Spatially Confined Gap-Mode Excited Molecular Vibrations in Poly(p-Phenylene Vinylene)

This study explores excitation of molecular vibrations in oriented poly(p-phenylene vinylene) or PPV chains by spatially confined gap-mode of a nanoslit antenna fabricated in ≈200-nm-thick gold film. Semi-infinite nanoslit antenna of ≈10 nm width and >200 μm length supports Fabry-Perot resonator like gap-modes, exhibiting strongly confined and spatially diffracting electromagnetic fields on a sub-5 nm length scale that is comparable to the PPV chain width. Excitation of Raman active phenylene and vinylene vibration modes in PPV by such a unique gap-mode facilitates exploration of molecular vibrations selection rules beyond the long wavelength approximation. Raman spectroscopy results presented here reveal clear broadening in vinylene modes of PPV chains due to the relaxation of momentum-space selection rules induced by strong spatial confinement of the gap-mode. Broadening is accompanied by a small red shift in Raman peak position associated with a much higher gap-mode wavevector distribution compared to the free space propagating mode. The experimental results demonstrate potential applications of extremely narrow (≈λ/100), essentially 1D illumination conditions generated by a tunable width nanoslit antenna in probing and engineering nanoscale light-matter interactions beyond long-wavelength approximation over a broad spectral range.