Optical nonlinearities of methyl red in various solid matrices

An azo dye, methyl red (MR), was introduced into glass films and bulks and into polymer films. The glasses were either pure silicate or composed of a silicate network with the addition of long chain organic molecules. Some glasses were composites of silicate and methyl methacrylate (MMA) or poly(vinyl acetate) (PVAc). The polymer films, which consisted of PVAc or poly(vinyl alcohol) (PVA), were deposited on glass plates. The induced transparency of the samples was measured by a pump and probe method, using the 488 nm line of an argon-ion laser for both the pump and probe beams. The nonlinear effect was observed with the neutral and basic forms of MR (orange and yellow, respectively). The magnitude of the effect decreased with increasing density of the host as follows: polymer matrices, composite glass, sol-gel glass. These results, as well as measurements of the excited state spectra, are consistent with the hypothesis that the observed nonlinearities are the result of a trans-cis isomerization of MR, for optical excitation in the milliwatt range.