Bandwidth Expansion for Lippmann-Bragg Holographic Photopolymers

When two rays of monochromatic laser light intersect, there is both constructive and destructive interference between the light waves. This interference creates areas of high and low light intensity. When this light is focused on a urethane material containing photo-initiators and mobile monomer, the light initiates free radical chain growth. The mobile monomer forms polymer chains, and changes the refractive index of the material, creating a hologram pattern within the material.

A reflection hologram that has a high reflectance over a broad range of wavelengths is desirable for many applications. A broader bandwidth allows for an inexpensive light source like an LED to be used for applications like heads-up-displays. Thicker holograms, which have a higher reflectance, also have a very narrow bandwidth, thus obtaining a higher bandwidth typically requires layering. The wavelengths that the hologram reflects are functions of the spacing between interference patterns, or fringe spacing. A post-processing method called “chirping” is used to expand the bandwidth of these reflection holograms. By diffusing mobile monomer into a recorded hologram, we are able to swell the material and create a distribution of fringe spacing that increases the bandwidth.

Starting with a bandwidth of 2.5nm with a maximum reflectance of 2.25%, after one hour of monomer diffusion the bandwidth expanded to 9.7nm while the maximum reflectance dropped to 0.60%.  This trend was observed until 5 hours of chirping, at which point the bandwidth narrowed and the maximum reflectance returned to a higher value. As the monomer diffusion process nears steady state, there is an even distribution of swelling throughout the material, and the reflectance increases. The peak reflectance is now shifted to a higher wavelength (457.0nm to 466.6nm) as the fringe spacing is larger. The change in mass of the hologram was 8.05% which matches closely with this 8.35% swelling observed. This confirms this post processing technique is able to reliably tune the wavelengths, and range of wavelengths a hologram can reflect.