Dye-assisted gain of strongly confined surface plasmon polaritons in silver nanowires

Noble metal nanowires are excellent candidates as subwavelength optical components in miniaturized devices due to their ability to support the propagation of surface plasmon polaritons (SPPs). Nanoscale data transfer based on SPP propagation at optical frequencies has the advantage of larger bandwidths but also suffers from larger losses due to strong mode confinement. To overcome losses, SPP gain has been realized, but so far only for weakly confined SPPs in metal films and stripes. Here we report the demonstration of gain for subwavelength SPPs that were strongly confined in chemically prepared silver nanowires (mode area = λ²/40) using a dye-doped polymer film as the optical gain medium. Under continuous wave excitation at 514 nm, we measured a gain coefficient of 270 cm^-1 for SPPs at 633 nm, resulting in partial SPP loss compensation of 14%. This achievement for strongly confined SPPs represents a major step forward toward the realization of nanoscale plasmonic amplifiers and lasers.