Schematic diagram of mechanism for occurrence of the upconverted luminescence. Horizontal lines in each parabola denote vibrational sublevels where |g〉 and |e〉 denote the electronic ground and excited states, respectively. The electronic excitation and de-excitation of the molecule are induced by the absorption and emission of the surface plasmon, respectively. These electron transitions are accompanied by the vibrational excitations, and the vibrational excitations assist the occurrence of the upconverted luminescence.
Miwa et al. Nanoscale Research Letters 2013 8:204 doi:10.1186/1556-276X-8-204