13 - Habi Photoinitiators in Photopolymer Films Developed for Holographic Applications

Abstract

Bis-imidazole photoinitiators are well-suited to photopolymer holographic films because these photoinitiators are sensitive, can be stored indefinitely, can be sensitized to all visible wavelengths, and are readily compatible with binder systems. It was the coincident development of bis-imidazole photoinitiators that catalyzed the rapid growth of photopolymer films in DuPont—holographic films in particular. The ready sensitization of bis-imidazoles has been exploited in a wide variety of holographic photopolymer films sensitized to all major visible commercial lasers, including panchromatic sensitization for the production of full-color holograms. DuPont has developed materials with excellent photospeed across the visible spectrum, extending from the UV at about 350 nm out to far red at about 700 nm. Typical sensitivities range in the few to tens of mJ/cm2, which is competitive with most other photopolymer systems and even approaches the photospeed of the ultra-fine grained silver halide materials that are employed to record similar structures. The choice of monomers, binders, and other polymers or oligomers determines the bulk refractive index and refractive index change. In addition, bulk physical properties such as diffusion lengths are determined by the choice and proportion of film components. This allows the design of films that record primarily small grating periods (reflection gratings and reflection holograms) or primarily larger periodic structures. It is also possible to record a wide range of pattern spacings and structures in films suitable for both transmission and reflection holography.