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–supercontinuum probe technique in the energy region of 1.6 – 3.2 eV. Two types of films were used corresponding to 600°C (1st) and to 700°C (2nd) heating. The samples were excited by optical pulses with a duration of 50 fsec and energy of ħωpump = 2.34 eV. The temporal evolution of the photoinduced response is characterized by two relaxation times τ1 and τ2. For the 1st film type, the spectral dependence of the ultrafast relaxation time shows nonmonotonous behavior, it decreases from τ1 ≈ 170 fsec to 70 fsec at 1.6 < ħωprobe < 1.9 eV and increases from τ1 ≈ 80 fsec to 140 fsec at 1.9 < ħωprobe < 3.2 eV. The spectral region­1.6–1.95 eV corresponds to photoinduced darkening while the region 1.95–3.2 eV corresponds to photoinduced bleaching. For the 2nd film type, only photoinduced bleaching is observed, and the ultrafast relaxation time decreases monotonously from τ1 ≈ 200 fsec to τ1 ≈ 70 fsec. The second relaxation time τ2 is of ∼1 psec for both types of films. © 1998 John Wiley & Sons, Ltd.