Amplification of long-range surface plasmons by a dipolar gain medium

Surface plasmon-polaritions, collective electron oscillations coupled to light waves at the surface of a metal, show unique properties that are valuable in a broad range of scientific fields. However, the intrinsic propagation loss of these waves poses a fundamental problem to many potential applications. To overcome this drawback, researchers have explored the possibility of loss compensation by means of surface plasmon-polarition amplification. Here we provide the first direct measurement of gain in propagating plasmons using the long-range surface plasmon-polariton supported by a symmetric metal stripe waveguide that incorporates optically pumped dye molecules in solution as the gain medium. The measured mode power gain is 8.55 dB mm(-1). Furthermore, it is shown that this new class of amplifier benefits from reduced spontaneous emission into the amplified mode, potentially leading to low-noise optical amplification.