Linear and third order nonlinear optical properties of substituted oligothiophenes

Low band gap polythiophene derivatives synthesized using the method of Stil le coupling have shown good third order nonlinear optical properties. In or der to gain more insight into the structure/ property relationship in these materials, we measured the optical properties of a number of oligothiophen e derivatives. In addition to linear absorption spectra, we investigated th e third order nonlinear optical properties using Z-scan at a number of wave lengths. As in other conjugated oligomers, the properties depend strongly o n chain length. We also observe significant dependence on the substituent t ype, number, and position, with strong enhancement of the oscillator streng th for asymmetric substitution. The resonance nonlinear absorption shows st rong saturation behavior, in general changing to induced absorption off-res onance. The nonlinear refractive indices were negative near resonance, sugg esting the possibility of thermal effects. While off-resonant nonlinear ref ractive indices were small, in the five substances for which they could be measured, the values were found to be negative, suggesting that resonant en hancement is still playing a role. (C) 1999 American Institute of Physics. [S0021-9606(99)70306- 5].