  This paper demonstrates an arbitrary tunable spaser based on double-Fano
resonance of a plasmonic nanostructure consisting of two sets of disk-ring
(TSDR) nanostructure. TSDR nanostructure supports two Fano resonances, which
can be served as the lasing mode and the pumping mode of a spaser. These two
mode can be tuned in a very wide wavelength range because of the good
tunability of the two nanorings. The tuning range of the lasing mode and the
pumping mode can be reached to 710 nm and 620 nm. These results represent a
significant step in the pursuit of ultimate spasers and propose a approach to
manipulate lasing mode and pumping mode over a broad spectral range.
