Spontaneous emission in long-range surface plasmon-polariton amplifiers

Measurements of amplified spontaneous emission are conducted on a long-range surface plasmon-polariton amplifier consisting of a symmetric metallic waveguide incorporating a gain medium in the form of optically pumped dye molecules in solution. An effective input noise power per unit bandwidth of 7.47 x 10(-4) fW/Hz is measured at lambda = 876 nm, which is 3.3 times larger than the theoretical minimum for phase-insensitive optical amplifiers. A semiclassical theoretical model describing the amplified spontaneous emission of long-range surface plasmonpolaritons (and other types of surface plasmon modes) is proposed, finding good agreement with experimental results. It is shown that the amplifier's low noise follows from a low spontaneous emission rate of long-range surface plasmonpolaritons.