ULTRAFAST OPTICAL-RESPONSE OF IR-TREATED POLYACRYLONITRILE FILMS

An ultrafast photoinduced optical response of infrared (heat) treated polyacrylonitrile films (PAN-IR) was studied by a femtosecond pump-sup ercontinuum probe technique in the energy region pf 1.6 - 3.2 eV. Two types of films were used corresponding to 600 degrees C (Ist) and to 7 00 degrees C (2nd) heating. The samples were excited by optical pulses with a duration of 50fsec and energy of (h) over bar w(pump) = 2.34eV . The temporal evolution of the photoinduced response is characterized by two relaxation times tau(1) and tau(2). For the 1st film type, the spectral dependence of the ultrafast relaxation time shows nonmonoton ous behavior, it decreases from tau(1) approximate to 170fsec to 70fse c at 1.6 < (h) over bar w(probe) < 1.9eV and increases from tau(1) app roximate to 80fsec to 140fsec at 1.9 < (h) over bar w(probe) < 3.2eV. The spectral region 1.6-1.95eV corresponds to photoinduced darkening w hile the region 1.95-3.2eV corresponds to photoinduced bleaching. For the 2nd pm type, only photoinduced induced bleaching is observed, and the ultrafast relaxation time decreases monotonously from tau(1) appro ximate to 200fsec to tau(1) approximate to 70fsec. The second relaxati on time tau(2) is Of similar to 1 psec for both types of films. (C) 19 98 John Wiley & Sons, Ltd.