Propagation of surface-plasmon polaritons (SPPs) with optical gain over macroscopic distances has been demonstrated in a collboration by the University of Iceland (Reykjavik, Iceland), Harvard Medical School (Boston, MA), the University of Cologne (Cologne, Germany), and the Fraunhofer Institute for Applied Optics and Precision Engineering (Jena, Germany).
To overcome the substantial ohmic losses normally seen at the plasmon-dielectric interface, the researchers fabricated symmetric dielectric-metal-dielectric waveguides supporting coupled SPP modes on the top and bottom interfaces of a metal film. Because mode confinement and propagation loss for these long-range SPP modes decreases with metal thickness, amplified spontaneous emission (ASE) for the propagating SPPs can be observed. The waveguides consisted of a 4 nm thick gold layer and a 1 µm layer of a fluorescent poly(phenylene vinylene) derivative blended with a large-bandgap poly(spirofluorene) polymer and an alkyl compound that were sandwiched between two 20 µm thick transparent polymer layers on a silicon substrate. Experiments that gradually increased 532 nm pump power to the waveguide produced ASE near 600 nm. The measured net optical gain was 8 cm-1, which corresponds to a 3000-fold signal increase after 1 cm of propagation. Contact Malte C. Gather at [email protected].