Low mode-locking threshold induced by surface plasmon field enhancement of gold nanoparticles.

We demonstrate a novel method to reduce the mode-locking threshold of erbium-doped fiber laser (EDFL) based on saturable absorber (SA). The SA was prepared by mixing gold nanoparticles (GNPs) and single-wall carbon nanotubes in sodium carboxymethylcellulose. The mode-locking threshold of EDFL was adjusted through simple changing the concentration of GNPs in the SA. The variation range of the threshold was as large as 21.5 mW. A lowest threshold of ~16 mW was obtained with the concentration of GNPs as 0.006 mmol/ml. The largest decreased ratio of the initial threshold was 47.5%. Surface plasmon field enhancement effect was speculated as the main reason for the reduced mode-locking threshold.