PHOTOSTABILITY STUDIES ON HIGH DIPOLE NONLINEAR OPTICAL ADDUCTS OF TCNQ IN POLYMER OPTICAL WAVEGUIDES

Abstract

The photostabilities of nonlinear optical organic dyes doped as guests into Poly(methyl mathacrylate), (PMMA), waveguide films have been investigated using measurements of planner waveguide transmission over time. Direct photodecomposition of the dyes is observed when the doped films are exposed to wavelengths lying within the main absorption bands. The dyes exhibit a rapid photochemical degradation that is enhanced when the experiments are repeated in an air environment at the irradiation between 450 nm and 515 nm. Excitation of the zeroth-order transverse magnetic mode (TM0) by prism couplers produced complete degradation within 30 minutes. The dye photostabilities are compared to that of a commonly studied laser dye, 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran, DCM, which exhibits a photo-oxidation mechanism. The experiments in air lead to comparable degradation after 3 to 4 hours but the dye proves to be exceptionally photostable in vacuum. Further interesting results are reported on the formation of crystallites in the films when irradiated at 632.8 nm where linear absorption in the films is low. Microscopic examination of the waveguide films at the input coupling position confirms the formation of crystallites for all the films studied. Cross coupling to higher order modes from the light scattering of crystallites is revealed by scanning m-line images of the waveguides. This is associated with a marked refractive index decrease in the films. An air and humidity environment is found to favor the crystallite formation in the dye-doped waveguides.