<title>Photomechanical multistability in a polymer optical fiber Fabry-Perot device</title>

Abstract

We report on the progress of constructing a mesoscopic photomechanical unit (MPU)-a miniaturized device that acts as a sensor, all-optical logic unit, and mechanical positioner/actuator. We have to date made MPU's in polymer optical fiber (PMMA doped with DR1) that are 1 cm-4 cm long and have a diameter of 110 micrometers . This MPU is powered with a 18 mW laser diode at 685 nm or a 30 mW laser diode at 690 nm. These MPU's exhibit optical and mechanical bistability, multistability and regions of unstable oscillating/chaotic behavior. We report on both theoretical and experimental confirmation of these effects. In our MPU's, the fiber ends are shaped in an approximately retroreflector geometry to provide a higher reflectivity. The ends then form two reflectors that define a Fabry-Perot cavity. The refractive index, absorption and the length all change (through the photothermal heating mechanism) with light intensity: a nonlinear response. We also discuss the role of light induced isomerization. Aside from regions of multistability we also find unstable oscillating/chaotic behavior that is possibly caused by the mixing of the various nonlinear effects, stimulated Brillouin scattering and/or from the coupling of weaker Fabry- Perot cavities in the fiber caused by fiber imperfections. Polymers are better suited for devices based on the photomechanical mechanism since their coefficient of thermal expansion is typically an order of magnitude greater than most other materials. For example PMMA has a coefficient of thermal expansion roughly 90 times greater than that of fused silica and twice that of aluminum. We will also discuss how such devices can be built into optical logic units and smart materials.