Third-Order Nonlinear Optical Properties of Sulfur-Rich Compounds

The molecular third-order optical nonlinearity γ_R (second hyperpolarizability or nonlinear refractive index) was measured for a series of sulfur-rich molecules: sulfur (S₈), carbon-sulfur compounds, and metal thiolate clusters. Z-scan techniques (pulse width 27 ps, wavelength 527 nm) were used to measure these properties in solution by comparing the solution to the pure solvent. Our approach is an efficient way to evaluate a number of different compounds and to quickly direct synthetic strategies for improved nonlinear and linear optical properties. The optical nonlinearities were evaluated in terms of figures of merit, |W|/I₀ and [T]^-1, where |W|/I₀ is the ratio of nonlinear refraction to linear absorption and [T]^-1 is the ratio of nonlinear refraction to nonlinear absorption. Among the carbon-sulfur compounds, C₆S₈O₂ had the largest figures of merit: |W|/I₀ = 4.3 × 10^-13 m² W^-1 and [T]^-1 > 5. The metal thiolate cluster with the largest second hyperpolarizability was [Zn₁₀S₄(SPh)₁₆]^4- (γ_R = -7.8 × 10^-56 C m⁴ V^-3, -6.3 × 10^-31 esu). This cluster exhibited no measurable linear or nonlinear absorption, so the figures of merit were effectively infinite. Previous work on the second hyperpolarizability of sulfur-rich compounds examined species that were hampered by substantial linear absorption coefficients. The present work shows that high figures of merit can be achieved without significant linear or nonlinear absorption.