Optimization of the Electronic Third-order Nonlinearity of Cyanine-like Molecules for All Optical Switching

All optical switching (AOS) applications require materials with a large nonlinear refractive index (n(2)) but relatively small linear and nonlinear absorption loss. The figure-of-merit (FOM), defined as the ratio between the real and imaginary parts of the second hyperpolarizability (gamma), is widely used to evaluate the operating efficiency of AOS materials. By using an essential-state model, we describe the general dispersion behavior of gamma of symmetric organic molecules and predict that the optimized wavelength range for a large FOM is near its linear absorption edge for cyanine-like dyes. Experimental studies are normally performed on organic solutes in solution which becomes problematic when the solvent nonlinearity dominates the total signal. This has been overcome using a Dual-arm Z-scan methodology to measure the solution and solvent simultaneously on two identical Z-scan arms and discriminating their small nonlinear signal difference. This technique significantly reduces the measurement uncertainty by correlating the excitation noise in both arms, leading to nearly an order-of-magnitude increase in sensitivity. Here we investigate the n(2) and two-photon absorption (2PA) spectra of several classes of cyanine-like organic molecules and find that the results for most molecules agree qualitatively and quantitatively with the essential-state model. Many cyanine-like molecules show a relatively small FOM due to the presence of large 2PA bands near the linear absorption edge; however, an exception is found for a thiopyrylium polymethine molecule of which the maximum FOM can be > 400, making it an excellent candidate for AOS.