  We have investigated second harmonic generation (SHG) from Ag-coated LiNbO3
(LN) core-shell nanocuboids and found that giant SHG can occur via deliberately
designed double plasmonic resonances. By controlling the aspect ratio, we can
tune fundamental wave (FW) and SHG signal to match the longitudinal and
transverse plasmonic modes simultaneously, and achieve giant enhancement of SHG
by more than five orders of magnitude in comparison to a bare LN nanocuboid and
by about one order of magnitude to the case adopting only single plasmonic
resonance. The underlying key physics is that the double-resonance nanoparticle
enables greatly enhanced trapping and harvesting of incident FW energy,
efficient internal transfer of optical energy from FW to SHW, and much improved
power to transport the SHG energy from the nanoparticle to the far-field
region. The proposed double-resonance nanostructure can serve as an efficient
subwavelength coherent light source through SHG and enable flexible engineering
of light-matter interaction at nanoscale.
