Nonlinear refractive indices of polydiacetylene microcrystals

Polydiacetylene microcrystals in a water suspension or a polymer matrix were prepared by a simple precipitation procedure in order to obtain a thin samples with good optical quality maintaining the characteristics of crystalline materials. The nonlinear refraction, n2, and nonlinear absorption, (beta) , around the exciton peak were measured by z-scan method using a tunable picosecond laser. The signs of the n2 are positive/negative at wavelengths shorter/longer than the exciton peak and strong saturable absorption, which (beta) is negative, are observed around the peak. The largest n2 was obtained to be plus 0.024 cm2/GW (Re(chi (3) equals plus 1.4 multiplied by 10-9 esu) at 640 nm for a gelatin composite film containing 24 wt percent of polydiacetylene microcrystals.

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