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. 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.