Polymethine and squarylium molecules with large excited-state absorption

We study nonlinear absorption in a series of ten polymethine dyes and two s quarylium dyes using Z-scan, pump-probe and optical limiting experiments. B oth picosecond and nanosecond characterization were performed at 532 nm, wh ile picosecond measurements were performed using an optical parametric osci llator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acryl ate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. F or several of the dyes we measure a relatively large ground-state absorptio n cross section, sigma(01), which effectively populates an excited state th at possesses an extremely large ESA cross section, sigma(12). The ratios of sigma(12)/sigma(01) are the largest we know of, up to 200 at 532 nm, and l ead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased t o up to 3 ns in PUA. This lifetime is less than optimum for sensor protecti on applications for Q-switched inputs, and intersystem crossing times for t hese molecules are extremely long, so that triplet states are not populated . These parameters show a significant improvement over those of the first s et of this class of dyes studied and indicate that further improvement of t he photophysical parameters may be possible. From these measurements, corre lations between molecular structure and nonlinear properties are made. We p ropose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans-cis conformatio nal change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degra dation at high inputs. (C) 1999 Elsevier Science B.V. All rights reserved.