Photogeneration of Nonlinear Excitations in Semiconducting Polymers: Fast Response Nonlinear Optical Phenomena

Abstract

In traditional three-dimensional semiconductors, the four-fold (or six-fold, etc.) coordination of each atom to its neighbor through covalent bonds leads to a rigid structure. In such systems, therefore, the electronic excitations can usually be considered in the context of this rigid structure leading to the conventional concepts of electrons and holes as the dominant excitations. The situation in semiconductor polymers is quite different; the two-fold coordination makes these systems generally more susceptible to structural distortion. As a result, the dominant “electronic” excitations are inherently coupled to chain distortions. Thus, solitons, polarons and bipolarons are the excitations of major importance in this novel class of one-dimensional polymer semiconductors. These semiconducting polymers are, therefore, fundamentally and inherently nonlinear. Moreover, the major structural rearrangements (to form the solitons, bipolarons, etc.) which lead to the nonlinear optical properties are known to occur on time scales of order 10⁻¹³ seconds [1].