New optical limiting materials: synthesis, structures and nonlinear absorption of cubic cage shaped clusters

Abstract

Mixed metal clusters (n-Bu4N)3[WCu3Br4S4] (I), (n-Bu4N)3[WAg3Br4S4] (II), (n-Bu4N)3[MoAg3BrCl3S4] (III) and (n-Bu4N)3[MoAg3BrI3S4] (IV) were synthesized by solid state reactions. The cluster anions assume cubic cage shaped structures and possess strong optical limiting capability. At very low fluences the molecules respond linearly to the incident light obeying Beer's law. As light fluence rises their molecular absorptivities increase rapidly exhibiting limiting effect with threshold and saturation fluence of 1.3 J/cm2 and 0.7 J/cm2 for compound I, 0.7 J/cm2 and 0.5 J/cm2 for compound II, 0.6 J/cm2 and 0.3 J/cm2 for compound III, and 0.5 J/cm2 and 0.3 J/cm2 for compound IV respectively. The threshold and saturation values of compound IV are about 3 and 2 times better than those of C60 measured under identical conditions. The optical limiting capability of the clusters is derived from excited state absorption, a process that turns on within nanoseconds and off over milliseconds.