THE NONLINEAR-OPTICAL SUSCEPTIBILITIES OF POLYDIACETYLENES

We present a general formalism of the NLO susceptibilities by using th e Genkin-Mednis approach for a one-dimensional electron-lattice system . Based on the SSH model, we obtained an analytic expression for the t hird-order nonlinear optical susceptibilities chi((3))(omega) for poly diacetylenes (PDAs) after considering the case of conjugated chains, i n which there is no adjustable parameter. The results are in good agre ement with experiments on PDAs. Furthermore, the origin of the second peak in the chi((3))(omega) spectrum of PDAs is discussed, and a trans ition mechanism is proposed. It is believed that when the external lig ht field is applied along the centrosymmetric polymer backbone, the pa rity symmetry of the system is broken, there occurs a mixing between t he even-and odd-parity exciton stales, leading to the transition from the ground state to the A(g) excited stale, and accordingly a second s harp peak appears in the chi((3))(omega) spectrum which could be attri buted to the three-photon resonance with the A(g) exciton generation p rocess at around 2.2 eV. (C) 1997 Elsevier Science B.V.