Abstract

The nonlinear mechanisms underlying the third order nonlinearity of any material can be studied by using nonlinear spectroscopy over an appropriate range of photon energy. Traditionally in the field of organic materials, third harmonic generation has been the principal probe used. A number of THG experiments have been successfully interpreted in terms of a limited number of contributing molecular states.[1] For conjugated polymers the best results to date have been obtained in terms of 4 "essential" states, the ground state (1A_g), the one photon o exciton state (1B_u), a dominant two photon state (mA_g) and a second one photon state (nB_u) which is weakly coupled to the ground state, but strongly coupled to the strong mA_g two photon o state. In the past we have successfully interpreted our THG data using these four states.[1] However, when we extended our measurements to other nonlinear spectroscopic techniques such as non-degenerate three wave mixing and two photon absorption, we were no longer able to obtain satisfactory fits to all of our data simultaneously. We describe here a combined experiment-theory program in which we have studied the nonlinear spectra of a variety of nonlinear polymers in an effort to find a better interpretation of their nonlinear mechanisms.

© 1993 Optical Society of America