The refractive index is a key characteristic of polymer materials in optical applications. For organic
polymers, typical refractive indices are in the range of 1.35 to 1.65. Extending the refractive index beyond
the limits is of fundamental scientific interest and would enhance the utility of polymers in many
applications. Polymeric thin films fabricated by plasma enhanced chemical vapor deposition (PECVD)
have been investigated in the fields of electronics and optics and their utility is becoming more widespread
in a variety of applications. Outstanding attributes of the PECVD photonic films include a smooth surface,
dense crosslinking structure, robustness, environmental resistance, optical transparency in either visible or
IR regions, and good adhesion to many optical window and substrate materials. In recent years, our
laboratory has fabricated novel polymer optical coatings and films by PECVD. One focus of this research
has been to expand the achievable maximum refractive index. This goal has been sought using two
approaches including increasing the conjugation and crosslinking of chemical moieties of the bulk film and
incorporation of metal ions into the structure. The techniques of XPS, FTIR, HRSEM, and ellipsometry
were used to characterize both the optical properties and the chemical structure of plasma polymerized
benzene, ferrocene, and metal-phthalocyanine thin films. The structure-property relationship and the effect
of PECVD processing conditions are also discussed in this presentation.
Two polymer systems including polymer elastomers and gels have been studied as host materials for optical limiting applications. Both systems have high laser damage thresholds (LDT), typically 20 to 35 times higher than commercial PMMA bulk materials. For the polymer elastomers, Epotek optical epoxy 301-2 and 310, the LDT increases with an increase of the molecular flexibility. We speculate that the thermo-mechanical fracture may be the mechanism for the laser induced damage. For the hydrogel system, the LDT increases with increasing water content. The mobility of the water plays a key role in determining the LDT by facilitating laser energy dissipation and self-healing. It appears that the polymer elastomer and hydrogel systems both have potential for high power laser applications.
Conventional guest-host optical limiting materials utilize either a liquid solvent or solid as the matrix for nonlinear absorbing chromophore dopants. Concentration gradients of the chromophore in the matrix can improve optical limiting performance. However, low viscosity liquid solutions can not retain a concentration gradient while polymer solid matrices damage at low laser fluences. We report on a novel approach of using an elastic polymer and viscoelastic gels for guest- host optimal limiting matrices. We achieve high bulk laser damage thresholds in the hosts and maintain a concentration gradient of the chromophore. By softening the epoxy we significantly enhance its bulk laser damage threshold. We characterize this effect by measuring the damage threshold as a function of viscoelastic properties. In addition, optical limiting was demonstrated in all the hosts doped with nonlinear phthalocyanine chromophores.