plasmonic behavior due to their metal island structure which does not fit well with any of the models mentioned above.…”

Multispectral Holographic Intensity and Phase Imaging of Semitransparent Ultrathin Films

“…When modeling the optical response of ultrathin films assuming a specific ñ , e.g., that of bulk material such as bulk gold or a thick gold film, − this can lead to critical discrepancies between the modeled and fabricated thin film devices, such as changing the perceived colors of thin film stacks or affecting the plasmonic behavior . To counter this, there have been efforts to incorporate thickness-dependent effects into the optical design process. , Therefore, a proper characterization of fabricated ultrathin films is of utmost importance to determine their actual ñ (i.e., both n and k ), and to improve the design and modeling process.…”

“…37 To counter this, there have been efforts to incorporate thickness-dependent effects into the optical design process. 38,39 Therefore, a proper characterization of fabricated ultrathin films is of utmost importance to determine their actual ñ(i.e., both n and k), and to improve the design and modeling process.…”