Unbiased gold nanoparticles are negatively charged in aqueous solution but not hydrated. Optical spectroscopy of voltage-clamped single gold nanoparticles reveals evidence that anion adsorption starts at positive potentials above the point of zero charge, causing severe but reversible plasmon damping in combination with a spectral red shift exceeding the linear double layer charging effect. Plasmon damping by adsorbate is relevant for the use of nanoparticles in catalysis, in biodiagnostics, and in surface enhanced Raman scattering.

We demonstrate random lasing with star-shaped gold nanoparticles ("nanostars") as scattering centers embedded in a dye-doped gain medium. It is experimentally shown that star-shaped gold nanoparticles outperform those of conventional shapes, such as spherical or prolate nanoparticles. The nanoparticles are randomly distributed within a thin film of gain medium, forming resonators which support coherent laser modes. Driven by single-pulsed excitation, the random lasers exhibit coherent lasing thresholds in the order of 0.9 mJ/cm(2) and spectrally narrow emission peaks with linewidths less than 0.2 nm. The distinguished random laser comprising nanostars is likely to take advantage of the high plasmonic field enhancements, localized at the spiky tips of the nanostars, which improves the feedback mechanism for lasing and increases the emission intensity of the random laser.

We report a cavity quantum electrodynamic enhancement of stimulated emission in polymer microlasers. The enhancement of the stimulated emission cross section is shown through the demonstration of laser action in the 40μm diameter cylindrical microcavities whose total molecular population density is below that theoretically required for the onset of lasing. The lower bound of the enhancement factor is estimated to be 3.9 based on the noncavity value of the stimulated emission cross section and the measured cavity quality factor.

The spectral shape of the phosphorescence emission of organometallic porphyrin molecules is shown to be altered when these chromophores are incorporated into hybrid nanostructures with gold nanorods. This result shows that triplet-singlet transitions, which are (at least partially) dipolar forbidden, can be modified by the dipolar resonances of gold nanoparticles. By choosing nanorods of increasing aspect ratios, it is possible to match the long axis plasmon resonance of the nanorods to a specific phosphorescence transition. Consequently, the emission colour of the hybrids can be tuned.

Near-infrared optical gain in luminescent polymeric slab asymmetric waveguides has been investigated by use of amplified spontaneous emission. Upon nanosecond photopumping, the waveguides have shown a small-signal gain coefficient of 37.2±2.1cm−1 at 820nm for a pump fluence of 1.57mJ∕cm2 (314kW∕cm2). The loss coefficient and transparency fluence have been found to be 7.3±1.0cm−1 at 820nm and 0.14mJ∕cm2 (28kW∕cm2), respectively. Furthermore, we show that a small-signal gain of 19.7±2.3dB is achievable in a 1.2-mm-long waveguide.