Modeling photobleaching of optical chromophores: Light-intensity effects in precise trimming of integrated polymer devices

We present a model for the photobleaching of nonlinear optical (NLO) chromophores, via photo-oxidation, using either high-intensity or low-intensity light sources. A closed-form expression is derived for calculating the temporal evolution of bleaching-induced refractive index change, averaged over the thin-film depth. The averaged values are appropriate for analytically calculating corresponding, changes in the effective index of optical waveguides. This applies to precise trimming of the resonant wavelength and coupling in chromophore-doped polymer microring resonator (MRR) devices. In high-intensity (few kW/cm(2)) laser photobleaching experiments for trimming of MRR coupling, the observed refractive index changes are nearly I order of magnitude less than those predicted by low-intensity bleaching curves, for the same total exposure energy. An in-depth review of the photophysics and. photochemistry of photo-oxidation of aromatic molecules is presented here, revealing that a high-intensity excitation source is likely to cause chromophore ground-state depopulation, due to the long lifetime of the triplet state. This theory also explains why photobleaching efficiency can be increased by pulsing the excitation, because the chromophore ground-state is allowed to replenish between bleaching pulses. For the first time, to our knowledge, the concept of saturated absorption is applied to model the effect that high-intensity light has on photobleaching-induced index change. The values of the photostability figure-of-merit and saturation intensity for it-conjugated NLO chromophores, such as CLD-1, are obtained here by fitting experimental data. We also note that light-intensity is known to affect the rate of varied photoreactions, such as laser ablation of polymers and human tissue and plant photosynthesis, which share striking photophysical similarities with photobleaching.