Nanoplasmonics and surface enhanced spectroscopy

Emission enhancement of plasmonically modified YAG structures co-doped by Ce3+, Bi3+ and Yb3+ ions at low temperature
M. Kushlyk1, V. Tsiumra1,2, Ya. Zhydachevskyy2,3, V. Haiduchok3,4, I.I. Syvorotka3,4, D. Sugak3, 4, M. Baláž5, A. Suchocki2
1 Ivan Franko National University of Lviv, 79017, 107 Tarnavskoho Str., Ukraine 
2 Institute of Physics PAS, Al. Lotników 32/46, Warsaw 02-668, Poland 
3Lviv Polytechnic National University, 79646, 12 Bandera, Lviv, Ukraine
4SRC „Carat“, 202 Stryjska Str., Lviv 79031, Ukraine
5 Institute of Geotechnics SAS, Watsonova 45, 04001 Košice, Slovakia 

In recent years, much attention has been paid to the plasmonic effect of metal nanoparticles formed on the surface of the down-converting phosphors. Combination of thin conversion layer, with silver plasmonic nanostructures leads to increase donor absorption and emission capacity. 
High enhancement factors are caused by a close matching of the maximum of plasmon extinction and Bi3+ emission bands. The enhancement in PL intensity can be explained by the interaction between the spontaneous recombination and surface plasmon in metal nanoparticles.
According to the FDTD simulation, near surface of NPs exhibits dipole and quadrupole resonance modes LSPR which high enhancement factor can lead to the high DC efficiency. Quadrupole mode of Ag plasmon resonance could be used for Bi3+ emission enhancement. However, because the relatively short penetration depth of SPR electric field of NPs gives rise to the limitations of luminescence enhancement efficiency. 
Enhancement of Bi3+ emission in YAG epitaxial films with Ag NPs during cooling the samples below 200 K was observed. We obtained maximum value of enhancement factor near 170% at 4 K temperature. Such enhancement arises as the result better matching of quadrupole mode of SPR maximum with Bi3+ emission wavelength. The observed redshift of quadrupole peak is suggested to be caused by temperature changes of Ag, SiO2 and YAG dielectric constants.
It is shown that the low temperatures leads to an increase of: the EM field intensity around the NP; the probability of spontaneous recombination; the penetration depth of the LSPR into the substrate and the adjustment of the position of the LSPR. Simultaneous action of all these factors leads to Bi3+ emission intensity enhancement and as a result increase energy transfer to the Yb ion