3RD-ORDER NONLINEAR-OPTICAL PROPERTIES OF A LADDER POLYMER OBTAINED BY PYROLYSIS OF POLYACRYLONITRILE

We have investigated the nonlinear optical properties of a ladder poly mer obtained by pyrolysis of polyacrylonitrile in the presence of oxyg en. Undoped and FeCl3 doped polymer are found to be electrically condu cting. Upon doping, the polymer spectre show changes indicative of the presence of polarons. Subpicosecond degenerate four-wave mixing measu rements have been performed at 602 nm yielding effective chi((3)) valu es of the order of 10(-11) esu for most samples and indicating the pre sence of longer-lived excitations contributing to the third-order nonl inearity. The characteristic relaxation time constant for these nonlin earity contributions is in the range of 2-7 ps. The influence of dopin g on the nonlinearity is discussed in terms of contributions from both the undoped chain sections and chemically-induced polarons. (C) 1998 Elsevier Science Ltd. All rights reserved.