T. Tumkur, J. K. Kitur, and M. A. Noginov, "Quest for low-threshold stimulated emission of surface plasmons polaritons,"
in CLEO: 2013,
OSA Technical Digest (online)
(Optical Society of America, 2013), paper QTh4B.6.
The stimulated emission threshold of SPPs can be reduced by an order of magnitude if the operation range is changed from visible to near-IR and metamaterial substrates are used instead of silver films.