Hae-Wook Yoo, Lee J. Richter, Hee-Tae Jung, Chris A. Michaels
We report surface plasmon polariton (SPP) mediated Raman microscopy on dielectric films in contact with a Ag layer at 785 nm with spatial resolution approaching the optical diffraction limit and reasonable spectral acquisition times. The excitation and scattered photons couple to the SPP of the Ag film through a hemispherical ZnSe solid immersion lens (SIL). SPP mediated microscopy enables in-situ studies of material in intimate contact with metal electrodes when optical access to the dielectric material by conventional means is unavailable. We use the prototypical organic solar cell system of a 1:1 blend of poly(3-hexylthiophene) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) to demonstrate the new microscopy. Calculations of the electric field intensity at the metal-dielectric interface are presented that yield insight into the optimal angular coupling into the Ag SPP and the optimal Ag film thickness. SPP Raman spectra of pure P3HT films are reported for a series of samples with Ag films of varying thickness, confirming excitation via the SPP. SPP Raman images of thermally annealed blend films are presented and compared to conventional confocal Raman images.