Nonlinear optical properties of J-aggregated merocyanine dye microcrystals in polymer matrices

We found that microcrystals of an amphiphilic merocyanine dye (MCSe-C₁₈) dispersed in water could be stably converted to the J-aggregate form. The absorption maximum shifts to 610 nm (J1 state) or 630 nm (J2 state) from 550 nm in the non-J state and shows the sharp excitonic peak with a half-width at room temperature of roughly 40 nm or 30 nm, respectively. The maximum of the modulus of χ⁽³⁾ was observed to be approximately 2.5 × 10^-12 esu for the J2 state microcrystals (10 wt.%) in a poly(vinyl alcohol) matrix, which is 20 times larger than that of non-J state microcrystals (10 wt.%). The change in the phase of χ⁽³⁾ from 0 deg. at 700 nm to 180 deg. at 550 nm can be explained by the 3-photon resonance of the excitons. The figure of merit |χ⁽³⁾|/α reaches a maximum value of 5.7 × 10^-12 esu·μm at a wavelength of 670 nm. This is 1.5 and 20 times greater than at non-resonant (720 nm) and excitonic peak (630 nm) wavelengths, respectively. From this result, we surmise that the enlargement of nonlinear optical properties can be obtained by sharpening the absorption peak and using a wavelength near band edge.