Ultrafast optical response of carbon films

Abstract

The ultrafast photoinduced optical response of heat (infrared) treated polyacrylonitrile films (PAN-IR) is studied by the femtosecond pump-supercontinuum probe (PSCP) technique in the probe energy region 1.6-3.2 eV. The samples were excited by pulses with duration of 50 fs and photon energy of (h) over bar omega(pump) = 2.34 eV. Two types of films were used corresponding to 600 and 700 degrees C heating. The structure of the films was investigated with atomic force microscopy, micro-Raman spectroscopy, and optical absorption spectroscopy. The size of the graphitic clusters was evaluated as similar to 13 Angstrom for the first and > 17 Angstrom fur the second sample. The study of the photoinduced response shows that its temporal evolution is characterized by two-stage relaxation with fast and slow relaxation rates. For the first type of film, the spectral dependence of the fast relaxation rate shows nonmonotonic behavior with a maximum at (h) over bar omega(probe) approximate to 1.95 eV. The region 1.6-1.95 eV shows photoinduced darkening, while the region 1.95-3.2 eV shows photoinduced bleaching. For the second type of film, only photoinduced bleaching is observed, with the fast relaxation I att: increasing monotonically. From the analysis of the transient spectra and relaxation rates, two types: of optical transitions are proposed. The first is the normal intramolecular absorption when photoexcited electrons stay within the same cluster. The second one is a charge-transfer transition, when the photoexcited electron is transferred from one cluster to another closest one. For large size clusters, the contribution from these transitions vanishes and only intramolecular excitations survive.