Modeling Photobleaching of Optical Chromophores: Light-Intensity Effects in Precise Trimming of Integrated Polymer Devices

Abstract

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 chromophoredoped 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 1 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 π-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. † Part of the "Larry Dalton Festschrift".

Photobleaching induced time-dependent light emission from dye-doped polymer nanofibers

“…To investigate 5 the temporal behavior of absorbance with different excitation powers P ex , an additional experiment has been performed. The result shows that, with increasing P ex , the absorbance decreases more quickly and the photobleaching is more obvious, which is consistent with the reported results 15,16 . More details and an 10 additional Figure S2 have been added in the Supplementary Information.…”

Lasing in Si3N4-organic hybrid (SiNOH) waveguides

“…Focusing the pump to a smaller cross section has two effects on this mechanism: First, the length of the active WG sections within the pump spot is smaller, such that laser operation requires a higher density of excited single-state dyes to compensate the cavity losses. Second, a higher photon density may also lead to further transitions from the first excited singlet states to higher singlet states, from where the transition to a triplet state is more likely [44]. We believe that a systematic investigation of these effects can lead to additional approaches to increase the device lifetime.…”

“…With a certain probability, such a singlet excited state can undergo an intersystem crossing to a triplet excited state, which requires another intersystem crossing to relax to the singlet ground state and which thus has a rather long lifetime in the microsecond range [42]. Photodegradation can either directly result from a chemical reaction of excited triplet-state dyes with their molecular environment [43] or can be indirectly caused through generation of highly reactive singlet oxygen molecules during one of the intersystem crossings [44]. Focusing the pump to a smaller cross section has two effects on this mechanism: First, the length of the active WG sections within the pump spot is smaller, such that laser operation requires a higher density of excited single-state dyes to compensate the cavity losses.…”

On-Chip Real-Time Chemical Sensors Based on Water-Immersion-Objective Pumped Whispering-Gallery-Mode Microdisk Laser

“…However, at each constant temperature, the lasing wavelength minor shifts linearly which can be seen from the enlarged view of spectral shift at 20 °C (inset in Figure 5a). This blue shift of laser wavelength is attributed to the decreased RI of the polymer-dye composites due to photobleaching [36,37,38,39]. As the spectral shift is linear over time [38], the spectral shift can be corrected by eliminating its slope (red fitting line in the inset of Figure 5a).…”