Picosecond Degenerate Four-Wave Mixing Study of Nonlinear Optical Properties of the Poly-N-Vinyl Carbazole: 2,4,7-Trinitrofluorenone Composite Polymer Photoconductor

摘要

The role of charge carriers in determining optical non-linearities in inorganic semiconductors and photoconductors has been widely investigated An evaluation of optical nonlinearities derived from charge carriers in organic semiconductors and photoconductors is lacking. Our initial motivation for the present work was to assess the contributions to X(3) derived from charge carrier dynamics in organic systems. For our study of third-order optical nonlinearity in an organic photoconductor, we have chosen poly-N-vinyl carbazole (PVK) as the photosystem which has been widely studied. Resonant third-order nonlinear optical susceptibility x(3) of poly-vinyl-n-carbazole: 2,4,7- trinirofluorenone composite polymer photoconductor has been measured at 602 nm for various compositions by a picosecond degenerate four-wave mixing. The origin of effective third-order nonlinearity of this system is attributed to the change-transfer excitation which creates thermalized correlated electron-hole pairs. The optical nonlinearity of this polymeric system is characterized by a long relaxation time of hundreds of picoseconds. A progressive enhancement of the signal intensity and hence effective X(3) accompanied by an increase in the decoy rate of the degenerate four-wave mixing signal has been observed with an increase in the mole fraction of trinitrofluorenone. Reprints. (AW)