Raman scattering gain in DOO-PPV films

We have investigated the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV) thin films under high intensity ps pulsed laser excitation (Ipump > 1 MW/ cm2). We observed that the emission spectrum consist of a spectrally narrowed emission (SNE) band (FWHM ~ 10 nm) and superimposed sharp spectral lines. High resolution spectral measurements conducted at room and liquid nitrogen temperatures show that the spectral width of these lines is less than 1 nm. The lines peak positions are independent of film thickness, however, they shifts upon changing the excitation wavelength. Also, the excitation intensity dependence of these narrow lines is found to be different from that of the SNE band. We argue that the narrow lines are due to Raman scattering gain of the 1st, 2nd and 3d order Raman active modes of DOO-PPV.