Towards optical gain in plasmonic metamaterials

Manipulating and guiding light at physical dimensions below the diffraction limit of conventional optics necessitates optical elements appreciably smaller than the design wavelength.  In this respect, the localised plasmonic resonances (LPR) of silver or gold nanoparticles present excellent advantages in terms of field confinement and enhancement. For nanoparticle synthesis, self-assembled anodic aluminium oxide (AAO) templates producing quasi-hexagonally ordered gold nanorods oriented perpendicular to a substrate benefit from low size dispersion and ease of fabrication. Nanorod resonances are dependent on their length, diameter, surrounding permittivity and near-field coupling between adjacent rods providing strong electromagnetic field enhancement.  Such metamaterials, having a tunable plasmonic response and benefiting from homogeneous nanorod orientation are ideal for designing and implementing nano-plasmonic waveguides.  These waveguides suffer no limitation in bending losses in comparison with conventional approaches (dielectrically loaded surface plasmon polaritonic waveguides, metallic stripe waveguides etc.).  Here we demonstrate the ability to create such nanoscale plasmonic elements by focused ion beam milling the nanoporous template.  Plasmon-polariton propagation is, however, greatly attenuated by ohmic losses in the metal but may be significantly reduced by embedding the plasmonic metal in gain media.  By tuning the resonant plasmonic properties of the system coincident with the photo-emissive properties of the gain molecules, losses may be compensated via coherent electromagnetic coupling.  Finally, we present investigations of plasmon-polariton loss compensation in arrays of gold nanorods in a two-colour pump-probe arrangement. Gain media consists of a red dopant 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) initially dispersed in a polystyrene matrix with a view to implementing a guest-host system, based on Forster energy transfer from a green host tris(8-hydroxy-quinolinato)aluminum (Alq3) to the red DCM dopant.