Effect of Axial Substitution on the Optical Limiting Properties of Indium Phthalocyanines

Abstract

Nonlinear absorption, refraction, and optical limiting by a series of monoaxially chloro- and aryl-substituted indium(III) phthalocyanines are described. The absorption cross sections and temporal evolution of the low-lying excited states are also reported. A large nonlinear absorption that increased with wavelength between 500 and 590 nm was observed in each material. The nanosecond nonlinear absorption and the optical limiting are shown to be dominated by a strong excited state absorption from an orientationally averaged triplet state. We derive and experimentally confirm the relation between the molecular absorption cross sections and the fluence-dependent nonlinear absorption coefficients. The effective nonlinear refraction on the nanosecond time scale was reduced because the electronic contribution to the nonlinear refractive index was of the opposite sign from the thermal contribution. An optical limiter using the new material, p -(trifluoromethyl)phenylindium(III) tetra- tert -butylphthalocyanine [( t -Bu) 4 PcIn( p -TMP)], showed a much lower threshold for optical limiting and a much lower transmission at high fluences than previously reported indium phthalocyanine limiters. This improved optical limiting was due both to the larger nonlinear absorption coefficient and to the design of the limiter device. The optical properties of the In phthalocyanine moiety were found to be surprisingly robust to structural changes in the axial position.