Photorefraction and complementary grating competition in bipolar transport molecular material

Abstract

A comprehensive investigation of the photorefractive properties of an unusual molecular material is reported. This material is a glassy solid that is composed of a multifunctional molecule consisting of a sexithiophene covalently linked with a nonlinear optical chromophore, a methine dye. A net photorefractive gain coefficient of 70.5 ${\mathrm{cm}}^{\mathrm{-}1}$ and a diffraction efficiency of 18.9% (130 μm thick) were observed. It was found that this material exhibits the competition of complementary holographic gratings that are formed by the space-charge field of two types of photocarriers. Competition of complementary holographic gratings was revealed from the cancellation and revelation of the two types of gratings and is discussed based on a bipolar two-trap photorefractive model. Two transport channels and two trapping centers for photogenerated electrons and holes, respectively, are responsible for the formation of the two complementary gratings. The mobility and the number density of traps for the two types of charged carriers are different; time-of-flight results indicate that the holes have higher mobility than the electrons. A slow, secondary (weak) grating formed by electrons is 180° out of phase with respect to that of the fast, principal one formed by holes. This reduces the net space-charge field, and a cancellation in the index grating was exhibited during the grating formation process. The slow grating could be revealed, and an oscillationlike behavior was shown under the irradiation of a uniform light. The oxidation and reduction potentials of the charge-carrier species explain the microscopic mechanism for the bipolar transport channels. The buildup dynamics of the gratings are discussed in detail.