The nonlinear optical susceptibilities of polydiacetylenes

We present a general formalism of the NLO susceptibilities by using the Genkin-Mednis approach for a one-dimensional electron-lattice system. Based on the SSH model, we obtained an analytic expression for the third-order nonlinear optical susceptibilities χ⁽³⁾(ω) for polydiacetylenes (PDAs) after considering the case of conjugated chains, in which there is no adjustable parameter. The results are in good agreement with experiments on PDAs. Furthermore, the origin of the second peak in the χ⁽³⁾(ω) spectrum of PDAs is discussed, and a transition mechanism is proposed. It is believed that when the external light field is applied along the centrosymmetric polymer backbone, the parity symmetry of the system is broken, there occurs a mixing between the even- and odd-parity exciton states, leading to the transition from the ground state to the A_g excited state, and accordingly a second sharp peak appears in the χ⁽³⁾(ω) spectrum which could be attributed to the three-photon resonance with the A_g exciton generation process at around 2.2 eV.