Copolymerization approach to control the optical properties of multilayer films

Background

Today’s optical devices trigger a need for thinner and lighter optical materials without any compromise in image quality. This has garnered interest in gradient refractive index (GRIN) materials, which can be used for compact optical design and enhance focusing power, correct aberrations, and reduce the number of components needed for an effective optical system. GRIN materials include both those with a continuous index gradient as well as those comprising of discrete layers with different refractive indices. To date, there are only a few flexible and all-dry processes available that can fabricate copolymers with large overall refractive index changes and control the thickness of the deposited films.

Technology Overview

By finely tuning the feed gas ratio, the University of Rochester researchers fabricated uniform, transparent, and highly reproducible single-layer polymer thin films with a wide but controllable range (1.35 to 1.58) of refractive indices and excellent optical transparency in the visible range using two sets of monomers: high refractive index and low refractive index. They then developed three distinct types of ultrathin four-layer thin films with a completely arbitrary stack structure and different spatially-varied refractive index profiles. All constituted one layer of a) high index homopolymer, b) copolymer of lower refractive index, c) copolymer of even lower refractive index, and d) lowest index homopolymer. The optical performance and thickness profiles of their materials were characterized by in-situ spectroscopic ellipsometry, and results were validated by ex-situ ellipsometry with multiple angles of incidence to ensure the high accuracy of the measurements.

Benefits

The technology enables the design of elaborate optical coating designs with a single polymer material; examples include a multilayer consisting of discrete layers or a graded index layer. Through copolymerization, the polymer index can be continuously varied from the index of the low index component to the index of the high index component. The film composition (thus, index) is easily adjusted during the deposition process, providing for independent control of each deposited layer. In situ ellipsometry is integrated with the deposition chamber, providing nanometer-level resolution in thickness. Sample-to-sample variation in the refractive index for a given composition is negligible, indicative of excellent reproducibility. Additionally, thickness ranged from 20.9 to 21.1 nm in a 50.2 cubic centimeter scanned area (8 cm diameter); a film thickness variation of only 0.2 nm between the extrema proved they were synthesized with exquisite thickness control, outstanding conformality, and high spatial uniformity.