New Optical Limiting Materials: Synthesis, Structures and Nonlinear Absorption of Cubic Cage Shaped Clusters
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S. SHI, W. JI AND X.C. XIN Optical Crystal Laboratory and Physics Department, National University of Singapore, Singapore 0511.
ABSTRACT
Mixed metal clusters (n-Bu4N)AWCu 3Br4S4] (I), (n-Bu 4N)3 [WAg 3Br 4S4] (II), (n-Bu 4N) 3[MoAg 3BrCl 3S4] (111) and (n-Bu4N) 3[MoAg 3BrI 3S4] (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 2 compound I, 0.7 J/cm2 and 0.5 J/cm2 for compound H, 0.6 J/cm2 and 0.3 J/cm for compound 2 2 threshold and saturation The IV respectively. J/cm for compound III, and 0.5 J/cm and 0.3 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.
INTRODUCTION The current development of optical signal detection techniques (such as signal acquisition, remote sensing, and night vision) is often frustrated by the intrinsic incompetence in self-protection of highly sensitive optical detectors. Every detector has a finite power window. It is often the case that the more sensitive a detector is toward a weak signal, the more fragile it is toward an undesired strong signal. This paradox situation calls for research on optical limiting (OL) materials that are transparent to weak and opaque to strong signals. Unfortunately, most of the materials do not have such a desired optical property. By contraries, they often become more transparent under high fluences of light because of the depletion of electronic ground state. Two-photon absorption has been discovered to be able to 2 produce optical limiting effect, but only in the range of very high light intensity." Recently fullerene C6o has been found to possess such unusual optical limiting capability by one-photon mechanisms and hence received tremendous attention.3 '4 As part of our search for optical limiting materials, we have studied the nonlinear optical properties of a number of inorganic clusters. In this report an account will be given to the synthesis, structures and optical limiting effect of four transition metal clusters, (n-Bu 4N) 3[WCu 3Br4S4] (I), (n-Bu 4N) 3[WAg 3Br4 S4] (H), (n-Bu4 N)3[MoAg 3BrC13S4] (MI) and (n-Bu 4N) 3[MoAg 3BrI 3S4] (IV). Evidence of excited state absorption and heavy atom effect will 363
Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
also be discussed. EXPERIMENTAL
Compounds I to IV were synthesized by similar solid state reactions. A typical synthetic procedure is illustrated with compound I. A well-ground mixture of 1.0 mmol of ammonium tetrathiotungstate ((NH 4)2WS4), 4.0 mmol of copper(l) bromide (CuBr), and 3.0 mmol of tetrabut
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