CO₂ laser induced refractive index changes in optical polymers

We study the infrared photosensitivity properties of two optical polymer materials, benzocyclobutene (BCB) and epoxy OPTOCAST 3505, with a 10.6 μm CO₂ laser. We discover that the CO₂ laser radiation can lower the refractive index of BCB by as much as 5.5 × 10^-3, while inducing no measurable index change in the epoxy. As confirmed by Fourier transform infrared spectroscopy, the observed index change in BCB can be attributed to photothermal modification of chemical bonds in the material by the CO₂ laser radiation. Our findings open up a new possibility of processing polymer materials with a CO₂ laser, which could be further developed for application in the areas of post-processing and direct-writing of polymer waveguide devices. © 2011 Optical Society of America.