Nonlinear absorption in CuPc-SiO2 Composite

Recently macrocyclic organic dyes have attracted a lot of attention as possible candidates for applications in optical power limiters and pulse shaping devices. The fundamental requirement for power limiting is that the system must exhibit fluence dependent reverse saturable absorption. Copper Pthalocyanines possess a larger excited state absorption cross section than a ground state absorption cross section. They are thus suitable candidates for limiting applications. Most of the nonlinear optical studies have been done for solutions of the dyes1. For applications such as intracavity pulse shaping devices a solid material is more attractive than a solution2. In this paper we report nonlinear absorption studies of Copper Pthalocyanine, CuPc, in solution in sulfuric acid as well as incorporated in a Silica matrix host. The concentration of the solution was 5.75 x 10-4 Molar. The composite had a concentration of 1 x 10-4 Molar in the sol. Upon drying of the gel we estimate a similar concentration for the composite as in the solution given the reduction in volume. The thickness of the composite is of 4 mm. The solution was held in 1 mm thick spectroscopic grade glass cuvettes. The CuPc/silica composite was obtained by the Sol-gel method from the hydrolysis of tetramethoxysilane (TMOS). The reaction of TMOS with acidic water (pH=2) was promoted by means of 20 kHz ultrasound energy. The resulting gel was kept drying at room temperature for two weeks. We observe marked differences in the linear absorption spectrum of the solution and the composite. These changes are attributable to aggregation of the Copper Pthalocyanine molecules in the pores of the silica matrix as well as to the interaction between the dye molecules and the matrix. The linear absorption spectrum for both samples is shown in Fig. 1.