Nano Lett. 11, 4295–4298 (2011)

The localized surface plasmon resonances of metallic nanoparticles have so far been used to enhance lasing in random lasers, but not to provide feedback. Tianrui Zhai and co-workers from Beijing University of Technology in China have now demonstrated a waveguide–plasmonic feedback scheme that offers high-quality confinement of the radiation for efficient amplification, low-threshold lasing and directional output. The scheme is essentially a waveguide on a silica substrate and comprises a 20 mm × 20 mm layer of randomly distributed gold nano-island structures spin-coated with a solution of poly(methyl methacrylate) (PMMA) doped with a red dye. The researchers chose the dye and nano-islands such that there was an excellent overlap between the photoluminescence spectrum of the dye and the plasmonic resonance spectrum of the gold nano-islands. When pumped by a green (532 nm) pulsed laser operating with a pulse energy of 50 mJ, a large part of the scattered light was reflected totally at the PMMA–air interface and subsequently scattered by the gold nanostructures within the active waveguide, leading to amplification by stimulated emission in a manner similar to that of a conventional microcavity. The team observed that random lasing commenced at a pump threshold of around 0.1 MW cm−2. Because the lasing wavelength can be tuned by changing the size of the gold nano-islands, the researchers say that multicolour or even white light may be achievable.